LLP2025: Fifteenth workshop of the Long-Lived Particle Community

2 Jun 2025, 07:00 → 6 Jun 2025, 19:00 Europe/Madrid

València - ADEIT

Emma Torro Pastor (Univ. of Valencia and CSIC (ES)), José Francisco Zurita (IFIC - Univ. of Valencia and CSIC (ES))

 

 

Short url: https://indico.cern.ch/e/LLP_June_2025

Mattermost channel: https://mattermost.web.cern.ch/llpcommunity/channels/llp-2025

-------------- LLP2025 --------------

The fifteenth workshop of the Long-Lived Particle Community will occur from 2 to 6 June, 2025, organized by Instituto de Física Corpuscular (IFIC) in València, Spain.

The event will take place at ADEIT, located in the city center of València.

The call for abstracts is now open until April 7.

Registration is now open, until May 13 (early bird registration ends on May 2)

 

HOTEL BOOKING SCAM: WARNING!!

Please be aware that there is a scam relating to hotel reservations for LLP 2025: some of the registered participants are receiving emails from Global Travel about hotel reservations. Note that no travel agency has been contacted to manage or support the management of hotel reservations for LLP 2025. In case of doubt please contact us directly.

 

 

A reminder of the CERN code of conduct.

 

Scientific committee (llp2025-sc@cern.ch):

Juliette Alimena
James Beacham
Lisa Benato
Matthew Citron
Albert de Roeck
Jan Hajer
Audrey Kvam
Federico Leo Redi
Mason Proffitt
Neha Santpur
Andrii Usachov
Carlos Vázquez Sierra

Local organizing committee:

Chandan Hati
Martin Hirsch
Jacobo López-Pavón
Vasiliki A. Mitsou
Laura Molina
Arantza Oyanguren
Emma Torró (chair)
Óscar Vives
Bryan Zaldívar
José Zurita (chair)

 

This workshop is partially supported by:

• Ministerio de Ciencia y Innovación
• Generalitat Valenciana, ASFAE and GenT
• IFIC, Severo Ochoa Center
• CSIC
• University of Valencia, Departments of Atomic, Molecular and Nuclear Physics and Theory Physics

 

 

Monday 2 June

09:00 → 09:30 Registration

09:30 → 10:10 Introduction and welcome

Conveners: Emma Torro Pastor (Univ. of Valencia and CSIC (ES)), Dr José Francisco Zurita (IFIC - Univ. of Valencia and CSIC (ES))

09:30 Introduction to the LLP Community

Speaker: Dr José Francisco Zurita (IFIC - Univ. of Valencia and CSIC (ES))

LLP_2025_Community.pdf

09:40 Welcome from the IFIC authorities

Speaker: Nuria Rius

IFIC_LLP.pdf

09:50 Welcome from the GVA authorities

Speaker: Rafael Sebastian (DG Science and Research. Consellería de Educación, Cultura, Universidades y Ocupación, GVA)

10:00 Details on logistics

Speaker: Emma Torro Pastor (Univ. of Valencia and CSIC (ES))

10:10 → 11:10 Overview talks I

Conveners: Emma Torro Pastor (Univ. of Valencia and CSIC (ES)), Dr José Francisco Zurita (IFIC - Univ. of Valencia and CSIC (ES))

10:10 LHCb overview and latest results

The LHCb detector at the LHC offers unique coverage of forward rapidities. The experiment also has a flexible trigger that enables low-mass states to be recorded with high efficiency and a precision vertex detector that enables excellent separation of primary interactions from secondary decays. This allows LHCb to make significant and world-leading contributions in these regions of phase space in the search for long-lived particles that are predicted by dark sectors that accommodate dark matter candidates. This talk will cover an overview of LHCb searches, with a focus on dark photons and axion-like particles, alongside the prospects for the current Run 3 at the LHC. The talk will also discuss advancements in the GPU-based trigger HLT1 that boosts the sensitivity for these searches.

Speaker: Andrii Usachov (Nikhef and VU (NL))

10:40 CMS: Overview of recent LLP results

CMS: Overview of recent LLP results

Speaker: Bryan Cardwell (University of Virginia (US))

11:10 → 11:40 Coffee

11:40 → 12:40 Overview talks II

Conveners: Chandan Hati, Dr Vasiliki Mitsou (IFIC - CSIC and Univ. of Valencia (ES))

11:40 ATLAS: Overview of recent LLP results

remote

Speaker: Dr Louie Dartmoor Corpe (Laboratoire de Physique Clermont Auvergne (LPCA))

12:10 The MoEDAL-MAPP experiment – Status Report

The Monopole and other Exotics Detector At the LHC (MoEDAL) was installed at IP8 of the LHC in 2010. MoEDAL has been focused on searches for magnetic monopoles and other highly ionizing particles (HIPs) predicted by Beyond the Standard Model (BSM) theories, including supersymmetry, neutrino mass models, and left-right symmetry. It has produced pioneering results that overcome limitations of previous accelerator-based searches for monopoles, performed the first search for dyons at an accelerator, and placed the leading limits on highly electrically charged objects. MoEDAL continues searching for HIPs in Run-3 with a lower threshold, higher luminosity, and the center-of-mass energy of 13.6 TeV. Additionally, the collaboration is preparing the MoEDAL Apparatus for Penetrating Particles (MAPP) in order to search for feebly interacting particles (FIPs). Installed in the UA83 tunnel, 100 meters from IP8, it is expected to begin data collection in 2025. MAPP is projected to be sensitive to particles with charge, or effective charge, as low as 0.001e. To extend the MAPP's sensitivity to FIPs with higher masses, the collaboration is preparing an Outrigger detector. The Outrigger is awaiting approval and is expected to begin data collection in 2026.  MAPP and its Outrigger will also have some sensitivity to neutral long-lived particles (LLPs). In Phase-2, the MAPP-2 upgrade is planned for the HL-LHC and will be installed in the UGC1 gallery near IP8, significantly improving MoEDAL-MAPP’s ability to detect neutral LLPs decaying into charged particles and photons.

Speaker: Dr Igor Ostrovskiy (The University of Alabama)

12:40 → 13:00 Dedicated experiments I

Conveners: Chandan Hati, Dr Vasiliki Mitsou (IFIC - CSIC and Univ. of Valencia (ES))

12:40 Latest results from the NA64 experiment

NA64 is a fixed target experiment at the CERN Super Proton Synchrotron accelerator searching for Dark Sectors employing high energy electron, positron and muon beams. In this talk, we report its latest results on sub-GeV Dark Matter searches with the 2016-2022 statistics. With the new data, NA64 is starting to probe for the first time the very interesting region of parameter space motivated by benchmark Light Dark Matter models. The experiment can also probe a variety of well-motivated New Physics scenarios that will be briefly covered in this talk such as: ALPs, inelastic DM, B − L and $L_{\mu}-L_{\tau}$ Z' boson searches. Moreover, in this contribution we will also present the first results of NA64 running in positron and muon modes as well as the future plans of the experiment.

