Nonuniversality of heavy quark hadronization in elementary high-energy collisions

Abstract

It has been traditionally hypothesized that the heavy quark (charm $c$ and bottom $b$) 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, ${\mathrm{\Lambda }}_{QCD}$. However, this universality hypothesis has recently been challenged by the observation that the $c$- and $b$-baryon production relative to their meson counterparts in minimum bias proton-proton ($pp$) collisions at the CERN Large Hadron Collider (LHC) energies is significantly enhanced as compared to the electron-positron ($e^{+}{e}^{-}$) collisions. The conception of nonuniversality is unambiguously reinforced by the latest measurement of the charged-particle multiplicity dependence of the $b$-baryon–to–meson yield ratio, ${\mathrm{\Lambda }}_b/B$, by the LHCb experiment in $\sqrt{s}=13\mathrm{TeV}pp$ collisions at the LHC, evolving continuously from the saturation value in minimum bias $pp$ collisions toward the small value in $e^{+}{e}^{-}$ collisions as the system size gradually reduces. We address the multiplicity dependence of $b$-baryon production in the canonical statistical hadronization model with input $b$-hadron spectrum augmented with many hitherto unobserved states from quark model predictions. We demonstrate that the decreasing trend of the ${\mathrm{\Lambda }}_b/B$ toward low multiplicities can be quantitatively understood from the canonical suppression on the yield of ${\mathrm{\Lambda }}_b$, 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.