XIII International Conference on Kaon Physics -- Kaon 2025 Mainz

8 Sept 2025, 09:00 → 12 Sept 2025, 15:10 Europe/Zurich

Alte Mensa (Johannes Gutenberg University Mainz)

Letizia Peruzzo (Universität Mainz), Rainer Wanke (Universität Mainz)

The 2025 edition of the International Conference on Kaon Physics will be hosted from 8th to 12th September at the Johannes Gutenberg Univeristy Mainz (Mainz Germany). The XIII edition continues the KAON conference series, originated in 1988, and offers opportunities to discuss experimental and theoretical aspects of kaon physics within the high-energy physics community.

Topics:

• Rare Decays
• CP and T Violation
• CKM Matrix and Flavor Mixing
• Precision Standard Model Tests
• CPT and Quantum Mechanics
• Lepton Universality and Flavor Violation
• Lattice Gauge Theory
• Chiral Perturbation Theory
• Physics Beyond the Standard Model
• Future Opportunities in Kaon Physics

Previous KAON Conferences:

• KAON2022 University of Osaka, Japan
• KAON2019 INFN and University of Perugia, Italy
• KAON2016 University of Birmingham, UK
• KAON2013 University of Michigan, USA
• KAON2009 Tsukuba, Japan
• KAON2007 Frascati, Italy
• KAON2005 Northwestern University, USA
• KAON2001 Pisa, Italy
• KAON99 University of Chicago, USA
• Workshop on Kaon Physics 1996, Orsay, France
• Workshop on Rare Kaon Decay Physics 1991, KEK, Japan
• Rare Decay Symposium 1988, Vancouver, Canada

 

Host Institutions:

Johannes Gutenberg University Mainz:

• Institute of Physics https://www.iph.uni-mainz.de
• PRISMA+ Cluster of Excellence https://www.prisma.uni-mainz.de

 

ConferencePhoto.jpg

ConferencePhoto.pdf

Kaon25Logo.png

KAON25Poster.pdf

PictureMainzCity.pdf

UniMainzCampusMap.pdf

UniMainzDirections.pdf

Monday 8 September

11:00 → 13:00 Registration

13:00 → 14:00 Lunch Break

14:00 → 14:15 Welcome

14:00 Welcome

Speaker: Rainer Wanke (Universität Mainz)

KAON_Welcome_Sep25.pdf

14:15 → 15:30 FCNC Decays: Part 1

14:15 Overview of NA62 K+ ➞ π+ ν ν̄ results

The $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay is a golden mode for flavour physics. Its branching ratio is predicted with high precision by the Standard Model to be less than $10^{-10}$, and this decay mode is highly sensitive to indirect effects of new physics up to the highest mass scales. A new measurement of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This new dataset was collected after modifications to the beamline and detectors and at a higher instantaneous beam intensity with respect to the previous 2016--2018 data taking. Using the NA62 datasets from 2016--2022, a new measurement of $\mathcal{B}(K^{+}\rightarrow\pi^{+}\nu\bar{\nu}) = \left( 13.0^{+ 3.3}_{- 2.9} \right)\times10^{-11} $ is reported, and for the first time the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay is observed with a significance exceeding $5\sigma$.

Speaker: Dr Joel Swallow (INFN - Laboratori Nazionali di Frascati)

KAON2025_NA62_JSwallow_vP.pdf

14:40 Latest status of the KL ➞ π0 ν ν̄ search at the J-PARC KOTO experiment

The KOTO experiment at the J-PARC is dedicated to searching for the rare decay $K_L\rightarrow \pi^0\nu\bar{\nu}$.
In the analysis of the data taken in 2021, no signal candidates were observed, and the world-best limit of $2.2×10^{-9}$ on the branching ratio was set at the 90% confidence level.
After the data taking in 2021, we improved our detectors to further suppress the charged kaon background, which was one of the main backgrounds in the analysis of the data taken in 2016-2018. The charged particle detector was upgraded to make it more sensitive to charged kaons in the beam. In addition, the permanent magnet was installed in the beam line to reduce the flux of charged kaons.
The DAQ system was also upgraded to improve its rate capability. A new trigger was implemented to take more control samples for the precise evaluation of the photon inefficiency in veto detectors, which is crucial to evaluate the number of $K_L \rightarrow 2\pi^0$ backgrounds. After these upgrades, we have accumulated the physics data during 2024-2025.
In this presentation, we will report the current status of the analysis of the 2024-2025 data, which has approximately twice the statistics of the previous analysis.

Speaker: Keita ONO (Osaka University)

kaon2025_ono_final.pdf

15:05 K ➞ π ν ν̄: updated SM predictions and its distributions at NA62

I present resent updates on the SM predictions of the rare K->pi nu nu modes and discuss how the distributions at NA62 can be used to disentangle the contributions from SM-like operators with contributions from NP operators.

Speaker: Emmanuel Stamou (TU Dortmund)

kpinunu_distributions.pdf

15:30 → 16:00 Coffee Break

16:00 → 17:35 FCNC Decays: Part 2

16:00 Correlations between K and B Physics

New Physics is generically expected to mostly couple to third generation fermions to avoid stringent bounds, e.g. from Kaon physics, but should induce correlated deviations also in other flavour observables. Investigating and quantifying such correlations is important to understand the flavour structure of New Physics and to steer the model building. We consider examples of EFT scenarios that are both specific enough to induce interesting correlations while being generic enough to allow us to explore different directions and structures in flavour space as Rank-One flavour violation or $U(2)^5$ flavour symmetry.

Speaker: Claudio TONI (LAPTH, Annecy)

Kaon25_Toni.pdf

16:25 Measurements of EW penguin and LFV B decays with missing energy at Belle and Belle II

The Belle and Belle II experiments have collected a 1.2 ab$^{-1}$ sample of $e^+ e^-\to B\bar{B}$ collisions at a centre-of-mass energy corresponding to the $\Upsilon(4S)$ resonance. These data, with low particle multiplicity and constrained initial state kinematics, are an ideal environment to search for rare electroweak penguin $B$ decays and lepton-flavour-violating $B$ decays to final states with missing energy from neutrinos.
Results from $b\to s\nu\bar{\nu}$ processes and their interpretation are presented. In addition, we present searches for the processes $B\to K^{(*)}\tau^+\tau^-$. Finally, we present our search for the lepton-flavour violating decay $B^0\to K^{*0}\tau^{\pm}\ell^{\mp}$, where $\ell$ is an electron or muon.

Speaker: Meihong Liu (Jilin University & DESY)

Kaon08092025_meihong_v1.pdf

16:50 s ➞ d 𝓁 𝓁 processes beyond the SM: kaons vs hyperons vs high p_T at LHC

We discuss the constraints on new physics affecting s to d l lbar processes from rare kaon and hyperon decays as well as high pT observables at LHC. Although, in general, the constraints from rare kaon decays are the strongest ones, we emphasise the complementarity of all observables in order to cover the complete parameter space of dimension six Wilson coefficients.

Speaker: German Valencia (Monash University (AU))

valencia_kaons.pdf

17:10 Theory K ➞ π 𝓁 𝓁

xxx

Speaker: Nazila Mahmoudi (Universite Claude Bernard Lyon I (FR))

Nazila_Mainz_Kaon2025.pdf

17:35 → 18:50 Hadronic Decays

17:35 K ➞ π π Transitions

After a brief introduction, I review the status and prospects of kaon decay calculations using lattice QCD by the RBC and UKQCD collaborations. The focus is on direct CP violation parameterized by $\epsilon’$. Attention is paid to the sources of error in the calculations and how to reduce them. The total uncertainty for the standard model value is about 2.5 times larger than the experimental world average . RBC/UKQCD are pursuing two independent calculations, one with G-parity boundary conditions, and the other with ordinary periodic boundary conditions. These have different relative strengths and weaknesses which will be highlighted. I will also describe a planned future lattice calculation of the beyond the standard model value of $\epsilon’$ within the standard model effective field theory (SMEFT) framework.

