Chi_2 production in polarized pp collisions at RHIC: measuring \Delta G and testing the color octet model

TL;DR

This paper investigates $$ charmonium production in polarized and unpolarized proton-proton collisions at RHIC, proposing methods to distinguish production mechanisms and measure the polarized gluon distribution in the proton.

Contribution

It introduces a way to differentiate color singlet and octet production mechanisms via angular distributions and proposes using these distributions to measure  in polarized collisions.

Findings

Angular distribution in unpolarized collisions distinguishes production mechanisms.

Polarized angular distribution can measure the polarized gluon distribution .

Effective theory based on QCD multipole expansion applied to charmonium production.

Abstract

We consider the production and decay of the $\chi_2$ charmonium state in polarized and unpolarized $pp$ collisions at RHIC in the framework of an effective theory based on the QCD multipole expansion. We find that the angular distribution in the decay of the produced charmonium, $\chi_2 \to J/\psi + \gamma$, in the unpolarized case allows us to distinguish clearly between the color singlet and color octet production mechanisms. Once the production mechanism is known, the angular distribution in the polarized case can be used to measure the polarized gluon distribution in the proton, $\Delta G(x)$.