One-loop helicity amplitudes for top quark pair production in Randall-Sundrum model

Abstract

In this paper, we show how to calculate analytically the one-loop helicity amplitudes for the process qq → tt induced by KK gluon, using the spinor helicity formalism. A minimal set of Feynman rules which are uniquely fixed by gauge invariance and the color representation of the KK gluon are derived and used in the calculation. Our results can be applied to a variety of models containing a massive color octet vector boson. Warped extra-dimension model proposed by Randall and Sundrum (RS) [1] is a popular model that can solve the hierarchy problem. When allowing the Standard Model (SM) field propagating in the extra dimension, the RS model provides many novel points of view for some problems, like hierarchy of fermion masses and the unification of gauge couplings. The first signal of the RS model may be the observation of KK gluon (first Kaluza-Klein excitation mode of SM gluon) as a resonant in the tt final state at the early LHC, simply because its production rate is large compare to the other KK particles [2]. Detailed study of total cross sections and the invariant mass distribution of tt production induced by KK gluon have been made in the literatures [2-8] at the Leading Order (LO) in QCD.Recently CDF collaboration reported a large top quark forward-backward asymmetry with integrated luminosity 5.3 fb −1 [9]. This 3.4 σ discrepancy with SM predictions [10,11] has motivated a lot of work in the theory community, cf. ref.[12] and references therein.One attractive explanation of the large anomalous forward-backward asymmetry is provided by the RS model. Within the RS framework, a realistic model has been constructed that has been shown to be able to explain the forward-backward asymmetry of top quark sector as well as bottom quark sector simultaneously [13]. This is realized by a careful choice of fermion localizations, so that there is a relatively light KK gluon with mass around 1.5TeV and large parity violation in the first generation of quark couplings. The authors of Ref. [13] have shown at the LO in QCD that the model they constructed can explain well the total asymmetry observed at the Tevatron by CDF [9], as well as the asymmetry at large top quark pair invariant mass region and large rapidity region [9].It's well known that LO prediction suffers from large scale uncertainty, and therefore is not appropriate for precision measurement, e.g., extraction of couplings between quark and KK gluon. Furthermore, tt production is known to have large K-factor at the Next-to-Leading Order (NLO) in QCD [14][15][16]. The large K-factor is partially taken into account in the Ref.[13] using a formalism proposed in [17]. Aiming at the precise prediction for the KK gluon mediated tt production, Ref.[18] calculated the interference of SM box diagrams and tree diagram of tt production induced by KK gluon. Furthermore, Ref.[19] consider the effects of KK gluon on tt production by gluon-gluon fusion. It was found that the contributions, although non-zero, are highly suppressed. For this reason, we focus our

Renormalization group-induced phenomena of top pairs from four-quark effective operators

“…in refs. [10][11][12][13]51]. We emphasize that our qualitative discussions will still be valid regardless of whether missing diagrams exist or not.…”

“…The SM higher-order calculations have been made significant progresses recently [1][2][3][4][5][6][7][8][9], and more is coming. Higher-order corrections to even some new physics models have also been calculated [10][11][12][13] (but not for all models). It is also useful to study higher-order physics more model-independently and systematically using the effective operators which will aid the latter purpose.…”

Kaluza-Klein gluons at 100 TeV: NLO corrections

“…To remove this mixing we choose a gauge in which ∂ z A µ = 0 and A 5 = 0 at z = R and z = R , and we add gauge-fixing terms to the effective four-dimensional Lagrangian. In the general R ξ gauge, the action becomes [47]:…”