Observation of the Charged-Particle Multiplicity Dependence of σψ(2S)/σJ/ψ in p -Pb Collisions at 8.16 TeV

Abstract

Bound states of charm and anticharm quarks, known as charmonia, have a rich spectroscopic structure that can be used to probe the dynamics of hadron production in high-energy hadron collisions. Here, the cross section ratio of excited <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mo stretchy="false">(</a:mo><a:mrow><a:mi>ψ</a:mi><a:mo stretchy="false">(</a:mo><a:mrow><a:mn>2</a:mn><a:mi>S</a:mi></a:mrow><a:mo stretchy="false">)</a:mo></a:mrow><a:mo stretchy="false">)</a:mo></a:math> and ground state <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mo stretchy="false">(</g:mo><g:mrow><g:mi>J</g:mi><g:mo>/</g:mo><g:mi>ψ</g:mi></g:mrow><g:mo stretchy="false">)</g:mo></g:math> vector mesons is measured as a function of the charged-particle multiplicity in proton-lead (<k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mi>p</k:mi><k:mi>Pb</k:mi></k:mrow></k:math>) collisions at a center-of-mass (CM) energy per nucleon pair of 8.16 TeV. The data corresponding to an integrated luminosity of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mn>175</m:mn><m:mtext> </m:mtext><m:mtext> </m:mtext><m:msup><m:mi>nb</m:mi><m:mrow><m:mo>−</m:mo><m:mn>1</m:mn></m:mrow></m:msup></m:math> were collected using the CMS detector. The ratio is measured separately for prompt and nonprompt charmonia in the transverse momentum range <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mn>6.5</o:mn><o:mo>&lt;</o:mo><o:msub><o:mi>p</o:mi><o:mi mathvariant="normal">T</o:mi></o:msub><o:mo>&lt;</o:mo><o:mn>30</o:mn><o:mtext> </o:mtext><o:mtext> </o:mtext><o:mi>GeV</o:mi></o:math> and in four rapidity ranges spanning <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:mo>−</r:mo><r:mn>2.865</r:mn><r:mo>&lt;</r:mo><r:msub><r:mi>y</r:mi><r:mi>CM</r:mi></r:msub><r:mo>&lt;</r:mo><r:mn>1.935</r:mn></r:math>. For the first time, a statistically significant multiplicity dependence of the prompt cross section ratio is observed in proton-nucleus collisions. There is no clear rapidity dependence in the ratio. The prompt measurements are compared with a theoretical model which includes interactions with nearby particles during the evolution of the system. These results provide additional constraints on hadronization models of heavy quarks in nuclear collisions.