# NRQCD Confronts LHCb Data on Quarkonium Production within Jets

## TL;DR

This paper compares NRQCD-based theoretical calculations with LHCb data on $J/\psi$ production within jets, showing NRQCD models outperform default PYTHIA simulations in matching the observed $z(J/\psi)$ distribution.

## Contribution

It demonstrates that NRQCD calculations, combined with resummation techniques, better describe LHCb jet data than standard PYTHIA simulations, highlighting the importance of nonperturbative effects.

## Key findings

- NRQCD models agree better with LHCb data than PYTHIA.
- Resummed FJF formalism provides accurate $z(J/\psi)$ predictions.
- High-$p_T$ LDME fits align well with experimental results.

## Abstract

We analyze the recent LHCb measurement of the distribution of the fraction of the transverse momentum, $z(J/\psi)$, carried by the $J/\psi$ within a jet. LHCb data is compared to analytic calculations using the fragmenting jet function (FJF) formalism for studying $J/\psi$ in jets. Logarithms in the FJFs are resummed using DGLAP evolution. We also convolve hard QCD partonic cross sections, showered with PYTHIA, with leading order Non Relativistic Quantum Chromodynamics (NRQCD) fragmentation functions and obtain consistent results. Both approaches use Madgraph to calculate the hard process that creates the jet initiating parton. These calculations give reasonable agreement with the $z(J/\psi)$ distribution that was shown to be poorly described by default PYTHIA simulations in the LHCb paper. We compare our predictions for the $J/\psi$ distribution using various extractions of nonperturbative NRQCD long-distance matrix elements (LDMEs) in the literature. NRQCD calculations agree with LHCb data better than default PYTHIA regardless of which fit to the LDMEs is used. LDMEs from fits that focus exclusively on high transverse momentum data from colliders are in good agreement with the LHCb measurement.