Speaker: Thomas Blum

talk.pptx

18:00 Radiative Kaon decays from a large-Nc perspective

Making predictions for radiative kaon decays is an arduous theoretical
task. While chiral perturbation theory provides a suitable framework, it is seldom predictive for lack of knowledge about the low-energy constants involved. In this presentation it will be discussed how QCD with a large number of colours can be helpful in improving our qualitative and quantitative understanding of these decay amplitudes. The amplitudes for the decay modes $K \to \pi \ell^+ \ell^-$ will be used as illustrative examples.

Speaker: Marc KNECHT

MK_KAON_2025.pdf

18:25 Scattering π–π and K–π

xxx

Speaker: Felix Benjamin Erben

Erben_Kaons_Mainz25.pdf

19:00 → 21:00 Welcome Reception

Tuesday 9 September

09:00 → 09:50 Other Flavours

09:00 B anomalies: status and prospects from LHCb

The study of $b$-quark decays plays a central role in exploring the origin of flavour. Thanks to its large mass, it allows for precise theoretical predictions, and its ability to decay into all four lighter quarks and all three charged leptons provides access to a broad range of flavour observables. Over the last 15 years, LHCb has been collecting an increasingly larger dataset of beauty hadron decays and has tested the Standard Model up to ever higher energy scales. This exploration uncovered a set of intriguing anomalies in semileptonic $b \to c$ and $b \to s$ transitions. In this talk, I will present recent LHCb results and discuss the prospects enabled by the experiment’s recent upgrade.

Speaker: Martino Borsato (Universita & INFN, Milano-Bicocca (IT))

KAON2025_MBorsato.pdf

09:25 Search for BSM in final states with leptons with ATLAS

Many BSM scenarios predict the existent of new, heavier leptonic states, or particles that can decay to final states with many leptons. This talk will discuss recent ATLAS results for the search of BSM scenarios in final states with leptons with the ATLAS detector. Relevance will be given to searches for the production of the so-called vector-like leptons, heavy neutral leptons, leptoquarks, and of the supersymmetric partners of the leptons (the sleptons).

Speaker: Shalini Epari (University of Montreal)

Presentation_KAON25_Shalini.pdf

09:50 → 10:40 CKM Matrix

09:50 Current Status of Vcb and Vub

The CKM matrix elements Vcb and Vub are key inputs for the prediction of flavour-changing processes and for testing the Standard Model at the precision frontier. Despite significant progress on both experimental and theoretical fronts, persistent tensions remain between their inclusive and exclusive determinations. In this talk, I will review the current status of these determinations, with a particular focus on their phenomenological impact, including implications for rare kaon decays.

Speaker: Marzia Bordone (University of Zürich and CERN)

Bordone_Kaon.pdf

10:15 Measurements of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix at Belle and Belle II

The Belle and Belle II experiment have collected a 1.2 ab$^{-1}$ sample of $e^+e^-\to B\bar{B}$ decays at a centre-of-mass energy corresponding to the $\Upsilon(4S)$ resonance. The SuperKEKB collider is asymmetric, providing a boost to the $B$ mesons in the laboratory frame, so we can perform measurements of time-dependent $C\!P$ violation. Among the new results, we measure $CP$-violating parameters related to the determination of the least well-known angle of the unitarity triangle $\alpha$ using the decay $B^0\to\rho^+\rho^-$. In addition, we present both inclusive and exclusive measurements of semileptonic and lepton $B$ decay that can determine magnitude of the matrix elements $\left|V_{cb}\right|$ and $\left|V_{ub}\right|$.

Speaker: Sebastiano Raiz

CKM_measurements_Belle2.pdf

10:40 → 11:10 Coffee Break

11:10 → 13:00 New Physics: Part 1

11:10 Missing Energy in K Decays

Speaker: Michele Tammaro

KAON25_Tammaro.pdf

11:35 New physics searches with kaon and pion decays at NA62

Searches for the decays $K^{+}\rightarrow\pi^{+}X$ and $\pi^{+}\rightarrow e^{+}N$ are presented using data collected in 2016-2022 and 2017-2024, respectively. Results are interpreted to constrain a range of new physics scenarios covering all four portal model scenarios. Upper limits on the $K^{+}\rightarrow\pi^{+}X$ branching ratio are established at the $10^{-11}$ level, providing constraints on dark photon, scalar and ALP couplings. From the search for heavy neutral lepton production in $\pi^{+}\to e^+N$ decays of beam pions, upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the $10^{-8}$ level over the heavy neutral lepton mass range 95--126~MeV/$c^2$.

Speaker: Mauro Raggi (CERN and Sapienza)

KtoPiX_Kaon25.pdf

11:55 Performing Novel Searches at KOTO

Due to the high sensitivity required to study the $K_L\rightarrow \pi^0\nu\overline{\nu}$ decay, KOTO is in a unique position to study rare $K_L$ interactions, such as sub-GeV quark coupling to dark matter. One avenue to study this is the mode $K_L \rightarrow \pi^0\pi^0X$, where $X\rightarrow\gamma\gamma$, and another is the ``dark pair'' $K_L\rightarrow XX$ where $X \rightarrow \gamma\gamma$. The former was studied in the E391a experiment at KEK in the $X$ mass region 194.3-219.3 MeV. In the analysis of 2021 data, the sensitivity is improved up to an order of magnitude on the previous search while the scope is increased to include $X$ mass in the range 160-220 MeV/c$^2$. The analysis includes the first search for the rare decay $K_L\rightarrow \pi^0\pi^0\gamma\gamma$ which is described by ChPT (Funck, 1993). The results of the analysis on $K_L\rightarrow \pi^0\pi^0 X$ and $K_L\rightarrow \pi^0\pi^0\gamma\gamma$ on data taken in 2021 will be discussed in this presentation. The latter analysis was performed on the data taken in 2018, and is also the worlds first search. No events were observed in the mass range of 40–110 MeV/c$^2$ and 210–240 MeV/c$^2$. This allowed KOTO to set upper limits on the branching fractions as BR⁡($K_L\rightarrow XX) < (1-4)\times 10^{-7}$ and BR⁡($K_L\rightarrow XX) < (1-2)\times 10^{-6}$ at 90% C.L. for the two masses regions respectively. This presentation will go into details on the results of these novel analyses, as well as discuss future KOTO analysis prospects.

Speaker: Joseph Redeker

Kaon2025_DMSearch_KOTO.pdf

12:15 Overview of new physics searches in beam dump at NA62

The NA62 experiment at CERN, designed to measure the highly-suppressed decay $K^{+} \to \pi^{+}\nu\bar{\nu}$, has the capability to collect data in a beam-dump mode, by removing the kaon production target and moving the upstream collimators into a "closed'' position. In this configuration 400 GeV protons are dumped on an absorber and New Physics (NP) particles, including dark photons, dark scalars and axion-like particles, may be produced and reach a decay volume beginning 80 m downstream of the absorber. More than $10^{17}$ protons on target have been collected in "beam-dump'' mode by NA62 in 2021. Results from analysis of this data, with a particular emphasis on Dark Photon and Axion-like particle Models, are presented. We also report recent results on the first NA62 search for long-lived NP particles decaying in flight to hadronic final states based on a blind analysis of a sample of $1.4 \times 10^{17}$ protons on dump collected in 2021.

Speaker: Jonathan Leon Schubert (Max Planck Society (DE))

KAON25_DumpModeSearches_NA62_Schubert_minimalAnimation.pdf

12:35 Searching for new physics with low energy pions: the PIENU and PIONEER experiments

The PIENU experiment at TRIUMF has provided, to date, the most precise experimental determination of R$^{\pi}_{e/\mu}$ , the ratio of pions decaying to positrons relative to muons. While more than an order of magnitude less precise that the Standard Model (SM) calculation, the PIENU result is a precise test of the universality of charged
leptons interaction, a key principle of the Standard Model (SM), constrains a large range of new physics scenario, and allows dedicated searches for exotics such as sterile neutrinos. I’ll go over a short overview of R$^{\pi}_{e/\mu}$ measurements and introduce the next generation precision pion decay experiment in the making: PIONEER.
This newly proposed experiment aims at pushing the boundaries of precision on R$^{\pi}_{e/\mu}$ and expanding the physics reach by improving on the measurement of the very rare pion beta decay $\pi^+\rightarrow \pi^0 e^+ \nu$. This will provide a new and competitive input to the determination of |Vud|, an element of the Cabibbo- Kobayashi-Maskawa (CKM) quark-mixing matrix.

