Associated production of $Z$ bosons and $b$-jets at the LHC in the combined $k_T$ + collinear QCD factorization approach

TL;DR

This paper investigates Z boson production with b-jets at the LHC using a combined k_T + collinear QCD approach, incorporating various subprocesses and comparing results with experimental data and NLO predictions.

Contribution

It introduces a combined QCD factorization method that interpolates between small and large x physics, including multiple subprocesses and double parton scattering effects.

Findings

Good agreement with CMS and ATLAS data

Highlights importance of subleading quark contributions

Demonstrates uncertainties and comparison with NLO predictions

Abstract

We consider the production of $Z$ bosons associated with beauty quarks at the LHC using a combined $k_T$ + collinear QCD factorization approach, that interpolates between small $x$ and large $x$ physics. Our consideration is based on the off-shell gluon-gluon fusion subprocess $g^* g^* \to Z Q\bar Q$ at the leading order ${\cal O}(\alpha\alpha_s^2)$ (where the $Z$ boson further decays into a lepton pair), calculated in the $k_T$-factorization approach, and several subleading ${\cal O}(\alpha \alpha_s^2 )$ and ${\cal O}(\alpha \alpha_s^3 )$ subprocesses involving quark-antiquark and quark-gluon interactions, taken into account in conventional (collinear) QCD factorization. The contributions from double parton scattering are discussed as well. The transverse momentum dependent (or unintegrated) gluon densities in a proton are derived from Catani-Ciafaloni-Fiorani-Marchesini (CCFM) evolution equation. We achieve reasonably good agreement with the latest data taken by CMS and ATLAS Collaborations. The comparison of our results with next-to-leading-order pQCD predictions, obtained in the collinear QCD factorization, is presented. We discuss the uncertainties of our calculations and demonstrate the importance of subleading quark involving contributions in describing the LHC data in the whole kinematic region.