Higgs Pairs Workshop 2025

11 May 2025, 16:00 → 17 May 2025, 19:30 Europe/Rome

Isola d'Elba

Welcome to Higgs Pairs Workshop

We are proud to announce that the INFN Pisa together with the Universities of Pisa and Siena will host the Higgs Pairs Workshop in the Hotel Hermitage, La Biodola, located on the wonderful Isola d'Elba, from May 11th to 17th,  2025.

We foresee a thorough review of the latest theoretical results on Higgs boson pair production and on searches for new physics in the scalar sector, with final results from LHC Run 2, results from the ongoing LHC Run 3, and the prospects and potential of the di-Higgs searches at the future colliders.

The workshop will provide an unique opportunity for critical discussions. The main goal is to provide intense and fruitful discussions between experimentalists and theoreticians. 

Contributions will be organised in parallel and plenary sessions on the current research topics related to the di-Higgs sector, with invited and submitted talks.

The workshop will be held in person, and is limited to 280 participants, due to maximum number of the available hotel rooms. Once this limit is reached, additional registrants will be placed on a first-come first-served waiting list.

Organizing Institution

Partnership

Sponsors

Warning : the detailed program is being finalised so adjustments of the schedule can be expected

Conference Photo1.jpeg

Conference Picture2.jpeg

HH2025 Bulletin 2.pdf

HH2025 Bullettin 1.pdf

Higgs_pairs2025poster.pdf

Preliminary_Schedule.pdf

Privacy.pdf

SessionsSchedule.png

Sunday 11 May

16:00 → 19:30 Registration

19:30 → 20:30 Welcome cocktail

Monday 12 May

09:00 → 12:30 Plenary: Opening Session

Conveners: Ramona Groeber (Università di Padova and INFN, Sezione di Padova), Stefano Manzoni (CERN)

09:00 Welcome

Speakers: Agnese Ciocci (Università di Siena / INFN), Alberto Annovi (INFN Sezione di Pisa)

Opening_HH2025_20250512.pdf

09:10 Welcome from the Programme Committee

Speaker: Ramona Groeber (Università di Padova and INFN, Sezione di Padova)

Higgs Pairs 2025 Intro.pdf

09:15 Theory status for non resonant HH

Speaker: Joshua Davies (University of Liverpool)

davies.pdf

09:45 Overview of CMS HH non-resonant results

Speaker: Agni Bethani (Universite Catholique de Louvain (UCL) (BE))

HH_CMS_HHworkshop2025.pdf

10:10 Overview of ATLAS HH non-resonant results

Speaker: Katharine Leney (Southern Methodist University (US))

HiggsPairs_ATLAS_nonresHH_KLeney.pdf

10:35 Coffee break

11:05 Theory status for resonant HH and Scalar+H

Speaker: Tania Robens (Rudjer Boskovic Institute (HR))

robens_dihiggs.pdf

11:35 Overview of ATLAS HH and Scalar+H resonant searches

Speaker: Minori Fujimoto (CPPM, Aix-Marseille Université, CNRS/IN2P3 (FR))

ATLAS_resonant_HH_summary.pdf

12:00 Overview of CMS HH and Scalar+H resonant searches

Speaker: Loukas Gouskos (Brown University (US))

lg_resonanthh_overview_cms_hh2025.pdf

12:30 → 15:30 Lunch break

15:30 → 19:30 Plenary: Experimental performances

Conveners: Andrea Carlo Marini (Universita & INFN Pisa (IT)), Marco Valente (CERN)

15:30 Trigger performance for HH at ATLAS and CMS

Speakers: Liaoshan Shi (UCL (GB)), Silvio Donato (Universita & INFN Pisa (IT))

Trigger performance for HH at ATLAS and CMS.pdf

16:00 Jet and missing transverse momentum performance at ATLAS and CMS

Speakers: Charles Lewis (University of Washington (US)), Fabio Iemmi (Chinese Academy of Sciences (CN))

JETMET_HiggsPairs2025_ATLAS+CMS .pdf

16:30 Performance in tracking and flavour tagging at ATLAS and CMS

Speakers: Angela Maria Burger (Centre National de la Recherche Scientifique (FR)), Angela Zaza (Universita e INFN, Bari (IT))

talk_HiggsPairs_tracking_tagging-1.pdf

17:00 Coffee break

17:30 Tau performance at ATLAS and CMS

Speakers: Konstantin Androsov (Texas A & M University (US)), Sinead Farrington (University of Edinburgh)

sinead_diH_130525.pdf

sinead_diH_130525.pptx

tau_CMS.pdf

18:00 Photons and leptons performance at ATLAS and CMS

Speakers: Filippo Errico (Sapienza Universita e INFN, Roma I (IT)), Rosy Nikolaidou (Université Paris-Saclay (FR))

FE_RN_HiggsPairs2025.pdf

18:30 ATLAS object performance at HL-LHC

Speaker: Lorenzo Santi (CERN)

250516 - ATLAS object performance at HL-LHC.pdf

18:50 CMS object performance at HL-LHC

Speaker: Nicola Amapane (Universita e INFN Torino (IT))

250512_Amapane.pdf

Tuesday 13 May

09:00 → 12:30 Plenary: non-resonant HH

Conveners: Fabio Monti (CERN), Valentina Cairo (CERN)

09:00 ATLAS and CMS non-resonant HH searches in the 4b final state

Speakers: Marina Kolosova (University of Florida (US)), Yizhou Cai (Nanjing University (CN))

HiggsPairs2025_bbbb.pdf

09:30 ATLAS and CMS non-resonant HH searches in the 2b2γ final state

Speakers: Adelina D'Onofrio (INFN Napoli (IT)), Irene Dutta (Fermi National Accelerator Lab. (US))

HHPairs2025_bbgg_nonResonant (5).pdf

10:00 ATLAS and CMS non-resonant HH searches in the 2b2τ final state

Speakers: Carlo Schiavi (INFN e Universita Genova (IT)), Silvia Goy Lopez (CIEMAT - Centro de Investigaciones Energéticas Medioambientales y Tec. (ES))

