Top Quark Threshold Production at a gamma-gamma Collider at Next-to-Next-to-Leading Order

Abstract

The next-to-next-to-leading order (NNLO) QCD corrections to top quark threshold pair production in gamma-gamma collisions are computed. The NNLO effects turn out to be significant. Threshold cross sections in gamma-gamma and e+e- collisions are compared.

Figures (4)

• Figure 1: Examples of non-abelian hard corrections to $\gamma\gamma\to t\overline t$ computed in this paper. In addition, we include diagrams similar to (f) with gluon, Faddeev-Popov ghost and fermion loops inserted in all one-loop corrections.
• Figure 2: $R_S^{++}$ at LO, NLO and NNLO. Top quark pole mass $m_t = 175.05~{\rm GeV}$ width $\Gamma_t = 1.43~{\rm GeV}$ are used. The hard renormalization scale and the factorization scale are equal to the top quark mass ($\alpha_s(m_t) = 0.109$). The soft renormalization scale is $60~{\rm GeV}$ ($\alpha_s(60~{\rm GeV}) =0.127$) but we also show the NNLO curve for the soft renormalization scale $30~{\rm GeV}$ to demonstrate large changes with the coupling constant scale variation.
• Figure 3: $R_S^{++}$ parameterized with the kinetic mass $m_{\rm kin}(15~{\rm GeV}) = 173.10~{\rm GeV}$. Other parameters are the same as in Fig. \ref{['fig:plot']}. Dashed, dashed-dotted and solid curves are LO, NLO and NNLO approximations, respectively.
• Figure 4: The ratio of $R^{++}_S$ and $R_e$ in the threshold region at LO, NLO and NNLO (with two different soft renormalization scales) parameterized by kinetic mass $m_{\rm kin}(15~{\rm GeV})$.