Observation of Coherent <math display="inline"><mrow><mi>ϕ</mi><mo stretchy="false">(</mo><mn>1020</mn><mo stretchy="false">)</mo></mrow></math> Meson Photoproduction in Ultraperipheral PbPb 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>5.36</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>

Bein, Samuel;Benecke, Anna;Bethani, Agni;Bruno, Giacomo;CMS;et.al.
(2025) Physical Review Letters — Vol. 135 (2025)

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Abstract
The first observation of coherent <math display="inline"><mrow><mi>ϕ</mi><mo stretchy="false">(</mo><mn>1020</mn><mo stretchy="false">)</mo></mrow></math> meson photoproduction off heavy nuclei is presented using ultraperipheral lead-lead collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. The data were collected by the CMS experiment and correspond to an integrated luminosity of <math display="inline"><mrow><mn>1.62</mn><mtext> </mtext><mtext> </mtext><mi mathvariant="normal">μ</mi><msup><mrow><mi>b</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>. The <math display="inline"><mrow><mi>ϕ</mi><mo stretchy="false">(</mo><mn>1020</mn><mo stretchy="false">)</mo></mrow></math> meson signals are reconstructed via the <math display="inline"><mrow><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>K</mi></mrow><mrow><mo>−</mo></mrow></msup></mrow></math> decay channel. The production cross section is presented as a function of the <math display="inline"><mrow><mi>ϕ</mi><mo stretchy="false">(</mo><mn>1020</mn><mo stretchy="false">)</mo></mrow></math> meson rapidity in the range <math display="inline"><mrow><mn>0.3</mn><mo>&lt;</mo><mo stretchy="false">|</mo><mi>y</mi><mo stretchy="false">|</mo><mo>&lt;</mo><mn>1.0</mn></mrow></math>, probing gluons that carry a fraction of the nucleon momentum (<math display="inline"><mi>x</mi></math>) around <math display="inline"><msup><mn>10</mn><mrow><mo>-</mo><mn>4</mn></mrow></msup></math>. The observed cross section exhibits little dependence on rapidity and is significantly suppressed, by a factor of <math display="inline"><mo>∼</mo><mn>5</mn></math>, compared to a baseline model that treats a nucleus as a collection of free nucleons. Theoretical models that incorporate the nuclear shadowing effect generally provide a better description of the <math display="inline"><mi>ϕ</mi><mo stretchy="false">(</mo><mn>1020</mn><mo stretchy="false">)</mo></math> data than those incorporating gluon saturation. This study establishes a powerful new tool for exploring nuclear effects and nuclear gluonic structure in the small-<math display="inline"><mi>x</mi></math> regime at a unique energy scale bridging the perturbative and nonperturbative quantum chromodynamics domains.
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Bein, S., Benecke, A., Bethani, A., Bruno, G., Cappati, A., de Favereau de Jeneret, J., Delaere, C., Giammanco, A., Guzel, A. O., Jain, S., Jain, S., Lemaitre, V., Lidrych, J., Mastrapasqua, P., Turkcapar, S., CMS, & et al. (2025). Observation of Coherent <math display=“inline”><mrow><mi>ϕ</mi><mo stretchy=“false”>(</mo><mn>1020</mn><mo stretchy=“false”>)</mo></mrow></math> Meson Photoproduction in Ultraperipheral PbPb 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>5.36</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>. Physical Review Letters, 135. https://doi.org/10.1103/2ssw-wwyy (Original work published 2025)