NonResonant2b2tau.pdf

10:25 LOC Updates

Speakers: Alberto Annovi (INFN Sezione di Pisa), Prof. Maria Agnese Ciocci (Universita di Siena & INFN Pisa (IT))

tuesdaynews.pdf

10:30 Coffee break

11:00 News on precision computations of HH: EW corrections

Speaker: Huai-Min Yu (Peking Univeristy)

gg_HH_Elba.pdf

11:25 MC generators for HH in gluon fusion and VBF

Speaker: Gudrun Heinrich (KIT)

HiggsPairs2025_GH.pdf

11:50 EFTs for HH

Speaker: Christoph Englert

2025_HH_Englert.pdf

12:30 → 15:30 Conference PHOTO + Lunch Break

15:30 → 18:30 Plenary: non-resonant HH

Conveners: Alessandra Betti (Sapienza Università e INFN, Roma I (IT)), Nan Lu (University of Science and Technology of China (CN))

15:30 ATLAS and CMS non-resonant HH searches in the 2b2l/2b2V final states

Speakers: Abhisek Datta (University of California Los Angeles (US)), Anyes Taffard (University of California Irvine (US))

HH_bbVV_non_resonant_Higgs_Pairs_Workshop.pdf

16:00 ATLAS and CMS non-resonant HH search in multi-leptons final states

Speakers: Ana Sculac (University of Split. Fac.of Elect. Eng., Mech. Eng. and Nav.Architect. (HR)), Anamika Aggarwal (Johannes Gutenberg Universitaet Mainz (DE))

HiggsPairs_HHMLtalk.pdf

16:30 Di-Higgs combination, single-Higgs constraint and EFT interpretation in ATLAS and CMS

Speaker: Marco Delmastro (CNRS/IN2P3 LAPP)

Delmastro_2025-05-13_HiggsPairs_v3.pdf

17:00 Conference PHOTO + Coffee break

17:30 CMS results with EFT and UV completion

Speaker: Torben Lange (National Institute of Chemical Physics and Biophysics (EE))

ElbaHiggsPairsTLange.pdf

17:50 HL-LHC projections for di-Higgs non-resonant searches at ATLAS and CMS

Speaker: Davide Zuolo (University of Colorado - Boulder (US))

LoZuolo_DiHiggsWorkshop2025_NonResonantProj_final.pdf

18:20 Reminder: social event at 21:30

Speaker: Sandra Leone (Universita & INFN Pisa (IT))

tuesdaynews.pdf

18:20 → 19:20 Poster: Session

List of poster contributions

21:30 → 22:30 Social Event

Wednesday 14 May

09:00 → 11:00 Plenary: Rare productions and quartic couplings

Conveners: Alex Cerri (Università di Siena / INFN), Paolo Francavilla (Universita & INFN Pisa (IT))

09:00 Triple-Higgs searches in ATLAS

Speaker: Alessandra Betti (Sapienza Università e INFN, Roma I (IT))

ABetti_HiggsPairs2025_HHH.pdf

09:20 Resonant BSM for HHH

Speaker: Dr Andreas Papaefstathiou (Kennesaw State University, GA, USA)

HiggsPairs_May2025.pdf

09:45 Triple Higgs boson production in EFTs from on-shell amplitude techniques

With the measurement of double Higgs production within reach, it is justified to expand our interest to more exotic processes, such as three Higgs boson production. This rare process is crucial for advancing our understanding of the Higgs boson, its interactions (including self-coupling), the Higgs potential, and the mechanism of electroweak symmetry breaking (EWSB). This talk explores the application of on-shell amplitude techniques to study triple Higgs production via gluon fusion and vector boson fusion in EFTs. We construct the relevant kinematic structures, match our results to both HEFT and SMEFT, and analyze the implications. The comparison provides valuable insights into Higgs dynamics and the interplay between EFT frameworks. The talk based on ongoing work.

Speaker: Michal Ryczkowski

Ryczkowski_HH_Elba_2025.pdf

10:05 Connecting Multi-Higgs production and the Electroweak Phase Transition

Exploring the Higgs sector via multi-Higgs production searches is a main goal for run-3 and high-lumi LHC. Can these searches inform us about the electroweak phase transition and matter-antimatter asymmetry?
We address this question in the context of the TRSM (Two-Real-Singlet Model), which has known benchmark points enhancing multi-Higgs production.
We update the triple-Higgs production benchmark points to include refined perturbativity bounds and explore the type of electroweak phase transition that occurs in the early universe; whether continuous or the first-order discontinuous phase transition desired for matter-antimatter asymmetry.
After presenting our work, I outline lessons on correlating the type of electroweak phase transition and the enhancement of di-Higgs or triple Higgs production, highlighting the importance of the theory's vacuum expectation value of today and the symmetries of the model.

Speaker: Osama Karkout (Nikhef National institute for subatomic physics (NL))

EWSB_HHH_ELBA.pdf

10:25 Search for HH in rare production modes (VHH, ttHH) at ATLAS and CMS

Speaker: Jessy Daniel (Centre National de la Recherche Scientifique (FR))

HiggsPairWorkshop_110525.pdf

10:50 Conference Photos

ConferencePhotos.pdf

11:00 → 19:00 Excursions

Thursday 15 May

09:00 → 12:30 Plenary: Resonant HH and scalar+H

Conveners: Giuseppe Bagliesi (Universita & INFN Pisa (IT)), Marco Valente (CERN)

09:00 Overview of BSM models not yet excluded (REMOTE)

REMOTE PRESENTATION

Speaker: Georg Ralf Weiglein (Deutsches Elektronen-Synchrotron (DE))

Hpairs_models_25.pdf

09:30 ATLAS and CMS HH/SH resonant searches in the 4b final state

Speakers: Lidija Zivkovic (Institute of physics Belgrade (RS)), Santeri Laurila (CERN & Helsinki Institute of Physics (FI))

HHPairs2025_ATLAS_CMS_Resonant4b.pdf

10:00 ATLAS and CMS HH/SH resonant searches in the 2b2γ final state

Speakers: Elise Anne Marie Jourd'Huy (Centre National de la Recherche Scientifique (FR)), Grigorii Tolkachev (CPPM, Aix-Marseille Université, CNRS/IN2P3 (FR))

