J/ψ Production and Polarization within a Jet

Abstract

We study the production and polarization of J/ψ mesons within a jet in proton-proton collisions at the LHC. We define the J/ψ-jet fragmentation function as a ratio of differential jet cross sections with and without the reconstructed J/ψ in the jet. We demonstrate that this is a very useful observable to help explore the J/ψ production mechanism, and to differentiate between different NRQCD global fits based on inclusive J/ψ cross sections. Furthermore, we propose to measure the polarization of J/ψ mesons inside the jet, which can provide even more stringent constraints for the heavy quarkonium production mechanism. Introduction.Understanding the J/ψ production mechanism is one of the most active and challenging subjects in strong interaction physics [1,2]. The most common approach to calculating the J/ψ production cross section in hadronic collisions is the non-relativistic QCD (NRQCD) factorization formalism [3]. In this approach, the heavy quark and anti-quark pair, QQ, is produced at short-distance which can be calculated perturbatively due to the large heavy quark masses. Such a QQ state will then hadronize into a physical J/ψ meson. This transition is non-perturbative, but it can be characterized through a set of universal NRQCD long-distance matrix elements (LDMEs). Global analyses of the world's data on J/ψ production with the next-to-leading order (NLO) calculations in powers of strong coupling constant α s have been performed by several groups [4][5][6][7][8]. Even though all these groups can describe the inclusive J/ψ production cross section, i.e., the p T spectrum, they have not been able to fully explain the polarization of high-p T heavy quarkonia produced at the Tevatron [9][10][11][12] and the LHC [13,14]. Despite of numerous attempts made in the past, the J/ψ polarization remains a puzzle.

Transverse momentum dependent distributions in e+e− and semi-inclusive deep-inelastic scattering using jets

“…A number of other jet observables that account for transverse momentum dependence have recently been considered. The main focus has been on the transverse momentum of hadrons fragmenting in jets, in both inclusive [4] and semi-inclusive [5,6] processes. In the same context, refs.…”

The groomed and ungroomed jet mass distribution for inclusive jet production at the LHC

“…The functions G c (z, p T , R, τ, µ) are the semi-inclusive jet mass functions (siJMFs), which describe how a parton c initiates the signal jet that carries a momentum fraction z of that parton, and at the same time the jet mass τ is observed. Following earlier work [52,[56][57][58][59], the siJMFs are defined at the operator level as follows…”

Transverse-Momentum-Dependent Distributions with Jets

“…The factorization analysis in Refs. [11,12] showed that the standard TMD fragmentation functions enter when the transverse momentum of a hadron is measured with respect to the standard jet axis (SJA). If instead the Winner-Take-All (WTA) axis [13,14] is used, a new and rather different TMD is obtained: it is insensitive to soft radiation and has a DGLAP-like evolution equation [15].…”