Speaker: Chloe Malbrunot (TRIUMF)

PIONEER_PIENU_Malbrunot_KAON2025.pdf

13:00 → 14:00 Lunch Break

14:00 → 16:00 Guided Visit MAMI/TRIGA: MAMI/TRIGA Visit

14:00 INFO

Speaker: Letizia Peruzzo (Johannes Gutenberg Universitaet Mainz (DE))

Visit_Info.pdf

16:00 → 16:30 Coffee Break

16:30 → 17:10 New Physics: Part 2

16:30 Probing QCD-axion couplings with Three-Body Kaon decays

We investigate kaon three-body decays to ππa and μμa, where a is a new low-mass pseudoscalar. The ππa channel occurs at tree level, while μμa is loop-suppressed but turns out to be finite. These modes take advantage of the excellent tracking performance at LHCb. We outline an estimation strategy for the sensitivity that future searches could achieve, based on the present uncertainties in the leading irreducible backgrounds. Our results suggest that dedicated analyses could probe Peccei–Quinn scales in the range 10⁴–10⁶ TeV. A comparison with existing searches supports this conclusion, and we point out that reconstruction efficiency maps naturally align high efficiencies with regions of larger yields in Dalitz plots.

Speaker: Diego Guadagnoli (LAPTh Annecy)

Talk_KAON_09_2025_w_tr.pdf

16:50 A search for the dark photon in the OKA experiment

A search for the massless dark photon in the decay $K^{+} \to \pi^{+} \pi^{0} \overline{\gamma}$ is performed with the OKA detector exposed to 17.7 GeV/c RF separated secondary beam of the U70 Proton Synchrotron.
The missing mass analysis is performed to search for the stable invisible dark photon in the final state.
No signal is observed, and a preliminary result for the 90% C.L. upper limit on the branching ratio of the decay Br<$0.9\cdot10^{-6}$ is obtained, based on a realistic matrix element of the process. Currently it is the best limit for this decay channel.

Speaker: Alexander Sadovskiy (NRC “Kurchatov Institute”-IHEP)

Kaon2025_v5.pdf

17:10 → 18:55 LFV & LNV

17:10 Muon to electron Transitions

$\mu \to e$ flavour change is tightly constrained by $\mu \to e \gamma$, $\mu \to 3e$ and $\mu - e$ conversion on nuclei. Furthermore, the experimental sensitivity to these processes will improve by orders of magnitude in coming years.
This motivates the bottom-up, EFT study of what can be learned from these three processes, which will be described in this talk and allows to address questions such as "Can these processes exclude models?", or "If $\mu-e$ flavour change exisits, will we see it?"

Speaker: Sacha Davidson (Centre National de la Recherche Scientifique (FR))

DavidsonKAON.pdf

17:35 Search for cLFV with COMET experiment at J-PARC

The COMET experiment at J-PARC facility in JAPAN is designed to search for charged lepton flavour violation (cLFV), one of the most promising way of looking for physics beyond the Standard Model. Specifically, it will search for the coherent, neutrinoless conversion of a muon to an electron in the field of an aluminium nucleus, a process that is forbidden in the Standard Model and highly suppressed in most of its minimal extensions that accommodate neutrino oscillations.

With an expected single event sensitivity (SES) of the order of
$10^{−17}$, COMET aims to improve the current limit on the conversion rate by four orders of magnitude. This presentation will review the physics motivation, the experimental design, and the current status of the experiment. The focus will be on recent progress in the construction of the facility and of the detectors for the first phase of the experiment, COMET Phase-I, foreseen to start data taking in 2027.

Speaker: Dr Cristina Carloganu (LPC/IN2P3/CNRS)

250910-KAON2025-Carloganu.pdf

17:55 Searches for dark sector particles, and LFV in τ decays at Belle and Belle II

The Belle and Belle II experiments have collected a combined sample of $1.6~\mathrm{ab}^{-1}$ of $e^+e^-$ collision data at centre-of-mass energies near the $\Upsilon(nS)$ resonances. These data have constrained kinematics and low multiplicity, which allow searches for dark sector particles in the mass range from a few MeV to 10 GeV. Using a 365 fb$^{-1}$ sample collected by Belle II, we search for inelastic dark matter and an $Z^{\prime}$ that decays to invisible particles. In addition, we search for the dark photon decay $A^{\prime}\to \mu^+\mu^-$ using the full Belle II data set of $500~fb^{-1}$ Using a 711 fb$^{-1}$ sample collected by Belle, we search for $B\to h + \mathrm{invisible}$ decays, where $h$ is a $\pi$, $K$, $D$, $D_{s}$ or $p$, and $B\to Ka$, where $a$ is an axion-like particle. The combined Belle and Belle II data sample contains approximately 1.5 billion $e^+e^-\to \tau^+\tau^{-}$ events, which we use to search for lepton-flavour violating decays. We present searches for $\tau\to\ell\gamma$, tau decay to three charged leptons, $\tau^-\to K_{\rm S}^0\ell^{-}$, and $\tau^-\to \ell^-\alpha$, where $\alpha$ is an invisible scalar particle.

Speaker: Arthur Thaller

thaller_kaon25_v4.pdf

18:15 Searches for lepton flavour and number violation at NA62

The NA62 experiment at CERN has collected a large sample of $K^+$ decays in flight during Run 1 in 2016-2018 and the ongoing Run 2 which started in 2021. Dedicated lepton-pair trigger lines have enabled a broad rare and forbidden decay programme. A review of searches for lepton flavour and number violating $K^+$ and $\pi^0$ decays with the NA62 lepton-pair dataset collected in 2016-2018 is presented. Ten final states have been investigated, improving on the world data in each case. Upper limits of the decay branching fractions are obtained typically at the $10^{-11}$ level, and the sensitivities are not limited by backgrounds. Prospects of the searches with the full NA62 dataset are also discussed.

Speaker: Evgueni Goudzovski (University of Birmingham)

goudzovski_090925.pdf

18:35 Search for rare processes and LFV decays of the Higgs boson at ATLAS

The Standard Model predicts several rare Higgs boson processes, including decays into a Z boson and a photon, a low-mass lepton pair and a photon, or a meson and a photon. Observing these rare decays would offer new and complementary insights into the Higgs boson's coupling structure beyond the more commonly studied channels. In addition, searches for lepton-flavor-violating decays of the Higgs boson are performed, where any observation would provide unambiguous evidence of physics beyond the Standard Model. This talk presents several recent results from the ATLAS experiment based on proton-proton collision data collected in Run2 at \sqrt{s} = 13 TeV, , with the inclusion of available Run 3 results where relevant.

Speaker: Magda Diamantopoulou (Carleton University (CA))

HiggsMD_05092025.pdf

Wednesday 10 September

09:00 → 09:25 Other Flavours: Wednesday Part

09:00 Flavour Model Building

The flavor puzzles remain among the most compelling open questions in particle physics. The pronounced hierarchies in charged fermion masses and mixings define the Standard Model (SM) flavor puzzle, highlighting a deep structural mystery pointing beyond the SM. Meanwhile, the lack of deviations in precision measurements of flavor-changing neutral currents imposes tight constraints on TeV-scale new physics, creating the new physics flavor puzzle. This talk highlights recent progress in flavor model building, focusing on approaches to address these puzzles in the quark sector.

Speaker: Admir Greljo (Universitaet Basel (CH))

Greljo-Kaon-2025.pdf

09:25 → 10:10 Rare Decays

09:25 Rare kaon decays at LHCb

Rare decays are fundamental probes for physics beyond the Standard Model, and the expanding LHCb program on strange physics provides unique and complementary information with respect to the beauty and charm sectors. Recent results of rare kaon decays at LHCb will be summarized and prospects for strange physics with LHCb Upgrade I, where the change in trigger strategy will allow for a much higher selection efficiency of kaon decays, will be outlined.

