Forward trijet production in p-p and p-Pb collisions at LHC

Marcin Bury,Krzysztof Kutak,Andreas Van Hameren,Piotr Kotko

doi:10.1007/jhep09(2020)175

Abstract

We calculate various azimuthal angle distributions for three jets produced in the forward rapidity region with transverse momenta pT> 20 GeV in proton-proton (p-p) and proton-lead (p-Pb) collisions at center of mass energy 5.02 TeV. We use the multiparton extension of the so-called small-x Improved Transverse Momentum Dependent factorization (ITMD). We study effects related to change from the standard kT -factorization to ITMD factorization as well as changes as one goes from p-p collision to p-Pb. We observe rather large differences in the distribution when we change the factorization approach, which allows to both improve the small-x TMD gluon distributions as well as validate and improve the factorization approach. We also see significant depletion of the nuclear modification ratio, indicating a possibility of searches for saturation effects using trijet final states in a more exclusive way than for dijets.

Highlights

JHEP09(2020)175 the high energy factorization (HEF) focuses on the small Bjorken x limit, not neglecting the powers kT /μ [8,9,10,11]
In CGC theory, such a contribution is absent for massless two-particle production, but appears in heavy quark production [44] and will appear in higher multiplicity processes as it has been already observed in the correlation limit for three-parton final state [45] basing on the quark-initiated three-jet production formulae in CGC [46]
Before we present our results for the cross section, let us first discuss in more detail the basic TMD gluon distributions Fq(g2), Fg(g1), Fg(g2), Fg(g6) calculated in [53] from the Kutak-Sapeta (KS) dipole gluon distribution Fq(g1), as well as the Weizsacker-Williams gluon distribution Fg(g3) calculated in [56]

Summary

Numerical results

Before we present our results for the cross section, let us first discuss in more detail the basic TMD gluon distributions Fq(g2), Fg(g1), Fg(g2), Fg(g6) calculated in [53] from the Kutak-Sapeta (KS) dipole gluon distribution Fq(g1), as well as the Weizsacker-Williams gluon distribution Fg(g3) calculated in [56]. In addition to the absolute differential cross sections, a very useful observable is the nuclear modification ratio that quantifies saturation effects. It is defined generically as dσp+P b. We see that in addition to the already discussed suppression and enhancement of the p-Pb cross section (per nucleon), the normalisation of ITMD* is significantly larger than for HEF in the correlation region. We attribute this feature to a visibly different shape and larger normalization of the TMD gluons not present in the HEF formalism. At present, such modules are not available for the saturation framework, in particular for ITMD (for a recent progress in matching HEF and parton shower see eg. [59])

Conclusions

A ITMD factorization in the color connection representation