Charge Asymmetry in Hadroproduction of Heavy Quarks

Abstract

A sizeable difference in the differential production cross section of top and antitop quarks, respectively, is predicted for hadronically produced heavy quarks. It is of order αs and arises from the interference between charge odd and even amplitudes respectively. For the TEVATRON it amounts up to 15% for the differential distribution in suitable chosen kinematical regions. The resulting integrated forward-backward asymmetry of 4-5% could be measured in the next round of experiments. At the LHC the asymmetry can be studied by selecting appropriately chosen kinematical regions.12.38. Bx, 12.38.Qk, 13.87.Ce, 14.65.Ha Top quark production at hadron colliders has become one of the central issues of theoretical [1] and experimental [2] research. The investigation and understanding of the production mechanism is crucial for the determination of the top quark couplings, its mass and the search for new physics involving the top system. A lot of effort has been invested in the prediction of the total cross section and, more recently, of inclusive transverse momentum distributions [1].In this work we will point to a different aspect of the hadronic production process, which can be studied with a fairly modest sample of quarks. Top quarks produced through light quark-antiquark annihilation will exhibit a sizeable charge asymmetry -an excess of top versus antitop quarks in specific kinematic regions -induced through the interference of the final-state with initial-state radiation (Fig. 1 a, b) and the interference of the box with the lowest-order-diagram ( Fig. 1 c, d). The asymmetry is thus of order α s relative to the dominant production process. In suitable chosen kinematical regions it reaches up to 15%, the integrated forward-backward asymmetry amounts to 4-5%. Top quarks are tagged through their decay t → b W + and can thus be distinguished experimentally from antitop quarks through the sign of the lepton in the semileptonic mode and eventually also through the b-tag. A sample of hundred to two hundred tagged top quarks should in fact be sufficient for a first indication of the effect. Top production at the TEVATRON is dominated by quark-antiquark annihilation, hence the charge asymmetry will be reflected not only in the partonic rest frame but also in the center of mass system of proton and antiproton. The situation is more intricate for proton-proton collisions at the LHC, where no preferred direction is at hand in the laboratory frame. Nevertheless it is also in this case possible to pick kinematical configurations which allow the study of the charge asymmetry.The charge asymmetry has also been investigated in [3] for a top mass of 45 GeV. There, however, only the contribution from real gluon emission was considered requiring the introduction of a physical cutoff on the gluon energy and rapidity to avoid infrared and collinear singularities. Experimentally, however, only inclusive top-antitop 1

Effective-field-theory approach to top-quark production and decay

“…The SM prediction is dominated by O(α 3 S ) QCD interference effects and is 5% in the lab frame [46,47,48,49]. There is a discrepancy of about 2σ between theory and experiment.…”

Top quark forward–backward asymmetry

“…Within QCD it was shown that [3] higher order correction can give an asymmetry of 5%. If the data is real, then the contribution from new physics is of order 15%.…”

“…If the asymmetry is true, it will indicate the presence of new physics, because within the SM the asymmetry is only up to about 5% [3]. So far, a few explanations [4,5,6] have been put forward to account for such a large asymmetry.…”

AFBtmeets LHC

“…One is thus forced to rely on the subdominant qq and qg initial states, making use of the strong correlation between the rapidity of the tt system and the direction of the incoming quark. Event-by-event, a positive forward-backward asymmetry in qq(qg) → tt + X translates into a positive charge asymmetry in the forward regions |y| > y 0 [5,6]. This forward-central charge asymmetry is (assuming CP conservation) a natural LHC observable.…”