Speaker: Dr Luis Miguel Garcia Martin (EPFL - Ecole Polytechnique Federale Lausanne (CH))

KaonsAtLHCb_Kaons.pdf

09:50 Measurement of the K+ ➞ μ+ ν μ+ μ– decay at NA62

The first observation of the $K^+\to\mu^+\nu\mu^+\mu^-$ decay with the 2016-2024 dataset collected by the NA62 experiment is presented. The branching ratio of the process is measured relative to the normalisation channel $K^+\to\pi^+\mu^+\mu^-$, taking advantage of similar final states and both decays being collected by the same trigger stream. The dominant background is the $K^+\to \pi^+\pi^+\pi^-$ decay with three in-flight $\pi^\pm\to \mu^\pm\nu$ decays. The observation of the signal mode is possible due to complete kinematic information about the event provided by a beam spectrometer for the kaon and a downstream spectrometer for the muons. This, together with sub-ns timing resolution of the NA62 detector systems, allows to control a background process with a branching ratio higher by six orders of magnitude than the signal.

Speaker: Adam Mateusz Tomczak (University of Birmingham (GB))

KtoMnMM_Tomczak_KAON25_v3-1.pdf

10:10 → 10:30 Hyperons

10:10 Latest results on hyperon decays at LHCb

The latest results in hyperon decays from the LHCb collaboration will be shown.The $\Sigma^+ \to p \mu^+ \mu^-$ decay is observed for the first time at the LHCb experiment. This is a flavour changing neutral current sensitive to physics beyond the Standard Model, which could modify its properties. In particular the HyperCP experiment years ago presented an evidence of this decay with a hint of a possible unknown intermediate particle. This was excluded by LHCb already in 2018. This new measurement presents a highly significant observation and a measurement of its integrated branching fraction and dimuon invariant mass distribution. This is the rarest baryon decay ever observed.
Additionally, the sensitivity of these observables to Chiral Perturbation Theory parameters will be discussed. Prospects for additional observables, such as a CP violation measurement, will also be presented. Finally, the latest measurement on $\Lambda \to p \mu \nu$ branching fraction and test of LFU in semileptonic hyperon decays will be shown.

Speaker: Gabriele Martelli (Universita e INFN, Perugia (IT))

KAON2025_100925_GMartelli.pdf

10:30 → 11:00 Coffee Break

11:00 → 12:45 CP, T & CPT

11:00 Electric Dipole Moments

I briefly review electric dipole moments as precision tests of CP violation. I will discuss the complementarity to LHC constraints in an effective theory framework, and present some recent results in the two-Higgs doublet model

Speaker: Joachim Brod (University of Cincinnati)

edm.pdf

11:25 RI-(S)MOM to MS-bar Conversion for B̂_K at Two-loop Order

The Kaon bag parameter, $\hat{B}_K$, is a key non-perturbative ingredient in the search for new physics through CP-violation. It parameterizes the QCD hadronic matrix element of the effective weak $\Delta S = 2$ four-quark operator which can be computed non-perturbatively on the lattice. In this talk I will present the conversion factors for $\hat{B}_K$ between the RI-(S)MOM and the $\overline{\text{MS}}$ schemes at two-loop order. In addition, I will discuss our result for $\hat{B}^{(f=3)}_K = 0.7627(60)$, which comprises the complete set of 3- and 4-flavour lattice results and implies an updated value for the indirect CP violation in the Kaon system $|\epsilon_K|= 2.171(65)_{\text{pert.}}(71)_{\text{non-pert.}}(153)_{\text{param.}} ×10^{−3}$. This talk is based on preprint arXiv:2411.19861.

Speaker: Sandra Kvedaraite (University of Granada)

KAON25 Talk.pdf

11:45 The "back from the future" quantum effect in entangled neutral kaons at KLOE/KLOE-2

A novel quantum phenomenon associated to
a peculiar time correlation between entangled neutral kaons produced at a 𝜙-factory is discussed: the past state of the first decayed kaon, when it was still entangled before its decay,
is post-tagged by the result and the time of the future observation on the other kaon decay.
This surprising “back from the future” effect is fully observable and preliminary results obtained from the analysis of data collected
by the KLOE experiment at the DAΦNE collider are presented, showing experimental evidence of this new effect.

Speaker: Antonio Di Domenico (Sapienza Universita e INFN, Roma I (IT))

didomenico_KAON2015_9_web.pdf

12:05 Updates on CP symmetry tests at KLOE-2

Measurement of $K_S\to3\pi^0$ decay branching ratio and determination of the ratio of amplitudes for $K_S\to3\pi^0$ to $K_L\to3\pi^0$ characterizes the CP violation. The 2013 KLOE results are now being updated with the usage of the KLOE-2 data sample increasing the available statistics to 5fb$^{-1}$. In this talk the status of the data analysis will be presented.

The ratio of strengths of direct and indirect CP violation, namely $\epsilon'/\epsilon$, is another observable used for CP symmetry study. As it was never determined simultaneously for its real and imaginary part, such measurement would be highly appreciated. The idea of such measurement together with the analysis preparation will be reported.

Speaker: Szymon Gamrat

20250910(1).pdf

12:25 New proposal to search for T-violating μ+ polarization in K+ ➞ π0 μ+ ν decays using stopped kaons

Time reversal symmetry has long been a subject of interest from pre-modern physics time, since it implies the reversibility of motion. In the $K^+ \to π^0 \mu^+ \nu$ ($K_{\mu3}$) decay, the transverse muon polarization ($P_T$) is defined as the polarization component perpendicular to the decay plane. A non-vanishing value of $P_T$ provides clear evidence for T-violation under the condition that spurious effects from final state interactions are negligibly small. We are now proposing a new T-violation experiment to achieve $ΔP_T~10^(-5)$ at the J-PARC Hadron Hall without using a magnetic spectrometer. The most important characteristic of the new experiment is the measurements of the muon momentum vector, the $π^0$ momentum vector, and the muon polarization by the same highly segmented sequential electro-magnetic calorimeter surrounding the $K^+$ stopping target. Here it should be noted that one of key issues in the experiment is the choice of a scintillation material which can preserve the muon spin polarization for a reasonably long time [1].
A test experiment to measure residual muon polarization in CeF3, LaF3, PrF3, and NdF3 scintillating crystals was performed using a 100% polarized muon beam at J-PARC MLF. In the longitudinal field of 140 Gauss, the muon polarization in these materials was obtained to be 90% at room temperature, which is high enough to perform the new T-violation experiment [1-3]. Since the calorimeter should be placed very close to the $K^+$ beam line, a single rate for each module will be very high and the timing resolution should be better than 1 ns to reduce accidental background effects. The timing resolution using a CeF3 crystal with the size of $20\times 20 \times 20$ mm$^3$ was obtained to be ~100 ps using solar-blind phototubes. The time interval of the two CeF3 detector signals generated by the cosmic ray passage was measured. The timing resolution is sufficiently good, and the accidental backgrounds must be harmless in the proposed T-violation experiment.
The $\mu^+$ polarization can be determined by the delayed $e^+$ signals from the $\mu^+$ decay detected by the calorimeter module around the muon stop. The experimental method to measure the $e^+$ asymmetry determination by selecting the $π^0$-forward and backward events is adopted to suppress systematic uncertainties. Furthermore, the analyzing power in the polarization measurement should be improved by measuring the $e^+$ energy using the calorimeter [3] because the magnitude of the $e^+$ asymmetry depends on the $e^+$ energy, while only the energy　integrated asymmetry is obtained in the standard polarization measurement. The dedicated analysis method has been developed by separating events into partial energy regions and optimizing weight parameters to averaging the $e^+$ asymmetry in each bin.
In this talk, some details of the future T-violation experiment, the results of the test experiment to determine the residual polarization in CeF3, LaF3, PrF3, and NdF3 materials, the timing resolution of the CeF3 detector, and an increase in the analyzing power in the polarization measurement will be reported.

References
[1] S. Shimizu et al., Nucl. Instrum. Methods A 945 (2019) 162587.
[2] K. Horie et al., Nucl. Instrum. Methods A 1037 (2022) 166932.
[3] Horie et al., Nucl. Instrum. Methods A 1066 (2024) 169606.

