Higgs, Supersymmetry and Dark Matter After Run I of the LHC

271 Pages·2017·8.67 MB·English

by  Béranger Dumont (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Béranger Dumont

Higgs,
Supersymmetry
and Dark Matter
After Run I of the
LHC
Springer Theses
Recognizing Outstanding Ph.D. Research
Aims and Scope
The series “Springer Theses” brings together a selection of the very best Ph.D.
theses from around the world and across the physical sciences. Nominated and
endorsed by two recognized specialists, each published volume has been selected
foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield
of research. For greater accessibility to non-specialists, the published versions
includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor
explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill
provide a valuable resource both for newcomers to the research fields described,
and for other scientists seeking detailed background information on special
questions. Finally, it provides an accredited documentation of the valuable
contributions made by today’s younger generation of scientists.
Theses are accepted into the series by invited nomination only
and must fulfill all of the following criteria
(cid:129) They must be written in good English.
(cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences,
Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience,
Chemical Engineering, Complex Systems and Biophysics.
(cid:129) The work reported in the thesis must represent a significant scientific advance.
(cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis
must be gained from the respective copyright holder.
(cid:129) They must have been examined and passed during the 12 months prior to
nomination.
(cid:129) Each thesis should include a foreword by the supervisor outlining the signifi-
cance of its content.
(cid:129) The theses should have a clearly defined structure including an introduction
accessible to scientists not expert in that particular field.
More information about this series at http://www.springer.com/series/8790
é
B ranger Dumont
Higgs, Supersymmetry
and Dark Matter After Run I
of the LHC
Doctoral Thesis accepted by
the University of Grenoble, France
123
Author Supervisor
Dr. BérangerDumont Dr. SabineKraml
CTPU-IBS Laboratory of Subatomic Physics
IBSCenter for TheoreticalPhysics &Cosmology, CNRS/IN2P3
ofthe Universe University of Grenoble
Yuseong-gu,Daejeon Grenoble
Korea,Republic of (SouthKorea) France
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-44955-5 ISBN978-3-319-44956-2 (eBook)
DOI 10.1007/978-3-319-44956-2
LibraryofCongressControlNumber:2016949591
©SpringerInternationalPublishingSwitzerland2017
Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart
of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations,
recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission
orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar
methodologynowknownorhereafterdeveloped.
The use of general descriptive names, registered names, trademarks, service marks, etc. in this
publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom
therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse.
The publisher, the authors and the editors are safe to assume that the advice and information in this
book are believed to be true and accurate at the date of publication. Neither the publisher nor the
authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor
foranyerrorsoromissionsthatmayhavebeenmade.
Printedonacid-freepaper
ThisSpringerimprintispublishedbySpringerNature
TheregisteredcompanyisSpringerInternationalPublishingAG
Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland
And here, poor fool! with all my lore
I stand, no wiser than before
Johann Wolfgang von Goethe, Faust, 1808
We shed as we pick up, like travellers who
must carryeverything intheirarms, andwhat
we let fall will be picked up by those behind.
The procession is very long and life is very
short. We die on the march. But there is
nothing outside the march so nothing can be
lost to it.
Tom Stoppard, Arcadia, Act I, Sc. 3, 1993
’
Supervisor s Foreword
The years spent on a doctoral thesis are a very special time, even more so if they
coincide with the discovery of a century and one can contribute to its under-
standing. Such was the case for Béranger.
Béranger set out in 2011 to do his Ph.D. in theoretical particle physics on the
topicof“Darkmatterthelaboratoryandinthesky”.Hisfirstpublicationconcerned
the phenomenology of a mainly right-handed sneutrino (a spin-0 partner of the
neutrino in supersymmetric theories) as a candidate for the dark matter in the
universe. This work consisted of a global analysis by means of Markov Chain
MonteCarlosampling,includingall relevant constraints fromcollider searchesfor
supersymmetric particles as well as direct and indirect dark matter searches. One
of the key elements here was the careful treatment of nuclear and astrophysical
uncertainties when evaluating the constraints from the XENON experiment. Two
distinct cases emerged that satisfied all constraints: heavy sneutrino dark matter
with mass of order 100 GeV, as well as light sneutrino dark matter with mass of
about 3–6 GeV; we discussed the implications of both cases for dark matter
searches, as well as for SUSY and Higgs searches at the LHC. While originally
thoughttobethefoundationuponwhichtobuildthethesis,thisworkisnowjusta
small part of Chap. 3 of this thesis.
In2012,thediscoveryoftheHiggsbosonattheCERNLHCmarkedamilestone
in particle physics. The Higgs boson was the last missing piece of the Standard
Model,itsexistencebeingcrucialforthebreakingoftheelectroweaksymmetryand
for giving mass to the weak gauge bosons. The most burning question after the
discovery was whether we were dealing with the Higgs boson predicted by the
standard theory, or whether the newly discovered particle would carry hints for a
more fundamental theory. Béranger immediately took the opportunity to join my
collaborationwithJohnF.Gunion,oneofthepioneersofHiggsphenomenology,to
work on this problem. Mastering all the technical subtleties on his own, he
developed a fitting code which thoroughly took into account all published LHC
