QCD corrections to t\bar t b \bar b productions via photon-photon collisions at linear colliders

TL;DR

The paper computes the complete one-loop QCD corrections to gamma-gamma -> ttbar bbbar production at a photon-photon collider within the SM. It handles the challenging hexagon and pentagon loop amplitudes and combines them with a two-cut phase-space slicing treatment for real gluon emission to obtain a finite NLO cross section. The results show the NLO corrections generally increase the LO cross section, with K-factors ranging from 1.70 to 1.14 as the center-of-mass energy goes from 400 GeV to 2 TeV, and they improve the renormalization-scale stability while altering the transverse momentum distributions of the final-state quarks. These precise predictions for a multi-particle final state are important for background treatment and precision top-quark studies at future linear colliders such as the ILC.

Abstract

We calculated the complete next-to-leading order(NLO) QCD corrections to the $t\bar t b \bar b$ production process at a $γγ$ collider in the standard model(SM). The calculation of the one-loop QCD correction includes the evaluations of the hexagon and pentagon amplitudes. We studied the NLO QCD corrected total cross section, the distributions of transverse momenta of final top- and bottom-quark states, and the dependence of the cross section on renormalization scale $μ$. It shows that NLO QCD correction generally increases the LO cross section in our chosen parameter space, and the K-factor varies from 1.70 to 1.14 when colliding energy goes up from $400 GeV$ to $2 TeV$. We find that the correction distinctly changes the distributions of transverse momenta of the final top- and bottom-quark states, and the NLO QCD correction obviously improves the independence of the cross section for process $γγ\to t\bar t b\bar b$ on the renormalization scale.

Figures (6)

• Figure 1: The t-channel tree-level Feynman diagrams at the ${\cal O}(\alpha_{ew}\alpha_s)$ order for $\gamma\gamma \to t\bar{t} b\bar{b}$ .
• Figure 2: The hexagon Feynman diagrams for $\gamma\gamma \to t\bar{t} b\bar{b}$ process.
• Figure 3: (a) The cross section parts $\sigma_4(=\sigma_{tree}+\sigma_{soft}+\sigma_{virtual})$, $\sigma_5(=\sigma_{hard})$ and the NLO QCD corrected cross section($\sigma_{NLO}$) of the $\gamma\gamma \to t\bar{t} b\bar{b}$ process as the functions of the soft cutoff $\delta_s(=2~\Delta E_7/\sqrt{s})$ with $\mu=\mu_0=m_t+m_b$ and $\sqrt{s}=800~{\rm GeV}$. (b) The enlarged plot of Fig.\ref{['fig3ab']}(a) for the NLO QCD corrected cross section($\sigma_{NLO}$) with integration error versus $\delta_s$.
• Figure 4: (a) The LO and NLO QCD corrected cross sections for the process $\gamma\gamma \to t\bar{t} b\bar{b}$ as the functions of c.m.s. colliding energy($\sqrt{s}$), (b) the corresponding K-factor versus $\sqrt{s}$.
• Figure 5: The LO and NLO QCD corrected cross sections for the $\gamma\gamma \to t\bar{t} b\bar{b}$ process as the functions of renormalization scale $\mu/\mu_0(\mu_0 \equiv m_t+m_b=147.2~GeV)$.