Measurement of forward $J/ψ$ production cross-sections in $pp$ collisions at $\sqrt{s}=13$ TeV

TL;DR

This study delivers forward ${J/\psi}$ production cross-sections in $pp$ collisions at $\sqrt{s}=13$ TeV using LHCb data, separating prompt from ${J/\psi}$-from-$b$ production within $2.0<y<4.5$ and $p_T<14$ GeV/$c$. It reports double-differential and integrated cross-sections, determines the non-prompt fraction, and extrapolates to the full ${b\bar b}$ cross-section, all under a zero-polarization assumption. The results are in good agreement with NRQCD for prompt production and with FONLL for ${J/\psi}$ from $b$, and they show the 13 TeV cross-sections are larger and the $p_T$ spectrum harder than at lower energies. The work also provides cross-section ratios across energies and contributes to constraining heavy-quark production mechanisms in QCD.

Abstract

The production of $J/ψ$ mesons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV is studied with the \lhcb detector. Cross-section measurements are performed as a function of the transverse momentum $p_\mathrm{T}$ and the rapidity $y$ of the $J/ψ$ meson in the region $p_\mathrm{T}<14\mathrm{GeV}/c$ and $2.0<y<4.5$, for both prompt $J/ψ$ mesons and $J/ψ$ mesons from $b$-hadron decays. The production cross-sections integrated over the kinematic coverage are $15.03\pm 0.03\pm 0.94$ $μ$b for prompt $J/ψ$ and $2.25\pm 0.01\pm 0.14$ $μ$b for $J/ψ$ from $b$-hadron decays, assuming zero polarization of the $J/ψ$ meson. The first uncertainties are statistical and the second systematic. The cross-section reported for $J/ψ$ mesons from $b$-hadron decays is used to extrapolate to a total $b\bar{b}$ cross-section. The ratios of the cross-sections with respect to $\sqrt{s}=8$ TeV are also determined.

Figures (11)

• Figure 1: Invariant mass (left) and pseudo decay time (right) distributions for the kinematic bin $2<\hbox{$p_{\rm T}$}\xspace<3{\mathrm{\,Ge V\!/}c}\xspace$, $3.0<y<3.5$, with fit results superimposed. The solid (red) line is the total fit function, the shaded (green) area corresponds to the background component. The prompt ${\mathrm{J}\xspace / \uppsi\xspace }$ contribution is shown in cross-hatched area (blue), ${\mathrm{J}\xspace / \uppsi\xspace }$-from-$b$ in a solid (black) line and the tail contribution due to the association of ${\mathrm{J}\xspace / \uppsi\xspace }$ with the wrong PV is shown in full filled (magenta) area. The tail contribution is not visible in the invariant mass plot.
• Figure 2: Double differential cross-section for prompt ${\mathrm{J}\xspace / \uppsi\xspace }$ mesons as a function of $p_{\rm T}$ in bins of $y$. Statistical and systematic uncertainties are added in quadrature.
• Figure 3: Double differential cross-section for ${\mathrm{J}\xspace / \uppsi\xspace }$-from-$b$ mesons as a function of $p_{\rm T}$ in bins of $y$. Statistical and systematic uncertainties are added in quadrature.
• Figure 4: Fractions of ${{\mathrm{J}\xspace / \uppsi\xspace }}\xspace$-from-$b$ mesons in bins of ${\mathrm{J}\xspace / \uppsi\xspace }$$p_{\rm T}$ and $y$. Statistical and systematic uncertainties are added in quadrature.
• Figure 5: Differential cross-sections as a function of $\hbox{$p_{\rm T}$}\xspace$ integrated over $y$ for (left) prompt ${\mathrm{J}\xspace / \uppsi\xspace }$ and (right) ${\mathrm{J}\xspace / \uppsi\xspace }$-from-$b$ mesons.