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First measurement of the charge asymmetry in beauty-quark pair production at a hadron collider

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 Publication date 2014
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The difference in the angular distributions between beauty quarks and antiquarks, referred to as the charge asymmetry, is measured for the first time in $bbar{b}$ pair production at a hadron collider. The data used correspond to an integrated luminosity of 1.0fb$^{-1}$ collected at 7TeV center-of-mass energy in proton-proton collisions with the LHCb detector. The measurement is performed in three regions of the invariant mass of the $bbar{b}$ system. The results obtained are: begin{eqnarray} A_{C}^{bbar{b}}(40 < M_{bbar{b}} < 75,rm{GeV/c^2}) &=& 0.4 pm 0.4(rm{stat}) pm 0.3(rm{syst})% ewline A_{C}^{bbar{b}}(75 < M_{bbar{b}} < 105,rm{GeV/c^2}) &=& 2.0 pm 0.9(rm{stat}) pm 0.6(rm{syst})% ewline A_{C}^{bbar{b}}(M_{bbar{b}} > 105,rm{GeV/c^2}) &=&1.6 pm 1.7(rm{stat}) pm 0.6(rm{syst})% end{eqnarray} where $A_{C}^{bbar{b}}$ is defined as the asymmetry in the difference in rapidity between jets formed from the beauty quark and antiquark. The beauty jets are required to satisfy $2<eta<4$, $E_{rm T} > 20$GeV, and have an opening angle in the transverse plane $Deltaphi>2.6$rad. These measurements are consistent with the predictions of the Standard Model.



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371 - Rachik Soualah 2012
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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