Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Cécile Caillol
Scalar Boson
Decays to Tau
Leptons
In the Standard Model and Beyond
Springer Theses
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é
C cile Caillol
Scalar Boson Decays to Tau
Leptons
In the Standard Model and Beyond
Doctoral Thesis accepted by
é
Universit Libre de Bruxelles, Brussels, Belgium
123
Author Supervisor
Dr. CécileCaillol Prof. Barbara Clerbaux
University of Wisconsin-Madison UniversitéLibre deBruxelles (ULB)
Madison Brussels
USA Belgium
and
CERN
Meyrin
Switzerland
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-70649-8 ISBN978-3-319-70650-4 (eBook)
https://doi.org/10.1007/978-3-319-70650-4
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’
Supervisor s Foreword
Understanding the nature of the electroweak symmetry breaking (EWSB) mecha-
nism is one of the key issues in particle physics today and is at the center of the
research work of Cécile Caillol. The precise construction of our comprehension of
elementary particles of matter and elementary forces between them is developed
during the 60s and 70s, as the result of a ping-pong game between experimental
observationsandtheorydeductions,leadingtotheconceptionofthestandardmodel
of particle physics (SM). In 1964, the EWSB mechanism was proposed by Brout,
Englert, and Higgs, to accommodate the existence of massive elementary particles
inthemodel,andpredictedtheexistenceofanadditionalparticle,thescalarboson
H. The latest triumph in the field arrived with its discovery in 2012. This was an
important achievement: The H boson had been the missing piece of the SM for
more than 40 years.
TheH bosondiscoverywasannouncedinJuly2012bythetwogeneral-purpose
experiments, ATLAS and CMS, at the Large Hadron Collider (LHC) at CERN.
Fromitsveryconception,theLHCwasindeeddesignedtobeadiscoverymachine.
ThefirstrunoftheLHCtookplaceinyears2010–2012atcenter-of-massenergyof
7 and 8 TeV. The first part of Cécile Caillol’s work addresses the study of the
couplings between the scalar boson H and particles of matter (fermions) in par-
ticular the tau lepton, using the Run-1 dataset collected by the CMS experiment.
The associated production of a scalar boson with a Z or a W boson was studied.
When combined with all the other production modes, an evidence for H !¿¿
coupling was observed. This was a key result of CMS announced in 2013 and
published in Nature Physics: It is indeed one of the two accessible tests of the
coupling of H to fermions.
However, despite its incredible success, the SM is not a complete theory, as it
does not provide a framework to describe several important observations in the
universe, for example, the matter–antimatter asymmetry observed today in the
universe, orthe nonzero mass of theneutrino as deduced from neutrino oscillation
phenomena. It does not provide candidates to account for the dark matter and the
dark energy in the universe, as established by cosmological observations. The SM
suffers, in addition, from several conceptual problems, in particular the so-called
v
vi Supervisor’sForeword
“fine-tuning”problem,relatedtothepresenceofthescalarparticleinthemodel.It
isnowgenerallyacceptedthattheSMneedstobeextendedtoaddressandexplain
the limitations presented above.
This is all the paradoxical situation of particle physicists today: desperately
searching for new physics beyond the standard model (BSM), both at the experi-
mental level (trying to see hints of new phenomena, as the discovery of new
particles or new interactions) and at the theoretical level (brainstorming for new
ideas to accommodate the many open questions). On the theory side, a way to
attack theproblem istointroduce inthemodel newparticles andnew symmetries,
at a certain scale. Several interesting extensions of the SM have been proposed,
such as the introduction of a new symmetry, the supersymmetry (SUSY). In that
case, the particle spectrum is at least twofold, with an extended scalar sector,
providing a rich phenomenology, with many different production mechanisms and
final states. Its simplest version is called the minimal supersymmetric model
(MSSM).Thedirectsearchforsupersymmetricparticlesisoneofthekeytopicsat
the LHC. Another natural direction to follow is the study of generic extensions
of the SM scalar sector, the so-called two-Higgs-doublet models (2HDM),
assuming two doublets of scalar complex fields. On the experimental side, a
detailedstudyofthenatureofthescalarbosonisextremelyimportantinthecontext
ofsearchesforBSMphysics.Isthenewscalarparticlejustdiscoveredbehavingas
expected by the minimal version of the SM? Or is there room for new physics? Is
thisnewparticletheonlyelementaryscalarparticleinnatureoraretheremoretobe
discovered? In parallel to the discovery and the study of properties of the scalar
boson(s), the search for other new particles, as expected from new physics, is also
crucial.