Speaker: Balint Radics (York University)

13:00 → 15:00 Lunch

15:00 → 16:20 Dedicated experiments II

Conveners: Arantza Oyanguren (IFIC - Valencia), Martin Hirsch

15:00 Recent results from the SND@LHC experiment

SND@LHC is a compact and stand-alone experiment to perform measurements with neutrinos and long lived particles produced at the LHC in a hitherto unexplored pseudo-rapidity region of 7.2 < 𝜂 < 8.6, complementary to all the other experiments at the LHC. The experiment is located 480 m downstream of IP1 in the unused TI18 tunnel. The detector is composed of a hybrid system based on an 800 kg target mass of tungsten plates, interleaved with emulsion and electronic trackers, followed downstream by a calorimeter and a muon system. The configuration allows to efficiently distinguish between all three neutrino flavours, opening a unique opportunity to probe physics of heavy flavour production at the LHC in the region that is not accessible to ATLAS, CMS and LHCb. This region is of particular interest also for future circular colliders and for predictions of very high-energy atmospheric neutrinos. The detector concept is also well suited to searching for Feebly Interacting Particles via signatures of scattering in the detector target. The first phase aims at operating the detector throughout LHC Run 3 to collect a total of 290 fb−1. The experiment has been running successfully during 2022 and 2023 and has published several results. This talk will focus on the experience gained from the first measurements and on the overall physics goals of SND@LHC.

Speaker: Zhibin Yang (EPFL - Ecole Polytechnique Federale Lausanne (CH))

15:20 Neutrino results from FASER

The FASER experiment at the LHC is designed to search for light, weakly-coupled new particles, and to study high-energy neutrinos. The experiment has been running since 2022, and has collected nearly 200/fb of pp collision data. FASER has released several neutrino results including the first observation of electron and muon neutrinos at a particle collider, the first measurement of the muon and electron neutrino interaction cross sections in the TeV energy range, and the first differential measurement with muon neutrinos and anti-neutrinos. This talk will summarise the FASER experiment, the neutrino results, and discuss future prospects for FASER neutrino results.

Speaker: Wissal Filali (Bonn (DE))

15:40 Long lived BSM particle searches at FASER

The FASER experiment at the LHC is designed to search for light, weakly-coupled new particles, and to study high-energy neutrinos. The experiment has been running since 2022, and has collected nearly 200/fb of pp collision data. FASER has released a search for long-lived dark photons, and long lived axion-like-particles (also interpreted in several other scenarios). This talk will summarise the long-lived BSM particle search program and discuss future prospects.

Speaker: Sinead Eley (University of Liverpool (GB))

16:00 First NA62 search for long-lived new physics particle hadronic decays

The NA62 experiment at CERN, designed to measure the highly-suppressed decay $K^{+} \rightarrow \pi^{+}\nu\bar{\nu}$, has the capability to collect data in a beam-dump mode, where 400 GeV protons are dumped on an absorber. In this configuration, 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. A search for NP particles decaying in flight to hadronic final states is reported, based on a blind analysis of a sample of $1.4 \times 10^{17}$ protons on dump collected in 2021.

Speaker: Jan Jerhot (Max Planck Society (DE))

16:20 → 16:50 Coffee

16:50 → 18:30 Theory and pheno I

Conveners: James Beacham (Max Planck Society (DE)), Dr José Francisco Zurita (IFIC - Univ. of Valencia and CSIC (ES))

16:50 Heavy Neutral Leptons in Ice, Water and Rock

Heavy neutral leptons (HNLs) are well-motivated candidates for physics beyond the Standard Model. These heavy right-handed neutrino states can couple to their left-handed Standard Model counterparts through various portals, the simplest of which are mass-mixing and transition magnetic moments. This talk discusses searches for HNLs in two different experimental settings. First, neutrino telescopes such as IceCube and KM3NeT can search for mass-mixed and dipole-portal HNLs through a characteristic double cascade signature. We present the first attempt at an HNL-induced double cascade search in IceCube and discuss the status of upcoming improved double cascade searches. Second, far-forward experiments at the Large Hadron Collider can search for HNLs using the neutrino flux produced in proton-proton collisions. To this end, we introduce two new collider neutrino detector concepts leveraging the local natural environment: the Surface-based Integrated Neutrino Experiment (SINE) and the UNDerwater Integrated Neutrino Experiment (UNDINE). We discuss prospects for HNL searches at SINE and UNDINE using delayed event signatures and di-muon final states.

Speaker: Dr Nicholas Kamp (Harvard University)

17:10 UHE muons at KM3NeT in the context of the anomalous ANITA-IV events

Recently, the KM3NeT collaboration announced the detection of the neutrino with the highest energy ever measured, 220 PeV. Nevertheless, the observation of this event is in 2.5-3.5σ tension with the non-observation of such events in 12 years at IceCube.
In fact, this is not the first anomalous measurement of an ultra-high-energy event. Namely, the ANITA-IV collaboration measured four 1 EeV events, compatible with a tau neutrino, and in strong tension with IceCube.
In this talk I will review under which conditions can BSM (and, in particular, LLPs) reconcile these tensions, and the potential of ultra-high-energy neutrino telescopes at constraining models with LLPs.

Speaker: Antoni Bertólez-Martínez (Departament de Física Quàntica i Astrofísica, Universitat de Barcelona)

LLP 2025 - Bertolez.pdf

17:30 Probing long-lived HNLs via top-philic interactions at the LHC

Heavy Neutral Leptons (HNLs) provide a well-motivated extension of the Standard Model, offering a window into neutrino mass generation and potential new physics at the TeV scale. In this talk, we explore the phenomenology of HNLs at the LHC in the framework of $N_R$SMEFT, focusing on top-philic dimension-six operators. We identify two key HNL production channels: direct production in association with single top quarks and production via exotic top decays. For long-lived HNLs, we estimate the sensitivity reach of both main LHC detectors (such as ATLAS) and far-detector experiments (including MATHUSLA and ANUBIS) in the high-luminosity era. Our results show that ATLAS can probe new physics scales up to 12 TeV for certain operators, while far detectors are sensitive up to 4.5 TeV, covering complementary regions of HNL parameter space. These findings highlight the role of top quark interactions in unveiling long-lived particles and expanding the discovery potential for HNLs at the LHC.

Speaker: Rebeca Beltran (IFIC (CSIC-UV))

17:50 Probing HNL properties through their lifetime distribution

Extensions of the standard model of particle physics with Heavy Neutral Leptons (HNLs) can simultaneously generate the light neutrino masses as well as the baryon asymmetry of the Universe. In the minimal seesaw model, with only two HNLs, these particles form a pseudo-Dirac pair, degenerate in mass with nearly identical couplings to the SM leptons. The HNLs therefore follow a simple exponential decay law, with the only parameter that can be extracted at leading order being the relationship between their coupling and lifetime.

If the seesaw scenario is extended by a third HNL, this decay law can be significantly modified, deviating from the simple pseudo-Dirac picture, and leading to a much richer phenomenology. This can in turn be used to recover more of the the HNL parameters, including their mass differences, even in scenarios where these are too small to be distinguished kinematically.

Speaker: Juraj Klaric (Universiteit van Amsterdam, Nikhef)

18:10 Lepton Number Violation in Symmetry-protected Seesaw Models

Type I seesaw models, which provide an explanation for the observed small neutrino masses, additionally predict the existence of heavy neutral leptons (HNLs). In collider-observable scenarios, these HNLs must form nearly mass-degenerate pseudo-Dirac pairs, as captured by symmetry-protected seesaw models. The mass splitting within these pairs induces heavy neutrino-antineutrino oscillations, leading to observables that are sensitive to lepton number violation. We explore the potential of current and future collider experiments to quantify the amount of lepton number violation.

