Nonuniversality of heavy quark hadronization in elementary high-energy collisions

It has been traditionally hypothesized that the heavy quark (charm <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>c</a:mi></a:math> and bottom <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>b</b:mi></b:math>) fragmentation is universal across different collision systems, based on the notion that hadronization as a soft process should occur at the characteristic nonperturbative quantum chromodynamics (QCD) scale, <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi mathvariant="normal">Λ</c:mi><c:mrow><c:mi>Q</c:mi><c:mi>C</c:mi><c:mi>D</c:mi></c:mrow></c:msub></c:math>. However, this universality hypothesis has recently been challenged by the observation that the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mi>c</e:mi></e:math>- and <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mi>b</f:mi></f:math>-baryon production relative to their meson counterparts in minimum bias proton-proton (<g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mi>p</g:mi><g:mi>p</g:mi></g:mrow></g:math>) collisions at the CERN Large Hadron Collider (LHC) energies is significantly enhanced as compared to the electron-positron (<h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:msup><h:mi>e</h:mi><h:mo>+</h:mo></h:msup><h:msup><h:mi>e</h:mi><h:mo>−</h:mo></h:msup></h:mrow></h:math>) collisions. The conception of nonuniversality is unambiguously reinforced by the latest measurement of the charged-particle multiplicity dependence of the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mi>b</i:mi></i:math>-baryon–to–meson yield ratio, <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mrow><j:msub><j:mi mathvariant="normal">Λ</j:mi><j:mi>b</j:mi></j:msub><j:mo>/</j:mo><j:mi>B</j:mi></j:mrow></j:math>, by the LHCb experiment in <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mrow><l:msqrt><l:mi>s</l:mi></l:msqrt><l:mo>=</l:mo><l:mn>13</l:mn><l:mspace width="0.16em"/><l:mi>TeV</l:mi><l:mspace width="4pt"/><l:mi>p</l:mi><l:mi>p</l:mi></l:mrow></l:math> collisions at the LHC, evolving continuously from the saturation value in minimum bias <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:mi>p</o:mi><o:mi>p</o:mi></o:mrow></o:math> collisions toward the small value in <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mrow><p:msup><p:mi>e</p:mi><p:mo>+</p:mo></p:msup><p:msup><p:mi>e</p:mi><p:mo>−</p:mo></p:msup></p:mrow></p:math> collisions as the system size gradually reduces. We address the multiplicity dependence of <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mi>b</q:mi></q:math>-baryon production in the canonical statistical hadronization model with input <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"><r:mi>b</r:mi></r:math>-hadron spectrum augmented with many hitherto unobserved states from quark model predictions. We demonstrate that the decreasing trend of the <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:msub><s:mi mathvariant="normal">Λ</s:mi><s:mi>b</s:mi></s:msub><s:mo>/</s:mo><s:mi>B</s:mi></s:mrow></s:math> toward low multiplicities can be quantitatively understood from the canonical suppression on the yield of <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:msub><u:mi mathvariant="normal">Λ</u:mi><u:mi>b</u:mi></u:msub></u:math>, as caused by the requirement of strict conservation of baryon number in sufficiently small systems. We have therefore proposed a plausible scenario for understanding the origin of the nonuniversality of heavy quark fragmentation in elementary collisions. Published by the American Physical Society 2024