The second part of Cécile Caillol’s thesis investigates several BSM searches in
the scalar sector. In extended models, a search for a heavy scalar boson decaying
intoapairoftauleptonsisperformed.Thisisthemostpowerfulchanneltouncover
an MSSM scalar sector at the LHC in some parameter space. In addition, Cécile
Caillol participated to three BSM searches performed for the first time using the
8 TeV data at the LHC: a search for a heavy A boson in the A!Zh!‘‘¿¿
channelinterpretedintheMSSMandina2HDMmodel,asearchforalightscalar
produced in association with a b-quark pair, bbA!bb¿¿, and a search for exotic
decays of the H scalar boson into light scalars in the H !aa!ll¿¿ decay
channel. In all these original analyses, no hint for a modified or extended scalar
sector has been observed. However, the scalar sector is known for providing par-
ticles difficult to detect. The quest for new physics at the LHC is continuing, and
much hope is placed on the new high-energy run at a center-of-mass energy of
13 TeV, with hopefully discoveries. Running beyond 2022, with ten times more
data,thehigh-luminosityLHCphasewillbeneededformoredetailedstudiesofthe
scalar sector and of any new physics that could be discovered meanwhile. Many
important results are thus expected to come from the LHC in the future.
Let me finish with a word for non-specialist in the field. In addition to the
detailed description of the original results of her work, Cécile Caillol’s thesis
provides a well-thought introduction on the SM model and on its possible
Supervisor’sForeword vii
extensions.ThelastchapterofthethesissummarizesthekeyachievementsofCMS
concerning scalar boson measurements in the SM and beyond and outlines future
prospects in this area. I trust that these sections will be very useful to guide
non-expertsinthefield.Itisanexcitingtimeintheareaofparticlephysicstoday,in
particular on what concerns the scalar sector, and I am pleased that Springer is
publishing Cécile Caillol’s outstanding thesis on this subject.
Brussels, Belgium Prof. Barbara Clerbaux
August 2017
Abstract
Thisthesispresentsastudyofthescalarsectorinthestandardmodel(SM)aswell
as different searches for an extended scalar sector in theories beyond the standard
model (BSM). All analyses have in common the fact that at least one scalar boson
decays to a pair of tau leptons. The results exploit the data collected by the CMS
detector during LHC Run-1, in proton–proton collisions with a center-of-mass
energy of 7 or 8 TeV.
Theparticlediscoveredin2012,H,lookscompatiblewithaSMBrout–Englert–
Higgs boson, but this statement is driven by the H !(cid:2)(cid:2) and H !ZZ decay
modes.TheH !¿þ¿(cid:2) decaymodeisthemostsensitivefermionicdecaychannel
andallowstotesttheYukawacouplingsofthenewparticle.ThesearchfortheSM
scalar boson decaying to tau leptons, and produced in association with a massive
vector boson W or Z, is described in this thesis. Even though a good background
rejection can be achieved by selecting the leptons originating from the vector
boson,Run-1dataarenotsensitivetothesmallproductioncrosssectionspredicted
in the SM for the scalar boson. The combination with the gluon–gluon fusion and
vectorbosonfusionproductionsearchesleadstoanevidenceforthedecayoftheH
boson to tau leptons.
Many BSM models, such as the minimal supersymmetric SM (MSSM) or
models with two scalar doublets (2HDM), predict the existence of several scalar
bosons. The decays of these bosons to tau leptons can be enhanced in some sce-
narios depending on the model parameters, which makes the di-tau decay mode
powerfultodiscoverBSMphysics.Foursearchesforanextendedscalarsectorare
detailed in this thesis. The first analysis searches for a pseudoscalar boson with a
mass between 220 and 350 GeV, decaying to an SM-like scalar boson and a Z
boson,inthefinalstatewithtwolightleptonsandtwotauleptons.Second,asearch
for the exotic decay of the new particle H to a pair of light pseudoscalar bosons,
which is still allowed by all measurements made up to now, in the final state with
two muons and two tau leptons is performed. Third, a mass region almost never
ix
x Abstract
explored at the LHC is probed by the search of a light pseudoscalar, with a mass
between25and80 GeV,decayingtotauleptonsandproducedinassociationwithb
quarks.ThelastanalysisdescribesthesearchforaheavyresonanceintheMSSM,
decayingtoapairoftauleptons.Noneoftheseanalyseshasfoundanyhintofnew
physics beyond the SM, but stringent limits on the cross section of such signals
could be set.
