Deuteron polarizations in the proton-deuteron Drell-Yan process for finding the gluon transversity

The gluon transversity distribution in the deuteron is defined by the matrix element between linearly polarized deuteron states, and it could be investigated in proton-deuteron collisions in addition to lepton-deuteron scattering. The linear polarization of photon is often used, whereas it is rarely used for the spin-1 deuteron. Therefore, it is desirable to express deuteron-reaction cross sections in term of conventional deuteron spin polarizations for actual experimental measurements. In this work, we investigate how proton-deuteron Drell-Yan cross sections are expressed by the conventional polarizations for finding the gluon transversity distribution. In particular, we show that the gluon transversity can be measured in the proton-deuteron Drell-Yan process by taking the cross section difference between the deuteron spin polarizations along the two-transverse axes. Since the gluon transversity does not exist for the spin-$1/2$ nucleons, a finite gluon transversity of the deuteron could indicate an “exotic” mechanism beyond the simple bound system of the nucleons in the deuteron. Therefore, the gluon transversity is an interesting and appropriate observable to shed light on a new hadronic mechanism in nuclei.

Figure 1

Proton-deuteron reactions in the center-of-momentum frame, where the deuteron moves in the $-z$ direction.

Figure 2

Proton-deuteron reactions in the laboratory frame, where the polarized deuteron is at rest with spin ${\overrightarrow{s}}_{+1}$. The spin vector ${\overrightarrow{s}}_{+1}$ is rotated with the angle ${\alpha }_y$ (${\alpha }_x$) around the axis $y$ ($x$).