measurementsinthevariousHiggsproductionanddecaychannels.Thisenabledus
to carry out a whole series of studies, including “Higgs couplings at the end of
vii
viii Supervisor’sForeword
2012” published in JHEP1302, 053 (2013), “Status of invisible Higgs decays”
publishedinPhys.Lett.B723,340(2013)and“GlobalfittoHiggssignalstrengths
and couplings, and implications for extended Higgs sectors” published in Phys.
Rev. D88, 075008 (2013), which all very quickly became top-cited papers. In
particular, the last one has been cited nearly 200 times by now.
Based on this, in January 2013 and only in his second year of Ph.D., Béranger
wasaskedtogiveaplenarytalkattheCERNworkshop“LikelihoodsfortheLHC
searches” about how theorists use the Higgs results published by the experimental
collaborations. This can be considered a rare honour, and Béranger gave an
excellent presentation. Moreover, this set off an initiative which culminated in the
document“OnthepresentationoftheLHCHiggsresults”,arXiv:1307.5865,which
putforthconclusionsandsuggestionsregardingthepresentationoftheLHCHiggs
results that may help to maximize their impact and their utility to the whole High
Energy Physics community. The document was co-signed by 17 physicists, but
Béranger’s contributions were among the most crucial ones.
Of course we were not the only group to work on the theoretical understanding
of the Higgs measurements. In fact, it was a highly competitive undertaking,
somethinglikeagoldrushinourfield.Itwasahighlyexcitingtime,andapleasure
to work with Béranger and see him develop. While most of the other groups kept
their computer codes private, Béranger realised that it would be highly valuable to
have a public code for fitting the Higgs data, written in a modern programming
language and based on modern programming standards. Indeed the wealth of
experimental results makes it quite a complex task to assess the compatibility of a
nonstandard Higgs boson with all the available data. Together with a fellow Ph.D.
student, Jeremy Bernon, Béranger thus set out to write such a program for public
use. The outcome is Lilith, a Python library in which the Higgs likelihood in
whatever model theuser feeds inisevaluatedbased onexperimental resultsstored
inaneasilyextensibleXMLdatabase.Theprogram,publishedinEur.Phys.J.C75,
no. 9, 440 (2015) and available from Launchpad, is easy to use and very fast. All
this now forms Chap. 2 and the major part of this thesis. Very clearly and peda-
gogicallywritten,itcanserveasatextbookintroductiontounderstandingtheHiggs
measurements at Run 1 of the LHC.
In parallel, in his final year of the Ph.D., Béranger worked on (re-)interpreting
theresultsoftheLHCsearchesforsupersymmetricparticles.Tostartwith,together
with authors of the MadAnalysis5 package (a C++ package for sophisticated
physics analyses of Monte Carlo events) he extended the program to be able to
simultaneously deal with multiple sub-analyses. Moreover, he helped devise a
novel user-friendly treatment for the kinematic selection criteria. Béranger also
wrote the first implementation of an LHC analysis, which then served as the
template for all other analyses that now form the “MadAnalysis5 Public Analysis
Database”.ThisisdescribedinthesecondpartofChap.3ofthethesis,thusclosing
thecirclewithwherehestarted.Again,thechapterisveryclearlywritten,carefully
explaining the subtleties involved in (re-)interpreting the LHC searches, starting
from the so-called simplified-model approach all the way to full recasting of the
experimental analyses.
Supervisor’sForeword ix
At theend,Béranger entitledhis thesis “Higgs, supersymmetry and darkmatter
afterRunIoftheLHC”.Thetitleiswellchosen,ashisthesisindeeddescribesthe
legacy of Run I, to which he significantly contributed on the theory side. As said
before, it was a highly exciting time. Béranger brought in curiosity, dedication, a
verycareful approach,andhewasnever afraidoffacingchallenges, neither onthe
intellectual nor on the technical side. In his modest and softly-spoken manner he
stoodhisgroundagainststrongopinions,seniorprofessors,andthehighpressureto
achieve and to publish fast.
Forme,thesethreeyearswerefullofdelight.WhatremainsistowishBéranger
luck for his future career, and the reader joy in indulging in this thesis.
Grenoble, France Dr. Sabine Kraml
August 2016
Abstract
Two major problems call for an extension of the Standard Model (SM): the hier-
archy problem in the Higgs sector and the dark matter in the Universe. The dis-
covery of a Higgs boson with mass of about 125 GeV was clearly the most
significant piece of news from CERN’s Large Hadron Collider (LHC). In addition
to representing the ultimate triumph of the SM, it shed new light on the hierarchy
problem and opened up new ways of probing new physics. The various measure-
mentsperformedatRunIoftheLHCconstraintheHiggscouplingstoSMparticles
as well as invisible and undetected decays. In this thesis, the impact of the LHC
Higgs results on various new physics scenarios is assessed, carefully taking into
account uncertainties and correlations between them. Generic modifications of the
Higgs coupling strengths, possibly arising from extended Higgs sectors or higher
dimensional operators, are considered. Furthermore, specific new physics models
are tested. This includes, in particular, the phenomenological Minimal
SupersymmetricStandardModel.WhileaHiggs bosonhasbeenfound,nosignof
beyond the SM physics was observed at Run I of the LHC in spite of the large
number of searches performed by the ATLAS and CMS collaborations. The
implications of the negative results obtained in these searches constitute another
important part of this thesis. First, supersymmetric models with a dark matter
candidateareinvestigatedinlightofthenegativesearchesforsupersymmetryatthe
LHCusingaso-called“simplifiedmodel”approach.Second,toolsusingsimulated
events to constrain any new physics scenario from the LHC results are presented.
Moreover, during this thesis the selection criteria of several beyond the SM anal-
yses have been reimplemented in the MadAnalysis 5 framework and made
available in a public database.
xi