Speaker: Mr Bruno M. S. Oliveira (Centro de Física Teórica de Partículas (CFTP), Instituto Superior Técnico (IST), Universidade de Lisboa (UL))

18:30 → 19:30 Welcome Reception

Tuesday 3 June

09:30 → 11:00 Theory and pheno II

Conveners: Jan Hajer (CFTP, IST, Universidade de Lisboa), Martin Hirsch

09:30 Cosmology and Long-Lived Particles

Speaker: Jose Miguel No (IFT-UAM/CSIC)

10:00 Exploring Long-Lived Hidden Valleys

Hidden Valley (HV) scenarios featuring confining dark sectors can offer compelling collider signatures known as “dark-showers”. This confinement gives rise to dark mesons, some portions of which are expected to be long-lived particles (LLPs). We consider HV LLPs at lifetimes where it is necessary to reinterpret existing CMS searches targeting LLP decays in the muon system. Using a model-agnostic approach, we show that the existing CMS search is sensitive to the fundamental parameters of the confining dark sector itself through their influence on the LLP multiplicity and boost. The results can be used for model dependent exclusions on HV parameter space and motivates new pathways to explore HV phenomenology.

Speaker: Joshua Lockyer

10:20 Dark showers from sneaky dark matter

I will present a minimal composite dark matter model, based on a $SU(N_d)$ dark sector with $n_f$ dark quarks and a heavy t-channel mediator. For $n_f \ge 4$, the dark flavor symmetry guarantees the stability of a subset of the dark pions, which serve as our dark matter candidates. Their relic abundance is determined by co-scattering or co-annihilation with the remaining dark pions, which are unstable and decay. Due to their degenerate masses, the annihilation cross section is suppressed at low temperatures, thereby avoiding stringent constraints from indirect detection and opening up the GeV mass window. The decaying dark pions are naturally long lived.

Speaker: Adrian Carmona Bermudez (Universidad de Granada (ES))

10:40 Probing conversion-driven freeze-out at the LHC

Conversion-driven freeze-out is an appealing mechanism to explain the observed relic density while naturally accommodating the null results from direct and indirect detection due to a very weak dark matter coupling. Interestingly, the scenario predicts long-lived particles decaying into dark matter with lifetimes favorably coinciding with the range that can be resolved at the LHC. However, the small mass splitting between the long-lived particle and dark matter renders the visible decay products soft, thus challenging current search strategies. We consider four different classes of searches covering the entire range of lifetimes: heavy stable charge particles, disappearing tracks, displaced vertices, and missing energy searches. We discuss the applicability of these searches to conversion-driven freeze-out and derive current constraints highlighting their complementarity. For the displaced vertex search, we demonstrate how a slight modification of the current analysis significantly improves its sensitivity to the scenario.

Speaker: Lucas Magno Dantas Ramos

11:00 → 11:30 Coffee

11:30 → 13:10 ATLAS, CMS, LHCb results I

Conveners: Federico Leo Redi (Università degli Studi e INFN Milano (IT)), Lisa Benato (Austrian Academy of Sciences (AT))

11:30 Search for emerging jets in pp collisions at √s=13.6 TeV with the ATLAS experiment

Speaker: Luka Selem (CNRS ­­– LPSC)

11:50 Searches for displaced Scalar decays to dimuons: LHCb’s extended reach in Run 3

A detailed study exploiting novel trigger and reconstruction techniques developed to search for Beyond Standard Model (BSM) Long-Lived Particles (LLPs) with very displaced vertices is presented. Building on feasibility studies that have successfully reconstructed Standard Model decays occurring up to 8 m forward of the interaction point in LHCb’s magnet region, the search for LLP particles into charged final states exploits LHCb’s unique forward geometry and segmented tracking system—comprising the Vertex Locator, Upstream Tracker, and SciFi stations—to extend sensitivity into previously inaccessible regions.
The presentation will cover innovative trigger strategies implemented in LHCb’s software trigger for inclusively selecting very displaced dimuon pairs, alongside advanced offline selection methods utilising multivariate analysis methods to robustly suppress background while maintaining high signal efficiency. Preliminary sensitivity estimates for Dark Higgs -> mu mu search indicate that these approaches can achieve competitive performance compared to dedicated LLP experiments. Future prospects will also be discussed. This work aims to provide a comprehensive framework for enhancing LLP discovery potential at LHCb and offers insights that could be beneficial for the broader BSM LLP search community.

Speaker: Izaac Sanderswood (Univ. of Valencia and CSIC (ES))

12:10 CMS: Muon detector showers with B-parking dataset

A search for signatures of a dark analog to quantum chromodynamics is performed. The analysis targets long-lived dark mesons that decay into standard model particles with a high branching fraction to muons. A unique dataset with $10^{10}$ B meson events is used. It was collected by the CMS experiment at the CERN LHC in 2018 using displaced muon triggers, which have a high trigger efficiency for the signal models. Resonant dimuon signatures are searched for, with both pointing and non-pointing topologies. No significant excess is observed beyond the standard model expectation. Upper limits on the branching ratio of Higgs boson decays to dark partons are determined to be less than $10^{−3}$, at 95% confidence level, surpassing and extending existing limits for the mean proper lifetime of less than approximately 0.1 m for a mass as low as 2 GeV. First limits are set for extended dark shower models, probing the low-mass region down to 0.33 GeV.

Speaker: Daniel Diaz (Univ. of California San Diego (US))

12:30 Search for displaced leptons in 13 TeV and 13.6 TeV 3 $pp$ collisions with the ATLAS detector

Remote

A search for leptons displaced from the primary vertex is performed with the ATLAS detector at the Large Hadron Collider. The search includes the full proton-proton collision dataset collected during Run 2 at $\sqrt{s} = 13$ TeV and a partial dataset collected during Run 3 in 2022-2023 at $\sqrt{s} = 13.6$ TeV, corresponding to integrated luminosities of $140 fb^{−1}$ and $56.3 fb^{−1}$, respectively. Final states with displaced electrons or muons are considered, and novel triggers introduced in Run 3 are employed that use large impact parameter tracking to reconstruct displaced tracks with low momentum. In addition, photon reconstruction and multivariate techniques are employed to broaden the sensitivity to channels with large background rates or highly displaced electrons, respectively. The results are consistent with the Standard Model background expectations and are used to set model-independent limits on the production of displaced electrons and muons. The analysis is also interpreted in the context of a gauge-mediated supersymmetry breaking model with pair-produced long-lived sleptons, and a dark sector model with pair-produced chargino-like states. The results include 95% CL exclusions of selectrons with lifetimes from 4 ps to 60 ns and a mass of 150 GeV, and exclusions of selectrons, smuons, and staus with a lifetime of 0.3 ns for masses up to 740 GeV, 830 GeV, and 440 GeV, respectively. Dark charginos with masses up to 380 GeV are excluded for a mass difference with the neutral state of 40 GeV, and mass differences down to 17 GeV are excluded for dark charginos with a 100 GeV mass.

Speaker: Yvonne Ng (Univ. Illinois at Urbana Champaign (US))

12:50 CMS: Dark showers in B-parked data

Among the novel models that provide explanation to the nature of dark matter, hidden valley models include a dark sector that extends the Standard Model (SM) with a non-Abelian gauge group, similar to the SM quantum chromodynamics. They predict the production of dark showers, which are dark hadrons with high multiplicity, which can result in a rich phenomenology of displaced decays back into SM particles. In this talk we present the most recent results from CMS that probe the dark sector using B-parked data collected at the LHC.

