Exclusive J/$\psi$ and $\psi$(2S) production in $pp$ collisions at $\sqrt{s} = 7$ TeV

[to restricted-access page]

Abstract

Exclusive $J/\psi$ and $\psi(2S)$ vector meson production has been observed in the dimuon channel using the LHCb detector. The cross-section times branching fractions to two muons with pseudorapidities between 2.0 and 4.5 are measured to be $\sigma_{pp\rightarrow J/\psi (\rightarrow \mu^{+} \mu^{-})} (2.0 <\eta_{\mu^{\pm}}< 4.5) = 307 \pm 21 \pm 36 \text{pb}, \sigma_{pp\rightarrow \psi(2S) (\rightarrow \mu^{+} \mu^{-})} (2.0 <\eta_{\mu^{\pm}}< 4.5) = 7.8 \pm 1.3 \pm 1.0 \text{pb},$ where the first uncertainties are statistical and the second are systematic. The measurements are found to be in good agreement with results from previous experiments and theoretical predictions. The $J/\psi$ photoproduction cross-section has been measured as a function of the photon-proton centre-of-mass energy. The results are consistent with measurements obtained at HERA and confirm a similar power law behaviour for the photoproduction cross-section.

Figures and captions

 Number of tracks in the forward region for dimuon triggered events which in the backward region have (left) one or more tracks or (right) no tracks. Fig1a.pdf [14 KiB] HiDef png [107 KiB] Thumbnail [118 KiB] *.C file Fig1b.pdf [14 KiB] HiDef png [102 KiB] Thumbnail [112 KiB] *.C file Invariant mass distributions in the regions of (left) the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ and (right) $\psi {(2S)}$ mass peaks for events with exactly two tracks, no photons and a dimuon with $p_{\rm T}$ below 900${\mathrm{ Me V /}c}$. The overall fits to the data are shown by the full curves while the dashed curves show the background contributions. Fig2a.pdf [23 KiB] HiDef png [156 KiB] Thumbnail [133 KiB] *.C file Fig2b.pdf [18 KiB] HiDef png [171 KiB] Thumbnail [151 KiB] *.C file Feynman diagrams displaying (a) exclusive ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ photoproduction and (b) inelastic ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ photoproduction where a small number of additional particles are produced due to gluon radiation and (c,d) proton dissociation. Fig3.pdf [27 KiB] HiDef png [122 KiB] Thumbnail [124 KiB] *.C file Transverse momentum distribution for exclusive ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ candidates with exactly two tracks and no photons. The points represent the data. The fit contains an exclusive signal component (short-dashed green curve) as estimated by SuperChic and an inelastic background component (long-dashed red curve) as estimated from data. Fig4.pdf [20 KiB] HiDef png [192 KiB] Thumbnail [152 KiB] *.C file (a) Transverse momentum distribution in ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ events with no backward tracks and no photons when there are three (black squares) and eight (red open circles) forward tracks. The results of fitting a Novosibirsk function to the three and eight forward track data are represented by the dashed black and the full red curves respectively. The values for (b) $P_{0}$, (c) $\sigma$ and (d) $\alpha$ of the ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ $p_{\rm T}$ distribution are shown as a function of the number of forward tracks. Fig5a.pdf [22 KiB] HiDef png [231 KiB] Thumbnail [197 KiB] *.C file Fig5b.pdf [14 KiB] HiDef png [101 KiB] Thumbnail [97 KiB] *.C file Fig5c.pdf [14 KiB] HiDef png [96 KiB] Thumbnail [88 KiB] *.C file Fig5d.pdf [14 KiB] HiDef png [96 KiB] Thumbnail [88 KiB] *.C file Dependence of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ photoproduction cross-section on the centre-of-mass energy of the photon-proton system. The blue (red) triangles represent the data from H1 (ZEUS) [3,4]. The black dots and squares are derived from the LHCb differential cross-section as a function of rapidity. The dashed and full lines are the power law dependences determined from the HERA and LHCb data, respectively. The uncertainty on the LHCb power law determination is shown by the shaded band. Fig6.pdf [18 KiB] HiDef png [454 KiB] Thumbnail [180 KiB] *.C file Animated gif made out of all figures. PAPER-2012-044.gif Thumbnail

Tables and captions

 Summary of selection requirements. Table_1.pdf [46 KiB] HiDef png [63 KiB] Thumbnail [31 KiB] tex code Relative systematic uncertainties on the measurement. Table_2.pdf [41 KiB] HiDef png [79 KiB] Thumbnail [36 KiB] tex code Comparison of cross-section times branching fraction measurements (pb) with theoretical predictions. Table_3.pdf [78 KiB] HiDef png [55 KiB] Thumbnail [26 KiB] tex code Cross-section measurements (nb) as a function of ${ J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ rapidity. Table_4.pdf [42 KiB] HiDef png [58 KiB] Thumbnail [29 KiB] tex code

Created on 14 September 2019.