Speaker: Suguru Shimizu (University of Osaka)

kaon2025_shimizu.pdf

12:45 → 13:00 Conference Photo

ConferencePhoto.pdf

13:00 → 14:30 Lunch Break

14:30 → 18:30 Excursion

ConferencePhoto.jpg

KAON_Kloster_Eberbach.pdf

Thursday 11 September

09:00 → 11:10 Future: Part 1

09:00 Overview on present and future Flavor Physics

Overview on Flavour Physics

Speaker: Gino Isidori (University of Zurich (CH))

Kaon25.pdf

09:35 Unleashing NA62's full physics potential: status and prospects

he NA62 experiment at CERN collected a large sample of in-flight $K^+$ decays during Run 1 (2016–2018) and resumed data taking for Run 2 in 2021, with operations expected to continue through 2026. This talk will present the current status of several NA62 analyses - including precision tests of the CKM matrix based on a dedicated low-intensity data sample collected in 2024–2025 - and discuss their prospects using the full Run 1+2 data set.

Speaker: Karim Massri (Lancaster University (GB))

KAON2025_NA62_prospects.pdf

10:00 KOTO status and prospects

The J-PARC KOTO experiment, which searches for the rare neutral kaon decay $K_L \to \pi^0 \nu \overline{\nu}$, is now in the stage of stable data accumulation. As for the 30GeV Main Ring accelerator, the beam power of the slow extraction reaches more than 90kW recently, which is close to an immediate goal intensity. On the experimental side, the detector and the DAQ system are well established, and no major changes are planned at the moment. We can thus foresee what can be achieved in coming years realistically. In this talk, we summarize the KOTO status and discuss prospects particularly on the sensitivity reach of the $K_L \to \pi^0 \nu \overline{\nu}$ search and other physics opportunities.

Speaker: Tadashi Nomura

koto-250911-rel.pdf

10:25 Current status and future prospects of J-PARC Hadron Experimental Facility

The Hadron Experimental Facility (HEF) at J-PARC utilizes a slowly extracted 30-GeV proton beam and provides a variety of secondary particle beams such as pions, kaons, and autiprotons for particle and nuclear physics experiments. The KL beam line serves a neutral kaon beam for a kaon rare decay experiment, KOTO. The K1.8 and K1.8BR beam lines provide charged secondary beams with the momentum up to 2.0 and 1.1 GeV/c, respectively, mainly for strangeness nuclear physics experiments. In addition, a part of the primary protons are split to the B line and delivered directly to hadron physics experiments. Moreover, 8-GeV proton beam is also provided for the mu-e conversion search experiment, COMET. The primary beam power of 93 kW has been achieved so far, and the R&D of a new production target capable of 150-kW beam power is now underway.

Recently, the extension project of the HEF (HEF-ex) are discussed. In this project, the area of the experimental hall will be doubled and the second target station and new secondary beam lines will be constructed, including the KL2 for a next-generation neutral kaon rare decay experiment.

In this paper, the current status of the HEF is briefly summarized and the future plan including the HEF-ex project is presented.

Speaker: Prof. Hitoshi Takahashi (Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization)

j-parc_hadron.pdf

10:45 KOTO II experiment to measure the branching ratio of KL ➞ π0 ν ν̄ at J-PARC

We proposed the KOTO II experiment at J-PARC to measure the branching ratio of the rare decay $K_L\to\pi^0\nu\bar{\nu}$ with 82 members from 11 countries. KOTO II is a next-generation experiment aiming at the operation in the 2030s. A new beamline with a 5-degree extraction angle and a new detector with a 12-meter-long signal decay region and a 3-meter diameter calorimeter were designed. A discovery of the decay with $5\sigma$ significance is achievable for the SM value of the branching ratio $3\times 10^{-11}$. An indication of new physics is possible when a measured branching ratio differs by 40% or more from the SM value.The design, expected performance, and current status of the KOTO II experiment will be presented.

Speaker: Hajime Nanjo (Osaka University (JP))

KAON2025_nanjo.pdf

11:10 → 11:40 Coffee Break

11:40 → 13:00 Future: Part 2

11:40 Kaon experiments at next-Generation XFEL facilities

X-ray Free-Electron Laser (XFEL) facilities are state-of-the-art instruments that produce extremely intense, coherent, and ultrafast X-ray pulses. By combining principles of accelerator and laser physics, they offer unprecedented spatial and temporal resolution, enabling breakthroughs in areas such as protein structure determination, atomic-scale imaging, high-energy density physics, and plasma studies.
Several next-generation XFEL facilities are planned worldwide over the coming decade. This talk will explore the concept, advantages, and physics potential of a kaon experiment operating at one of these future XFELs, highlighting the novel opportunities such an environment could offer for ultra-rare kaon physics.

Speaker: Cristina Lazzeroni (University of Birmingham (GB))

KAON2025_Lazzeroni.pdf

12:00 Round Table Discussion

Round Table Discussion on Future of Kaon Physics

Speakers: Andreas Juettner, Andrzej Buras (Munich), Augusto Ceccucci (CERN), Gino Isidori (University of Zurich (CH))

Kaons_2025_Round_Table.pdf

13:00 → 14:00 Lunch Break

14:00 → 15:30 Poster Session

14:00 High-Precision Kaonic Atom Measurements with SIDDHARTA-2 at the DAΦNE Collider

Low-energy QCD, the theory within the Standard Model that describes the strong interaction, still lacks fundamental experimental input to advance its understanding. Among these contributions, kaonic atom X-ray spectroscopy stands out as a unique gateway for probing the interaction between antikaons and nucleons at threshold energy. This research has significant implications for particle and nuclear physics, as well as astrophysics, particularly in understanding neutron stars and their equation of state.

Kaonic atoms can also be exploited to perform precision QED measurements, such as those providing the charged kaon mass—an open puzzle in particle physics.

By combining the exceptional quality of the low-energy kaon beam provided by the DAΦNE collider at INFN-LNF (Italy) with cutting-edge experimental techniques, such as fast and highly precise X-ray spectroscopy detectors like Silicon Drift Detectors, the SIDDHARTA-2 collaboration has performed groundbreaking measurements of a series of kaonic atom X-ray transitions, including the first-ever measurement of kaonic deuterium.

I will introduce the SIDDHARTA-2 scientific case, the experiment, and the results obtained in measuring various kaonic atoms, such as helium-4 and neon, along with a preliminary outcome on kaonic deuterium.

I will also present future plans, including the EXKALIBUR proposal, as the experiments at the DAΦNE collider represent a unique opportunity to finally unravel the secrets of the strong interaction in the strangeness sector.

Speaker: Francesco Clozza (INFN-LNF)

14:25 Novel CZT Detectors for kaonic atoms spectroscopy

Kaonic atoms spectroscopy provides essential observables for investigating low-energy strong interactions in systems with strangeness. I shall present an overview of the SIDDHARTA-2 collaboration’s efforts in this field, with a particular focus on the development and first use of a novel Cadmium-Zinc-Telluride (CZT) detector system for studying intermediate-mass kaonic atoms.
This innovative detection system, applied for the first time in fundamental physics research at a collider, extends the accessible energy range of kaonic atoms spectroscopy to the hundreds of keV. Initial tests have demonstrated that its excellent energy resolution, efficient background rejection, and precise timing capabilities make it highly suitable for exotic atom measurements.
During the first data-taking campaign at the DAΦNE collider in Italy, the collaboration successfully measured kaonic fluorine and kaonic aluminium transitions, highlighting the detector’s potential for advancing kaonic atom studies. These results pave the way for further applications at DAΦNE and J-PARC in Japan.
Ultimately, these developments aim to refine our understanding of kaon-multinucleon low-energy strong interactions by enabling high-precision measurements of intermediate-mass kaonic atoms.

Speaker: Francesco Artibani (INFN-LNF)

15:05 A fast, compact, highly granular small-angle calorimeter for the KOTO II beam-hole photon veto

The lead-aerogel Cherenkov detector used as the beam-hole photon veto (BHPV) has performed well in KOTO, but for high-intensity operation at KOTO II, a detector with better time resolution would be desirable to reduce losses from the interactions of beam particles in accidental coincidence. As an alternative, a compact, high-granularity small-angle calorimeter (SAC) is under study. The SAC design utilizes layers of ultrafast lead tungstate (PWO-UF) crystals offering superior time resolution and radiation tolerance, with high granularity enabling effective neutron/photon discrimination to reduce accidental vetoes. Tests with PWO-UF and PbF₂ crystals have demonstrated promising photoelectron yields and timing performance, and radiation exposure studies have confirmed the durability of PWO-UF. A new prototype with full-size crystals and fast, radiation-hard photomultipliers is now under construction for testing at the CERN PS in late summer. Preliminary results from this beam test will be presented.

