Cross Sections for Leptophobic Topcolor Z' decaying to top-antitop

We present numerical calculations of the production cross section of a heavy Z' resonance in hadron-hadron collisions with subsequent decay into top-antitop pairs. In particular, we consider the leptophobic topcolor Z' discussed under Model IV of hep-ph/9911288 which has predicted cross sections large enough to be experimentally accessible at the Fermilab Tevatron and the Large Hadron Collider at CERN. This article presents an updated calculation valid for the Tevatron and all proposed LHC collision energies. Cross sections are presented for various Z' widths, in p-pbar collisions at \sqrt{s}=2 TeV, and in p-p collisions at \sqrt{s}=7, 8, 10 and 14 TeV.

Electroweak symmetry breaking is a cornerstone for our understanding of particle physics. However, despite the spectacular phenomenological success of the Standard Model (SM), the fundamental mechanism of electroweak symmetry breaking remains a mystery. Various new models have been proposed to explain this mechanism. One such class of models is based upon topcolor [1,2] which can generate a large top-quark mass. The topcolor model also predicts a Z ′ [3].
The physics in production and decay of the Z ′ are discussed in Ref. [3] under different model assumptions. In this paper, we consider the Z ′ from Model IV which represents a novel class and has predicted cross sections large enough to be experimentally accessible at hadron colliders at the Fermilab Tevatron and the Large Hadron Collider (LHC) at CERN. Such Z ′ resonances couple strongly only to the first and third generations of quarks, and have no significant couplings to the leptons. They are, therefore, leptophobic and topophyllic.
This article presents an updated calculation valid for the Tevatron and all proposed LHC collision energies.
The leptophobic topcolor Z ′ decaying to tt has been searched for at both the Tevatron [4][5][6] and the LHC [7]. The Tevatron searches used a previous calculation of the Z ′ cross section [3]. The LHC search conducted by the CMS collaboration have used the cross section calculation presented here to set limits on the Z ′ mass. This article is primarily intended to document that Z ′ cross section used for a pp collision energy of √ s = 7 TeV. For current and future LHC searches, we present the Z ′ cross section at the current LHC pp collision energy of √ s = 8 TeV and the potential future collision energies of 10 and 14 TeV. We also present the cross section in pp collisions at √ s = 2 TeV for comparison with Tevatron searches.

II. MODEL
As discussed in Ref. [3], non-standard models can be constructed in which the U (1) Y → U (1) 1 ×U (1) 2 and the generations are grouped differently. Model IV is the case where quark generations (1, 3) ⊃ U (1) 2 . The dominant part of the interaction Lagrangian for Model IV is: where g 1 is the SM coupling constant, and we require the following: f 1 > 0 (attractivett channel) and/or f 2 < 0 (repulsivebb channel). Also, cot θ H >> 1 to avoid finetuning. The parton-level subprocess cross section that we use in the next section is obtained from this Lagrangian.

III. CROSS SECTION FOR Z ′ PRODUCTION AND DECAY
The total lowest-order cross section for a Z ′ produced in hadron collisions and decaying into top-antitop pairs from Model IV discussed above, is given by: where dσ/dm, the differential cross section at tt invariant mass m, is given by Hereσ(qq → Z ′ → tt) is the parton-level subprocess cross section. The kinematic variable τ is related to the initial-state parton fractional momenta inside the first hadron x h1 and the second hadron x h2 by τ = x h1 x h2 = m 2 /s. The boost of the partonic system y b is given by y b = (1/2) ln(x h1 /x h2 ). The partonic "luminosity function" is just the product of parton distribution functions: where q(x, µ) (q(x, µ)) is the parton distribution function of a quark (anti-quark) evaluated at fractional momenta x and renormalization scale µ.
The parton-level subprocess cross section,σ(qq → Z ′ → tt), for a leptophobic, b r -phobic, and topophyllic, Z ′ is given by: where cot 4 θ H can be obtained from the total decay-width of the resonance [9]: The subprocess cross section in Eq. 5 is for spin and color summing on both initial and final state legs, while most parton distributions assume spin and color averaged on the initial-state legs and spin and color summing on the final-state legs. Therefore, the subprocess cross section given by Eq. 5 must be multiplied by a factor of 1 spins when used with parton distributions from PDFLIB [10] and other standard sources. We have taken this into account when calculating the cross section. We have also used m t = 172.5 GeV/c 2 , and cos 2 θ W = 0.768. For a default parton distribution set we have chosen CTEQ6L [11]. This is a modern parton distribution set appropriate for leading-order calculations and is available in PDFLIB [10]. For a default renormalization scale we choose µ = m/2, half the tt invariant mass. This scale has the benefit that it reduces to the usual µ = m t at top production threshold, but also increases with increasing tt invariant mass. With these choices, the total cross section for the production of a leptophobic and topophyllic Z ′ and its subsequent decay into tt pairs is presented in Sections IV and V for the Tevatron and the LHC.

IV. CROSS SECTIONS AT THE TEVATRON
We calculate numerically the lowest order cross section for the process pp → Z ′ → tt at the Tevatron at √ s = 2 TeV, using Eq. 2 and taking into account the spincolor factor in Eq. 7. The only remaining parameter of the topcolor model that affects the cross section is the mixing angle cot 2 θ H , or equivalently the width Γ Z ′ which is related to it. We calculate the cross section for different choices of Γ Z ′ , equal to 1.2% and 2% of M Z ′ , both of which qualify as narrow resonances at the Tevatron. The integration in Eq. 2 is performed using the full available phase space of 2m t < m < √ s. The results are tabulated in Table I and displayed in Fig. 1.

V. CROSS SECTIONS AT THE LHC
We perform the numerical calculation of the lowest order cross section for the process pp → Z ′ → tt at the  LHC for different values of √ s between 7 − 14 TeV, using Eq. 2 and taking into account the spin-color factor in Eq. 7. We calculate the cross section for different choices of Γ Z ′ , equal to 1%, 1.2%, 2%, and 10% of M Z ′ . The first three widths qualify as narrow resonances at the LHC, and the integration in Eq. 2 is performed using the full available phase space of 2m t < m < √ s. The integration    by both fixing an error in the reported width of the leptophobic topcolor Z ′ , and using m t = 172.5 GeV/c 2 and CTEQ6L parton distributions in an improved calculation procedure. This note documents the first calculations of the cross section for a leptophobic topcolor Z ′ at the LHC.

VII. ACKNOWLEDGMENTS
We thank Chris Hill for sending us the correction to the equation for the topcolor Z ′ width, found by James Ferrando and Mads Frandsen. We also thank Jan Steggemann and Ia Iashvili for several useful discussions.