Measurement of $J/\ddot{I}ˆ$-pair production in $pp$ collisions at $\sqrt{s}=13$ TeV and study of gluon transverse-momentum dependent PDFs

Abstract

The production cross-section of $J/\ddot{I}ˆ$ pairs in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV is measured using a data sample corresponding to an integrated luminosity of 4.2 fb${}^{-1}$ collected by the LHCb experiment. The measurement is performed with both $J/\ddot{I}ˆ$ mesons in the transverse momentum range $0<p_{\text{T}}<14$ GeV/$c$ and rapidity range $2.0<y<4.5$. The cross-section of this process is measured to be 16.36$\pm$0.28(stat)$\pm$0.88(syst) nb. The contributions from single-parton scattering and double-parton scattering are separated based on the dependence of the cross-section on the absolute rapidity difference $\hat{I}”y$ between the two $J/\ddot{I}ˆ$ mesons. The effective cross-section of double-parton scattering is measured to be $\ddot{I}{ƒ}_{\text{eff}}=$13.1$\pm$1.8(stat)$\pm$2.3(syst) mb. The distribution of the azimuthal angle $\ddot{I}{•}_{\text{CS}}$ of one of the $J/\ddot{I}ˆ$ mesons in the Collins-Soper frame and the $p_{\text{T}}$-spectrum of the $J/\ddot{I}ˆ$ pairs are also measured for the study of the gluon transverse-momentum dependent distributions inside protons. The extracted values of $⟨\cos 2\ddot{I}{•}_{\text{CS}}⟩$ and $⟨\cos 4\ddot{I}{•}_{\text{CS}}⟩$ are consistent with zero, but the presence of azimuthal asymmetry at a few percent level is allowed.

All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2023-022.html (LHCb public pages)