2nd COMETA General Meeting

28 Apr 2025, 08:55 → 30 Apr 2025, 13:00 Europe/Warsaw

Aula (IFJ PAN Krakow)

Alessandra Cappati (Universite Catholique de Louvain (UCL) (BE)), Arnaud Ferrari (Uppsala University (SE)), Claudius Krause (HEPHY Vienna (ÖAW)), Flavia de Almeida Dias (Nikhef National institute for subatomic physics (NL)), Giovanni Pelliccioli (Università degli Studi di Milano-Bicocca), Ilaria Brivio (University & INFN Bologna), Kadri Ozdemir (Izmir Bakircay University (TR)), Karlis Dreimanis (Riga Technical University (LV)), Karolos Potamianos (University of Warwick (GB)), Magdalena Slawinska (Polish Academy of Sciences (PL)), Matteo Presilla (KIT - Karlsruhe Institute of Technology (DE)), Pietro Govoni (Universita & INFN, Milano-Bicocca (IT)), Ramona Groeber (Università di Padova and INFN, Sezione di Padova), Rene Poncelet (IFJ PAN Krakow), Riccardo Finotello (CEA Paris-Saclay), Richard Ruiz (Institute of Nuclear Physics (IFJ) PAN), Valentina Cairo (CERN)

COMETA (COmprehensive Multiboson Experiment-Theory Action) is a COST Action devoted to improving measurements of multi-boson production processes at the LHC and their interpretation.

It brings together theorists, experimentalists and Machine Learning experts, with the goal of creating a tightly interconnected community that can boost the development of innovative approaches to multi-boson measurements. The network also aims at creating an inclusive and informal environment, where young scientists and researchers based in underfunded european countries can gain international visibility.

COMETA promotes a broad scientific program including:

• BSM/EFT interpretation of multiboson measurements
• precise SM predictions and event generation (MC+PS) for multiboson production processes
• development of theory and tools to measure signals with polarised W and Z bosons
• development of advanced Machine Learning tools in collaboration with professional ML practitioners
• combined analyses of multiboson processes (H, HH, VV, VVV, VBS...) within the experimental collaborations

The network currently includes 240+ researchers from over 34 countries within and outside Europe.

For more information or to join the network, see:
- www.cost.eu/actions/CA22130/  
- foswiki.web.cern.ch/COMETA/

 

The Call for Abstracts is now closed.

Registration is open until March 8th.

 

Previous General Meeting (2024): indico.cern.ch/event/1334055/

Please note: photographs and video recordings will be taken by members of the local organising team and the COMETA outreach team. If you do not feel comfortable with this, please contact the organisers via email before the day of the event.

Short URL for the event: https://indico.cern.ch/e/cometa-gm2

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Monday 28 April

WG1 session

1 Di-Higgs production in gluon fusion: top-mass renormalisation

Speaker: Sebastian Eryk Jaskiewicz (University of Bern)

2 Status and prospect for polarised-boson calculations

Speaker: Diana Mareen Hoppe (Technische Universitaet Dresden (DE))

3 SMEFT predictions in multi-boson processes

Speaker: Hesham El Faham (The University of Manchester)

10:30 Coffee break

WG2 session

4 Quantum Convolutional Neural Networks for Jet Images Classification

Speaker: Alice Di Tucci

11:40 Q&A

5 Point Cloud Deep Learning Methods for Particle Shower Reconstruction in the DHCAL

Speaker: Maryna Borysova (Weizmann Institute of Science & KINR, NAS of Ukraine)

12:25 Q&A

12:30 Lunch

WG5 session

6 Report from WG5

Speakers: Flavia de Almeida Dias (Nikhef National institute for subatomic physics (NL)), Karlis Dreimanis (Riga Technical University (LV))

7 Outreach and Media Reach

Speaker: Richard Ruiz (Institute of Nuclear Physics (IFJ) PAN)

