Probing Gluon Fluctuations in Nuclei with the First Energy-Dependent Measurement of Incoherent J/ψ Photoproduction in Ultraperipheral PbPb Collisions

Abstract

Incoherent <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>J</a:mi><a:mo>/</a:mo><a:mi>ψ</a:mi></a:math> photoproduction in heavy ion ultraperipheral collisions (UPCs) provides a sensitive probe of localized, fluctuating gluonic structures within heavy nuclei. This Letter reports the first measurement of the photon-nucleon center-of-mass energy (<c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>W</c:mi><c:mrow><c:mi>γ</c:mi><c:mi mathvariant="normal">N</c:mi></c:mrow></c:msub></c:math>) dependence of this process in PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV, using <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mrow><f:mn>1.52</f:mn><f:mtext> </f:mtext><f:mtext> </f:mtext><f:msup><f:mrow><f:mi>nb</f:mi></f:mrow><f:mrow><f:mo>−</f:mo><f:mn>1</f:mn></f:mrow></f:msup></f:mrow></f:math> of data recorded by the CMS experiment. The measurement covers a wide <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:msub><h:mi>W</h:mi><h:mrow><h:mi>γ</h:mi><h:mi mathvariant="normal">N</h:mi></h:mrow></h:msub></h:math> range of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mo>≈</k:mo><k:mn>40</k:mn><k:mi>–</k:mi><k:mn>400</k:mn><k:mtext> </k:mtext><k:mtext> </k:mtext><k:mi>GeV</k:mi></k:mrow></k:math>, probing gluons carrying a fraction <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>x</m:mi></m:math> of nucleon momentum down to an unexplored regime of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mn>6.5</o:mn><o:mo>×</o:mo><o:msup><o:mn>10</o:mn><o:mrow><o:mo>−</o:mo><o:mn>5</o:mn></o:mrow></o:msup></o:math>. Compared to baseline predictions neglecting nuclear effects, the measured cross sections exhibit significantly greater suppression at lower <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>x</q:mi></q:math>. Additionally, the ratio of incoherent to coherent photoproduction is found to be constant across the probed <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:msub><s:mi>W</s:mi><s:mrow><s:mi>γ</s:mi><s:mi mathvariant="normal">N</s:mi></s:mrow></s:msub></s:math> and <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mi>x</v:mi></v:math> range, disfavoring the establishment of the black disk limit. This Letter provides critical insights into the <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"><x:mi>x</x:mi></x:math>-dependent evolution of fluctuating gluonic structures within nuclei and calls for further advancements in theoretical models incorporating nuclear shadowing and gluon saturation.