Speaker: Kai Hong Law (Imperial College (GB))

13:10 → 15:10 Lunch

15:10 → 16:30 Dedicated experiments III

Conveners: Carlos Vazquez Sierra (Universidade da Coruña (ES)), Oscar Manuel Vives Garcia (Univ. of Valencia and CSIC (ES))

15:10 Searches for Long-Lived Particles with MATHUSLA

The Large Hadron Collider (LHC) may be producing a significant number of neutral Long-Lived Particles (LLPs) with masses above a GeV, yet these elusive particles could be escaping detection by the main experiments. To bridge this gap, we propose the MATHUSLA detector (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles), a dedicated surface-based experiment positioned above CMS, designed to operate during the High-Luminosity LHC era. MATHUSLA consists of multiple layers of plastic scintillators with wavelength-shifting fibers coupled to silicon photomultipliers, surrounding a large, air-filled decay volume. The Conceptual Design Report (CDR) of a 40-meter MATHUSLA detector (MATHUSLA40) has been published on arxiv in March 2025. We will present the design of MATHUSLA40, as well as background studies of rare Standard Model processes and provide measurements of the "demonstrator modules" at the University of Victoria and the University of Toronto.

Speaker: Runze Ren

15:30 Update on the CODEX-b Experiment

The COmpact DEtector for EXotics at LHCb (CODEX-b) is a particle physics detector dedicated to displaced decays of exotic long-lived particles (LLPs), compelling signatures of dark sectors Beyond the Standard Model, which arise in theories containing a hierarchy of scales and small parameters. The CODEX-b detector is a cube with 10m per side with two internal sections, planned to be installed near the LHCb interaction point. It is built of a new generation of high performance RPCs triplet chambers, derived from the ATLAS upgrade RPC technology, providing a space x time resolution of a few mm x 300 ps per individual detector layer. It will have a near-zero background environment, hence complementing the new-searches program of other detectors like ATLAS or CMS. A demonstrator detector, CODEX-𝛽, has been installed to take data beginning in 2025. It will validate the design and physics case for the future CODEX-b. CODEX-𝛽 will be responsible for validating the background estimations for CODEX-b, demonstrating integration in the LHCb readout system, and showing the suitability of the baseline tracking and its mechanical support. This talk will present the latest developments and will focus on the status and plans for CODEX-𝛽.

Speaker: Emilio Xose Rodriguez Fernandez (Universidade de Santiago de Compostela (ES))

15:50 Exploring the Projected Sensitivity of the ANUBIS detector to exotic LLP models

Despite the success of the Standard Model (SM) there remains behaviour it cannot describe, in particular the presence of non-interacting Dark Matter, which composes a significant fraction of the Universe’s matter. Many models that describe dark matter can generically introduce exotic Long-Lived Particles (LLPs). The proposed ANUBIS experiment is designed to search for these LLPs within the ATLAS detector cavern alongside the ATLAS detector, located approximately 20-30 m from the IP. We report on the potential sensitivity of ANUBIS to a selection of LLP models, i.e. Higgs Portal and Heavy Neutral Leptons, as well as future planned studies.

Speakers: Aashaq Shah (University of Cambridge (GB)), Cayetano Fernandez Ruiz (University of Cambridge (GB)), Michael Revering (University of Cambridge (GB)), Oleg Brandt (University of Cambridge (GB)), Paul Nathaniel Swallow (University of Cambridge (GB)), Théo REYMERMIER

16:10 SHIFT: Shifted Interaction on a Fixed Target

SHIFT is a proposed extension of the LHC research program by installing a gaseous fixed target ≈150 meters upstream of CMS, delivering collisions at the center of mass energy of ≈115 GeV. The particles produced in such collisions, or their decay products, travel through the rock and other material on their path, potentially reaching the CMS detector where they can be registered and studied.

Preliminary studies show that SHIFT improves CMS sensitivity to new particle couplings by 2–3 orders of magnitude by maximizing decay volume and acceptance while using intervening material as shielding that does not affect weakly interacting LLPs or muons they decay to. Compared to other projects like SHiP, FASER, CODEX-b, and MATHUSLA, SHIFT achieves unique sensitivity at a fraction of the cost by using an existing detector.

Speaker: Jeremi Niedziela (Deutsches Elektronen-Synchrotron (DE))

16:30 → 17:00 Coffee

17:00 → 18:10 Theory and pheno III

Conveners: Jacobo Lopez Pavon (IFIC, CSIC-Universitat de València (Spain)), Juliette Alimena (DESY)

17:00 Status of GeV-scale LLPs

In this talk, I explore the search for feebly interacting new physics particles (FIPs) in the GeV mass range from a theoretical perspective. I begin by emphasizing the complementarity between astrophysical and cosmological probes in exploring the FIP parameter space. I then survey the landscape of intensity frontier experiments designed to detect FIPs, addressing the principal challenge - theoretical uncertainties in FIP phenomenology, which can substantially influence experimental constraints and sensitivities. Regarding the cosmological probes, I discuss the impact of FIPs on BBN and CMB, focusing on the epoch around neutrino decoupling, a critical period at the edge of current observational capabilities. Special attention is given to recent advances in our understanding of how primordial neutrinos are affected by FIP decays into high-energy neutrinos and metastable particles, such as pions and kaons.

Speaker: Maksym Ovchynnikov (CERN)

17:30 Leptophilic ALPs in Laboratory Experiments

In this talk I will review the collider phenomenology of leptophilic axion-like particles (ALPs), i.e. pseudoscalar particles that couple only to charged leptons. Loops of charged leptons induce effective interactions of the ALPs with photons, which depend on the momenta of the interacting particles and differ between pseudoscalar and derivative lepton couplings. I will systematically discuss the form of the interaction with photons for general external momenta and identify the regimes when it can be safely approximated by an effective coupling constant. I will use these results to derive novel constraints from LEP and calculate state-of-the-art limits from E137 and NA64 for four different scenarios, in which the ALPs couple either to a single lepton generation or universally to all, for both pseudoscalar and derivative lepton couplings. I will collect complementary bounds from astrophysics, flavour, and other laboratory experiments to chart the allowed parameter space of leptophilic ALPs in the MeV-GeV mass range.

