The Higgs boson (H) plays a prominent role in the Standard Model (SM) of particle physics: the related Higgs field, which permeates the Universe, is responsible for the spontaneous breaking of electroweak symmetry and the origin of the masses of the elementary particles. Its role in the theory and its atypical properties (notably its spin 0) make it a fundamental subject of study, that might also have potential cosmological implications.
Since March 2021 a "Higgs'' team, working on the measurement of the properties of the Higgs boson with data from the ATLAS (A Toroidal Large ApparatuS) experiment at the Large Hadron Collider (LHC) at CERN and on the preparation of the Future Circular Collider (FCC) project has joined the Particles group.
The ATLAS experiment (Figure 1) is a multipurpose detector, 46 m long, 25 m high, weighing 7000 tonnes, located about 100 m below ground around one of the collision points of the LHC's counter-rotating beams. It consists of six different detecting subsystems arranged concentrically, to record the momenta and directions of the particles produced in the collisions. The measured quantities are used to investigate a wide range of physics processes, such as strong interactions, electroweak production of W and Z bosons, top quark and Higgs boson properties, and to search for hypothetical phenomena beyond the Standard Model such as extra dimensions or dark matter particles.
After the first two data-taking phases (Run 1: 2010-2012, Run 2: 2015-2018), the LHC is now entering its third data-taking period (Run 3: 2022-2025), which will be followed by a 2-year long shutdown for detector and collider upgrades that will lead to the so-called high-luminosity phase of the LHC (HL-LHC: 2028-2040).
Since the beginning of the LHC data taking, our team has focused on the physics of the Higgs boson. It contributed to its discovery at the LHC Run 1 using its decays into two photons , and to the demonstration of its Yukawa couplings to the fermions of the SM through the observation of its decays into bottom quark pairs with data from Run 2 . We also measured the Higgs boson mass with diphoton decays and searched for the rare Higgs boson decays to a Z boson and a photon.
Recently, our team has worked on the following analyses of the full set of proton-proton (pp) collisions collected by ATLAS during the LHC Run2:
- The measurement of the Higgs boson production cross sections using the H→bb  and H→γγ  decays. The H→bb decay has a large branching ratio (58%) but suffers from large background from QCD multihadron production, while the H→γγ decay has small branching ratio (0.22%) but much lower backgrounds. The cross sections are measured as a function of several kinematic quantities which can be used to probe different Higgs production mechanisms and to look for deviations from the predictions of the SM. All the measurements are found to be in agreement with the Standard Model predictions and constraints are set on the Yukawa couplings of the Higgs boson to the bottom and charm quarks and on anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework.
- The most sensitive search for the (yet unobserved) rare Higgs boson decay to Zγ . No significant excess with respect to the expected background was found in the data, and an upper limit at 95% confidence level on the production cross-section times the branching ratio for pp → H → Zγ was set at 3.6 times the SM prediction, while 2.6 times was expected in the presence of the SM Higgs boson. The best-fit value for the signal yield normalised to the Standard Model prediction was 2.0 ± 1.0; a similar excess was found by CMS, which might hint to a possible deviation from the SM expectation that will be further investigated with more data in Run 3.
- The search for the production of pairs of Higgs bosons (HH). This process is the portal to measuring the Higgs boson self-coupling parameter λ and thus the potential of the Higgs boson. We focused on the search for di-Higgs production in the final state bbγγ , one of the two most sensitive ones, which combines the large branching ratio to bb for one Higgs boson with the good signal/background ratio of the γγ decay for the other. The analysis of the full ATLAS Run 2 data found no excess compared to the background, and set an upper limit, with a confidence level (CL) of 95%, of 130 fb on the production cross section of the pp → HH process, corresponding to 4.1 times the SM prediction. The result implies the following constraint on the Higgs boson self-coupling modifier κλ = λ/λSM: -1.5 < κλ < 6.7 at a 95% CL.
The quality of the results of the ATLAS experiment requires proper functioning of the detector as well as a detailed understanding of the objects used in the analyses. Recently our team has been focusing on:
- The improvement of the performance of identification of prompt photons and of rejections of candidate photons coming either from the decay of a neutral meson in a hadronic shower, or from the electromagnetic shower initiated by an electron, for the future LHC data taking (Run 3, 2022-2025).
- The characterization of the reconstruction and tagging performance of jets of particles resulting from the hadronization of a b quark, with two measurements in situ of (i) the calibration of the energy scale of the b-quark jets using ttbar events followed by a t → Wb →qqb decay and the known top quark mass , and (ii) the efficiency of identifying a b-quark jet, using a fit to the momentum distribution of a muon in a jet relative to the axis of the jet+muon system.
- The measurement of radiation damage to the ATLAS tracker pixel sensors ,  and its implementation in the ATLAS detector simulation. With increasing luminosity radiation damage effects are not negligible, and it will be important to account for them in the simulations for the future LHC data taking phases.
- The construction and software development of the future silicon vertex detector ("ITk'') of the ATLAS experiment for the HL-LHC. We are working on the development of the digitization code of the ITk pixel sensors, and on the construction of the ITk pixel detector, having contributed to the market survey process of the various vendors of sensors and to the preparation of the next phase of pre-production and testing of modules.