Speaker: Matthew Moulson (INFN e Laboratori Nazionali di Frascati (IT))

15:05 A new calorimetric PID algorithm for the K+ ➞ π+ ν ν̄ decay at NA62

The first observation of the golden mode $K^+\to\pi^+\nu\bar{\nu}$ with a significance exceeding $5\sigma$ was recently achieved using data collected in 2016-2022 by the NA62 experiment at the CERN SPS. With the 2023-2024 dataset, double the sample size is expected with respect to the 2016-2022 dataset. For analysis of the latest data NA62 is exploring recent advancements in machine learning to improve sensitivity and background rejection. A new calorimetric particle identification algorithm utilising a combined dense and convolutional neural network is presented. Improved background rejection is achieved with an updated boosted decision tree.

Speaker: Chandler Baynham Kenworthy (University of Birmingham (GB))

15:05 Chasing hints from flavor physics at the energy frontier with the ATLAS detector

Hints from the flavor sector—such as anomalies in B-meson decays and inconsistencies in CKM unitarity—offer clues about what new physics beyond the Standard Model might look like. Among the most motivated scenarios are models predicting vector-like leptons and/or leptoquarks. This poster highlights recent results from the ATLAS experiment targeting these possibilities, using data collected during Run 2 of the LHC (2015–2018). These searches leverage state-of-the-art analysis techniques and robust background estimation methods, and in some cases reveal intriguing excesses that motivate further investigation with future data from the experiment.

Speaker: Shalini Epari (University of Montreal)

15:05 Development of a Calorimeter for the KOTO ll experiment

The KOTO ll experiment is planned as a next-generation successor to the KOTO experiment. We have been studying a Shashlyk-type calorimeter for the KOTO ll experiment. The Shashlyk-type calorimeter consists of alternating layers of lead and plastic scintillator tiles, penetrated by wavelength-shifting (WLS) fibers. To investigate its performance, a test setup composed of 10 scintillator tiles and 36 WLS fibers (without lead plates) was constructed. The light yield and timing resolution of the test counter were evaluated with cosmic rays. With these results, the energy and the timing resolutions of the calorimeter were estimated using simulations by taking into account the attenuation of photons propagating in the WLS fibers. We will report on these results.

Speaker: Mei Homma

15:05 Generative AI for High-Resolution Shower Reconstruction

We present an application of generative AI to improve shower profile modeling in grid-style detectors, with a focus on the CsI calorimeter used in the KOTO experiment. The core idea is to employ generative models to reconstruct electromagnetic shower patterns at a resolution finer than the detector’s actual segmentation. This approach enables the generation of high-resolution profiles that recover spatial features often lost in measurements. These enhanced profiles have the potential to improve analyses that rely on shower shape information, such as the reconstruction of the incident particle’s direction and particle identification.

Speaker: Mr Yu-Sheng Liu (National Kaohsiung Normal University)

15:05 Improvements of the High Level Trigger in the KOTO experiment

We are searching for the rare decay $K_L\to\pi^0\nu\bar{\nu}$ in the J-PARC KOTO experiment,
The High Level Trigger (HLT) in the DAQ system was developed with GPUs in 2024. The HLT is composed of two types of computers, Computing Nodes and Disk Nodes. Computing Nodes select events and compress waveforms using GPUs. Disk Nodes store the data temporarily, and the data on the Disk node is transferred to the computing cluster on the KEK 70 km far from the J-PARC.
In 2024, the number of Disk Nodes was one, and the performance of writing to disks on the Disk Node made a bottleneck in some special data taking with high rate.
In the data taking in 2025, an additional Disk node was installed to solve this issue. After this addition of the Disk Node, we evaluated the capability of the whole HLT to handle the trigger rate with beam data.
A new GPU algorithm to monitor a hit rate on a detector was implemented in the HLT. Accidental losses resulting from variations in beam intensity during the beam spill can be evaluated by comparing hit rates between two accidental triggers: a random trigger and a beam-intensity biased random trigger. This monitor can be used to make a quick feedback to the accelerator staffs, and it was actually used to select a better beam configuration in our data taking.
We report the capability of the trigger rate and the new monitoring system on the HLT.

Speaker: Daiki Ogawa (Osaka University)

15:05 Measurement of the K+ ➞ π+ π- μ+ ν decay at NA62

Motivation, prospects, and analysis status of the rare decay $K^+ \to \pi^+ \pi^- \mu^+ \nu$ at NA62 using the data collected in 2017-2018 will be presented.

Speaker: Tomas Velas (Comenius University (SK))

15:05 Measurement of the πo ➞ 4e decay at NA62

Motivation, prospects, and analysis status of the rare decay $\pi^0 \to e^+ e^- e^+ e^-$ at NA62 using the data collected in 2017-2024 will be presented.

Speaker: Michal Lelak (Charles University (CZ))

15:05 Numerical evaluation of improvement of statistical uncertainties for muon polarimeter to search for T-violating μ+ polarization in K+ ➞ πo μ+ ν decay

Time reversal symmetry has long been a subject of interest from pre-modern physics time, since it implies the reversibility of motion. In the $K^+ \to \pi^o\mu^+ \nu$ ($K_{\mu3}$) decay, the transverse muon polarization (𝑃𝑇) is defined as the polarization component perpendicular to the decay plane. A non-vanishing value of 𝑃𝑇 provides clear evidence for T-violation under the condition that spurious effects from final state interactions are negligibly small. We are now proposing a new T-violation experiment to achieve $\Delta PT \sim 10^{-5}$ at the J-PARC Hadron Hall without using a magnetic spectrometer. The most important characteristic of the new experiment is the measurements of the muon momentum vector, the $\pi^o$ momentum vector, and the muon polarization by the same highly segmented sequential electro-magnetic calorimeter surrounding the $K^+$ stopping target. Here it should be noted that one of key issues in the experiment is the choice of a scintillation material which can preserve the muon spin polarization for a reasonably long time [1-3].
The $\mu^+$ polarization can be determined by the delayed $e^+$ signals from the $\mu^+$ decay detected by the calorimeter module around the muon stop, as shown in Fig.1. The experimental method to measure the $e^+$ asymmetry by selecting events with $\pi^o$ going forward and backward direction is adopted to suppress systematic uncertainties.

Furthermore, the analyzing power in the polarization measurement should be improved by measuring the $e^+$ energy using the calorimeter [3] because the magnitude of the $e^+$ asymmetry depends on the $e^+$ energy, as shown in Fig.2, while only the energy-integrated asymmetry is obtained in the standard polarization measurement. The dedicated analysis method has been developed by separating events into partial energy regions and optimizing weight parameters to averaging the $e^+$ asymmetry in each bin. We will apply this method in practice for a muon spin imaging experiment which will be conducted at TRIUMF in Canada this November, and we will also present details of this application as an application of the present method. In this talk, some details of the future T-violation experiment, a new polarimeter system, and significance of the present method for improving statistical uncertainties in the polarization measurement will be reported.

References
[1] S. Shimizu et al., Nucl. Instrum. Methods A 945 (2019) 162587.
[2] K. Horie et al., Nucl. Instrum. Methods A 1037 (2022) 166932.
[3] K. Horie et al., Nucl. Instrum. Methods A 1066 (2024) 169606.

Speaker: Ryunosuke Imai (The University of Osaka)

15:05 Search for semi-leptonic decays of new physics particles at NA62 in beam dump mode

The NA62 experiment at CERN is designed to measure the highly suppressed decay $K^+\to \pi^+\nu\bar\nu$. It also has the capability to collect data in a beam-dump mode. In this configuration, $400\,\mathrm{GeV}$ protons are dumped on an absorber and New Physics (NP) particles may be produced and reach a decay volume beginning $80\,\mathrm{m}$ downstream of the absorber. A total of $6\times10^{17}$ protons on target were collected in “beam-dump” mode by NA62 in dedicated runs in 2021–2024. We present early insights from the analysis of this data, in the search for long-lived NP particles decaying in flight to semi-leptonic final states. Such a search allows probing Heavy Neutral Lepton extensions to the Standard Model. In the targeted parameter space of such NP models, the observed baryon asymmetry of the universe and small neutrino masses could be explained simultaneously.

