Proposition de thèse 2016 de stage m2

The Higgs boson is the unique scalar (spin zero and positive parity) particle in the Standard Model (SM) of the elementary particle physics. In the SM a complex scalar (Higgs) field leads to the spontaneous breaking of the SM electroweak symmetry through the Brout-Englert-Higgs mechanism, giving mass to the W and Z bosons and also allows the fermions to acquire mass through Yakawa couplings.

The Higgs boson with a mass of 125 GeV was discovered in 2012 by the ATLAS and CMS experiments located at the Large Hadron Collider (LHC) at CERN [1]. Since the discovery of Higgs boson, the era of verification that the observed state is consistent with the Higgs boson predicted by the Standard Model has begun, searching for deviations which may be signs of physics Beyond the Standard Model (BSM). There are several observations which indicate the existence of BSM physics: the existence of dark matter and baryon asymmetry in the universe, non-zero neutrino masses, and cosmic inflation. Many models addressing these issues, e.g. Electroweak Singlet, or Two Higgs Doublet Model, predict signals in the Higgs sector. One can directly search for new heavy particles in high-energy collisions. And as well one can perform precision measurements which may allow one to observe indirectly new particles through virtual effects, for particles which are too heavy to be produced at the LHC.

This thesis proposes to make a comprehensive search for a hint of BSM in the Higgs sector using the Higgs boson decaying into four leptons. This will be performed within the framework of Effective Field Theory (EFT) and will use the approximately 150 fb⁻¹ of integrated luminosity expected in 2015-2018 of RUN-2 of the LHC using the ATLAS detector.

For Higgs boson coupling studies at LHC in RUN-1, the so-called kappa-framework was chosen to study the coupling strength of the Higgs to gauge-bosons and fermions. This framework is only valid at the O(5−10%) level. In RUN-2&3, the accumulated statistics will allow for more precise measurements and for this, and a more precise theoretical framework is also needed.

The Higgs Effective Field Theory (Higgs EFT) is a general framework which can study possible deviations from the SM in Higgs-boson couplings and also CP properties. The basic assumption in the EFT approach is that, at the electroweak scale, there are no other particles beyond those of the SM, and the scale of new physics Λ is large, well above the electroweak scale. Dimension-6 operators encode for possible effects of heavy BSM particles and are related to physical measurable observables

Searching for deviations from the SM prediction may uncover evidence for new BSM physics, and will otherwise set experimental constraints on a large class of BSM theories. Up to now, no fully comprehensive implementation of a Higgs EFT analysis framework has been constructed for experimental analyses. One of the first goals of this PhD thesis is to establish an analysis framework for the Higgs EFT in LHC RUN-2 for the combined analysis of Higgs boson on-shell coupling, off-shell coupling and spin/CP study. The understanding of the Higgs EFT will be facilitated by interactions with a theory expert, Adam Falkowski (LPT-Orsay).

Despite its small branching ratio, the Higgs boson (h) decay h → ZZ^(∗) → 4l (l=lepton (electron or muon)) channel provides the best sensitivity to Higgs boson studies with a very good signal-to-noise ratio. This channel has been part of the Higgs boson discovery and has provided information on the Higgs boson mass, decay width, coupling to Z-bosons and Higgs-boson spin/parity determination. It has also been one of the baseline decay processes for the BSM Higgs boson searches in RUN-1. From the EFT perspective, this channel allows one to probe dimension-6 operators that cannot be realistically accessed in other processes.

The kinematics of h → ZZ^(∗) → 4l events is illustrated in Figure 1. Figure 2 shows the four lepton invariant mass spectrum for RUN-1 data. The useful information for EFT analyses are three invariant masses, defined as M_4l,M_Z1 and M_Z2, five decay angles of Ω = (θ^∗,cosθ₁,cosθ₂,Φ₁,Φ), together with information on the Higgs boson production, i.e. transverse momentum (p_T) and rapidity (Y ). We will develop the experimental analysis method which makes use of all of the kinematics information of the Higgs-boson production and decay.

3 Le cadre du travail et l’équipe d’accueil

The ATLAS team at LAL has been playing a major role in the detector construction, operation and calibration of the Liquid Argon electromagnetic calorimeter, as well, is implicated in detector R&D for future LHC run in HL-LHC program. LAL is also strongly engaged in the Higgs physics analysis (including the Higgs boson decay in h → ZZ^(∗) → 4l) as well as new particle searches such as supersymmetry (SUSY). The PhD thesis candidate will work together with Lydia Iconomidou-Fayard, Arthur Schaffer and Reisaburo Tanaka of h → ZZ^(∗) → 4l team at LAL.

The RUN-2 of the LHC has started in spring 2015 and will last until the end of 2018 with target luminosity of 150 fb⁻¹ at a center-of-mass energy between 13 and 14 TeV. With the increase in energy relative to RUN-1, the cross section for Higgs production is about 2.3 larger. This will provide some 300 to 400 h → ZZ^(∗) → 4l events, allowing a significant step in the precision of Higgs physics at LHC. This time frame matches well the duration of the thesis (Oct. 2016 - Sep. 2019).

During the first year, the thesis candidate will spend a significant amount of time studying the optimal operators in Higgs EFT and coupling parametrization for experimental studies in the h → ZZ^(∗) → 4l process. This will involve the use of the program Rosetta [2], which characterizes the HEFT, interfaced to a Monte Carlo generator (MadGraph5_aMC@NLO [3]), and will be done with the help of the Rosetta program author Adam Falkowski (LPT-Orsay). As well, an experimental qualification task will be started, which lasts one year, on a topic such as the calibration of the Liquid Argon electromagnetic calorimeter. The second and the third year will concentrate on applying the results of the Higgs EFT studies to the analysis of the RUN-2 data.

References

[1] ATLAS Collaboration, G. Aad et al., “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC”, PLB 716 (2012) 1, http://www.sciencedirect.com/science/article/pii/S037026931200857X

[2] A. Falkowski, et al., “Rosetta: an operator basis translator for Standard Model effective field theory,” arXiv:1508.05895 [hep-ph], http://arxiv.org/abs/1508.05895