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Register a new userFirst measurement of the forward-backward asymmetry in bottom-quark pair production at high mass
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We measure the particle-level forward-backward production asymmetry in $bbar{b}$ pairs with masses $m(bbar{b})$ larger than 150 GeV/$c^2$, using events with hadronic jets and employing jet charge to distinguish $b$ from $bar{b}$. The measurement uses 9.5/fb of ppbar collisions at a center of mass energy of 1.96 TeV recorded by the CDF II detector. The asymmetry as a function of $m(bbar{b})$ is consistent with zero, as well as with the predictions of the standard model. The measurement disfavors a simple model including an axigluon with a mass of 200 GeV/$c^2$ whereas a model containing a heavier 345 GeV/$c^2$ axigluon is not excluded.
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We report a measurement of the forward-backward asymmetry, $A_{FB}$, in $bbar{b}$ pairs produced in proton-antiproton collisions and identified by muons from semileptonic $b$-hadron decays. The event sample was collected at a center-of-mass energy of $sqrt{s}=1.96$ TeV with the CDF II detector and corresponds to 6.9 fb$^{-1}$ of integrated luminosity. We obtain an integrated asymmetry of $A_{FB}(bbar{b})=(1.2 pm 0.7)$% at the particle level for $b$-quark pairs with invariant mass, $m_{bbar{b}}$, down to $40$ GeV/$c^2$ and measure the dependence of $A_{FB}(bbar{b})$ on $m_{bbar{b}}$. The results are compatible with expectations from the standard model.
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.
At the LHC, top quark pairs are dominantly produced from gluons, making it difficult to measure the top quark forward-backward asymmetry. To improve the asymmetry measurement, we study variables that can distinguish between top quarks produced from quarks and those from gluons: the invariant mass of the top pair, the rapidity of the top-antitop system in the lab frame, the rapidity of the top quark in the top-antitop rest frame, the top quark polarization and the top-antitop spin correlation. We combine all the variables in a likelihood discriminant method to separate quark-initiated events from gluon-initiated events. We apply our method on models including G-primes and W-primes motivated by the recent observation of a large top quark forward-backward asymmetry at the Tevatron. We have found that the significance of the asymmetry measurement can be improved by 10% to 30%. At the same time, the central values of the asymmetry increase by 40% to 100%. We have also analytically derived the best spin quantization axes for studying top quark polarization as well as spin-correlation for the new physics models.
We present new measurements of the inclusive forward-backward ttbar production asymmetry, AFB, and its dependence on several properties of the ttbar system. The measurements are performed with the full Tevatron data set recorded with the CDF II detector during ppbar collisions at sqrt(s) = 1.96 TeV, corresponding to an integrated luminosity of 9.4 fb^(-1). We measure the asymmetry using the rapidity difference Delta-y=y_(t)-y_(tbar). Parton-level results are derived, yielding an inclusive asymmetry of 0.164+/-0.047 (stat + syst). We observe a linear dependence of AFB on the top-quark pair mass M(ttbar) and the rapidity difference |Delta-y| at detector and parton levels. Assuming the standard model, the probabilities to observe the measured values or larger for the detector-level dependencies are 7.4*10^(-3) and 2.2*10^(-3) for M(ttbar) and |Delta-y| respectively. Lastly, we study the dependence of the asymmetry on the transverse momentum of the ttbar system at the detector level. These results are consistent with previous lower-precision measurements and provide additional quantification of the functional dependencies of the asymmetry.
The bottom quark forward-backward asymmetry $A_{rm{FB}}$ is a key observable in electron-positron collisions at the $Z^{0}$ peak. In this paper, we employ the Principle of Maximum Conformality (PMC) to fix the $alpha_s$-running behavior of the next-to-next-to-leading order QCD corrections to $A_{rm{FB}}$. The resulting PMC scale for this $A_{rm{FB}}$ is an order of magnitude smaller than the conventional choice $mu_r=M_Z$. This scale has the physically reasonable behavior and reflects the virtuality of its QCD dynamics, which is independent to the choice of renormalization scale. Our analyses show that the effective momentum flow for the bottom quark forward-backward asymmetry should be $mu_rll M_Z$ other than the conventionally suggested $mu_r=M_Z$. Moreover, the convergence of perturbative QCD series for $A_{rm{FB}}$ is greatly improved using the PMC. Our prediction for the bare bottom quark forward-backward asymmetry is refined to be $A^{0,b}_{rm FB}=0.1004pm0.0016$, which diminishes the well known tension between the experimental determination for this (pseudo) observable and the respective Standard Model fit to $2.1sigma$.
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