Speaker: Jonathan Leon Schubert (Max Planck Society (DE))

15:05 Search for the K+ ➞ π+ πo X decay at NA62

Search for the $K^+\to\pi^+\pi^0X$ decay is performed at the NA62 experiment at CERN. In the decay $X$ represents an invisible new-physics particle, such as an axion-like particle (ALP) or a massless dark photon (DP). The search is complementary to the extensive work already performed by NA62 to the two-body decay $K^{+} \rightarrow \pi^{+} X$, opening possibility to Beyond Standard Model scenarios. The poster will give an overview on the analysis technique and status, providing the expected sensitivity and expected upper limits for both ALP and massless DP cases.

Speaker: Marco Ceoletta (Universität Mainz)

15:05 Search for the Rare Decay KL ➞ πo e+ e- in KOTO Run92b Data with Four-Cluster Trigger

We report a study of the rare decay $K_L \to \pi^0 X$, $X \to e^+ e^−$ using KOTO Run92b data collected with a dedicated four-cluster trigger. We apply selection criteria to suppress dominant backgrounds such as $K_L \to \pi^0 \pi^+ \pi^-$, and $K_L \to e^+ e^− \gamma \gamma$, and evaluate the expected signal sensitivity. The analysis framework and background estimation strategies are presented, along with preliminary results and sensitivity projections for the rare decay search. This analysis can also search for the Standard Model rare decay $K_L \to \pi^0 e^+ e^−$, which is highly suppressed via flavor-changing neutral currents. Such studies can help the design of future KOTO II experiment.

Speaker: Yi-Ting Su (National Taiwan University)

15:05 Semileptonic decays with NA62 low-intensity data

The semileptonic decays $K^+ \to \pi^0 \ell^+ \nu(\gamma)$ provide a clean way to test the $e$-$\mu$ lepton universality and probe the first-row unitarity $|V_{ud}|^2 + |V_{us}|^2 + |V_{ub}|^2 = 1$ of the CKM matrix. This work aims to update the branching fractions of semileptonic kaon decays, along with the other main kaon decay channels, by one-track analysis using a minimum-bias low-intensity dataset collected by the NA62 experiment at CERN. The current status of the analysis will be presented.

Speaker: Atakan Tugberk Akmete (Universität Mainz)

15:05 Study of upstream background for the K+ ➞ π+ ν ν̄ decay measurement at NA62

The $K^+\to\pi^+\nu\bar{\nu}$ decay is a golden channel in flavor physics. It’s highly suppressed in the Standard Model and sensitive to various indirect effects of new physics. The branching ratio of the K+ → pi+ nu nubar decay has been recently observed with 5 sigma significance by the NA62 experiment at the CERN SPS. To further improve the sensitivity to this channel at NA62, the contribution of background originating upstream of the decay region needs to be studied in detail. Contributions to this background are predicted to arise from interactions between the charged particle beam and the silicon beam spectrometer, GTK3, as well as the residual gas in the vacuum tank. The study of this component with combined 2018–2024 data is presented. The result shows beam-GTK3 interaction contributes $\sim 5\%$ of the upstream background while no clear evidence of beam-gas interaction is found.

Speaker: Xiafei Chang (EPFL - Ecole Polytechnique Federale Lausanne (CH))

15:05 Study on Mechanical Structure and Elements in KOTO II detector system

The KOTO-II detector will require several new specialized mechanical systems and we seek presently to establish the most basic aspects of their designs. Among these are dimensions and structural features for a 20 m long vacuum system utilizing 8 cylindrical chambers as large as 4 m in diameter. A vacuum window using a 4 m diameter thin elastic membrane to isolate cylindrical sections of the vacuum system has been investigated and shown to be feasible. In addition, a conceptual design has been developed for an optical interface and vacuum-feedthrough with the capacity to route hundreds of optical fiber bundles to silicon photomultiplier (SiPM) arrays. Lastly, some consideration has been given to the support structure and rail system needed for the detector as these must not only provide structural stability, but also accommodate the piecewise assembly of the detector that would be expected in a new proposed extension to the Hadron Experimental Facility at J-PARC.

Speaker: Emile Augustine

15:30 → 16:15 Semileptonic & Radiative Decays: Part 1

15:30 Rare Semileptonic Kaon Decay

xxx

Speaker: Ryan Hill

KAON25_rhill.pdf

15:55 Application of K ➞ 3π amplitudes to semileptonic kaon decays

We implement a dispersive parameterization of the nonlocal form factors (NFFs) governing photon or $Z$-boson exchange in the semileptonic $K^+ \to \pi^+ \ell^+ \ell^-$ $(\ell=e,\mu)$ and $K^+ \to \pi^+ \nu \bar{\nu}$ decays, respectively, to improve the theoretical description of the spectrum and decay rate of the golden neutrino mode. The parameterization links the hadronic $K^+ \pi^- \to \pi^+ \pi^-$ amplitude in P-wave with the pion vector form factor (VFF) through unitarity. The phase of the hadronic amplitude, in turn, can be constrained by a fit to the Dalitz slope parameters of all CP-conserving $K \to 3\pi$ decays, assuming isospin symmetry and elastic rescattering of the pions. We develop the complete basis of reduced isospin amplitudes in the complex plane by iterating the Khuri-Treiman equations, and compare to recent results in the literature. Finally, we emphasize a relation between the charged-lepton and neutrino NFFs in terms of the subtractions in their dispersive representations.

Speaker: Jack Jenkins (Universität Siegen)

jenkins_kaon2025.pdf

16:15 → 16:45 Coffee Break

16:45 → 18:05 Semileptonic & Radiative Decays: Part 2

16:45 Kaon radiative leptonic decay rates from lattice QCD simulations at the physical point

We present a Lattice QCD calculation of the radiative decay rates of the kaon $K^- \to \ell^- \bar{\nu}_{\ell} \gamma^{(*)}$, with a real or virtual photon, and $\ell = e, \mu$. The simulations are performed with physical kaon and pion masses, using gauge ensembles generated by the Extended Twisted Mass Collaboration with $N_f = 2+1+1$ flavors of Wilson-Clover twisted mass fermions.
For real photon emission ($K^- \to \ell^- \bar{\nu}_\ell \gamma$), we compute the quark-disconnected diagrams for the first time, improving the precision of the two relevant form factors by a factor of two compared to previous lattice calculations.
We observe a significant tension between our results and experimental data.
The decay with a virtual photon ($K^- \to \ell^- \bar{\nu}_{\ell} \gamma^{*} \to \ell^- \bar{\nu}_{\ell} \ell'^+ \ell'^- $) is a powerful probe of New Physics, as its rate starts at $O(\alpha_{\mathrm{em}}^2)$. We compute the four form factors, parametrizing the hadronic part of the decay, over the full kinematical range. For photon virtualities $\sqrt{k^2} > 2m_\pi$, we overcome the issues related to analytic continuation by applying, for the first time in an exclusive decay channel, spectral density methods.

Speaker: Mr Roberto Di Palma (Università Roma Tre, Infn, sezione di Roma Tre)

rdp_Kaon25.pdf

17:05 Results of radiative kaon decay studies at NA62

The NA62 experiment at CERN reports preliminary results from the study of the rare radiative decay $K^+ \rightarrow \pi^+ \pi^+ \pi^- \gamma$, using a data sample collected with a multi-track trigger in 2017-2018. A measurement of the branching ratio as a function of the photon energy in the kaon rest frame is presented and compared with previous measurements and theoretical predictions.

Speaker: Zdenko Hives (Charles University (CZ))

KAON2025_Hives.pdf

17:25 Lattice QCD Calculation of Radiative Corrections to Meson Leptonic Decays

Precise tests of CKM unitarity provide a probe of physics beyond the Standard Model. Using the most precise determinations—$V_{ud}$ from superallowed nuclear $\beta$ decays and $V_{us}$ from kaon leptonic and semileptonic decays—the first‑row sum $|V_{ud}|^{2}+|V_{us}|^{2}+|V_{ub}|^{2}$ deviates from unitarity by $2\text{–}3\,\sigma$. This tension emphasizes the need for further reductions in both experimental and theoretical uncertainties. On the theoretical side, the leading uncertainty stems from radiative corrections to the relevant decays. We perform a lattice QCD calculation of the radiative corrections to meson leptonic decays with the infinite‑volume reconstruction (IVR) method. Compared with the traditional $\mathrm{QED}_{L}$ approach, IVR reduces the systematic uncertainties due to the finite‑volume effects from $\sim\!40\%$ to $0.08\%$. We also compute the branching ratios for the radiative leptonic decays $\pi\to \ell\nu_\ell\gamma$ and $K\to \ell\nu_\ell\gamma$ and compare the lattice results with existing experimental data.

