First Measurement of the Charge Asymmetry in Beauty-Quark Pair Production

The difference in the angular distributions between beauty quarks and antiquarks, referred to as the charge asymmetry, is measured for the first time in $b\overline{b}$ pair production at a hadron collider. The data used correspond to an integrated luminosity of $1.0{\text{fb}}^{-1}$ collected at 7 TeV 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 $b\overline{b}$ system. The results obtained are $A_{\mathrm{C}}^{b\overline{b}}(40<M_{b\overline{b}}<75\mathrm{GeV}/c^2)=0.4\pm 0.4\pm 0.3\%$, $A_{\mathrm{C}}^{b\overline{b}}(75<M_{b\overline{b}}<105\mathrm{GeV}/c^2)=2.0\pm 0.9\pm 0.6\%$, $A_{\mathrm{C}}^{b\overline{b}}(M_{b\overline{b}}>105\mathrm{GeV}/c^2)=1.6\pm 1.7\pm 0.6\%$, where $A_{\mathrm{C}}^{b\overline{b}}$ is defined as the asymmetry in the difference in rapidity between jets formed from the beauty quark and antiquark, where in each case the first uncertainty is statistical and the second systematic. The beauty jets are required to satisfy $2<\eta <4$, $E_{\mathrm{T}}>20\mathrm{GeV}$, and have an opening angle in the transverse plane $\mathrm{\Delta }\varphi >2.6\mathrm{rad}$. These measurements are consistent with the predictions of the standard model.

Figure 1

(top) Corrected mass of TOPO objects associated to bTAG jets in the final event sample. Less than 2% of jets are found to originate from charm. (bottom) Corrected mass of TOPO objects associated to subleading vs. leading jets in the final event sample. A small $c\overline{c}$ contribution is visible near (2,2)$\mathrm{GeV}/c^2$.

Figure 2

Reconstructed $\mathrm{\Delta }y$ distribution for all selected events after background subtraction and correction for qTAG impurity. The dashed line shows the distribution reflected about the vertical axis.

Figure 3

Migration matrix in $\mathrm{\Delta }y$ and $M_{b\overline{b}}$.

Figure 4

Corrected $\mathrm{\Delta }y$ distribution for all selected events. The statistical uncertainties are negligible. The systematic uncertainties are highly correlated from bin to bin and largely cancel in the determination of $A_{\mathrm{C}}^{b\overline{b}}$. The LO SM prediction obtained from Pythia [36, 37] is also shown.