8 Report from COMETA Colloquia

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

Submitted talks

9 Intermediate vector bosons and gauge symmetry

Since the late 1970ies/early 1980ies it is known that subtle field-theoretical aspects of gauge symmetry require a treatment of the (electro)weakly interacting particles going beyond standard perturbation theory. This is possible by augmenting perturbation theory by the Fröhlich-Morchio-Strocchi mechanism, which primarily affects the asymptotic initial states and final states. This has been corroborated in many theoretical investigations in the last decade. While a subleading effect at past experiments, current and future experiments will be able to probe these effects. This is discussed for examples of vector boson properties and interactions, as well as electroweak resummation effects. Indications for visible effects are found close to thresholds and at very high energies. The latter may substantially affect predictions for future high-energy lepton colliders.

Speaker: Prof. Axel Torsten Maas (University of Graz)

10 A study of gauge dependence in polarized W boson production at the LHC

In the Standard Model, the W and Z bosons acquire masses through the Higgs Mechanism, resulting in the longitudinal and transverse polarization states all being physical. However, extracting the helicity polarizations weak bosons is challenging due to their unstable nature. This is further complicated by the inherent gauge dependence in the definition of polarization vectors for massive gauge bosons. This talk presents an investigation into the gauge (in)dependence of helicity polarization in W + 1j production (with W → τ + ν) at the LHC, including full off-shell effects. Strategies for minimizing/mitigating the impact of gauge dependence are discussed.

Speaker: TRINA BASU

15:34 Coffee break

Submitted talks

11 ssWW VBS Polarisation Measurement - ATLAS

In 2023, the ATLAS experiment published the first differential cross-section measurement of same-charged W-boson scattering – an essential process for understanding electroweak symmetry breaking. Since W-bosons gain their mass and, consequently, their longitudinal polarization through the Higgs mechanism, studying the scattering of longitudinally polarized W-bosons offers a promising way to probe this mechanism and search for new physics beyond the Standard Model.

However, since W-bosons decay into a charged lepton and a neutrino, directly reconstructing their original polarization is not possible. To overcome this, the analysis presented in this talk employs neural networks to separate the longitudinal component of the same-charged W-boson scattering signal from other polarization states and background processes. This talk aims to give an overview of the analysis strategy and to discuss state of the art techniques used to include higher-order QCD and EW effects in the polarized signal prediction.

Speaker: Erik Bachmann (Technische Universitaet Dresden (DE))

12 A comparative study of Z production via vector boson fusion (VBF) using PYTHIA8 shower models

The production of Z bosons via vector boson fusion (VBF) is a standard candle process for probing Monte Carlo models as it can be easily compared with available real data. Accurate theoretical predictions for this process are essential, particularly when interfacing with parton shower models like Dipole and Vincia implemented in PYTHIA8. This presentation will synthesize findings from recent studies to elucidate the current understanding and challenges in modeling VBF-induced Z boson production. We will delve into the comparative analyses of different parton shower algorithms available in PYTHIA8 as explored in recent studies. These investigations highlight how variations in shower algorithms can impact the modeling of QCD radiation in VBF processes, affecting observables such as jet multiplicities and distributions.

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

13 Two-photon production of W and Z boson pairs at the LHC and LHeC.

In this talk we will summarize the status and prospects of studying the two-photon production of W and Z boson pairs at the LHC. As the proposed Large Hadron electron Collider (LHeC) offers a unique environment for studies of the electroweak gauge boson couplings - after a short introduction of 𝛾𝛾 interactions at the LHeC (see Ref. [1]), we will discuss the case of the two-photon production of W and Z boson pairs [2] and compare the sensitivities expected at the HL-LHC and LHeC.

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[1] L. Forthomme, H. Khanpour, K. Piotrzkowski, Y. Yamazaki, "High energy γγ interactions at the LHeC", talk at ICHEP2024, https://doi.org/10.22323/1.476.0329

[2] L. Forthomme, H. Khanpour, K. Piotrzkowski, Y. Yamazaki, "High energy γγ interactions at the LHeC", paper in preparation.

