Suppression of maximal linear gluon polarization in angular asymmetries

Abstract

Abstract:We perform a phenomenological analysis of the cos 2φ azimuthal asymmetry in virtual photon plus jet production induced by the linear polarization of gluons in unpolarized pA collisions. Although the linearly polarized gluon distribution becomes maximal at small x, TMD evolution leads to a Sudakov suppression of the asymmetry with increasing invariant mass of the γ * -jet pair. Employing a small-x model input distribution, the asymmetry is found to be strongly suppressed under TMD evolution, but still remains sufficiently large to be measurable in the typical kinematical region accessible at RHIC or LHC at moderate photon virtuality, whereas it is expected to be negligible in Z/W -jet pair production at LHC. We point out the optimal kinematics for RHIC and LHC studies, in order to expedite the first experimental studies of the linearly polarized gluon distribution through this process. We further argue that this is a particularly clean process to test the k t -resummation formalism in the small-x regime.

Transverse parton distribution and fragmentation functions at NNLO: the gluon case

“…The linearly polarized coefficients arise from helicity-flip contribution in gluon splitting, and are suppressed by one power of α s compared with the unpolarized ones. Their contributions to physical cross section have been discussed for diphoton production [46], Higgs production [15,[47][48][49], quarkonium production [50][51][52], γ * plus jet production [53,54], heavy quark pair [55] and dijet production [56]. In this paper we present results through O(α 2 s ) (two loops) for both polarizations.…”

Linearly polarized gluons at next-to-next-to leading order and the Higgs transverse momentum distribution

“…In particular, there are processes that are also sensitive to lpTMDPDF and that are addressed in the literature [31][32][33][34][35]. Among these it is worth a special mentioning the case of heavy-quark production [36][37][38][39][40][41][42], which is relevant at LHC, future Electron-Ion Collider (EIC) or the LHeC. Another important topic is the positivity bound for gluon TMDPDF derived in [13],…”

Probing the linear polarization of photons in ultraperipheral heavy ion collisions

“…In the saturation limit, the dipole type linearly polarized gluon distribution and the dipole type unpolarized gluon distribution remain identical, whereas the linearly polarization of Weizsäcker-Williams gluons is suppressed. Though it has been found promising to probe the linearly polarized gluon distribution by measuring cos 2φ azimuthal asymmetry for two particle production in various high energy scattering processes at RHIC, LHC, or a future Electron-Ion Collider(EIC) [3][4][5][6][7][8][9][10][11][12][13], this gluon distribution so far has not yet been studied experimentally.In analogy to the QCD case, one also can define a linearly polarized photon distribution for an unpolarized nucleon or nuclei target, which can be accessed by measuring the azimuthal asymmetries in di-lepton production in hadronhadron collisions [8]. However, it is not very practical to extract the polarized photon distribution in hadronic reactions due to the di-lepton Drell-Yan production background.…”