Distribution of linearly polarized gluons inside a large nucleus

The distribution of linearly polarized gluons inside a large nucleus is studied in the framework of the color glass condensate. We find that the Weizs\"acker-Williams distribution saturates the positivity bound at large transverse momenta and is suppressed at small transverse momenta, whereas the dipole distribution saturates the bound for any value of the transverse momentum. We also discuss processes in which both distributions of linearly polarized gluons can be probed.Comment: slightly extended discussion, typos fixed, matches journal versio

Semi-Exclusive Production of Photons at HERA

We study the feasibility of measuring semi-exclusive photon production \gamma + p \to \gamma + Y at HERA. The cross section of photons produced at large transverse momenta, recoiling off an inclusive system Y of limited mass, can without photon isolation cuts be simply expressed in terms of hard pQCD subprocesses and standard target parton distributions. With the help of event generators we identify the kinematic region where quark and gluon fragmentation processes can be neglected. The cross section in this semi-exclusive region is large enough to be measured with an upgraded HERA luminosity of L = 100/pb. The subprocesses of lowest order in \alpha_s are suppressed at low recoil masses M_Y, compared to higher order gluon exchange, i.e. BFKL contributions. The distinct M_Y-dependence makes it possible to determine experimentally the kinematic range where the higher order processes dominate.Comment: 11 pages, 7 figures, submitted to Eur. Phys. J. C, minor stylistic changes, references update

Connections between collinear and transverse-momentum-dependent polarized observables within the Collins-Soper-Sterman formalism

We extend the improved Collins-Soper-Sterman (iCSS) $W+Y$ construction recently presented in~\cite{Collins:2016hqq} to the case of polarized observables, where we focus in particular on the Sivers effect in semi-inclusive deep-inelastic scattering. We further show how one recovers the expected leading-order collinear twist-3 result from a (weighted) $q_T$-integral of the differential cross section. We are also able to demonstrate the validity of the well-known relation between the (TMD) Sivers function and the (collinear twist-3) Qiu-Sterman function within the iCSS framework. This relation allows for their interpretation as functions yielding the average transverse momentum of unpolarized quarks in a transversely polarized spin-$\frac{1}{2}$ target. We further outline how this study can be generalized to other polarized quantities.Comment: 14 pages, Version to be published in PL

Reviewing model calculations of the Collins fragmentation function

The Collins fragmentation function describes a left/right asymmetry in the fragmentation of a transversely polarized quark into a hadron in a jet. Four different model calculations of the Collins function have been presented in the literature. While based on the same concepts, they lead to different results and in particular to different signs for the Collins function. The purpose of the present work is to review the features of these models and correct some errors made in previous calculations. A full study of the parameter dependence and the possible modifications to these models is beyond the scope of the paper. However, some general conclusions are drawn

Transverse single spin asymmetry in the Drell-Yan process

We revisit the transverse single spin asymmetry in the angular distribution of a Drell-Yan dilepton pair. We study this asymmetry by using twist-3 collinear factorization, and we obtain the same result both in covariant gauge and in the light-cone gauge. Moreover, we have checked the electromagnetic gauge invariance of our calculation. Our final expression for the asymmetry differs from all the previous results given in the literature. The overall sign of this asymmetry is as important as the sign of the Sivers asymmetry in Drell-Yan.Comment: 9 page