1,264 research outputs found

    Measurement of top quark polarisation in t-channel single top quark production

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    A first measurement of the top quark spin asymmetry, sensitive to the top quark polarisation, in tt-channel single top quark production is presented. It is based on a sample of pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 fb−1\mathrm{fb^{-1}}. A high-purity sample of tt-channel single top quark events with an isolated muon is selected. Signal and background components are estimated using a fit to data. A differential cross section measurement, corrected for detector effects, of an angular observable sensitive to the top quark polarisation is performed. The differential distribution is used to extract a top quark spin asymmetry of 0.26±0.03(stat)±0.10(syst)0.26 \pm 0.03 \textrm{(stat)} \pm 0.10 \textrm{(syst)}, which is compatible with a p-value of 4.6%4.6\% with the standard model prediction of 0.44.Comment: presented at 9th International Workshop on Top Quark Physics Olomouc, Czech Republic, September 19ñ€“23, 201

    Üksiku t-kvargi omaduste mÔÔtmine CMS-detektoris

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    PĂ€rast hiljutist Higgsi bosoni avastamist on leitud kĂ”ik osakestefĂŒĂŒsika standardmudeli (SM) poolt ennustatud osakesed. Ometi on pĂ”hjust arvata, et tegu pole lĂ”pliku teooriaga, kuna mitmed nĂ€htused, nagu tumeaine vĂ”i neutriinode massid ei ole SM-ga seletatavad. Üks huvitav uurimisobjekt, mille tĂ€psete mÔÔtmiste abil saab kindlaks teha vĂ”imalikke kĂ”rvalekaldeid SM ennustustest, on t-kvark. T-kvark on raskeim avastatud elementaarosake ja laguneb seetĂ”ttu erinevalt teistest kvarkidest enne liitosakeste hadronite moodustamist, vĂ”imaldades erandlikult uurida paljast kvarki. Oma massi tĂ”ttu on t-kvark ka osake, mis omab kĂ”ige tugevamat vastasmĂ”ju Higgsi bosoniga. T-kvargid saavad tekkida kas kvargi- ja antikvargi paaridena tugeva vastastikmĂ”ju abil vĂ”i vĂ€iksemal mÀÀral ĂŒksikute (anti)kvarkidena nĂ”rga vastastikmĂ”ju kaudu. Seda tĂ”enĂ€osust, et vastav protsess aset leiab, iseloomustab tekkeristlĂ”ige, mis on kĂ”igi osakestefĂŒĂŒsika protsesside oluliseks omaduseks. Üksikute t-kvarkide teket vaadeldi esmakordselt alles 2009. aastal, samas on nende abil vĂ”imalik uurida mitmeid t-kvarkide omadusi paremini kui t-kvargi paaride abil. NĂ€iteks ei jĂ”ua t-kvargi spinn enne lagunemist muutuda ja seetĂ”ttu saab seda ĂŒksiku t-kvargi laguproduktide kaudu mÔÔta, kusjuures SM ennustab, et kĂ”ik tekkinud ĂŒksikud t-kvargid peaksid olema vasakukĂ€elised. Suur Hadronite PĂ”rguti (LHC) on 27 km pikkune maailma suurim ja kĂ”rgeima energiaga osakeste kiirendi, kus pĂ”rgatatakse vastassuundades liikuvaid prootonite kimpe masskeskme energiatel kuni 14 TeV. LHC-s tegutseb seitse eksperimenti, antud töö kasutab ĂŒldotstarbelise detektori CMS-iga tehtud mÔÔtmisi. Doktoritöös on mÔÔdetud ĂŒksiku t-kvargi ristlĂ”iked prootonite pĂ”rgetel masskeskme energiatel 7 TeV ja 8 TeV ning ĂŒksiku 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

    Measurement of the t(t)over-bar production cross section in the dilepton channel in pp collisions at √s=8 TeV

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    The top-antitop quark (t (t) over bar) production cross section is measured in proton-proton collisions at root s = 8 TeV with the CMS experiment at the LHC, using a data sample corresponding to an integrated luminosity of 5.3 fb(-1). The measurement is performed by analysing events with a pair of electrons or muons, or one electron and one muon, and at least two jets, one of which is identified as originating from hadronisation of a bottom quark. The measured cross section is 239 +/- 2 (stat.) +/- 11 (syst.) +/- 6 (lum.) pb, for an assumed top-quark mass of 172.5 GeV, in agreement with the prediction of the standard model

    Search for pair-produced resonances decaying to jet pairs in proton-proton collisions at √s=8 TeV

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    Results are reported of a general search for pair production of heavy resonances decaying to pairs of hadronic jets in events with at least four jets. The study is based on up to 19.4 fb(-1) of integrated luminosity from proton-proton collisions at a center-of-mass energy of 8 TeV, recorded with the CMS detector at the LHC. Limits are determined on the production of scalar top quarks (top squarks) in the framework of R-parity violating supersymmetry and on the production of color-octet vector bosons (colorons). First limits at the LHC are placed on top squark production for two scenarios. The first assumes decay to a bottom quark and a light-flavor quark and is excluded for masses between 200 and 385 GeV, and the second assumes decay to a pair of light-flavor quarks and is excluded for masses between 200 and 350 GeV at 95% confidence level. Previous limits on colorons decaying to light-flavor quarks are extended to exclude masses from 200 to 835 GeV

    Differential cross section measurements for the production of a W boson in association with jets in proton–proton collisions at √s = 7 TeV

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    Measurements are reported of differential cross sections for the production of a W boson, which decays into a muon and a neutrino, in association with jets, as a function of several variables, including the transverse momenta (pT) and pseudorapidities of the four leading jets, the scalar sum of jet transverse momenta (HT), and the difference in azimuthal angle between the directions of each jet and the muon. The data sample of pp collisions at a centre-of-mass energy of 7 TeV was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 5.0 fb[superscript −1]. The measured cross sections are compared to predictions from Monte Carlo generators, MadGraph + pythia and sherpa, and to next-to-leading-order calculations from BlackHat + sherpa. The differential cross sections are found to be in agreement with the predictions, apart from the pT distributions of the leading jets at high pT values, the distributions of the HT at high-HT and low jet multiplicity, and the distribution of the difference in azimuthal angle between the leading jet and the muon at low values.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio

    Juxtaposing BTE and ATE – on the role of the European insurance industry in funding civil litigation