Probing Small Bjorken-<math display="inline"><mi>x</mi></math> Nuclear Gluonic Structure via Coherent <math display="inline"><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></math> Photoproduction in Ultraperipheral Pb-Pb Collisions at <math display="inline"><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>NN</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>5.02</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>

Benecke, Anna;Bruno, Giacomo;Bury, Florian;Caputo, Claudio;Bethani, Agni;et.al.
(2023) Physical Review Letters — Vol. 131 (2023)

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Abstract
Quasireal photons exchanged in relativistic heavy ion interactions are powerful probes of the gluonic structure of nuclei. The coherent <math display="inline"><mi>J</mi><mo>/</mo><mi>ψ</mi></math> photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon (<math display="inline"><mrow><msubsup><mrow><mi>W</mi></mrow><mrow><mi>γ</mi><mi mathvariant="normal">N</mi></mrow><mrow><mi>Pb</mi></mrow></msubsup></mrow></math>) over a wide range of <math display="inline"><mrow><mn>40</mn><mo>&lt;</mo><msubsup><mrow><mi>W</mi></mrow><mrow><mi>γ</mi><mi mathvariant="normal">N</mi></mrow><mrow><mi>Pb</mi></mrow></msubsup><mo>&lt;</mo><mn>400</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>. Results are obtained using data at the nucleon-nucleon center-of-mass energy of 5.02 TeV collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of <math display="inline"><mrow><mn>1.52</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>nb</mi></mrow><mrow><mo>-</mo><mn>1</mn></mrow></msup></mrow></math>. The cross section is observed to rise rapidly at low <math display="inline"><msubsup><mi>W</mi><mrow><mi>γ</mi><mi mathvariant="normal">N</mi></mrow><mi>Pb</mi></msubsup></math>, and plateau above <math display="inline"><mrow><msubsup><mrow><mi>W</mi></mrow><mrow><mi>γ</mi><mi mathvariant="normal">N</mi></mrow><mrow><mi>Pb</mi></mrow></msubsup><mo>≈</mo><mn>40</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, up to <math display="inline"><mrow><mn>400</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>, entering a new regime of small Bjorken-<math display="inline"><mi>x</mi></math> (<math display="inline"><mrow><mo>≈</mo><mn>6</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>-</mo><mn>5</mn></mrow></msup></mrow></math>) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.
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Benecke, A., Bruno, G., Bury, F., Caputo, C., David, P., Delaere, C., Donertas, I. S., Giammanco, A., Jaffel, K., Jain, S., Lemaitre, V., Lidrych, J., Mondal, K., Tran, T. T., Vischia, P., Wertz, S., CMS, Bethani, A., & et al. (2023). Probing Small Bjorken-<math display=“inline”><mi>x</mi></math> Nuclear Gluonic Structure via Coherent <math display=“inline”><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></math> Photoproduction in Ultraperipheral Pb-Pb Collisions at <math display=“inline”><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>NN</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>5.02</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>. Physical Review Letters, 131. https://doi.org/10.1103/PhysRevLett.131.262301 (Original work published 2023)