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

Abstract

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 Zbb couplings can explain the 2σ 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.

Measurement of the forward-backward asymmetry in low-mass bottom-quark pairs produced in proton-antiproton collisions

“…The significant difference between the raw A FB and the particle level A FB comes from the interplay of the unfolding procedure and the small number of events in the highest m bb bin. The data are compared with the NLO theoretical prediction [27]. Figure 10 shows the comparison of the results with the theoretical prediction [27], which is calculated at the parton level using a different lower threshold for the lowest m bb bin.…”

“…There are several theoretical predictions predictions for A FB (bb) that cover the low bb invariant mass region investigated in this study [2,25,27]. The prediction presented in Ref.…”

“…The prediction presented in Ref. [27], which also includes mixed EW-QCD corrections is summarized in Table I. Near the Z pole, the SM bottom-quark asymmetry is maximal because it is dominated by tree-level exchange of EW gauge bosons.…”

“…Near the Z pole, the SM bottom-quark asymmetry is maximal because it is dominated by tree-level exchange of EW gauge bosons. The selection criteria used in this analysis match the criteria used in [27], except for the transverse momentum requirement on the b andb quark, p Tb,b : we require particle-level jets to have transverse energy E T > 20 GeV [28], while p Tb,b > 15 GeV/c is used in Ref. [27].…”

“…The selection criteria used in this analysis match the criteria used in [27], except for the transverse momentum requirement on the b andb quark, p Tb,b : we require particle-level jets to have transverse energy E T > 20 GeV [28], while p Tb,b > 15 GeV/c is used in Ref. [27]. The events with p Tb,b < 20 GeV/c can influence the bb asymmetry at low m bb , that is only the first bin in Table I, for which the prediction has the highest uncertainty.…”