XToSHTobbgg_ElbaHH25.pdf

10:30 Coffee break

11:00 Interference/NLO effects in resonant Higgs pair production

Speaker: Johannes Alf Braathen (DESY)

Braathen_HiggsPairs2025.pdf

11:30 ATLAS and CMS HH/SH resonant searches in the 2b2τ final state

Speakers: Allison Mccarn Deiana (Southern Methodist University (US)), Valeria D'Amante (Universita & INFN Pisa (IT))

HiggsPairs2025 (3).pdf

12:00 ATLAS and CMS HH/SH resonant searches in other final states

Speakers: Devin Michael Aebi (Texas A & M University (US)), Mr Soumyananda Goswami (Oklahoma State University (US))

CMS-ATLAS-HH_SH_RES_DEVIN_GOSWAMI.pdf

12:30 → 15:00 Lunch break

14:00 → 14:45 Parallel: Triple Higgs discussion (for YR5)

Conveners: Fabio Monti (CERN), Greg Landsberg (Brown University (US))

14:00 Discussion

Speaker: Greg Landsberg (Brown University (US))

HHH-Discussion.pdf

14:59 → 17:30 Plenary: Resonant HH and scalar+H

Conveners: Dario Buttazzo, Paolo Azzurri (Universita & INFN Pisa (IT))

14:59 News

Speakers: Alberto Annovi (INFN Sezione di Pisa), Maria Agnese Ciocci (Universita di Siena & INFN Pisa (IT))

thursday_news.pdf

15:00 Combination of HH and Scalar+H resonant searches at ATLAS and CMS

Speaker: Alexandra Carvalho Antunes De Oliveira (Peking University (CN))

HH_HY_resonant_HIggsPairs_final.pdf

15:25 Theory interplay between HH and diboson resonances

Speaker: Andrea Tesi (INFN)

Andrea_Tesi_HH.pdf

15:50 BSM Composite Higgs Models

Speaker: Stefania De Curtis (Universita e INFN, Firenze (IT))

HiggsPairs25.pdf

16:15 Summary of statistical excesses in di-Higgs and multi-boson searches

Speaker: Robert Les (Michigan State University (US))

Multiboson_Excesses.pdf

16:35 General CP-violating 2HDM in light of the excesses in di-photon searches at the LHC

The most general CP-violating two-Higgs-doublet model (2HDM) in the Yukawa alignment limit can produce significant di-photon branching ratios for the neutral scalars of the model. In the Higgs basis, where the two scalar doublets are identified as $H_1$ and $H_2$, the lagrangian term $Z_7 H_1^\dagger H_2 H_2^\dagger H_2 + {\rm h.c.}$ enters the branching ratios to di-photon of the new physics (mostly) CP-even scalar $H$ and the (mostly) CP-odd scalar $A$. While the $\Re[Z_7]$ contributes to $H\to\gamma\gamma$, the $\Im[Z_7]$ affects $A\to\gamma\gamma$ and can also be correlated to CP violating observables, such as non nonzero electric dipole moments.

In this framework, we derive bounds on electroweak scale scalars, using the recently observed statistically significant excesses at $95\,\text{GeV}$ and $152\,\text{GeV}$ as indicative benchmarks. In particular, the $152\,\text{GeV}$ scenario requires associated production of new physics scalars via Drell-Yan.

Based on: https://arxiv.org/abs/2412.00523

Speaker: Guglielmo Coloretti (University of Zurich (UZH) / Paul Scherrer Institute (PSI))

DiHiggs_2025_Coloretti.pdf

17:00 Coffee break

17:30 → 19:00 Plenary: LHCHWG YR

Conveners: Dr Andreas Papaefstathiou (Kennesaw State University, GA, USA), Fabio Monti (CERN)

17:30 The LHCHWG YR5

Speaker: Liza Brost (Brookhaven National Laboratory (US))

The LHCHWG YR5.pdf

17:45 Higgs pair production with higher power dependencies of the self-couplings

The precise determination of the Higgs boson self-couplings is essential for understanding the mechanism behind electroweak symmetry breaking. However, due to the limited number of Higgs boson pair events at the LHC, only loose constraints have been established so far. Current constraints are based on the assumption that the cross section is a quadratic function of the trilinear Higgs self-coupling within the κ framework. Incorporating higher-order quantum corrections from virtual Higgs bosons would significantly alter this function form, introducing new quartic and cubic power dependencies on the trilinear Higgs self-coupling. To derive this new function form, we propose a specialized renormalization procedure that tracks all Higgs self-couplings at each calculation step. Additionally, we introduce renormalization constants for the coupling modifiers within the κ framework to ensure the cancellation of all ultraviolet divergences. With the new function forms of the cross sections in both the gluon-gluon fusion and vector boson fusion channels, the upper limit of κλ3H = λ3H/λSM3H by the ATLAS (CMS) collaboration is reduced from 6.6 (6.49) to 5.4 (5.37). However, extracting a meaningful constraint on the quartic Higgs self-coupling λ4H from Higgs boson pair production data remains challenging. We also present the invariant mass distributions of the Higgs boson pair at different values of κλ, which could aid in setting optimal cuts for experimental analysis.

Speaker: Prof. Jian Wang (Shandong University)

WangJian_HH.pdf

18:05 top quark mass renormalisation scheme uncertainty

Speaker: Sebastian Jaskiewicz

hh25_jaskiewicz.pdf

18:35 Taming a leading theoretical uncertainty in HH measurements via accurate simulation for bbH production

We present a new simulation for Higgs boson production in association with bottom quarks ($bbH$) at next-to-leading order (NLO) matched to parton showers. The contributions proportional to the bottom-quark Yukawa coupling and top-quark Yukawa coupling (from gluon fusion) are both taken into account in a scheme with massive bottom quarks. The $bbH$ process constitutes a crucial background to measurements of Higgs-boson pair ($HH$) production at the LHC when at least one of the Higgs bosons decays to bottom quarks. So far, the modeling of $bbH$ induced one of the dominant theoretical uncertainties to $HH$ measurements, as the gluon-fusion component was described only at the leading order with uncertainties of O(100%). Including NLO corrections allows to reduce the scale dependence to O(50%). We provide an in-depth analysis of the $bbH$ background to $HH$ measurements and we propagate the effect of the new $bbH$ simulation to $HH$ searches in the $2b2\gamma$ and $2b2\tau$ final states.

