Forward-backward asymmetry of top quark production at the Tevatron in warped extra dimensional models

Abstract

The CDF and D0 experiments have reported on the measurement of the forward-backward asymmetry of top quark pair production at the Tevatron and the result is that it is more than 2 standard deviations above the predicted value in the Standard Model. This has to be added to the longstanding anomaly in the forward-backward asymmetry for bottom quark production at LEP which is 3 standard deviations different from the Standard Model value. The discrepancy in the bottom asymmetry can be accounted for by the contributions of Kaluza-Klein excitations of electroweak gauge bosons at LEP in warped extra dimensional models in which the fermions are localized differently along the extra dimension so that the gauge interactions of heavy third generation fermions are naturally different from that of light fermions. In this paper, we show that it is more difficult to elaborate a model generating a significant top asymmetry in a similar way -through exchanges of Kaluza-Klein gluons at the Tevatron -due to the indirect constraints originating from precision electroweak data.Apparently, something is indeed rotten in the kingdom of third generation quarks. Adding to the longstanding anomaly of the forward-backward (FB) asymmetry for bquark jets A b FB measured in Z boson decays at LEP [1, 2], which differs by 3 standard deviations from the Standard Model (SM) value [3], the CDF and D0 collaborations have reported results [4,5] on the measurement of the FB asymmetry of top quark pairs produced at the Tevatron, A t FB , that are not consistent with the SM expectation. In particular, the latest and most precise result from the CDF collaboration [4], using 3.2 fb −1 data, gives for this asymmetry in the pp laboratory frameIn the SM, this asymmetry is predicted to be vanishing at first order in QCD. Indeed, a very nice feature of the Tevatron is that it is almost a qq collider for top quark pair production as the process occurs mainly through virtual gluon exchange in qq annihilation, with only a small contribution from the initiated gluon-gluon fusion channel. As gluons have only vector-like couplings to quarks, the process does not generate an asymmetry between quarks and antiquarks and thus, A t FB is identically zero [6]. The asymmetry is then generated at next-toleading order (NLO) in QCD by diagrams involving an extra gluon radiation and (anti)quark-gluon annihilation as well as from the interference between the Born gluon exchange with one-loop box diagrams. These NLO contributions lead to the expected value in the SM of [7] A t FB = 0.05 ± 0.015 .In the absence of large higher order contributions [8], this leads to a 2 standard deviation between the experimentally measured and the theoretically predicted values. This is in contrast to the total pp → tt production cross section at the Tevatron which is measured to be [9]σ(tt) ex = 7.0 ± 0.63 pb ,in a good agreement with the SM expectation [10,11],As in the case of the LEP A b FB anomaly (see Ref.[12] and references therein), it is very difficult to explain this discrepancy, withou...

Forward–backward asymmetries of the bottom and top quarks in warped extra-dimensional models: LHC predictions from the LEP and Tevatron anomalies

“…[11] and A t FB [12] 1 . We propose a new version of the RS model where the whole asymmetry A t FB is in perfect agreement with the updated CDF data [5] and the two new excesses of A t FB integrated above M tt = 450 GeV and |∆y| = 1 [5] are addressed 2 .…”

“…2, in which are given these unfolded results, illustrates that the fit to data is improved in the RS realization compared to the SM situation. The reason is that if high absolute rapidities are selected, then large cos θ * values are considered, θ * being the scattering angle, so that the asymmetry generated by the KK gluon exchange is maximized [12] 5 .…”

“…Similarly, the effect of the KK gluon exchange at hadron colliders is expected to be large in the top quark sector. Indeed, the KK gluon exchange in the s-channel can soften [12] the discrepancy between the value of A t FB measured at the Tevatron [4] and its SM value. Therefore, the anomalies observed in the third generation quark sector can be interpreted as early signatures of warped models.…”

Top quark forward–backward asymmetry

“…So far, a few explanations [4,5,6] have been put forward to account for such a large asymmetry. In Ref.…”

AFBtmeets LHC

“…Ultimately, models which aim to explain the Tevatron measurements [11][12][13][14][15][16][17][18][19][20][21][22][23][24] may manifest themselves in a variety of ways at the LHC. In addition to modifications of A t F B , virtually all of the candidate theories result in an increase in tt production at large invariant mass, M tt , and the differential cross section dσ tt /dM tt can be a sensitive discriminant between various models [25][26][27].…”