Speaker: Marco Fedele

17:50 Underexplored Challenges in LLP/FIP Searches at Colliders and Cosmology

Current searches for Long-Lived Particles (LLPs) and Feebly Interacting Particles (FIPs) face critical systematic limitations in collider detector geometries and cosmological analyses. At the LHC, experiments like ATLAS, CMS, and LHCb employ displaced vertex triggers that approximate detection efficiency with step functions, neglecting the probabilistic survival probability $P_{\text{surv}}(L) = e^{-L/\gamma c\tau}$ for decays occurring at radii $r$. This leads to overestimated sensitivity for long-lived states ($c\tau > 10$ m), particularly at ATLAS/CMS, where edge effects near muon chambers are unaccounted for. LHCb’s extended decay volume (12 m baseline) improves reach for $c\tau \sim 1$–$10$ m, but momentum-dependent vertex resolution degrades for high-mass LLPs ($m \gtrsim 10$ GeV), introducing $\mathcal{O}(30\%)$ uncertainties in mass reconstruction. Furthermore, material interactions from beam-gas collisions generate $\mathcal{O}(10^3)$ fake displaced vertices per fb$^{-1}$ at LHCb’s low-$p_T$ thresholds ($p_T > 0.5$ GeV), necessitating non-Poissonian background treatments. Meanwhile, cosmological constraints on MeV-scale FIPs decaying during the CMB epoch ($z \sim 10^6$) assume instantaneous energy deposition, overlooking time-integrated spectral distortions $\Delta \mu \propto \int \Gamma_{\text{dec}} e^{-\Gamma_{\text{dec}} t} dt$, which weaken limits for $c\tau \sim 10^{12}$ m by $\sim 50\%$. Non-thermal production mechanisms, such as $\phi \to \chi\chi$ during reheating, evade BBN bounds via $\Omega_{\chi} \propto T_{\text{reh}}^3/m_{\phi}$ dependencies, yet remain absent from joint collider-cosmology frameworks. Sub-GeV FIPs with nucleon couplings $g_n \sim 10^{-10}$ further blur boundaries between collider and direct detection exclusions, leaving $\mathcal{O}(1)$ gaps in $m_{\chi} \sim 100$ MeV parameter spaces. To resolve these gaps, we advocate for adaptive trigger strategies combining LHCb’s vertexing with ATLAS/CMS calorimeter timing ($\Delta t > 2$ ns), time-integrated cosmological likelihoods for $c\tau > 10^{10}$ m, and unified beam-gas background models across experiments. This synthesis underscores the need for detector-specific statistical frameworks and cross-disciplinary correlations to close the LLP/FIP discovery gap.

Speaker: Brij Kishor Jashal (Rutherford appelton laboratory)

18:10 → 18:40 [CLOSED SESSION to SC and LOC] Logistics for LLP2026

Wednesday 4 June

09:30 → 11:10 Theory and pheno IV

Conveners: Chandan Hati, Jan Hajer (CFTP, IST, Universidade de Lisboa)

09:30 Long-lived Light Mediators in a Higgs Portal Model at the FCC-ee and FCC-hh

In the search for beyond the Standard Model (SM) physics, long-lived particles (LLPs) have emerged as potential candidates and are being explored in various ongoing experiments. Future lepton colliders, such as the FCC-ee, shall provide an excellent opportunity to probe LLPs, owing to their clean environment and improved particle identification. This study investigates the potential of the proposed Innovative Detector for an Electron-Positron Accelerator (IDEA) detector at FCC-ee in the detection of LLPs produced from $B$-meson and Higgs boson decays. We explore benchmark scenarios for different final states resulting from LLP decays, including a detailed analysis of the SM long-lived hadronic background. Additionally, we propose dedicated LLP detectors with different configurations, dimensions, and locations with respect to the IDEA detector. DELIGHT B, originally proposed as a dedicated LLP detector for the FCC-hh, stands out as the detector with the maximum efficiency for detecting LLPs produced at FCC-ee. We find that cylindrical detector configurations, if feasible to construct around the IDEA detector, would also enhance sensitivity for LLPs mostly decaying outside it. We also discuss briefly the potential of FCC-hh.

Speaker: Rhitaja Sengupta (BCTP and Physikalisches Institut der Universität Bonn, Germany)

09:50 Long-Lived Particle Searches at a Future Higgs Factory with the ILD experiment

Future e$^+$e$^-$ colliders provide a unique opportunity for long-lived particle (LLP) searches. We present a full simulation study of LLP searches using the International Large Detector (ILD), a detector concept for a future Higgs factory, with a gaseous time projection chamber as its main tracking device. Signatures of displaced vertices and kinked tracks are explored. We study challenging final states involving both very soft displaced tracks and boosted, nearly collinear tracks. Backgrounds from beam-induced interactions and other Standard Model processes are considered. We present expected exclusion limits for a model-independent analysis, as well as for Higgs boson decays to LLPs, for a range of LLP lifetimes.

Speaker: Jan Franciszek Klamka (University of Warsaw (PL))

10:10 Heavy Neutral Leptons without prejudice

We analyze prominent signatures in Higgs physics at colliders (LHC and FCC-ee) that arise when the Standard Model (SM) is extended by a Yukawa interaction with heavy neutral leptons (HNLs). This setup assumes no direct mapping between the Yukawa coupling and mixing with active neutrinos. At the LHC, we investigate HNL production through Higgs bosons, followed by decays through Higgs or gauge bosons into SM fermions, to assess the experimental reach at both primary detectors and far detectors like FASER and MATHUSLA. At the FCC-ee, we study HNL production from both Z and Higgs bosons at various center-of-mass energies to evaluate sensitivity limits. Additionally, we explore how precision Higgs measurements constrain beyond-the-SM parameters in the limit of zero neutrino mixing.

Speaker: Dr Kuldeep Deka (New York University Abu Dhabi)

10:30 The LOHENGRIN Experiment: Status and Theory Predictions

We present a proposal for a future light dark matter search experiment at the Electron Stretcher Accelerator ELSA in Bonn: LOHENGRIN.
It employs the fixed-target missing momentum based technique to search for long-lived dark-sector particles.
The LOHENGRIN experiment uses a high rate single electron beam of 3.2 GeV that is extracted from the ELSA accelerator and shot onto a thin target to produce mainly Standard Model bremsstrahlung and - in rare occasions - possibly new particles coupling feebly to the electron.
A well-motivated candidate for such a new particle is the dark photon, a massive gauge boson that arises from a new gauge interaction in a dark sector and kinetically mixes with the hypercharge gauge boson.
The LOHENGRIN experiment is estimated to reach sensitivity to couplings small enough to explain the relic abundance of dark matter in various models for dark photon masses between approximately 1 MeV and 100 MeV.
In the talk we will address the current status of the experiment and show detailed theory calculations that are implemented in a dedicated Monte Carlo code, Lohengrin++.

Speaker: Martin Schürmann

10:50 Improved Mass Bounds on HECOs via Dyson-Schwinger Resummation Technique

High-Electric-Charge compact Objects (HECOs) arise in various extensions of the Standard Model and are actively investigated at the LHC. Existing experimental searches constrain their masses by relying on tree-level predictions from Drell-Yan and photon-fusion production mechanisms. However, these conventional approaches lose validity due to the breakdown of perturbative QED at the high electric charges characteristic of HECOs. To address this limitation, we introduce a Dyson-Schwinger resummation framework that accommodates strong gauge couplings, enabling more robust calculations of HECO production cross sections. This method leads to significantly improved mass bounds derived from current ATLAS and MoEDAL data.

Speaker: Emanuela Musumeci (IFIC - Univ. of Valencia and CSIC (ES))

11:10 → 11:40 Photo. Coffee

11:40 → 13:00 Dedicated experiments IV

Conveners: Audrey Katherine Kvam (University of Massachusetts (US)), Dr Vasiliki Mitsou (IFIC - CSIC and Univ. of Valencia (ES))

11:40 The SHiP Experiment

The BDF/SHiP collaboration has proposed a general-purpose intensity-frontier experimental facility operating
in beam-dump mode at the 400 GeV CERN SPS accelerator to search for feebly interacting GeV-scale particles
and to perform measurements in neutrino physics. CERN is uniquely suited for this programme owing to the proton
energy and yield available at the SPS. In March 2024 the facility got approved at CERN and the final TDRs for the
BDF and the SHiP detectors are under preparation.
In this talk we will discuss the experimental methods, the SHiP detector set-up, the different sub-detectors, and
sensitivities to new particles such as axion-like particles, dark scalars, heavy neutral leptons, light dark matter particles
and more, as well as the presently foreseen schedule towards first data.