Speaker: Elena Mazzeo (Università degli Studi e INFN Milano (IT))

250515_HHWorkshop_ggHbb_modelling.pdf

20:00 → 00:00 Gala Dinner

Friday 16 May

09:00 → 12:30 Parallel: Future colliders

Conveners: Alex Cerri (Università di Siena / INFN), Dario Buttazzo (INFN Pisa (IT))

09:00 A Future Higgs Factory - The Circular Electron Positron Collider (CEPC)

The Circular Electron Positron Collider (CEPC) was proposed shortly after the discovery of the Higgs boson at the LHC, as a Higgs factory, with the possibility of being upgraded to a proton-proton collider in the future. The CEPC Study Group has been continuously working on the design of the accelerator, the R&D of critical technologies and components for both the collider and the instrumentation for the experiments. The group recently published the Technical Design Report of the e+e- collider and is in the process of completing a reference design of the detector system. The CEPC will be proposed to the Chinese government for approval for beginning the construction in the 2027-8 period. The CEPC is intended to be an early Higgs factory for the particle physics community. The author will report on the status, recent progress and the plan of the CEPC project.

Speaker: Mingshui Chen (Chinese Academy of Sciences (CN))

202505-HH-CEPC-Overview-v3.pdf

09:30 The FCC project

Speaker: Marina Cobal (Universita degli Studi di Udine (IT))

Elba_cobal_final.pdf

Elba_cobal_final.pptx

10:00 A Linear Collider Facility for CERN

Speaker: Jenny List (Deutsches Elektronen-Synchrotron (DE))

jlist_LCVision_HPairs_250516.pdf

10:30 Coffee break

11:00 Trilogy of event reconstruction at electron positron Higgs factory (REMOTE)

REMOTE PRESENTATION

The electron positron Higgs factory is regarded as the highest priority future collider facility, as it could bring decisive data from the Higgs portal to address the long standing Known Unknowns such as the origin of matter, the nature of Dark matter, the naturalness problem, etc. Meanwhile, these facilities, especially the circular ones like CEPC or FCC, could produce huge statistic of Massive Standard Model particles that enables the exploration at EW, Flavor, QCD, and direct New Physics signal search, which greatly enhances their physics merits.

Hadronic events are the bulk part of physics events at future electron positron Higgs factory. For instance, 97% of ZH signal decays into final state with jets, while the majority are actually full hadronic events. Therefore, an efficient reconstruction of those hadronic events is critical for the physics exploration at future Higgs factory, and, actually the entire high energy frontier.
To address this requirements, we propose and realize a trilogy for the event reconstruction at future Higgs factory using Artificial Intelligence and state of art detector design.
First, one-one correspondence reconstruction that aims at efficiently reconstruct and identify all the visible particles, or, to some extend, could be regarded as the confusion free Particle Flow reconstruction with Perfect particle identification;
secondly, jet origin identification that distinguish jets originated from 11 different kinds of colored particles; and thirdly, color singlet identification that aims at distinguish the color singlet origin of each reconstructed particles, for example, in a full hadronic ZH event or vvHH events, to identify from which boson a final state particle is generated.
We will present the current status of relevant performance studies, and discuss its impact on the physics exploration at future collider experiment.

Speaker: Manqi Ruan (Chinese Academy of Sciences (CN))

Trilogy Di-Higgs - v2.pdf

11:20 Analysis improvements for the Higgs self-coupling measurement at ILD

A precise determination of the Higgs self-coupling represents a cornerstone of the physics program of future colliders because it gives important insights into the shape of the Higgs potential and thus into the evolution of the early universe. This contribution will present an update of the analysis of di-Higgs production in $e^+e^-$ collisions at \mbox{500 GeV} using detailed simulations of the ILD detector concept, incorporating advancements through state-of-the-art particle ID, flavor tagging and ML-driven event selection. In particular, we will apply these tools to the $HH \rightarrow b\bar{b}b\bar{b}$ and $Z \rightarrow q\bar{q}$/$e^+$$e^-$/$\mu^+$$\mu^-$/$\bar{\nu}\nu$ channels. Based on the experience of previous analyzes, we extrapolate these to cover the contributions of other decay modes and from the $W^+W^-$ fusion production mode. We study the dependency of the results on the center-of-mass energy as well as on the value of the trilinear coupling realized in nature.

Speaker: Bryan Bliewert (Deutsches Elektronen-Synchrotron (DE))

Bliewert_HiggsPairs2025_ILD_ZHH.pdf

11:40 One Loop BSM Triple Higgs Couplings at future electron-positron colliders via double Higgs production

We analyze the impact of one-loop corrections to triple Higgs bosons on the di-Higgs production cross section at $e^+e^-$ colliders within the two Higgs doublet model (2HDM). In particular, we study the production cross section of two SM-like Higgs bosons together with a boson, also known as double Higgs-strahlung. The one-loop triple Higgs couplings are calculated using the one-loop effective potential, and in the case of the SM-like Higgs boson self-coupling $\lambda_{hhh}$, a full one-loop diagrammatic calculation is also considered. We show that one-loop corrections to the Higgs self-coupling can enhance the di-Higgs production cross section by up to a factor of about five with respect to the SM prediction for a center-of-mass energy of 500 GeV and 1 TeV. These large corrections, originating in the one-loop corrections to $\lambda_{hhh}$, arise from the large couplings of the SM-like Higgs boson with other heavy BSM Higgs bosons, while being in agreement with the main theoretical and current experimental constraints. In addition, we discuss the momentum effects from the full one-loop self-coupling prediction, and we show that they are small compared to the results obtained with the effective potential. We also analyze some scenarios where the one-loop corrections to the triple Higgs couplings can affect the resonant production of a heavy neutral Higgs boson, and discuss the implications for the structure of the resonance peak, as well as the accessibility of the BSM triple Higgs coupling $\lambda_{hhH}$ at $e^+e^-$ colliders.

