Scalar Boson Decays to Tau Leptons: in the Standard Model and Beyond PDF
Preview Scalar Boson Decays to Tau Leptons: in the Standard Model and Beyond
Springer Theses Recognizing Outstanding Ph.D. Research Cécile Caillol Scalar Boson Decays to Tau Leptons In the Standard Model and Beyond 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. 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More information about this series at http://www.springer.com/series/8790 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 LibraryofCongressControlNumber:2017957836 ©SpringerInternationalPublishingAG2018 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. 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Printedonacid-freepaper ThisSpringerimprintispublishedbySpringerNature TheregisteredcompanyisSpringerInternationalPublishingAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland ’ 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.
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