Speaker: Albert De Roeck (CERN)

12:10 First physics results from milliQan

We will report the status of the milliQan experiment at CERN. The milliQan "bar" detector was completed in June 2023 and has been taking physics data since then. The milliQan "slab" detector was completed in Fall of 2024 and is being commissioned. We will give an update on the readiness of the slab detectorfor physics data taking. Finally, we will present first physics results on the search for millicharged particles in 127 fb$^{-1}$ pp collision data recorded by the bar detector during LHC Run 3.

Speaker: Chris Hill (Ohio State University (US))

12:30 The Forward Physics Facility

The Forward Physics Facility (FPF) is a proposal developed to exploit the unique scientific potential made possible by the intense hadron beams produced in the far-forward direction at the high luminosity LHC (HL-LHC). Housed in a well-shielded cavern 627 m from the LHC interactions, the facility will enable a broad and deep scientific programme which will greatly extend the physics capability of the HL-LHC. Instrumented with a suite of four complementary detectors – FLArE, FASERν2, FASER2 and FORMOSA – the FPF has unique potential to shed light on neutrino physics, QCD, astroparticle physics, and to search for dark matter and other new particles. This talk describes some of the key scientific drivers for the facility (particularly sensitivity to LLPs), the engineering and technical studies that have been made in preparation for it, the design of its four complementary experiments, and the status of the project’s partnerships and planning.

Speaker: Juan Salvador Tafoya Vargas (University of California Davis (US))

13:00 → 15:00 Lunch

15:00 → 18:30 Free time

18:30 → 20:30 Walking Tour of Valencia

20:30 → 22:30 Workshop dinner at Only You

Thursday 5 June

10:10 → 11:10 ATLAS, CMS, and LHCb results II

Conveners: Andrii Usachov (Nikhef and VU (NL)), Mr Bryan Zaldivar (IFT Madrid)

10:10 Search for heavy neutral leptons in decays of W bosons using leptonic and semi-leptonic displaced vertices in $\sqrt{s}$ = 13 TeV pp collisions with the ATLAS detector

A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a W boson along with a muon or electron. Two channels are explored: a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with $140 fb^{−1}$ of $\sqrt{s} = 13$ TeV proton-proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as $10^{−7}$ are excluded at the 95% confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.

Speaker: Guglielmo Frattari (Brandeis University (US))

10:30 CMS: Compressed electroweakinos in events with a low-momentum, displaced track

Remote

A search is presented for higgsino dark matter (DM) in final states with a low momentum (soft), isolated track and large missing transverse momentum. In the minimal supersymmetric standard model (MSSM), charginos are most often produced in association with a nearly mass-degenerate neutralino or another chargino, and predominantly decay into the lightest neutralino (DM candidate) and a soft pion. For a mass difference $\Delta m^{\pm}$ less than 1 GeV, a discernible displacement of the pion's track with respect to the primary vertex can arise, reaching up to about 1 cm for the smallest allowed $\Delta m^{\pm}$. A parameterized multivariate classifier is employed to distinguish the signal track from background tracks, optimally targeting a range of Δm± by exploiting the track transverse momentum, impact parameter, and event topology to varying degrees depending on the assumed $\Delta m^{\pm}$. The analyzed data correspond to an integrated luminosity of 138 $\text{fb}^{−1}$ collected by the CMS experiment in proton-proton collisions at $\sqrt{s}= 13$ TeV. No evidence of new physics is observed, and limits are set at the 95% confidence level in the mass plane of the model. Assuming MSSM cross sections, values of $\Delta m^{\pm}$ between 0.28 and 1.15 GeV are excluded for a 100 GeV mass chargino, and chargino masses up to 185 GeV are excluded for $\Delta m^{\pm}$ of 0.55 GeV.

Speaker: Samuel Louis Bein (Universite Catholique de Louvain (UCL) (BE))

10:50 Results and prospects of an LHCb Run 3 HNL analysis with very displaced vertex reconstruction

Heavy Neutral Leptons (HNLs) are theoretical long-lived particles introduced to the Standard Model (SM) to explain, among other things, the observed, non-zero SM neutrino mass. In this presentation plans and potential sensitivity of an HNL analysis leveraging the Run 3 (2022-2026) data set of the Large Hadron Collider beauty (LHCb) experiment are discussed. In particular, a novel analysis strategy is presented wherein HNLs are reconstructed inclusively as a very displaced vertex; a vertex displaced between 0.5 and 8.0 meters from the proton-proton interaction point at LHCb. Over contemporary methods, this strategy yields a 40-fold increase in the number of considered HNL events and a 16-fold increase in the HNL lifetime to which we are sensitive. With the results of a sensitivity study, it is shown that these improvements directly translate to an increased reach in the HNL mass-coupling plane, and that with this strategy LHCb has discovery potential and the capacity to set world-best limits in HNL parameter space.

Speaker: Spencer Collaviti

11:10 → 11:40 Coffee

11:40 → 13:00 Theory and pheno V

Conveners: Dr José Francisco Zurita (IFIC - Univ. of Valencia and CSIC (ES)), Lisa Benato (Austrian Academy of Sciences (AT))

11:40 Placing Bounds on New Physics with Displaced Photons

If a neutral long-lived particle decays predominantly into photons and another stable particle, its presence can be reconstructed at the LHC in the form of displaced photons. These are recognized as such in terms of their delayed arrival time $t_\gamma$ and a non-pointing parameter $\Delta z_\gamma$.

In this work we present a detailed recast of a search for displaced photons in ATLAS, and show how it can be used to constrain new physics. In particular, we use the search to put bounds on the Dimension-5 Seesaw Portal (which can explain neutrino masses) and on a dark photon model coupling via a scalar portal to the Standard Model (which can explain dark matter).

Speaker: Dr Joel Jones-Perez

12:00 Long-lived particles in a scotogenic-like model

Neutrino masses and dark matter (DM) might have a common origin. The scotogenic model can be considered the proto-type model realizing this idea, but many other variants exist. In this talk, I will comment on long-lived particles predicted in a scotogenic-like scenario, containing a triplet scalar. We calculate the relic density and check for constraints from direct detection experiments. The parameter space of the model, allowed by these constraints, contains typically a long-lived or quasi-stable doubly charged scalar, that can be searched for at the LHC. We reinterpret existing searches to derive limits on the masses of the scalars of the model and estimate future sensitivities in the high-luminosity phase of the LHC.

Speaker: Giovanna Cottin (Pontificia Universidad Católica de Chile (CL))

12:20 Using BDTs as Surrogate Models for BSM searches in ATLAS

In this talk, I will present a novel approach for the reinterpretation of LHC searches for long-lived particles (LLPs) using Boosted Decision Trees (BDTs). This method allows us to estimate the likelihood that events from a new physics model would have been selected in a prior analysis, based solely on truth-level information. I will discuss the development and validation of method, including its integration within the HackAnalysis framework. Furthermore, I will explore its application to an extended version of the HAHM model with asymmetric LLPs (in mass and lifetime), and highlight the promising results obtained.