Speaker: Francisco Arco (DESY)

HH-arco.pdf

12:00 Looking for a SFOEWPT in the RxSM at the HL-LHC and LISA

We explore the real-singlet extension of the Standard Model without a Z2 symmetry (RxSM) as a framework to address the baryon asymmetry of the Universe and investigate modifications to the Higgs potential. First, we identify regions of parameter space that allow a Strong First-Order Electroweak Phase Transition (SFOEWPT) using the public tools BSMPT, CosmoTransitions, and TransitionListener, while incorporating relevant theoretical constraints as well as experimental bounds through HiggsTools. Additionally, we calculate the stochastic gravitational wave background and assess its potential observability at LISA. Next, we determine the one-loop corrections to the trilinear Higgs couplings involved in di-Higgs production ($\lambda_{hhh}$ and $\lambda_{hhH}$) using the public code anyH3. Finally, we evaluate the di-Higgs production cross section at the (HL-)LHC within the regions of the RxSM parameter space that permit an SFOEWPT, taking into account the one-loop corrections to the trilinear Higgs couplings. This result is compared with those obtained in the SM and in the RxSM at tree level, highlighting the significant impact of loop corrections on the trilinear couplings.

Speaker: Alain Verduras Schaeidt (DESY)

AVS_HP.pdf

09:00 → 10:30 Parallel: Performances

Conveners: Alessandra Betti (Sapienza Università e INFN, Roma I (IT)), Giuseppe Bagliesi (Universita & INFN Pisa (IT))

09:00 A calibratable jet-free HH(4b) search framework at the LHC

A calibratable experimental strategy is proposed to enhance the $HH(4b)$ search sensitivity via full-particle classification. Inspired by the competitive performance from the boosted-topology $HH$ analysis, which uses state-of-the-art jet neural networks to analyze $o(100)$ particles within large-$R$ jets, this approach aims to extend its strong signal-to-background discrimination power beyond the boosted regime to a broader phase space accessible through conventional $HH(4b)$ triggers.

The approach involves training a universal classifier to distinguish $X\rightarrow Y_1 Y_2 \rightarrow b\overline{b} b\overline{b}$ signals from QCD and $t\overline{t}$ multijet backgrounds across a wide range of $X$ and $Y_{1,2}$ mass values, and simultaneously estimating the $Y_{1,2}$ masses via a multiclass classification technique. Results demonstrate that the background suppression capability matches that of identifying two boosted $X\rightarrow b\overline{b}$ jets, revealing a scaling law governing signal and background yields in both cases. The framework is complemented by a robust signal calibration and validation procedure: event-level classier calibration is performed using an orthogonal dimuon-triggered phase space and an ``event hemisphere mixing’’ technique to construct fake $ZZ(4b)$ events; validation is then conducted using genuine $ZZ(4b)$ data passing the analysis trigger. With combined Run 2 and 3 datasets, the proposed strategy can achieve the first observation of the $ZZ(4b)$ process and deliver a search sensitivity for $HH(4b)$ comparable to HL-LHC projection. This approach holds a great premise to accelerate the pace of HH search at the LHC and advance our understanding of the Higgs self-coupling.

Speaker: Congqiao Li (Peking University (CN))

25.05.16_HH2025_JetFreeHH4b_CL.pdf

citable DOI

preview-hh4b.jpg

09:20 CMS Level-1 Trigger improvements for HH at the HL-LHC

The CMS Level-1 Trigger will be upgraded for the HL-LHC in order to cope with the increased pileup and upgraded detectors. In particular, the inclusion of the tracker will enable offline-like Particle Flow reconstruction at L1, which will allow preserving and even potentially extending the trigger acceptance. This talk will give a brief overview of the upgraded L1 Trigger, the expected performance and focus on improvements that will improve sensitivity to HH final states.

Speaker: Artur Lobanov (University of Hamburg (DE))

CMS_Phase2_Trigger_HH_Elba2025_v1.pdf

09:40 ATLAS advanced identification techniques for Boosted $H \to \tau\tau$

The identification of Lorentz-boosted Higgs boson is a critical challenge in advancing Higgs pair production studies. This talk provides a comprehensive overview of recent developments on techniques to identify boosted Higgs bosons decaying into tau lepton pairs ($H \to \tau\tau$), on offline reconstruction. Both hadronic ($H \to \tau_{\mathrm{had}}\tau_{\mathrm{had}}$) and leptonic ($H \to \tau_{\mathrm{had}}\tau_{\mu}$) decay channels are considered. The presentation explores the progress on several approaches, using machine learning-based algorithms adapted to the decay properties of tau leptons in the boosted regime. The presentation highlights the importance of the tagging strategy in the boosted regime to enhance the sensitivity of the Higgs pair production analyses.

Speaker: Gadi Ninio (Tel Aviv University (IL))

HH2025_boostedDitau_GadiNinio.pdf

10:00 Advances in Trigger Strategies for the Run 3 HH->bbtautau search in the ATLAS experiment

The HH searches are fundamentally constrained by statistical limitations. Enhancing the sensitivity to the HH → bbττ decay channel, the most sensitive process for Higgs pair production in ATLAS, necessitates innovative trigger strategies. Conventional trigger approaches adopted for the HH → bbττ channel in Run 2, focusing solely on di-τ triggers have faced challenges in capturing the full spectrum of events. The ATLAS collaboration is considering for the Run 3 analysis the usage of di-b-jet triggers in addition to improved di-τ triggers and implemented also a new b+tau trigger at the start of 2024 for increasing the trigger selection efficiency for the HH → bbττ signal. By this point, the ATLAS experiment has already recorded a remarkable integrated luminosity of 180 fb⁻¹ of Run 3 data, larger than the total Run 2 luminosity, with an additional 1.5 years of data collection remaining. This advancement is a crucial step toward overcoming current limitations and improving the efficiency of the HH → bbττ analysis.