14 Searching for new physics using hadronic decays of W and Z bosons in the ATLAS detector

Fully hadronic decays of pairs of vector bosons (WW, ZZ, WZ) into large-R jets may be very sensitive to currently undiscovered phenomena. As weak bosons are able to interact with spin-0, spin-1 and spin-2 particles, they act as a good tool for probing new physics beyond the Standard Model. Due to the large branching fraction of bosons to hadrons, studying the hadronic channels is advantageous, as it can yield a higher energy reach with respect to the semi and fully leptonic channels. In order to distinguish signal events from the large background of Standard Model dijet events, a novel machine-learning-based and mass-decorrelated large-R jet tagger, with optimized working points, is used. This analysis presents the latest ATLAS results for the search for heavy resonances decaying into vector bosons in the fully hadronic channel using the LHC full Run-2 data.

Speaker: Dylan Remberto Van Arneman (Nikhef National institute for subatomic physics (NL))

15 Vector boson fusion and associated production of Higgs bosons as an LHC signature of CP violation

Purely bosonic sources of CP violation (CPV) in typical extensions of the Standard Model (SM) scalar sector manifest in a series of tell-tale triboson couplings. The simultaneous observation of specific subsets among these couplings constitutes a robust and purely bosonic signal of CPV. In this work, we analyze the present and HL-LHC era observational prospects of such signals in the minimal setting of the complex two-Higgs-doublet model (C2HDM). The proposed program, of general application to extended scalar sectors, relies on the observability of carefully selected combinations of (resonant) gluon fusion, vector boson fusion and associated production processes featuring the prospective new spin-0 scalar states, as well as the decay of the heavier of these scalars into a h1Z final state, with h1 a new BSM scalar.

Speaker: Mr Rodrigo Capucha (CFTC, UL)

16 Search for Light Scalars in the TRSM at the LHC

Investigation for exotic decays of the scalar boson into a pair of light CP-even scalar particles, $ p p\to H_2 V \to H_1 H_1$, where the $H_1$-boson promptly decays into b-quarks and has a mass in the range of $20-60$ GeV, is presented within the framework of the Two Real Singlet Model (TRSM). This model extends the Standard Model by introducing two real scalar singlets that interact with Standard Model particles through their mixing with the Higgs boson, resulting in three CP-even Higgs states $h_i​ (i=1,2,3)$. The search is conducted in events where the Higgs boson is produced in association with a $W$- and $Z$-boson, leading to distinct signatures: either one charged lepton (electron or muon) and missing transverse energy or two charged leptons, accompanied by multiple jets from b-quark decays. A systematic scan over the TRSM parameter space is performed, taking into account all available theoretical and experimental bounds, to identify regions with potentially observable signals. Using Monte Carlo (MC) analysis, we evaluate the signal-to-background ratio for the final states $4b + l^+ + v_l$, $ 4b+l^-+\bar{v_l}$, and $\ 4b + l^+ l^-$ so that, the sensitivity of the ATLAS and CMS experiments at a collision energy of 13 TeV and an integrated luminosity of $139$ fb$^{-1}$ is assessed for these signals.

Speaker: Mr Mohamed OUCHEMHOU (Ruđer Bošković Institute, Bijenicka cesta 54, P.O. Box 180, 10002 Zagreb , Croatia)

17 How large can the light Yukawa couplings be?

First generation quark Yukawa couplings are notoriously difficult to measure. Various proposals for their measurement is based on multi-boson production such as Higgs pair production, Higgs off-shell production or triple V production. Those probes will not be able though to probe modifications below factors of order hundred. Given this in this talk, I will discuss how large the light quark Yukawa couplings can actually become in UV models. Concretely, large enhancements can be achieved in models with pairs of vector-like quarks. These models generate operators in Standard Model Effective Field Theory, both at tree-level and one-loop, that also modify, for example, the couplings of the quarks to the electroweak bosons. Therefore, constraints from electroweak precision tests and Higgs physics arise. Nevertheless, those models allow for rather large enhancement factors motivating to search for modifications in light quark Yukawa couplings in multi-boson processes.

