CMS-HIN-12-009

CMS-HIN-12-009 ; CERN-EP-2016-098
Coherent $\mathrm{ J } / \psi$ photoproduction in ultra-peripheral PbPb collisions at $\sqrt{s_{ \mathrm{NN}}} =$ 2.76 TeV with the CMS experiment
CMS Collaboration
23 May 2016
Phys. Lett. B 772 (2017) 489
Abstract: The cross section for coherent $\mathrm{ J } / \psi$ photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, $X_{\mathrm{n}}0_{\mathrm{n}}$ , is measured with the CMS experiment in ultra-peripheral PbPb collisions at $\sqrt{s_{ \mathrm{NN}}} =$ 2.76 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159 $\mu$ b $^{-1}$ , collected during the 2011 PbPb run. The $\mathrm{ J } / \psi$ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is $\mathrm{d}\sigma^{\text{coh}}_{X_{\mathrm{n}}0_{\mathrm{n}}}/{\mathrm{d} y}( \mathrm{ J } / \psi ) =$ 0.36 $\pm$ 0.04 (stat) $\pm$ 0.04 (syst) mb in the rapidity interval 1.8 $< \|{y}\| <$ 2.3 . Using a model for the relative rate of coherent photoproduction processes, this $X_{\mathrm{n}}0_{\mathrm{n}}$ measurement gives a total coherent photoproduction cross section of ${\mathrm{d}\sigma^{\text{coh}}}/{\mathrm{d} y}( \mathrm{ J } / \psi ) =$ 1.82 $\pm$ 0.22 (stat) $\pm$ 0.20 (syst) $\pm$ 0.19 (theo) mb. The data strongly disfavour the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent $\mathrm{ J } / \psi$ photoproduction in $\gamma$ +Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing.
Links: e-print arXiv:1605.06966 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1-a: Results from the simultaneous fit to dimuon invariant mass (a) and ${p_{\mathrm {T}}}$ (b) distributions from opposite-sign muon pairs with ${p_{\mathrm {T}}} <$ 1.0 GeV, 1.8 $< {\| y \| }<$ 2.3 and 2.6 $
png pdf	Figure 1-b: Results from the simultaneous fit to dimuon invariant mass (a) and ${p_{\mathrm {T}}}$ (b) distributions from opposite-sign muon pairs with ${p_{\mathrm {T}}} <$ 1.0 GeV, 1.8 $< {\| y \| }<$ 2.3 and 2.6 $
png pdf	Figure 2: Differential cross section versus rapidity for coherent ${\mathrm{ J } / \psi }$ production in ultra-peripheral PbPb collisions at $\sqrt {s_{ \mathrm{NN}}}=$ 2.76 TeV , measured by ALICE [29,30] and CMS (see text for details). The vertical error bars include the statistical and systematic uncertainties added in quadrature, and the horizontal bars represent the range of the measurements in $y$ . Also the impulse approximation and the leading twist approximation calculations are shown (see text for details).

Tables
png pdf	Table 1: Summary of systematic uncertainties for coherent ${\mathrm{ J } / \psi }$ events in the ${X_{\mathrm {n}}0_{\mathrm {n}}}$ configuration.

Summary

The coherent

$\mathrm{ J } / \psi$ photoproduction cross section in ultra-peripheral PbPb collisions at

$\sqrt{s_{ \mathrm{NN}}} =$ 2.76 TeV, in conjunction with at least one neutron on one side of the interaction point and no neutron activity on the other side, is measured to be

${\mathrm{d}\sigma^{\text{coh}}_{X_{\mathrm{n}}0_{\mathrm{n}}}}/{\mathrm{d} y}( \mathrm{ J } / \psi ) =$ 0.36

$\pm$ 0.04 (stat)

$\pm$ 0.04 (syst) mb in the rapidity interval 1.8

$< |{y}| <$ 2.3 . This measurement is extrapolated to the total coherent

$\mathrm{ J } / \psi$ cross section, resulting in

${\mathrm{d}\sigma^{\text{coh}}}/{\mathrm{d} y}( \mathrm{ J } / \psi ) =$ 1.82

$\pm$ 0.22 (stat)

$\pm$ 0.20 (syst)

$\pm$ 0.19 (theo) mb in the measured rapidity interval. These results complement recent measurements on coherent

$\mathrm{ J } / \psi$ photoproduction in ultra-peripheral PbPb collisions at

$\sqrt{s_{ \mathrm{NN}}} =$ 2.76 TeV by the ALICE collaboration. An impulse approximation model prediction is strongly disfavored, indicating that nuclear effects expected to be present at low

$x$ and

$Q^2$ values are needed to describe the data. The prediction given by the leading twist approximation, which includes nuclear gluon shadowing, is consistent with the data.

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