Speaker: Davide Fiacco (Sapienza Universita e INFN, Roma I (IT))

HH_Biodola_16_05_2025.pdf

10:15 Calibration of the GN2 b-tagging algorithm with ttbar and Z+jets events using 140 fb^-1 of ATLAS proton-proton collsion data at sqrt(s)=13 TeV

Jet flavour-tagging algorithms to identify hadronic jets containing b-hadrons are crucial tools for searches of Higgs boson pair production in the ATLAS experiment at the LHC, due to the large branching ratio of the H-->bb process. GN2, the state-of-the-art jet flavour-tagging algorithm developed by ATLAS, delivers a performance that significantly surpasses its predecessors and is expected to significantly enhance the sensitivity of Higgs pair production searches featuring b-quarks in their final states. This talk presents the calibration of GN2’s performance on b, c, and light-flavour jets on real data, using ttbar and Z+jets candidate events selected from 140 fb^-1 of proton-proton collision data collected at a center of mass energy of sqrt(s)= 13 TeV between 2015 and 2018.

Speaker: Martino Tanasini (Stony Brook University (US))

HiggsPairs 2025-3.pdf

09:00 → 12:30 Parallel: non-resonant HH

Conveners: Andrea Carlo Marini (Universita & INFN Pisa (IT)), Valentina Cairo (CERN)

09:00 A novel implementation of the Matrix Element Method at next-to-leading-order (NLO) for the measurement of the Higgs tri-linear coupling in di-Higgs production at the LHC

One of the LHC's priorities, following the discovery of the Higgs boson, is to observe the production of Higgs pairs and to measure the Higgs tri-linear coupling $\lambda_{3H}$.
Due to the rarity of di-Higgs production, measuring $\lambda_{3H}$ has proven to be highly challenging. Exclusion limits have been observed using a variety of approaches, including cut-based methods and boosted decision trees (BDTs).
To address this difficulty from a new perspective, our work explores the application of the Matrix Element Method (MEM), a technique that has demonstrated its effectiveness in multiple analyses in which measurements were performed in processes that were rare (at the time). One can mention the primordial role of the MEM in measuring the top-quark mass at Tevatron, or its role in the first evidence for single top production in the $s$-channel at the LHC for example.
The MEM is a statistically optimal multivariate method that maximizes the utilization of both the experimental and theoretical information available to an analysis, making it inherently well-suited to rare process searches and Standard Model measurements at particle accelerators like the LHC.
The MEM avoids the application of strict selection requirements to the data. It accounts for the entire dataset (with minimal preselection), which is very important when dealing with such rare processes.
Most MEM studies have been limited to leading-order (LO) accuracy, with extensions to next-to-leading-order (NLO) explored only in specific cases due to the additional complexities introduced by virtual and real contributions. Building a MEM at LO is already a highly challenging task, and incorporating NLO formalism substantially amplifies this difficulty due to the increased computational demands and complex theoretical requirements.
To contribute to the measurement of $\lambda_{3H}$ from LHC data in the $gg \to HH \to b\bar{b}\gamma\gamma$ channel, we developed a MEM framework by working on a new NLO implementation (which can be applied in many more analyses).
This MEM framework utilizes state-of-the-art Matrix Elements at NLO from the POWHEG-BOX-V2 and MG5_@NLO software packages. The framework is implemented within a modified version of MoMEMta, a software designed for managing multi-variable phase-space integration, which has been extended to incorporate this new NLO implementation.
To our knowledge, this work marks the first application of the MEM at NLO accuracy to the search for $HH$ and the measurement of $\lambda_{3H}$. This also represents the first application of the MEM using this new NLO formalism.
We have demonstrated that this NLO framework is functional and effective.
We also applied this framework to Monte Carlo (MC) simulated samples in a search for $\lambda_{3H}$, achieving promising results. This study aims to introduce this new approach to the community and position the MEM (at NLO) as a competitive alternative to other established methods to determine the Higgs self-coupling $\lambda_{3H}$.

Speaker: Matthias Jean Tartarin (Laboratoire des 2 Infinis - Toulouse, CNRS / Univ. Paul Sabatier (FR))

MatrixElementMethod@NLO HH 2025 TARTARIN.pdf

09:20 New results for HH→bbγγ with the ATLAS experiment

Speaker: Spyridon Merianos (Aristotle University of Thessaloniki (GR))

HH_bbyy_HiggsPairs_2025_Merianos_v5.pdf

09:40 Search for Higgs Pair Production Decaying into 2 Photons and 2 b Quarks at the High-Luminosity LHC

The Standard Model (SM) is currently the most predictive framework for describing interactions among the fundamental constituents of matter. It includes 19 free parameters, which are measured experimentally, notably with the ATLAS detector at the Large Hadron Collider (LHC) at CERN. These parameters encompass particle masses, phases, and coupling values. One of these couplings, the Higgs boson self-coupling, stands out due to its unique nature: Only a scalar can possess a self-coupling, and it is a key parameter in shaping the Higgs potential, explaining the origin of particle masses. Hence, precisely knowing the value of this coupling is crucial, either to strengthen the predictive power of the SM and constrain the shape of the Higgs potential, which has a direct impact on the stability of our universe, or to open the door to new physics. This coupling is involved in the simultaneous production of two Higgs bosons via a virtual Higgs boson, a rare process with a cross-section 1,000 times smaller than that of single Higgs boson production. This process presents a detection challenge, as the two Higgs bosons decay into various types of particles with different probabilities. A particularly sensitive signature for this measurement is the decay of a pair of Higgs bosons into two b-quarks and two photons ($HH\rightarrow b\bar{b}\gamma\gamma$). Although currently limited by the amount of data collected, this analysis has provided the best existing constraint on this value. An upgrade to the LHC, called the High Luminosity LHC (HL-LHC), aims to increase the data produced by a factor of 20 compared to the current measurement. A sensitivity projection study for this coupling measurement allows us to determine the expected constraint at the end of the HL-LHC program, based on the latest Run 2 legacy results provided by ATLAS (JHEP 01 (2024) 066). Moreover, this channel will play a major role in the combination with other channels as well as the one with CMS, providing the most precise measurement of the self-coupling and paving the way for the discovery of Higgs pair production.