Speaker: Abdelhamid Haddad (Laboratoire de Physique de Clermont-Auvergne (LPCA))

12:40 Probing exotic long-lived particles from the prompt side using the CONTUR method

A method to derive constraints on new physics models featuring exotic long-lived particles using detector-corrected measurements of prompt states is presented. The CONTUR workflow is modified to either account for the fraction of long-lived particles which decay early enough to be reconstructed as prompt, or to be sensitive to the recoil of such particles against a prompt system. This makes it possible to determine how many of signal events would be selected in the RIVET routines which encapsulate the fiducial regions of dozens of measurements of Standard Model processes by the ATLAS and CMS collaborations. New constraints are set on several popular exotic long-lived particle models in the very short-lifetime or very long-lifetime regimes, which are often poorly covered by direct searches. The probed models include feebly-interacting dark matter, hidden sector models mediated by a heavy neutral scalar, dark photon models and a model featuring photo-phobic axion-like particles.

Speakers: Bruna Pascual (Université Clermont Auvergne (FR)), Dr Louie Dartmoor Corpe (Laboratoire de Physique Clermont Auvergne (LPCA))

13:00 → 15:00 Lunch

15:00 → 16:50 Trigger streams

Conveners: Arantza Oyanguren (IFIC - Valencia), Matthew Daniel Citron (University of California Davis (US))

15:00 ATLAS: TLA/anomaly

Speaker: Tatiana Ovsiannikova (University of Washington (US))

15:30 CMS: scouting & parking

Speaker: Robert John Bainbridge (Imperial College (GB))

15:55 CMS: Anomaly Detection Triggers

Anomaly detection triggers offer a model-agnostic approach to capturing a wide range of beyond the Standard Model (BSM) signatures, including those from long-lived particles (LLPs). This talk presents an overview of the two machine learning-based anomaly detection triggers deployed in the CMS Level-1 trigger in 2024: AXOL1TL and CICADA. I will discuss their design, implementation, and integration into the CMS trigger. The talk will also outline future directions for enhancing sensitivity to LLPs with such triggers by incorporating Level-1 tracking and precision timing information.

Speaker: Jannicke Pearkes (University of Colorado Boulder (US))

16:10 BuSca: New Strategies for LLP Searches at 30 MHz at LHCb

The new fully software-based trigger of the LHCb experiment operates at a 30 MHz data rate, opening a search window into previously unexplored regions of physics phase space. The BuSca (Buffer Scanner) project at LHCb acquires and analyses data in real time, extending sensitivity to new lifetimes and mass ranges thanks to the recently deployed Downstream tracking algorithm. BuSca identifies hotspots indicative of potential new particle candidates in a model-independent manner, providing strategic guidance for developing new trigger lines. To control background, regions with minimal detector material interactions are selected, and pairs of same-sign tracks are used to suppress combinatorial background. This talk presents the results from the analysis of the first data.

Speaker: Valerii Kholoimov (Instituto de Física Corpuscular (Univ. of Valencia))

16:30 Discussion

16:50 → 17:20 Coffee

17:20 → 18:40 ATLAS, CMS, and LHCb results III

Conveners: Carlos Vazquez Sierra (Universidade da Coruña (ES)), Emma Torro Pastor (Univ. of Valencia and CSIC (ES))

17:20 Search for massive, long-lived charged particles with large specific ionisation and low-$\beta$ in $140~\mathrm{fb^{-1}}$ of $pp$ collisions at $\sqrt{s} = 13$~TeV using the ATLAS experiment

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of $140 fb^{−1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.

Speaker: Judith Höfer (IFIC (Univ. of Valencia and CSIC) (ES))

17:40 CMS: VLLs in the muon system

Remote

A first search is presented for vector-like leptons (VLLs) decaying into a light long-lived pseudoscalar boson and a standard model $\tau$ lepton. The pseudoscalar boson is assumed to have a mass of 2 GeV and to decay exclusively into a pair of photons. It is identified using the CMS muon system. The analysis is carried out using a data set of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment in 2016-2018, corresponding to an integrated luminosity of 138 $\text{fb}^{-1}$. Selected events contain at least one pseudoscalar boson decaying electromagnetically in the muon system and at least one hadronically decaying $\tau$ lepton. No significant excess of data events is observed compared to the background expectation. Upper limits are set at 95% confidence level on the vector-like lepton production cross section as a function of the VLL mass and the pseudoscalar boson mean proper decay length. The observed and expected exclusion ranges of the VLL mass extend up to 700 and 670 GeV, respectively, depending on the pseudoscalar boson lifetime.

Speaker: Daniel Guerrero (Fermi National Accelerator Lab. (US))

18:00 Limits on long-lived axion-like particles from previous searches for exotic decays of the Higgs boson with the ATLAS detector

Two ATLAS searches for anomalous decays of the Higgs boson into pseudo-scalar particles are used to set exclusion limits for models containing decays to long-lived axion-like particles (ALPs). Both searches use the full Run~2 data set of $140 fb^{−1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS experiment. The first search involves the decay of the Higgs boson to a Z boson and a light pseudo-scalar particle promptly decaying to two photons, where in this note long-lived ALPs with masses in the range 2-33 GeV and effective ALP-photon couplings, $|C_{γγ}|/\Lambda$, as small as $3\times10^{−6}$ TeV$^{−1}$ are excluded. The second search involves the decay of the Higgs boson to a pair of pseudo-scalar particles with finite lifetimes each decaying to two photons, where in this note ALPs with low masses in the range 0.01-0.1 GeV and effective ALP-photon couplings as small as 0.1 TeV{^−1} are excluded. Upper limits at 95% confidence level are provided on both the branching ratio of the Higgs boson to a Z boson and a non-promptly decaying ALP, and to a pair of non-promptly decaying ALPs, with values in the range 0.05%-20% depending on the ALP mass and couplings.

Speaker: Marzieh Bahmani (Humboldt University of Berlin (DE))

18:20 Search for long-lived particles in the ATLAS hadronic calorimeter in association with leptons or jets using 13 TeV collision data

A search for neutral long-lived particles (LLPs) decaying in the ATLAS hadronic calorimeter using $140 fb^{−1}$ of proton-proton collisions at $s=\sqrt{13}$ TeV delivered by the LHC is presented. The analysis is composed of three channels. The first targets pair-produced LLPs, where at least one LLP is produced with sufficiently low boost that its decay products can be resolved as separate jets. The second and third channels target LLPs respectively produced in association with a W or Z boson that decays leptonically. In each channel, different search regions target different kinematic regimes, to cover a broad range of LLP mass hypotheses and models. No excesses of events relative to the background predictions are observed. Higgs boson branching fractions to pairs of hadronically decaying neutral LLPs larger than 1% are excluded at 95% confidence level for proper decay lengths in the range of 30 cm to 4.5 m depending on the LLP mass, a factor of three improvement on previous searches in the hadronic calorimeter. The production of long-lived dark photons in association with a Z boson with cross-sections above 0.1 pb is excluded for dark photon mean proper decay lengths in the range of 20 cm to 50 m, improving previous ATLAS results by an order of magnitude. Finally, long-lived photo-phobic axion-like particle models are probed for the first time by ATLAS, with production cross-sections above 0.1 pb excluded in the 0.1 mm to 10 m range.

