Next-to-leading order corrections in exclusive meson production

TL;DR

Diehl and Kugler quantify next-to-leading order corrections in hard exclusive meson production within collinear factorization, employing a Gegenbauer expansion of meson DAs and modeled GPDs to explore radiative effects on cross sections and transverse-spin asymmetries. They find extremely large NLO corrections at small x_B driven by BFKL-type logarithms, signaling the need for resummation to achieve perturbative stability. At moderate to large x_B, NLO effects remain sizeable and channel-dependent, with A_UT in vector-meson production highly sensitive to the proton helicity-flip distributions E and their relative phase to H. The study underscores that reliable extraction of GPDs from data requires incorporating both radiative and power corrections, and that some channels (e.g., ρ) are more perturbatively stable than others (e.g., ω, φ), guiding future theoretical developments and phenomenology.

Abstract

We analyze in detail the size of next-to-leading order corrections to hard exclusive meson production within the collinear factorization approach. Corrections to the cross section are found to be huge at small xB and substantial in typical fixed-target kinematics. With the models we take for nucleon helicity-flip distributions, the transverse target polarization asymmetry in vector meson production is strongly affected by radiative corrections, except at large xB. Its overall size is very small for rho production but can be large in the omega channel.

Figures (25)

• Figure 1: Example graphs for the hard-scattering kernels $T_a$, $T_b$ and $T_g$ at order $\alpha_s^2$.
• Figure 2: The forward limits $e_a(x)$ of the nucleon helicity-flip distributions at $\mu= 2\operatorname{GeV}$ for different parton species $a$ in model 1.
• Figure 3: LO and NLO terms of the convolutions in the gluon and quark singlet sector at $Q = 4 \operatorname{GeV}$. The scales are set to $\mu_R = \mu_{GPD} = \mu_{DA} = Q$. The NLO terms are for Gegenbauer index $n=0$ unless specified explicitly. Here and in the following plots the label "NLO" denotes the $O(\alpha_s^2)$ part of the convolutions, whereas the sum of $O(\alpha_s)$ and $O(\alpha_s^2)$ terms is labeled by "LO+NLO".
• Figure 4: LO terms and the sum of LO and NLO terms of the convolutions in the quark non-singlet sector at $Q = 4 \operatorname{GeV}$, with $\mu_R = \mu_{GPD} = \mu_{DA} = Q$.
• Figure 5: Dependence on the common scale $\mu = \mu_R = \mu_{GPD}$ for the sum of convolutions in the gluon and quark singlet sector.