PROPOSITION DE THÈSE 2016
DE STAGE M2
+33(0)1 64 46 83 00
+33(0)1 69 46 83 97
Groupe du Laboratoire
Adresse et lieu du stage
téléphone
e-mail
ATLAS
LAL Université Paris-Sud- Bâtiment 200
BP 34 – 91898 Orsay cédex
01 64 46 83 78 / 01 64 46 83 03
R.D.Schaffer@cern.ch/tanaka@lal.in2p3.fr
Titre : La physique de précision du boson de Higgs en quatre leptons
en théorie effective des champs avec l’expérience ATLAS au LHC
Precision Higgs Physics with Higgs Boson decaying into four leptons
using effective field theory with the ATLAS experiment at LHC
Précisions sur le sujet proposé :
Thème: La physique du boson de Higgs
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.
2 Le but scientifique recherché
2.1 Higgs Boson Precision Measurements via Effective Field Theory
For Higgs boson coupling studies at LHC in RUN-1, the so-called kappaframework 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).
Fig. 1: Higgs boson production and decay
Fig. 2: Four lepton invariant mass
in pp collision in pp → h → ZZ → 4l.
Five angles (θ∗ ,cosθ1,cosθ2,Φ1,Φ) are
defined in the four lepton center-of-mass
frame.
spectrum in pp → ZZ(*) → 4l for center-ofmass ener- gies at 7 and 8 TeV measured
by the ATLAS experiment [Phys. Rev. D
91 (2015) 012006].
(*)
2.2 Experimental Studies of Higgs Boson Couplings
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 Higgsboson 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
M4l,MZ1 and MZ2, five decay angles of Ω = (θ∗ ,cosθ1,cosθ2,Φ1,Φ), together
with information on the Higgs boson production, i.e. transverse momentum
(pT) 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.
4 Le plan de travail envisagé
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
[3] P. Artoisenet et al.,“A framework for Higgs characterisation,” JHEP 11
(2013) 043,
http://link.springer.com/article/10.1007%2FJHEP11%282013%29043