T-odd effects in photon-jet production at the Tevatron

TL;DR

This work addresses the large $\cos 2\phi$ azimuthal asymmetry observed in Drell-Yan processes by invoking a T-odd, spin- and transverse-momentum dependent distribution $h_1^{\perp q}$ within a generalized TMD factorization framework. It extends the mechanism to photon-jet production in $p\bar{p}$ collisions, deriving the leading-order cross section in terms of TMDs and gauge-link induced color factors, and identifying a $\cos 2(\phi_\perp-\phi_\gamma)$ modulation arising from $h_1^{\perp q} h_1^{\perp \bar{q}}$ with convolutions $\mathcal{F}$, $\mathcal{G}$, and $\mathcal{H}$. Using a Gaussian ansatz for transverse momenta and LO PDFs, the authors estimate the observable ratio $R$ and find an expected azimuthal asymmetry $\mathcal{A}$ of order $5$–$15\%$ in Tevatron kinematics, suggesting that photon-jet measurements could provide a viable probe of the T-odd distribution $h_1^{\perp q}$. The results highlight a new experimental avenue to study transverse spin phenomena, with the caveat that factorization and resummation corrections warrant further theoretical work.

Abstract

The angular distribution in photon-jet production in p \bar{p} -> γjet X is studied within a generalized factorization scheme taking into account the transverse momentum of the partons in the initial hadrons. Within this scheme an anomalously large cos(2φ) asymmetry observed in the Drell-Yan process could be attributed to the T-odd, spin and transverse momentum dependent parton distribution function h_1^{\perp q}(x, p_{\perp}^2). This same function is expected to produce a cos(2φ) asymmetry in the photon-jet production cross section. We give the expression for this particular azimuthal asymmetry, which is estimated to be smaller than the Drell-Yan asymmetry but still of considerable size for Tevatron kinematics. This offers a new possibility to study T-odd effects at the Tevatron.

Figures (2)

• Figure 1: Cut diagrams for the subprocess $q \bar{q} \rightarrow \gamma g$.
• Figure 3: The ratio $R$ defined in (\ref{['eq:ratio']}) as a function of $y$, calculated according to (\ref{['eq:ratio2']}) for different values of $x_1$, $x_2$, $|\boldsymbol{K}_{\gamma\perp}|$ typical of the Tevatron experiments D0:Atr.