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Register a new userMeasurement of the charge asymmetry in top quark pair production in pp collisions at $sqrt s = 7 $ TeV using the ATLAS detector
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Rachik Soualah
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We present a measurement of the charge asymmetry in top-antitop production using data corresponding to an integrated luminosity of $0.70 fb^{-1}$ of proton-proton collisions at $sqrt s = 7$ TeV collected by the ATLAS detector. The top pair events decaying semileptonically (lepton+jets channel) to either an electron or muon, missing transverse energy and at least four jets are selected. The reconstruction of the $tbar{t}$ events was performed using a kinematic likelihood approach. The difference of absolute values of top and antitop rapidities is used to define the charge asymmetry: $A_{C} = (N(|Delta Y|>0) - N(|Delta Y|<0)) / (N(|Delta Y>0) + N(|Delta Y|<0))$. To allow comparisons with theory calculations, a Bayesian unfolding technique is applied to correct the measured $|Delta Y|$ distributions for acceptance and detector effects. The top charge asymmetry in both channels (e and mu) after correction is measured to be: $A_{C} = -0.009 pm 0.023 (stat) pm 0.032 (syst)$ (e+jets channel) and $A_{C} = -0.028 pm 0.019 (stat) pm 0.022 (syst)$ ($mu$+jets channel) giving a combined result of : $A_{C} = -0.024 pm 0.016 (stat) pm 0.023 (syst)$. These results are compatible with the Standard Model predictions of $A_{C}=0.006$.
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The ATLAS experiment at the LHC at CERN has analyzed 2010 and 2011 data looking for electroweak production of single top quarks in the lepton+jets and di-lepton final states. The production cross section for the t-channel process is measured to be 76 +41 -21 pb using 156 pb^-1 of 2011 data. A first limit is set on the Wt associated production process using lepton+jets and di-lepton events. The 95% CL upper limit on the Wt production cross section is 158 pb using 35 pb^-1 of 2010 data.
Measurements of normalized differential cross-sections of top quark pair ($tbar t$) production are presented as a function of the mass, the transverse momentum and the rapidity of the $tbar t$ system in proton-proton collisions at center-of-mass energies of $sqrt{s}$ = 7 TeV and 8 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb$^{-1}$ at 7 TeV and 20.2 fb$^{-1}$ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a $b$-hadron. The measured distributions are corrected for detector effects and selection efficiency to cross-sections at the parton level. The differential cross-sections are compared with different Monte Carlo generators and theoretical calculations of $tbar t$ production. The results are consistent with the majority of predictions in a wide kinematic range.
The production of Upsilon(1S), Upsilon(2S) and Upsilon(3S) mesons in proton-proton collisions at the centre-of-mass energy of sqrt(s)=7 TeV is studied with the LHCb detector. The analysis is based on a data sample of 25 pb-1 collected at the Large Hadron Collider. The Upsilon mesons are reconstructed in the decay mode Upsilon -> mu+ mu- and the signal yields are extracted from a fit to the mu+ mu- invariant mass distributions. The differential production cross-sections times dimuon branching fractions are measured as a function of the Upsilon transverse momentum pT and rapidity y, over the range pT < 15 GeV/c and 2.0 < y < 4.5. The cross-sections times branching fractions, integrated over these kinematic ranges, are measured to be sigma(pp -> Upsilon(1S) X) x B(Upsilon(1S)->mu+ mu-) = 2.29 {pm} 0.01 {pm} 0.10 -0.37 +0.19 nb, sigma(pp -> Upsilon(2S) X) x B(Upsilon(2S)->mu+ mu-) = 0.562 {pm} 0.007 {pm} 0.023 -0.092 +0.048 nb, sigma(pp -> Upsilon(3S) X) x B(Upsilon(3S)->mu+ mu-) = 0.283 {pm} 0.005 {pm} 0.012 -0.048 +0.025 nb, where the first uncertainty is statistical, the second systematic and the third is due to the unknown polarisation of the three Upsilon states.
The inclusive $D_s^{pm}$ production asymmetry is measured in $pp$ collisions collected by the LHCb experiment at centre-of-mass energies of $sqrt{s} =7$ and 8 TeV. Promptly produced $D_s^{pm}$ mesons are used, which decay as $D_s^{pm}tophipi^{pm}$, with $phito K^+K^-$. The measurement is performed in bins of transverse momentum, $p_{rm T}$, and rapidity, $y$, covering the range $2.5<p_{rm T}<25.0$ GeV$/c$ and $2.0<y<4.5$. No kinematic dependence is observed. Evidence of nonzero $D_s^{pm}$ production asymmetry is found with a significance of 3.3 standard deviations.
Forward top quark pair production is studied in $pp$ collisions in the $mu eb$ final state using a data sample corresponding to an integrated luminosity of 1.93 fb$^{-1}$ collected with the LHCb experiment at a centre-of-mass energy of 13 TeV. The cross-section is measured in a fiducial region where both leptons have a transverse momentum greater than 20 GeV and a pseudorapidity between 2.0 and 4.5. The quadrature sum of the azimuthal separation and the difference in pseudorapidities, denoted $Delta R$, between the two leptons must be larger than 0.1. The $b$-jet axis is required to be separated from both leptons by a $Delta R$ of 0.5, and to have a transverse momentum in excess of 20 GeV and a pseudorapidity between 2.2 and 4.2. The cross-section is measured to be $$sigma_{tbar{t}}= 126pm19,(mathrm{stat})pm16,(mathrm{syst})pm5,(mathrm{lumi}),,mathrm{ fb}$$ where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measurement is compatible with the Standard Model prediction.
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