Speaker: Arthur Lafarge (Université Clermont Auvergne (FR))

HH-to-bbyy_Projections-Higgs_Pairs_WS_2025.pdf

10:00 Top-Yukawa-induced corrections to Higgs pair production

After the discovery of the Higgs boson in 2012, the measurements of the Higgs self coupling is still a challenge for current and future experiments in particle physics.
Higgs-boson pair production via gluon fusion is a loop-induced process. In order to increase the accuracy of the theoretical predictions for this process, higher-order corrections are necessary to reduce theoretical uncertainties and to describe differential distributions reliably. The next-to-leading order (NLO) corrections involve the evaluation of two-loop Feynman diagrams. In particular, for electroweak (EW) corrections, many different mass scales appear in the calculation, such as the gauge boson, bottom, top quark, and Higgs boson masses. Further complications include numerical instabilities due to virtual thresholds which require careful treatment. In my talk, I will present results for the EW corrections induced by the top Yukawa coupling with contributions from light-quark loops without using any reduction techniques to master integrals. The calculations is done by keeping the masses as fully symbolic parameters, allowing, in the future, for a study of parametric and mass scheme/scale uncertainties.

Speaker: Sauro Carlotti

Sauro_Higgs_pairs.pdf

10:15 Light-Quark Electroweak Contributions to Double Higgs Production at Hadron Colliders

A precise determination of the Higgs boson self-coupling is a central objective in high-energy physics, offering critical insight into the structure of the Higgs potential and electroweak symmetry breaking. As the Hi-Lumi phase at the LHC aims to tighten constraints on this coupling, next-to-leading order (NLO) electroweak corrections to double Higgs production become a critical component of theoretical predictions.
In this talk, I will present the analytic evaluation of the light-quark electroweak contributions to double Higgs production at hadron colliders. I will discuss the computational challenges associated with these calculations and examine how these contributions compare to the full electroweak corrections.

Speaker: Mr Marco Bonetti (IAP Karlsruher Institut für Technologie)

BONETTI_HH.pdf

10:30 Coffee break

11:00 Boosted HH at CMS

We present searches for nonresonant Higgs boson (H) pair production in the highly Lorentz-boosted phase space with the CMS experiment at the LHC. The analyses are performed in the all-hadronic 4b and bbVV final states, with boosted H pairs identified using novel graph neural network and transformer algorithms for wide-radius jets. Constraints are placed on the strengths relative to the standard model of the H self-coupling and the quartic VVHH couplings.

Speaker: Raghav Kansal (California Institute of Technology)

25_05_15_Higgs_Pairs.pdf

11:20 Two-loop running effects in Higgs physics in Standard Model Effective Field Theory

We consider the renormalization group equations within the Standard Model Effective Field Theory and compute two-loop contributions proportional to the top quark Yukawa coupling for the operator generating an effective Higgs-gluon coupling, focusing on the Yukawa-like operator.
These two-loop running effects
are relevant for processes where the effective Higgs-gluon coupling contributes ata lower loop order compared to the Standard Model contribution and where a dynamical scale choice is adopted.
Such a situation arises, for instance, in the invariant mass distributin in Higgs pair production. We investigate the phenomenological impact of our computations, observing a large impact close to the threshold.
We also comment on the differences arising from using a fixed renormalization scale in comparison with a dynamical renormalization scale.

Speaker: Dr Stefano Di Noi (K.I.T.)

HiggsPairs_2025_DiNoi-indico.pdf

11:35 VBF-HH at NLO QCD in non-linear Higgs Effective Field Theory

We present a calculation of the NLO QCD corrections to Higgs boson pair production in vector boson fusion including anomalous couplings. Those couplings are generated by the relevant leading operators of the non-linear Higgs Effective Field Theory (HEFT). We use the Monte Carlo program Whizard interfaced with the amplitude provider GoSam to investigate EFT effects at the level of the total cross section and in various kinematic distributions.

Speaker: Marius Höfer (KIT)

2025_05_Elba_VBF-HH.pdf

11:50 Double Higgs production and the Higgs self coupling in Global SMEFT fits

The ongoing Standard Model Effective Field Theory (EFT) program at the LHC and elsewhere aims at charting the parameter space allowed for deviations from the SM coming from heavy Beyond Standard Model (BSM) physics. Possible hints of New Physics could appear as subtle correlated deviations in several observables but parameterized by a single parameter. Hence, the community has invested great effort in producing global fits of the SMEFT that consider hundreds of experimental data points from LHC experiments and others.

The SMEFiT collaboration, which I am part of, has produced the most ambitious global fit yet with more than 400 experimental data points, mostly from LHC Run 2, cutting-edge Bayesian statistical techniques, inclusion of NLO corrections and quadratic dependence on the WCs, automated support for future colliders and for UV models matched onto SMEFT (see 2404.12809 and 2309.04523).

Here, I will present one of our latest updates: the inclusion of double Higgs measurements from LHC Run 2 and its HL-LHC projections. This allowed us to fit the dimension-6 operator $c_H$ that modifies the Higgs potential for the first time and claim that double Higgs production is enough to constrain this operator in a large global fit. I will discuss possible correlations with other operators, RGE running effects, how FCC-ee could improve on double Higgs via 1-loop corrections, and the impact of this measurement on exploring concrete BSM models. Overall, we can assess the global impact of double Higgs measurements in the SMEFT framework now and in the future.
These results will be part of an upcoming paper (to be published before the workshop).

Speaker: Alejo Nahuel Rossia (University of Padua and INFN Sezione di Padova)

Rossia_smefit_HiggsPairs_2025.pdf

12:05 Impact of renormalisation group equation on Higgs production in the SMEFT

We study the impact of RGE running and mixing of SMEFT operators on differential observables in $Hj$,$t \bar{t} H$ and $HH$ production at the LHC. We focus on a subset of operators closed under the QCD-induced anomalous dimension matrix and explore the impact of employing a fixed or dynamical scale on the EFT predictions for the Higgs transverse momentum and Higgs pair invariant mass spectra. We then explore the impact of taking into account RGE effects on the constraints obtained on the Wilson coefficients through fits to current data, as well as projections for the HL-LHC.