Speaker: Barbara Anna Erdelyi (Università degli Studi di Padova and INFN Sezione di Padova)

Outreach event

Maps directions

Tuesday 29 April

WG3 session

18 SMEFT combinations in CMS

Speaker: Fabian Stager (University of Zurich (CH))

19 SMEFT combination in ATLAS

Speaker: Eleonora Rossi (University of Oxford (GB))

20 VBS combination in CMS

Speaker: Giacomo Boldrini (Centre National de la Recherche Scientifique (FR))

10:30 Coffee break

WG1 session

21 Double-Higgs VBF production in SMEFT

Speaker: Michal Ryczkowski (Università di Padova and INFN, Sezione di Padova)

22 BSM effects in multi-boson processes

Speaker: Roman Pasechnik (Lund university)

23 Triple Higgs production in EFTs from on-shell techniques

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

12:30 Lunch

Submitted talks

24 HEFT/SMEFT modeling for HH simulation by using machine learning techniques

A detailed double Higgs event simulation in the context of EFT models (HEFT and SMEFT) would impose unfeasible burden on computing resources to sample the parameter space densely enough. This talk focuses on developing techniques to model/parametrize the modifications of the cross section and the kinematics for double Higgs production in the whole parameter space of HEFT and SMEFT models, by using machine learning techiniques. A description of the modeling of relevant observables (such sh total cross section, di-Higgs invariant mass) by using neural network models is detailed. Figures of merit for the performance of the neural networks are derived and results of the complete study are presented. A likelihood-free approach based on machine leargning techniques is attempted to construct confidence sets, at a given confidence level value, in the HEFT and SMEFT parameter spaces.

Speaker: Nicola De Filippis (Politecnico/INFN Bari (IT))

25 The Art of Counting: where to cut-off SMEFT and HEFT in Higgs Pair Production

In searches for beyond the Standard Model (BSM) Physics, bottom-up effective field theories (EFTs) such as the Standard Model EFT (SMEFT) and Higgs EFT (HEFT) have become crucial theoretical tools. Besides specifying their particle content and the respective symmetries on which the higher-dimensional operators are built, the choice of a power counting in a small parameter is the third necessary ingredient. The latter dictates how the operators are organized and enables us to perform a truncation on Lagrangian and amplitude level, which is essential when computing physical observables. In this work, we discuss the power counting approaches to SMEFT and HEFT and study their application in the context of the Higgs pair production process.

Speaker: Konstantin Schmid (University and INFN Padova)

26 The interference issue in VBS with SMEFT dimension-8 operators

Data collected during Run 3 of the LHC will allow more precise studies of VBS processes and calculation of new limits on anomalous Quartic Gauge Couplings within the framework of SMEFT with dimension-8 operators. Within the SMEFT framework, the total cross section can be expressed as a sum of three components: the purely SM term, the interference between SM and EFT amplitudes, and the purely quadratic EFT term. From dimensional considerations one generally expects interference effects play the dominant role in BSM signal in order to ensure an accurate SMEFT description without invoking higher order corrections. This however may not necessarily be the case within the range of Wilson coefficients the LHC is currently able to probe. This issue has not been properly addressed in studies carried out so far, where no distinction was made between BSM signals originating from the interference and from the quadratic terms. We will present preliminary studies of possible BMS effects induced by SMEFT dimension-8 operators, using the same-sign WW VBS process as an example, where the relative contributions of interference and quadratic terms are examined separately. Using private samples of MadGraph generated VBS events with different dimension-8 operators, we study possible ways to enhance the sensitivity to the respective interference terms in the calculation of limits on aQGCs by exploring the observed BSM effects in the distributions of many kinematic variables. Preliminary studies done at the generator level may be extended to incorporate detector simulation in the future.

