Measurement of of the W → τv cross section and a search for a doubly charged Higgs boson decaying to t-leptons
Date
2012-11-19
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Abstract
Elementaarosakeste standardmudel kirjeldab hetkel teadaolevat fundamentaalfüüsikat ning on viimaste aastakümnete jooksul leidnud rohket eksperimentaalset kinnitust. Samas on põhjust arvata, et standardmudel ei ole fundamentaalne alusteooria, vaid pigem jäänuk üldisemast füüsikast, mis avaldub kõrgematel energiatel. Üheks kindlaks viiteks standardmudeli-järgse füüsika kohta on neutriinode nullist erinevad massid, mis on võrreldes teiste standardmudeli osakeste massidega väga väikesed.
LHC (Large Hadron Collider) on maailma suurim elementaarosakeste kiirendi, mis töötab kõrgemal energial kui ükski varasem eksperiment. LHC kiirendi üldisteks eesmaärkideks on nii uue füüsika ja Higgsi bosoni otsimine kui ka standardmudeli protsesside uurimine ja täppismõõtmised. Kiirendi paikneb 27 km. pikkuses ringikujulises tunnelis, mis koosneb magnetitest, milles ringlevad kaks vastassuunalist prootonkiirt. Prootonite interaktsioonipunktides paiknevad osakeste-detektorid, kus toimub põrgete edasine analüüs. Käesolevas doktoritöös kirjeldatud analüüsid on teostatud CMS (Compact Muon Solenoid) detektori poolt kogutud andmetega.
W-bosoni ristloõike mõõtmine τ-leptoni ja neutriino lagunemiskanalis on üks teadaolevatest standardmudeli protsessidest. τ-leptoniteks laguneva topeltlaetud Higgsi bosoni otsimine baseerub uuel füüsikateoorial, mis üritab seletada neutriinode väikseid, kuid nullist erinevaid masse. Analüüsitud eksperimentaalsed andmed on standardmudeliga kooskoõlas: mõõdetud W-bosoni ristloõige vastab teoorias arvutatud väärtusele ning topeltlaetud Higgsi bosoni võimaliku signaali otsing annab uue massi alampiiri, mis parandab oluliselt eelmist Tevatroni kiirendis saadud tulemust.
The standard model of particle physics describes known matter and fundamental interactions and has been experimentally verified to a great level of accuracy. However, it is generally believed that the SM is not a complete theory, but needs to be extended. One of the clear indications of physics beyond the standard model are nonzero neutrino masses, that are extremely small in comparison to the masses of other particles. The Large Hadron Collider (LHC) is the worlds largest particle collider that works at higher energies than ever achieved in previous experiments and tries to find answers to the most fundamental questions in modern physics like the origin of mass, dark matter or nonzero neutrino masses. The LHC is situated in the 27 km long circular tunnel and is composed of a ring of magnets that store two counter-rotating proton beams. The proton collisions are measured and further analyzed by several particle detectors that are situated at the collision points of the proton beams. The data collected by one of the large general-purpose experiments – Compact Muon Solenoid (CMS) – were used in the two analyses that are summarized in this thesis. Firstly, the measurement of the cross section of a W- boson decaying to a τ-lepton and a neutrino, is one of the known standard model processes. The measurement confirms the standard model and is a contribution to τ-lepton physics in CMS. The search for a doubly charged Higgs boson that decays to τ-leptons tests new beyond the standard model physical theory, that is related to the neutrino mass generation mechanism. Analyzed data was found to be in good agreement with the standard model prediction and a new lower bound on the doubly charged Higgs boson mass was set, which considerably improves the previous measurements.
The standard model of particle physics describes known matter and fundamental interactions and has been experimentally verified to a great level of accuracy. However, it is generally believed that the SM is not a complete theory, but needs to be extended. One of the clear indications of physics beyond the standard model are nonzero neutrino masses, that are extremely small in comparison to the masses of other particles. The Large Hadron Collider (LHC) is the worlds largest particle collider that works at higher energies than ever achieved in previous experiments and tries to find answers to the most fundamental questions in modern physics like the origin of mass, dark matter or nonzero neutrino masses. The LHC is situated in the 27 km long circular tunnel and is composed of a ring of magnets that store two counter-rotating proton beams. The proton collisions are measured and further analyzed by several particle detectors that are situated at the collision points of the proton beams. The data collected by one of the large general-purpose experiments – Compact Muon Solenoid (CMS) – were used in the two analyses that are summarized in this thesis. Firstly, the measurement of the cross section of a W- boson decaying to a τ-lepton and a neutrino, is one of the known standard model processes. The measurement confirms the standard model and is a contribution to τ-lepton physics in CMS. The search for a doubly charged Higgs boson that decays to τ-leptons tests new beyond the standard model physical theory, that is related to the neutrino mass generation mechanism. Analyzed data was found to be in good agreement with the standard model prediction and a new lower bound on the doubly charged Higgs boson mass was set, which considerably improves the previous measurements.
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Keywords
Suur Hadronite Põrkur, Higgsi bosonid, radioaktiivne lagunemine, tauleptonid, Large Hadron Collider, Higgs bosons, radioactive decay, tau leptons