Abstract
Following an explicit example, we present the chain of steps required for an event-by-event description of hadron production in high energy hadronic and nuclear collisions. We start from incoming nuclei, described in the color glass condensate effective theory, whose collision creates the gluon fields of the glasma. Individual gluons are then sampled from the gluon fields' Husimi (smeared Wigner) distributions and clustered using a new spacetime based algorithm. Clusters are fed into the Herwig event generator, which performs the hadronization, conserving energy and momentum. We discuss the physical implications of smearing and problems with the quasiparticle picture for the studied processes. We compute spectra of charged hadrons and identified particles and their azimuthal momentum anisotropies, and address systematic uncertainties on observables, resulting from the general lack of detailed knowledge of the hadronization mechanism.
Highlights
Strong multiparticle correlations that are long range in rapidity have been observed in high energy collisions of protons with protons or heavy nuclei at both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC) [1,2]
A comparison of experimental data with calculations employing frameworks with strong final state effects has in many cases shown good agreement [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], while calculations based on initial state effects, such as those within the color glass condensate (CGC) effective field theory (EFT) [18,19,20,21,22,23,24,25,26,27] have not been able to capture all systematic features of the data [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]
We compare to the hadron spectrum obtained from performing independent fragmentation of the smeared gluon spectrum using the next to leading order (NLO) Kniehl-KramerPotter (KKP) fragmentation functions [46] as described in [68]
Summary
Strong multiparticle correlations that are long range in rapidity have been observed in high energy collisions of protons with protons or heavy nuclei at both the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC) [1,2]. A comparison of experimental data with calculations employing frameworks with strong final state effects has in many cases shown good agreement [3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], while calculations based on initial state effects, such as those within the color glass condensate (CGC) effective field theory (EFT) [18,19,20,21,22,23,24,25,26,27] have not been able to capture all systematic features of the data [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45] While the former group employs hydrodynamic calculations, where hadronization is encoded in the equation of state and relies on the assumption of thermal equilibrium (with only small deviations due to viscous effects), the latter group has often compared experimental data with parton level results, or employed independent fragmentation, which should only be valid at high pT < 1–2 GeV (see e.g., [46] for details). We present distributions of the invariant masses of SAHARA and Herwig clusters in the Appendix
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