Bottom-quark forward-backward and charge asymmetries at hadron colliders

Predictions are made for the forward-backward and charge asymmetries in bottom-quark pair production at hadron colliders. Tree-level exchanges of electroweak (EW) gauge bosons dominate the Standard Model (SM) contribution to the asymmetry near the $Z$-pole. The mixed EW-QCD corrections are computed in an approximate way, and are found to be small in magnitude. These SM predictions are consistent with experimental results from CDF, D0, and LHCb. In particular, CDF and LHCb find that the asymmetry in the invariant mass bin containing the $Z$-pole is larger than in the adjacent bins, as predicted. Several beyond the Standard Model scenarios proposed for the top-quark forward-backward asymmetry, including a 100 GeV axigluon, are disfavored by this combination of SM predictions and measurements. On the other hand, modified $Zb\overline{b}$ couplings can explain the $2\sigma$ discrepancy in the bottom-quark forward-backward asymmetry at LEP1, while being consistent with the results of CDF and LHCb. It is also shown that $t$-channel $W$ exchange makes a non-negligible contribution to the charm-quark charge asymmetry.

Figure 1

$A_{FB}$ vs. $|\eta (B)|$ (left) and $p_T(B)$ (right). In both cases, data from D0 and their corresponding predictions [26], which were made using mc@nlo+Herwig, are shown in black and purple respectively. The SM predictions from this work are in red.

Figure 2

Top: CDF’s low mass measurements of $A_{FB}^{b\overline{b}}$ [25]. Bottom left: LHCb’s 7 TeV measurements of $A_C^{b\overline{b}}$ [29] Bottom right: CDF’s high-mass measurements of $A_{FB}^{b\overline{b}}$ [24]. The experimental data is shown in black. In all three plots, the SM predictions from this work are shown in red, and plotted in blue are predictions for the bottom asymmetry due to a 100 GeV axigluon.

Figure 3

CDF’s measurements of $A_{FB}^{b\overline{b}}$ [25] are plotted in black, and the SM predictions from this work are shown in red. Plotted in brown and green respectively are predictions for the bottom asymmetry due to a 105 GeV scalar weak doublet and a flavor octet of 150 GeV EW triplet vectors.

Figure 4

Left: Results of a fit to measurements of $A_{FB}^{(0,b)}$ and $R_b$ at LEP1. Regions favored at the 1 and $2\sigma$ levels are given in blue and orange respectively. Regions allowed by CDF and LHCb measurements of $A_{FB}^{b\overline{b}}$ and $A_C^{b\overline{b}}$ are shown in green and yellow. See the text for details on the parameter space allowed by CDF and LHCb. Right: Zoomed in version of the plot of the left, centered on the region allowed by LEP1.