Speaker: Giuseppe Ventura

Giuseppe_Higgs2025.pdf

10:30 → 11:00 Coffee Break

11:00 → 12:30 Parallel: resonant HH/SH

Conveners: Alessandra Betti (Sapienza Università e INFN, Roma I (IT)), Giuseppe Bagliesi (Universita & INFN Pisa (IT))

11:00 CMS ZH resonant searches

This presentation will cover the searches for BSM resonances decaying to a scalar particle H and a standard model (SM) Z boson at the CMS experiment. These searches are one the most powerful probes of the two-Higgs-doublet models at the LHC, and are complementary to the searches for scalars decaying to SM Higgs bosons (h). One of the major differences with respect to the resonant hh/Sh searches is that the H scalar could be sufficiently heavy to open the decay channel to a top-antitop quark pair. Searches in the most sensitive Z and H decay channels are probed at the CMS experiment and presented in this talk.

Speaker: Andrea Malara (Universite Libre de Bruxelles (BE))

25_05_16_HH_workshop.pdf

11:20 CMS HH/SH resonant searches in bbVV channels

The HH/SH resonant searches in the bbWW and bbZZ decay channels are presented in this talk. This channel is particularly important for beyond standard model searches because for certain values of the S mass, i.e. below the HH threshold and above the ZZ (or WW) threshold, this could be the SH decay channel with the largest branching ratio. Dedicated searches are carried out at the CMS experiment to cover multiple WW/ZZ decay channels with different reconstruction techniques depending on the Lorentz-boost of the resonances.

Speaker: Amitav Mitra (Johns Hopkins University (US))

AM_HiggsPairs_CMS_HH_HY_resonant_bbVV.pdf

11:40 CMS HH/SH resonant search with anomaly detection algorithm

A search for a new massive particle X decaying into a Higgs boson H and a second particle, Y using an anomaly detection algorithm is presented. The dataset collected by the CMS experiment in proton-proton collisions at 13 TeV center-of-mass energy is analyzed. The final states with Higgs bosons decaying to a bb quark pair and reconstructed a single large-radius jets are considered. This search focuses on the final states with the Y decay products reconstructed as a large-radius jets. The identification of the Y particle is enhanced by computing the anomaly score of its candidate jet using a variational autoencoder, allowing the simultaneous search for multiple Y decay modes with a single analysis.

Speaker: Matej Roguljic (Johns Hopkins University (JHU))

250516_Roguljic_HiggsPairs.pdf

12:00 Resonant HH production: interference effects and higher order loop corrections

In my talk, I will show that potentially large higher-order corrections to the trilinear Higgs coupling in Beyond the Standard Model (BSM) scenarios could enhance the interference effects between the non-resonant contribution to Higgs pair production and a resonantly produced heavy scalar. These interference effects have a significant impact on the expected shape of the differential cross section and the value of the total cross section, altering the predictions in the regions currently accessible to experiments. I will demonstrate that neglecting the interference between the contributions from heavy Higgs resonances and non-resonant (background) diagrams, as done by experimental collaborations, can lead to unreliable exclusion limits.

Speaker: Kateryna Radchenko Serdula (DESY)

HiggsPairs-Radchenko.pdf

12:30 → 15:00 Lunch break

15:00 → 19:00 Plenary: Future colliders

Conveners: Paolo Azzurri (Universita & INFN Pisa (IT)), Ramona Groeber (Università di Padova and INFN, Sezione di Padova)

15:00 the Higgs potential and gravitational waves (REMOTE)

REMOTE PRESENTATION

Speaker: Chiara Caprini (CERN)

Caprini_Higgs_pairs.pdf

15:30 Higgs self-couplings at FCCee and CEPC

Speakers: Abraham Tishelman-Charny (Brookhaven National Laboratory (US)), Jia Liu (Peking University)

Higgs_Self_Coupling_HiggsPairs2025_16_May_2025-v2.pdf

16:00 Higgs self-couplings at ILC and CLIC

Speaker: Jenny List (Deutsches Elektronen-Synchrotron (DE))

jlist_hhh_HPairs_250516.pdf

16:30 Higgs self-couplings at a future muon collider

Speaker: Xing Wang

slides_Xing_Wang.pdf

17:00 Coffee break

17:30 Higgs self-couplings at FCC-hh

Speaker: Angela Taliercio (Northwestern University (US))

di-Higgs Physics at FCC-hh Taliercio.pdf

18:00 Theory challenges for Higgs self coupling measurements at future lepton colliders (REMOTE)

REMOTE PRESENTATION

Speaker: Christophe Grojean (DESY (Hamburg) and Humboldt University (Berlin))

Grojean_h3.pdf

18:30 Theory challenges for Higgs self coupling measurements at future hadron colliders

Higgs-boson pairs are dominantly produced via gluon fusion at hadron colliders, i.e. via a loop-induced process. This process will constitute the first direct access to the trilinear Higgs self-interaction. In recent years the NLO QCD corrections involving the full top-mass dependence became available by means of numerical integrations, since analytical methods available so far are not capable to solve the two-loop integrals with up to five energy scales. I'll summarize the methods that have been adopted to achieve the results with a discussion of the outcome, i.e. with the particular emphasis on the scheme and scale dependence of the virtual top mass that induces the dominant theoretical uncertainties at present. Finally, I'll provide a summary and outlook on the extension of the efforts to the full electroweak corrections to the same process.

Speaker: Michael Spira (Paul Scherrer Institute (CH))

spira.pdf

19:00 → 19:30 Plenary: Final session

Conveners: Paolo Azzurri (Universita & INFN Pisa (IT)), Ramona Groeber (Università di Padova and INFN, Sezione di Padova)

19:00 EPS poster prize

Speakers: Gudrun Heinrich (KIT), Jacobo Konigsberg (University of Florida (US)), Marina Cobal (Universita degli Studi di Udine (IT)), Xinchou LOU (IHEP,CAS)

19:15 Workshop conclusion

Speaker: Stefano Manzoni (CERN)

HH2025_summary.pdf

Saturday 17 May

06:55 → 15:55 Departures (Bus departures at 7:00)