Speaker: Gordon Watts (University of Washington (US))

Friday 6 June

09:30 → 10:30 Dedicated experiments V

Conveners: Andrii Usachov (Nikhef and VU (NL)), Juliette Alimena (DESY)

09:30 Long-Lived Particle Searches at Beam-Dump Experiments

Long-lived particles (LLPs) could serve as portal to hidden sectors addressing different open problems of the Standard Model (SM). Among these, the evidence for neutrino masses and mixings—observed in neutrino oscillation experiments—stands out as particularly promising. A simple extension to the SM involves the inclusion of heavy right-handed neutrinos, commonly known as heavy neutral leptons (HNLs). In minimal models, the HNL production and decay are governed by weak interactions, suppressed by their mixing with active neutrino. This suppression leads to relatively long HNL lifetimes when their masses lie within the MeV–GeV range. The near detector complexes of current and upcoming neutrino oscillation experiments provide an optimal environment for searching visible HNL decays, given the high intensity of the beams, and the volume of the detectors involved. This talk will use two experiments as benchmarks to discuss sensitivity projections for the coming years. The first is ICARUS, the far detector of the Short Baseline Neutrino (SBN) program, exposed to the 120 GeV proton NuMI beam. The second is the gaseous argon TPC of the Deep Underground Neutrino Experiment (DUNE), planned for the Phase II of the experiment. Our analysis indicates that, thanks to the high-energy, high-intensity proton beam, and advanced detector capabilities of these experiments, the projected constraints on the HNL mixing as a function of its mass will be highly competitive with existing experimental limits. To wrap up, the DUNE expected sensitivity to additional beyond-the-SM extensions, including scalar, axion, and vector portals will be reviewed.

Speaker: Dr Josu Hernandez Garcia (Instituto de Física Corpuscular (IFIC), CSIC-UV)

09:50 Beyond Standard Model searches @protoDUNE-SPS

The Standard Model of particle physics has been remarkably successful in describing fundamental particles and interactions, yet it leaves us with profound mysteries, such as dark matter, the baryon asymmetry of the universe, and the origin of neutrino mass. These enigmas drive the search for physics beyond the Standard Model (BSM). Recent studies indicate that Liquid Argon Time projection Chambers (LArTPC) , as the Prototype of DUNE far detectors installed at CERN, have the potential to detect long-lived BSM particles from one of the targets (T2) in CERN’s north area exposed to the 400 GeV SPS beam. A key demonstration lies in observing neutrinos—well-known weakly interacting particles. Feasibility studies are ongoing with a test carried out using one of the ProtoDUNE detectors that could establish the potential of these detectors' for BSM searches. In this talk, we will highlight the status of our analysis and our plans for future BSM prospects.

Speaker: Animesh Chatterjee (CERN)

10:10 Probing for light new particles with the LUXE experiment

The proposed LUXE experiment (LASER Und XFEL Experiment) at DESY, Hamburg, using the electron beam from the European XFEL, aims to probe QED in the non-perturbative regime created in collisions between high-intensity laser pulses and high-energy electron or photon beams. This setup also provides a unique opportunity to probe physics beyond the standard model. In this talk we show that by leveraging the large photon flux generated at LUXE, one can probe axion-like-particles (ALPs) up to a mass of 350 MeV and with photon coupling of $3×10^{−6}$ GeV$^{−1}$. This reach is comparable to the background-free projection from NA62. In addition, we will discuss the ongoing optimisation of the experimental setup for the ALP search.

Speaker: Raquel Quishpe Quishpe (Karlsruhe Institute of Technology)

10:30 → 10:50 Theory and pheno VI

Conveners: Andrii Usachov (Nikhef and VU (NL)), Juliette Alimena (DESY)

10:30 Revealing the Origin of Neutrino Masses through Displaced Shower Searches in the CMS Muon System

Remote

We study the potential to probe the origin of neutrino masses, by searching for long-lived right-handed neutrinos (RHNs) $N$ in the $B-L$ model and in the RHN-extended Standard Model (SM) Effective Field Theory (EFT). Despite the small active-sterile mixing $|V_{\ell N}|^2$, RHNs are produced abundantly via SM and exotic Higgs production, as long as the Higgs mixing or EFT operator coefficient is sufficiently large. We reinterpret a search for displaced showers in the CMS muon system and we find that it is sensitive to parameter space at and below the seesaw floor, $|V_{\ell N}|^2 \approx 10^{-12}$ ($\ell = e$, $\tau$) for $m_N \approx 40$ GeV. With existing data constraining such well-motivated scenarios of neutrino mass generation, we determine the projected sensitivity at the HL-LHC, motivating dedicated searches for long-lived RHNs with decay lengths $\approx 10$ m.

Speaker: Wei Liu (Nanjing University of Science and Technology)

10:50 → 11:20 Coffee

11:20 → 12:40 ATLAS, CMS, and LHCb results IV

Conveners: Albert De Roeck (CERN), Laura Molina Bueno (Univ. of Valencia and CSIC (ES))

11:20 CMS: Search for long-lived particles using displaced vertices with low-momentum tracks and missing transverse momentum in proton-proton collisions at $\sqrt{s}=13$ TeV

A search for long-lived particles using final states including a displaced vertex with low-momentum tracks, significant missing transverse momentum, and a jet from initial state radiation is presented. The search uses the proton-proton collision data at a center-of-mass energy of 13 TeV collected by the CMS experiment at the CERN LHC in 2017 and 2018 with a total integrated luminosity of $100\text{ fb}^{-1}$. This search adopts specific supersymmetric (SUSY) coannihilation scenarios as benchmark signal models, characterized by a next-to-lightest SUSY particle (NLSP) with a mass difference of less than 25 GeV relative to the lightest SUSY particle (LSP). In the top squark coannihilation model, the NLSP is a long-lived top squark, while the LSP is a bino-like neutralino. In the bino-wino coannihilation model, the NLSPs are long-lived wino-like neutralino and prompt wino-like chargino, and the LSP remains a bino-like neutralino. The search excludes top squarks with masses less than 400-1100 GeV and wino-like neutralinos with masses less than 220-550 GeV, depending on the signal parameters. It sets the most stringent limits to date for the top squark coannihilation and bino-wino coannihilation models.

Speaker: Ang Li (Austrian Academy of Sciences (AT))

11:40 Search for events with one displaced vertex from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in pp collisions at $\sqrt{s} = 13$ TeV

A search for events with one displaced vertex from long-lived particles using data collected by the ATLAS detector at the Large Hadron Collider is presented, using $140 fb^{−1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded in 2015-2018. The search employs techniques for reconstructing vertices of long-lived particles decaying into hadronic jets in the muon spectrometer displaced between 3 m and 14 m from the primary interaction vertex. The observed number of events is consistent with the expected background and limits for several benchmark signals are determined. A scalar-portal model and a Higgs-boson-portal baryogenesis model are considered. A dedicated analysis channel is employed to target Z-boson associated long-lived particle production, including an axion-like particle and a dark photon model. For the Higgs boson model, branching fractions above 1% are excluded at 95% confidence level for long-lived particle proper decay lengths ranging from 5 cm to 40 m. For the photo-phobic axion-like particle model considered, this search produces the strongest limits to date for proper decay lengths greater than O(10) cm.

Speaker: Julian Friedrich Wack (University of Cambridge (GB))

12:40 → 13:00 Closing

12:40 Looking forward to LLP 2026

Speaker: TBC