Speaker: Xin-Yu Tuo (Brookhaven national laboratory)

KAON2025_xtuo.pdf

17:45 Measurement of the K+ ➞ e+ ν γ decay at NA62

Speaker: Angela Romano (University of Birmingham (GB))

AngelaRomano_KAON2025_11092025.pdf

18:30 → 23:00 Conference Dinner

KAON_Social_Dinner.pdf

Friday 12 September

09:00 → 09:20 Rare Decays

09:00 Precision measurements of kaon and pion decays at NA62

The NA62 experiment at CERN reports recent results from precision measurements of kaon and pion decays, using data samples collected in 2017-2018. A sample of $K^+ \rightarrow \pi^+ \gamma \gamma$ decays is collected using a minimum-bias trigger, and the results include measurement of the branching ratio, study of the di-photon mass spectrum, and the first search for production and prompt decay of an axion-like particle with gluon coupling in the process $K^+ \rightarrow \pi^+ A$, $A \rightarrow \gamma \gamma$. The radiative kaon decay $K^+ \rightarrow \pi^0 e^+ \nu \gamma$ (Ke3g) is studied with a data sample of O(100k) Ke3g candidates with sub-percent background contaminations. Results with the most precise measurements of the Ke3g branching ratios and T-asymmetry are presented. A sample of $\pi^0\rightarrow e^+ e^-$ decay candidates is collected using a dedicated scaled down di-electron trigger, and a preliminary result of the branching fraction measurement is presented.

Speaker: Michal Koval (Charles University (CZ))

KAON2025-Koval-Precision-NA62.pdf

09:20 → 10:05 Semileptonic & Radiative Decays: Part 3

09:20 Kaon Physics in the Standard Model Effective Field Theory Framework

In my talk I will present an overview of Kaon observables analyzed within the Standard Model Effective Field Theory (SMEFT) framework. Focusing on Kaon mixing, rare decays, and direct CP violation, I will discuss how these processes constrain higher-dimensional SMEFT operators. The treatment includes renormalization group evolution, matching to the low-energy effective theory, and the role of hadronic matrix elements. I will highlight recent developments in global SMEFT fits incorporating kaon data. Finally, I will outline prospects for tightening constraints with upcoming theoretical and experimental inputs.

Speaker: Jason Aebischer (CERN)

Kaon_JA.pdf

09:45 QED Overview

xxx

Speaker: Matteo Di Carlo

Kaon2025_mdc.pdf

10:05 → 10:55 FCNC Decays: Part 3

10:05 BK, Lattice, Matching, and Beyond

Precision flavour observables, such as CP violation in meson mixing, place stringent constraints on physics beyond the Standard Model. Matching the experimental accuracy of these observables requires equally precise theoretical inputs, combining high-order perturbative calculations with non-perturbative results from Lattice QCD. In this talk, I will present the two-loop matching calculations necessary for the NNLO RI/(S)MOM renormalisation of BK, the bag parameter relevant for εK. These results enable a consistent comparison of different determinations of BK, including both lattice and non-lattice approaches. I will discuss the resulting phenomenological implications and, in the outlook, highlight the role of other RI/(S)MOM matching calculations for Kaon physics.

Speakers: Martin Gorbahn, Martin Gorbahn (Liverpool University)

kaon25_new.pdf

10:30 The K ➞ π γ* γ* transitions at leading order and beyond

The rare radiative $K\to\pi\ell^+\ell^-$ decays ($\ell=e,\mu$) provide experimental access to the $K\to\pi\gamma^*$ transitions, and comparing the two channels allows for a stringent test of lepton-flavor universality. Although the underlying $K\to\pi\gamma^*$ conversions have been studied extensively, for instance the radiative corrections for the $K^+\to\pi^+\ell^+\ell^-$ decays involve the $K^+\to\pi^+\gamma^*\gamma^{(*)}$ transitions (with up to two virtual photons), not fully addressed in the literature. At the same time, the $K\to\pi\gamma^*\gamma^*$ transitions are essential for the description of the $K\to\pi e^+e^-\ell^+\ell^-$ decays, which represent a background to new-physics searches.

Unitarity corrections from $K\to3\pi$ have been proven important for the associated processes, and in the present work, these corrections to $K\to\pi\gamma^{(*)}\gamma^{(*)}$ transitions are revisited in the view of ongoing analyses of $K^+\to\pi^+\gamma\ell^+\ell^-$ decays at NA62 and extended to the doubly off-shell case.

Speaker: Tomáš Husek (Charles University and University of Birmingham)

Husek_KAON2025.pdf

10:55 → 11:25 Coffee Break

11:25 → 13:00 FCNC Decays: Part 4

11:25 Dispersive description of the K ➞ π 𝓁 𝓁 radiative amplitudes

A description of the two form factor $W_+$, $W_S$, associated with the radiative decays of the $K^+$ and the $K_S$, based on general properties of analyticity and unitarity is proposed. Starting from the simple consideration of the asymptotic behaviour of the two combinations $2W_+ - W_S$ and $W_+ + W_S$ we derive a dispersive representation involving only two parameters. Using the rich experimental information of the $K \to 3\pi$ amplitudes, extended beyond the low energy region using the Khuri-Treiman formalism, we show that the sign of the the $W_+$ form factor is unambiguously determined and its energy dependence is well reproduced. We also show that the yet unknown $\Delta{I}=1/2$ part of the $K_S \to \pi^+ \pi^- \pi^0$ can be determined from the value of $W_+(0)+W_S(0)$. The possibility of fixing the sign of $W_S$ from experiment is discussed.

Speaker: Bachir Moussallam

dispKpill-Kaon25.pdf

11:45 KL(S) ➞ μ μ

I will review recent studies of the sensitivity of future measurements in the $K\to\mu\mu$ system to short-distance parameters. I will introduce a method to present kaon CKM observables in a way that is independent of any B physics inputs, and discuss the arising picture and the status of the kaon program.

Speaker: Avital Dery

KAON_2025_DERY.pdf

12:10 Long-distance contribution to KL ➞ 𝓁 𝓁 in a dispersive approach

In this talk, we will present the improved Standard-Model predictions for $K_L \to \ell^+ \ell^-$ decays, making use of a dispersive representation of the underlying transition form factor that incorporates the constraints from the available data and the asymptotic behavior. The precise determinations of the Standard-Model branching fractions will be contrasted with the existing measurements. We will also derive the resulting constraints on physics beyond the Standard Model.

Speaker: Bai-Long Hoid (University of Mainz)

Kaon_Hoid.pdf

12:35 Computing the two-photon KL ➞ μ+ μ– decay amplitude from lattice QCD

The decay of a long-lived kaon to a pair of muons (KL2mu) could serve as a precision test of the Standard Model (SM) and a probe to physics beyond due to its sensitivity to physics at high energies. A precise determination of the size of the long-distance, two-photon contribution to KL2mu -- as well as its interference with the analytically-computable short-distance (SD) contribution -- are crucial for making a meaningful comparison between the SM prediction and experiment. In this talk, I will present the ongoing effort from the RBC/UKQCD collaboration to extract the two-photon decay amplitude of KL2mu using Lattice Quantum Chromodynamics. Both the size and the relative sign of the two-photon KL2mu decay amplitude with regard to its SD counterpart can be determined unambiguously from first principles with an accuracy of at least 10%. I will also discuss how to extend our framework to determine the CP-conserving two-photon contribution to the decay amplitude of a long-lived kaon into a neutral pion and a pair of muons.

Speaker: Dr En-Hung Chao (Columbia University)

kaon25-k2mumu.pdf

13:00 → 13:15 Concluding Remarks