The Future Circular Collider is a proposed integrated project of next-generation collider of about 90 km in length proposed to take over from the LHC (Figure 2).
It should start around 2040 with an initial electron-positron phase (FCC-ee) lasting around 8 years, to study in detail the properties of the electroweak bosons (W, Z), of the Higgs boson and of top quarks, running at center-of-mass energies of 91 GeV, 160 GeV, 240 GeV and 350-365 GeV, followed by a shutdown for machine and detector upgrades and then by a second data-taking phase (FCC-hh) in which proton beams would provide collisions at center of mass energies of 100 TeV or above.
Our team is contributing to the development of the physics case of the electron-positron phase of the future circular collider. The FCC-ee offers unprecedented opportunities to determine Higgs boson parameters, with over 10 million e+e- → ZH events and nearly 100 thousand WW → H events produced at center-of-mass energies around 240 and 365 GeV and with a much lower background than at the LHC, and without pileup effects. Another advantage over the LHC is related to the complete knowledge of the kinematics of the colliding e+e- particles, which makes it possible to identify the ZH events with the "recoil mass" technique, reconstructing the products of the decays of the Z boson and therefore - thanks to the conservation of energy and total momentum - the mass of the system recoiling against the Z boson.
In spring 2021 we carried out first studies of the precision that FCC-ee could expect on the cross section of the ZH process and on the mass of the Higgs boson  and on its hadronic couplings (to b and c quarks and gluons), using signal and background noise events generated and interfaced to a parametric simulation of the detector response. These initial studies will be updated according to the different technologies and projects proposed for the FCC-ee detectors, optimizing the selection criteria and the fit strategies to further improve the sensitivity of the analysis, in view of a "Conceptual Design Report'' by 2025 for the update of the European strategy for particle physics. This will mark the completion of the FCC-ee feasibility study, and will allow the CERN Council to decide whether the project can proceed with the start of the excavation of the FCC tunnel.
- Gregorio BERNARDI (DR, deputy team leader, responsible of the FCC activities)
- Marco BOMBEN (MdC)
- Romain BOUQUET (PhD 2019-2023)
- Giulia DI GREGORIO (postdoc 2022-2024)
- Ang LI (PhD 2020-2023)
- Tong LI (postdoc 2023-2024)
- Giovanni MARCHIORI (CR, team leader, responsible of the ATLAS activities)
- Keerthi NAKKALIL (PhD 2020-2023)
- Qiuping SHEN (PhD 2020-2024)
- Yulei ZHANG (PhD 2019-2023)
- ATLAS Collaboration public website: https://atlas.cern
- ATLAS public results: https://twiki.cern.ch/twiki/bin/view/AtlasPublic
- ATLAS page in inspire: https://inspirehep.net/experiments/1108541
- ATLAS glance page (only for ATLAS collaborators): https://atlas-glance.cern.ch/atlas/membership/institutes/profile?id=353
- FCC public website: https://fcc.web.cern.ch/Pages/default.aspx
- FCC page in inspire: https://inspirehep.net/experiments/1614720
 ATLAS Collaboration, “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC,” Phys. Lett. B716, 1–29 (2012), arXiv:1207.7214
 ATLAS Collaboration, "Observation of H->bb decays and VH production with the ATLAS detector", Phys. Lett. B 786 (2018) 59, arXiv:1808.08238
 ATLAS Collaboration, “Measurements of WH and ZH production in the H→bb decay channel in pp collisions at 13 TeV with the ATLAS detector,” Eur. Phys. J. C 81, 178 (2021), arXiv:2007.02873
 ATLAS Collaboration, "Measurements of the Higgs boson inclusive and differential fiducial cross-sections in the diphoton decay channel with pp collisions at √s=13 TeV with the ATLAS detector", arXiv:2202.00487
 ATLAS Collaboration, "A search for the Zγ decay mode of the Higgs boson in pp collisions at √s = 13 TeV with the ATLAS detector", Phys. Lett. B 809 (2020) 135754 arXiv:2005.05382
 ATLAS Collaboration, "Search for Higgs boson pair production in the final state with two bottom quarks and two photons in pp collisions at √s=13 TeV with the ATLAS detector", arXiv:2112.11876
 ATLAS Collaboration, "Energy scale calibration of b-tagged jets with ATLAS Run 2 data using ttbar lepton+jets events", ATLAS-CONF-2022-004
 Dawson, I.(Ed),Radiation effects in the LHC experiments: Impact on detector performance and operation, CERN-2021-001
 ATLAS Collaboration, "Measurements of sensor radiation damage in the ATLAS inner detector using leakage currents", JINST 16 (2021) P0802, arXiv:2106.09287
 P. Azzurri et al, "A special Higgs challenge: Measuring the mass and production cross section with ultimate precision at FCC-ee", https://arxiv.org/abs/2106.15438, to be published in EPJ+ special issue: A future Higgs and Electroweak factory (FCC): Challenges towards discovery, Focus on FCC-ee
APC activity report 2017-2021: https://apc.u-paris.fr/APC_CS/en/activity-report
For more details
Publications, theses/internships/post-docs supervised: