First results from Hybrid Hadronization in small and large systems
M. Kordell II, A. Angerami, S. A. Bass, S. Cao, Y. Chen, J. Coleman, L. Cunqueiro, T. Dai, L. Du, R. Ehlers, H. Elfner, D. Everett, W. Fan, R. Fries, C. Gale, F. Garza, Y. He, M. Heffernan, U. Heinz, B. V. Jacak, P. M. Jacobs, S. Jeon, W. Ke, E. Khalaj, B. Kim, A. Kumar, D. Liyanage, T. Luo, M. Luzum, A. Majumder, M. McNelis, J. Mulligan, C. Nattrass, D. Oliinychenko, L. G. Pang, C. Park, J.-F. Paquet, J. H. Putschke, G. Roland, B. Schenke, L. Schwiebert, C. Shen, A. Silva, C. Sirimanna, R. A. Soltz, Y. Tachibana, G. Vujanovic, X. -N. Wang, R. L. Wolpert, Y. Xu
FFirst results from Hybrid Hadronization in small and largesystems
Michael Kordell a , ∗ for the JETSCAPE collaboration a Cyclotron Institute, Texas A&M University,College Station TX 77843, USA
E-mail: [email protected] “Hybrid Hadronization” is a new Monte Carlo package to hadronize systems of partons. Itsmoothly combines quark recombination applicable when distances between partons in phasespace are small, and string fragmentation appropriate for dilute parton systems, following thepicture outlined by Han et al. [PRC 93, 045207 (2016)]. Hybrid Hadronization integrates withPYTHIA 8 and can be applied to a variety of systems from e + + e − to A + A collisions. It takessystems of partons and their color flow information, for example from a Monte Carlo partonshower generator, as input. In addition, if for A + A collisions a thermal background medium isprovided, the package allows sampling thermal partons that contribute to hadronization. HybridHadronization is available for use as a standalone code and is also part of JETSCAPE since the2.0 release.In these proceedings we review the physics concepts underlying Hybrid Hadronization and demon-strate how users can use the code with various parton shower Monte Carlos. We present calcula-tions of hadron chemistry and fragmentation functions in small and large systems when HybridHadronization is combined with parton shower Monte Carlos MATTER and LBT. In particular,we discuss observable effects of the recombination of shower partons with thermal partons. HardProbes20201-6 June 2020Austin, Texas ∗ Speaker © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - t h ] S e p ybrid Hadronization Michael KordellIn high energy collisions, from dilute systems such as in e + + e − events to the dense systemsin A + A events, the observed particles typically include some number of hadrons. The formationof these hadrons in hadronization is not well understood from first principles. However, there arehadronization models that can accurately describe observables related to hadron production. Themodels pertinent to this study are Lund string fragmentation [1] as implemented in the PYTHIA 8[2] event generator and quark coalescence / recombination [3].Lund string fragmention builds from the property of color flux tube formation in the QCDvacuum since color flux is expelled. This gives string-like behavior with gluons forming kinks inthese strings connecting color charges at large distances. These strings are then fragmented to formhadrons. String fragmentation enjoys a great deal of success in describing hadronic observables fordilute systems formed in e + + e − and p + p collisions.On the other hand, in a densely populated parton system quarks and quark-like constituentscould directly recombine into hadron and hadron resonances, similar to recombination in atomicphysics. Quark recombination can describe several key observables in A + A collisions such asenhanced baryon production and elliptic flow scaling.If we consider jet production in A + A events, we have a system composed of both dilute anddense portions. For such a case, neither string fragmentation nor quark recombination is expected tobe applicable over the entire event. This is the motivation for Hybrid Hadronization . It is a hybridof these two hadronization models that extrapolates smoothly between vacuum phenomenologyof string fragmentation and recombination in a densely populated environment, with a focus onhadronization of parton showers in these systems. Specifically, the motivation is to study in-medium effects of jet hadronization such as hadron chemistry, momentum diffusion, and mediumflow effects.The implementation of Hybrid Hadronization was initially developed as part of the JET col-laboration and is now part of the JETSCAPE framework [4]. The algorithm used [5] is basedon the instantaneous recombination model [3] and begins with some partons from some showerMonte-Carlo. The gluons present are split into q ¯ q pairs. Quarks that are close in coordinate andmomentum space have some probability to recombine into hadrons. Recombination probability isgiven by the overlap of the quark wavefunctions with the bound state hadron wavefunction and canbe calculated using the Wigner formalism assuming a Gaussian wave packet form for the quarks andfitting the hadron wavefunction widths to measured or predicted charge radii. Holes in the stringsproduced are naturally repaired using color flow information given from the shower Monte-Carlo.The remnant strings are then fragmented into hadrons using PYTHIA 8.In the presence of a medium, this procedure can be extended to include recombination ofshower partons with thermal partons. All partons in the jet that are ready to be hadronized mustexist at or outside the surface of the quark gluon plasma (QGP). If there are partons that existwithin the QGP, they must either be propagated to the surface by some shower Monte-Carlo orabsorbed by the medium. Sampled thermal quarks are added to the list of shower quarks to beconsidered for recombination. The same recombination procedure is applied and remnant stringsare fragmented as before. As we are only considering jet hadronization effects, we do not includepurely thermal hadrons for our analyses. This must be taken into account for any direct comparisonsto experimental data.The space-time structure of a heavy-ion event can influence a number of observables and is2 ybrid Hadronization Michael Kordellimportant to hadronization. The parton shower typically extends further in space-time than thefireball size; for a 500 GeV jet this can be more than 100 fm/ c . In Fig. 1 the space-time structureof 50 GeV quark jets is shown in vacuum and in the presence of a medium for partons that areready for hadronization and the produced hadrons immediately after hadronization. In vacuum, thepartonic and hadronic space-time structures look similar as to be expected, with the developmentof the lightcone on the diagonal. The presence of a medium drastically changes the distribution ofhadrons. At partonic level, the core of the jet punches through the medium and there is an intensedisk of jet partons at the hadronization hypersurface. At hadronic level, the core of the jet protrudesout of the brick and there is a thich halo of hadrons above the hypersurface. There is a smearing ofthe hadron position on the order of the hadron size, due to recombination of shower partons withthermal partons that lie outside of the lightcone. (a) The spacetime distribution of partons ready forhadronization ( Q < Q = (b) The spacetime distribution of hadrons immedi-ately after hadronization in a 50 GeV quark jet invacuum. (c)
The spacetime distribution of partons ready forhadronization ( Q < Q = T < T C ) in a50 GeV quark jet in a 4 fm brick. (d) The spacetime distribution of hadrons immedi-ately after hadronization in a 50 GeV quark jet in a4fm brick.
Figure 1:
Spacetime structure for 50 GeV quark jets immedately before and after hadronization.
For this study of Hybrid Hadronization, a QGP brick with a space-like hypersurface was3 ybrid Hadronization
Michael Kordellconsidered and the size was varied. Additionally, the thermal partons sampled had a flow velocitythat was varied. The jet initiating parton is a fixed energy 20 GeV quark, showered with MATTER[6] [7] and LBT [8] [9], and hadronized with Hybrid Hadronization. The traditional recombinationsignals of an enhanced baryon to meson ratio and flow were studied. It should be pointed out thatthese plots do not include purely thermal hadrons as this study is of the systematics of HybridHadronization. A comparison to experimental data will necessitate the inclusion of these. (a)
Ratio of fragmentation functions for 20 GeVquark jets where sampled thermal partons weregiven no flow velocity. (b)
Ratio of fragmentation functions for 20 GeVquark jets where sampled thermal partons weregiven a flow velocity v f low = . c . Figure 2:
Ratio of fragmentation functions for 20 GeV quark jets in bricks of varying sizes to vacuum.
In Fig. 2 the ratio of fragmentation functions for bricks of varying sizes to vacuum showan enhancement at low energy due to recombination with thermal partons. This enhancement ispushed out to a higher energy if the sampled thermal partons have a collective flow velocity. Thisenhancement increases with the size of the medium, which agrees with the naive expectation thatmedium signatures should increase for a larger medium. In Fig. 3 the ratio of proton to pionproduction shows the expected enhancement at low energy from recombination. This enhancementscales with the size of the medium, and is pushed out to higher energies for larger thermal partonflow velocity.In summary, there is a strong scaling of medium signatures with the size of the medium andclear signals for thermal partons imparting flow and increasing baryon production below 10 GeV/ c .These trends qualitatively agree with experimental observations, though a direct comparison wasnot included in this study. A tuning of MATTER and Hybrid Hadronization in small systems( e + + e − and p + p ) is underway with the expectation of an experimental comparison in the nearfuture. This work was supported by the US National Science Foundation under award nos. 1550221and 1812431. Portions of this research were conducted with the advanced computing resourcesprovided by Texas A&M High Performance Research Computing.4 ybrid Hadronization Michael Kordell (a)
Ratio of proton to pion yield for 20 GeV quarkjets where sampled thermal partons were given noflow velocity. (b)
Ratio of proton to pion yield for 20 GeV quarkjets where sampled thermal partons were given aflow velocity v f low = . c . Figure 3:
Proton to pion ratio for 20 GeV quark jets in bricks of varying sizes and in vacuum.
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