Exclusive $J/ψ$ production at the LHC in the $k_T$ factorization approach

Abstract

The recent LHCb data for exclusive $J/ψ$ peripheral production at 13 TeV motivate an improved `NLO' analysis to estimate the gluon distribution at low $x$ in which we re-calculate the rapidity gap survival factors and use a more precise expression for the photon flux. We comment on the difference between the $k_T$ and collinear factorization approaches.

Figures (2)

• Figure 1: The 'NLO' bands corresponds to the combined fits to the $W_+$ solutions extracted from the LHCb data for $pp\to p+J/\psi+p$ together with the HERA data for $\gamma^* p\to J/\psi+p$ data. The two fits, first omitting, and second including, the preliminary LHCb data at 13 TeV LHCb13 give bands that are essentially identical, and cannot be distinguished on the plot. The widths of the bands are given by propagating the errors on the fit parameters using the full covariance matrix obtained from our fit to the data. The width of the uncertainty bands is therefore controlled by the 1$\sigma$ uncorrelated uncertainties of the data and by the normalisation errors of the data sets.
• Figure 2: The narrow bands are the 'NLO' gluon PDF determined from fits using the 7 TeV and the preliminary 13 TeV LHCb data, for $\mu^2=Q^2=2.4, ~4.1$ and $6.4 ~\rm GeV^2$, where the bands result from the uncertainties on the fitted parameters. The results are essentially identical whether or not the 13 TeV data are included in the fit. The $x$ regions probed using the HERA data and the 7 TeV LHCb data are darkly shaded in the narrow bands. The extended $x$ region probed by the 13 TeV data is indicated by the less darkly shaded region of the $\mu^2=2.4$ GeV$^2$ band. For comparison, we show the spread of the central values of the gluon distributions obtained in three global parton analyses NNPDFMMHTCT14; together with shaded regions which show the uncertainties on $xg(x,\mu^2)$ in the NNPDF3.0 analysis.