Speaker: Monika Ghimiray (National Centre for Nuclear Research (PL))

27 Exotic $h \to Z a$ Higgs decays into $\tau$ leptons

Exotic Higgs decays are among the most promising areas to be explored at the High Luminosity Large Hadron Collider, given the unprecedentedly large amount of the Standard Model-like Higgs bosons that will be produced. In this context, we introduce a new search channel where a Higgs boson with a mass of 125 GeV decays to a Z boson and a light pseudoscalar (a), in the final state with two muons and two $\tau$ leptons ($ h\to Z a\to \mu\mu\tau\tau $). In the preliminary analysis, performed at a center-of-mass energy of $\sqrt{s}=$14 TeV, masses of the pseudoscalar boson between 14 and 30 GeV are probed, focusing on the kinematic region where the $ h\to Z a$ decay is on-shell. We analyze several collider variables to propose a suitable signal region and consider the different decay channels for the di-tau system, as sensitivity is strongly influenced by the $\tau\tau$ reconstruction efficiency. Model-independent limits are set at the 95$\%$ confidence level as a function of $m_a$. Upper limits are also established within the Axion-Like Particle (ALP) framework, based on a specific model motivated by the anomalous $g-2$ measurement.

Speaker: Dr Rosa María Sandá Seoane (Instituto de Física Teórica UAM-CSIC)

28 Prospect study for measurement of $Hb\bar{b}$ coupling at the LHC and FCC-hh

This study investigates the H + b + jets signature in proton-proton collisions to explore the structure of the $Hb\bar{b}$ couplings, with a focus on Higgs boson decays into photon pairs. The analysis includes both reducible and irreducible backgrounds, along with realistic simulations of the detector. Using multivariate techniques, especially Boosted Decision Trees (BDTs), kinematic variables are analyzed to improve signal-to-background separation. Our findings show that the H + b + jets process is key to measuring CP-even and CP-odd couplings of the Higgs-bottom quark interaction at the HL-LHC and FCC-hh.

We limit the new physics couplings, $c_b$ and $\tilde{c_b}$, through Monte Carlo simulations and assess the sensitivity of the H + b + jets channel at high luminosities. The results show that these colliders can reach previously unexplored parameter spaces, with $c_b$ and $\tilde{c_b}$ limits improving upon current experimental bounds. Additionally, we introduce a new asymmetry in lab-frame momenta to study CP violation in the $Hb\bar{b}$ coupling, offering a strong method for limiting CP-odd interactions.

Speaker: Zahra Abdy (AGH University of Krakow (PL))

15:25 Coffee break

Action reports

29 Report from WG1

Speakers: Giovanni Pelliccioli (Università degli Studi di Milano-Bicocca), Ramona Groeber (Università di Padova and INFN, Sezione di Padova)

30 Report from WG2

Speakers: Alessandra Cappati (Universite Catholique de Louvain (UCL) (BE)), Dr Claudius Krause (HEPHY Vienna (ÖAW)), Dr Riccardo Finotello (CEA Paris-Saclay)

31 Report from WG3

Speakers: Matteo Presilla (KIT - Karlsruhe Institute of Technology (DE)), Valentina Cairo (CERN)

32 Report from Grant Awarding Committee

Speaker: Kadri Ozdemir (Izmir Bakircay University (TR))

33 Discussion in preparation of Deliverables and Mid-term Report

Social dinner

Wednesday 30 April

WG2 session

34 Profile Likelihoods on ML-Steroids

Speaker: Theo Heimel (UCLouvain)

09:40 Q&A

35 CaloChallenge

Speaker: Dr Claudius Krause (HEPHY Vienna (ÖAW))

10:25 Q&A

10:30 Coffee break

WG3 session

36 non-resonant HH measurements in ATLAS and CMS

Speaker: Giulia Di Gregorio (CERN)

37 X->HH/SH measurements in ATLAS and CMS (remote)

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

38 HL-LHC prospects on HH in the ATLAS and CMS experiments

This talk aims at giving an overview of the latest HL-LHC prospect studies related to Higgs boson pair production in ATLAS and CMS, based on extrapolations of the Run-2 legacy results and prepared for the update of the European Strategy for Particle Physics.

Speaker: Arnaud Ferrari (Uppsala University (SE))