Top Quark Forward-Backward Asymmetry and Same-Sign Top Quark Pairs

Abstract

The forward-backward asymmetry for top quarks measured in proton-antiproton collisions at the Tevatron shows an interesting deviation from standard model expectations. Among possible interpretations, the exchange of a non-universal, flavor-changing Z ′ is of some interest as it naturally predicts a top quark in the forward region of rapidity. To reproduce the size of the Tevatron asymmetry, the couplings of the Z ′ to standard model quarks must be large, inevitably leading to production of same-sign top quark pairs at the Tevatron and at the Large Hadron Collider (LHC). We discuss the constraints on this model from (a) the Tevatron t t cross section, (b) the Tevatron t t invariant mass distribution, and the limits at the Tevatron on the same sign top quark pair cross section. We explore the discovery potential for tt and ttj production in early LHC experiments at 7 TeV and conclude that if a tt signal is not observed with 1 fb −1 of integrated luminosity, then a non-universal Z ′ alone cannot explain the Tevatron forward-backward asymmetry. Limits on the same sign cross section at the LHC from the CMS collaboration already disfavor this interpretation of the reported asymmetry.

Search for new physics in events with same-sign dileptons and b jets in pp collisions at $ \sqrt{s}=8 $ TeV

“…As a result, searches for anomalous production of same-sign dileptons can be very sensitive to new physics contributions. These include supersymmetry (SUSY) [1][2][3], universal extra dimensions [4], pair production of T 5/3 particles (fermionic partners of the top quark) [5], heavy Majorana neutrinos [6], and same-sign top-pair production [7,8]. More specifically, the signature of same-sign dileptons accompanied by bottom quarks can arise from SUSY processes where third-generation quark superpartners are lighter than other squarks [9][10][11][12][13], resulting in an abundance of top and bottom quarks produced in cascade decays.…”

Search for new physics in events with same-sign dileptons and b-tagged jets in pp collisions at $ \sqrt {s} = 7 $ TeV

“…[6], which is proposed as a possible explanation of the anomalous forward-backward asymmetry observed at the Tevatron [8][9][10]. This model introduces a new neutral boson with chiral couplings to u and t quarks.…”

“…The signal topologies with two b-quark jets in the final states are: like-sign top quark production in the Z model [6] and in the MxFV model [13]; production of two bottom squarks each decaying as b 1 → t χ − 1 . In the latter case χ − 1 → W − χ 0 1 , where χ 0 1 is the lightest supersymmetric particle (LSP).…”

“…Here, the signatures with two like-sign leptons, b-quark jets and missing transverse energy correspond to strongly produced SUSY processes with multiple W bosons appearing in the decay chains, either from top quarks or charginos. In addition to SUSY processes, the existence of a Z -boson with flavour-violating u-t quark coupling [6,7] would lead to like-sign top pair production, uu → tt via Z exchange, at the LHC. Such a boson has been proposed to explain the…”

Signs of Tops from Highly Mixed Stops

“…The single top signatures have been studied for instance in [28][29][30][31][32][33]. Same sign tops have been studied in new physics models attempting to explain the forward backward asymmetry [34][35][36][37][38][39][40], or in composite models [41,42]. In the context of supersymmetry, same-sign top final states can arise either in R-parity violating models [43][44][45] or in Rparity preserving theories through a gluino decay chain [46,47].…”