Two-loop form factors for $P$-wave quarkonium production and decay

TL;DR

This paper provides analytical two-loop form factors for P-wave quarkonium production and decay, including previously unknown processes, revealing complex pole structures and offering high-precision results for NNLO calculations.

Contribution

It delivers the first analytical two-loop form factors for several P-wave quarkonium processes, enhancing precision in theoretical predictions.

Findings

Analytical two-loop form factors for $ ext{γγ} o {^3 P_J^{[1]}}$

New two-loop corrections for $gg o {^3 P_J^{[1]}}$, $ ext{γ}g o {^3 P_J^{[8]}}$, and $gg o {^3 P_J^{[8]}}$

Complex NRQCD pole structure involving additional singularities

Abstract

We present the analytical results for the two-loop form factors needed for $\chi_{Q,J}$ production and decay. We consider the two-loop corrections to the process $\gamma \gamma \leftrightarrow {^3 P_J^{[1]}}$, that has been known only numerically before, and the processes $gg \leftrightarrow {^3 P_J^{[1]}}$, $\gamma g \leftrightarrow {^3 P_J^{[8]}}$ and $gg \leftrightarrow {^3 P_J^{[8]}}$, which have not been computed before. We observe that the NRQCD pole structure of the two-loop amplitude in the $gg$ channel is more involved for the spin-triplet $P$-wave case than for the pseudo-scalar $S$-wave case. It involves in addition to the standard Coulomb singularity also a new singularity whose cancellation requires the inclusion of the $gg \leftrightarrow {^3S_1^{[8]}}$ form factor. We give the high precision numerical results for the hard functions that can be used to compute $\chi_{Q,J}$ production and decay up to NNLO accuracy.