Measurement of single top quark properties with the CMS detector
Date
2016-09-22
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Abstract
Pärast hiljutist Higgsi bosoni avastamist on leitud kõik osakestefüüsika standardmudeli (SM) poolt ennustatud osakesed. Ometi on põhjust arvata, et tegu pole lõpliku teooriaga, kuna mitmed nähtused, nagu tumeaine või neutriinode massid ei ole SM-ga seletatavad.
Üks huvitav uurimisobjekt, mille täpsete mõõtmiste abil saab kindlaks teha võimalikke kõrvalekaldeid SM ennustustest, on t-kvark. T-kvark on raskeim avastatud elementaarosake ja laguneb seetõttu erinevalt teistest kvarkidest enne liitosakeste hadronite moodustamist, võimaldades erandlikult uurida paljast kvarki. Oma massi tõttu on t-kvark ka osake, mis omab kõige tugevamat vastasmõju Higgsi bosoniga.
T-kvargid saavad tekkida kas kvargi- ja antikvargi paaridena tugeva vastastikmõju abil või väiksemal määral üksikute (anti)kvarkidena nõrga vastastikmõju kaudu. Seda tõenäosust, et vastav protsess aset leiab, iseloomustab tekkeristlõige, mis on kõigi osakestefüüsika protsesside oluliseks omaduseks.
Üksikute t-kvarkide teket vaadeldi esmakordselt alles 2009. aastal, samas on nende abil võimalik uurida mitmeid t-kvarkide omadusi paremini kui t-kvargi paaride abil. Näiteks ei jõua t-kvargi spinn enne lagunemist muutuda ja seetõttu saab seda üksiku t-kvargi laguproduktide kaudu mõõta, kusjuures SM ennustab, et kõik tekkinud üksikud t-kvargid peaksid olema vasakukäelised.
Suur Hadronite Põrguti (LHC) on 27 km pikkune maailma suurim ja kõrgeima energiaga osakeste kiirendi, kus põrgatatakse vastassuundades liikuvaid prootonite kimpe masskeskme energiatel kuni 14 TeV. LHC-s tegutseb seitse eksperimenti, antud töö kasutab üldotstarbelise detektori CMS-iga tehtud mõõtmisi.
Doktoritöös on mõõdetud üksiku t-kvargi ristlõiked prootonite põrgetel masskeskme energiatel 7 TeV ja 8 TeV ning üksiku t-kvargi polarisatsioon energial 8 TeV.
After the recent discovery of the Higgs boson, all particles predicted by the standard model of particle physics (SM) have been found. However, since many phenomena, such as dark matter or neutrino masses, are not explained by SM, there is reason to believe that it is not the final theory. A particularly interesting object of study to check for possible deviations from SM predictions is the top quark. Top quark is the heaviest discovered particle. Its very high mass causes the top quark to decay before forming hadrons in contrast to other quarks, which always form composite particles. Thus, the top quark gives us a special opportunity to study a bare quark. Due to its mass, the top quark is also the particle with the strongest interaction with the Higgs boson. Top quarks are created either in quark-antiquark pairs through the strong interaction or less frequently as single top (anti)quarks through weak interaction. The probability of being created is measured by production cross section, which is an important property of all particle physics processes. Despite the smaller production cross section, single top production offers possibilities to study some of the top quark properties in a better way compared to pair production. For example, all single top quarks are created left-handed according to SM prediction, and as the top quark decays before the spin has time to change, the spin of the top quark can be measured through the decay products. Single top quark production was first observed only recently, in 2009. The Large Hadron Collider (LHC) is the world’s biggest and highest energy particle collider with a circumference of 27 km, colliding beams of protons circulating in opposite directions at centre-of-mass energies up to 14 TeV. There are seven experiments in operation at the LHC, this work utilises the data collected with the general purpose detector Compact Muon Solenoid (CMS). In this thesis, the measurement of single top quark cross section is performed at proton-proton collisions at the centre-of-mass energies of 7 and 8 TeV, and single top quark polarisation is measured at 8 TeV.
After the recent discovery of the Higgs boson, all particles predicted by the standard model of particle physics (SM) have been found. However, since many phenomena, such as dark matter or neutrino masses, are not explained by SM, there is reason to believe that it is not the final theory. A particularly interesting object of study to check for possible deviations from SM predictions is the top quark. Top quark is the heaviest discovered particle. Its very high mass causes the top quark to decay before forming hadrons in contrast to other quarks, which always form composite particles. Thus, the top quark gives us a special opportunity to study a bare quark. Due to its mass, the top quark is also the particle with the strongest interaction with the Higgs boson. Top quarks are created either in quark-antiquark pairs through the strong interaction or less frequently as single top (anti)quarks through weak interaction. The probability of being created is measured by production cross section, which is an important property of all particle physics processes. Despite the smaller production cross section, single top production offers possibilities to study some of the top quark properties in a better way compared to pair production. For example, all single top quarks are created left-handed according to SM prediction, and as the top quark decays before the spin has time to change, the spin of the top quark can be measured through the decay products. Single top quark production was first observed only recently, in 2009. The Large Hadron Collider (LHC) is the world’s biggest and highest energy particle collider with a circumference of 27 km, colliding beams of protons circulating in opposite directions at centre-of-mass energies up to 14 TeV. There are seven experiments in operation at the LHC, this work utilises the data collected with the general purpose detector Compact Muon Solenoid (CMS). In this thesis, the measurement of single top quark cross section is performed at proton-proton collisions at the centre-of-mass energies of 7 and 8 TeV, and single top quark polarisation is measured at 8 TeV.
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Keywords
kvargid, detektorid, quarks, detectors