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Programs in Physics & Physical Chemistry



Manuscript Title: SHAREv2: fluctuations and a comprehensive treatment of decay feed-down
Authors: G. Torrieri, S. Jeon, J. Letessier, J. Rafelski
Program title: SHAREv2
Catalogue identifier: ADVD_v2_0 Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 175(2006)635
Programming language: FORTRAN77.
Computer: PC, Pentium III, 512MB RAM. Not hardware dependent.
Operating system: Linux: RedHat 6.1, 7.2, FEDORA etc. Not system dependent.
Keywords: fluctuations, relativistic heavy-ion collisions, particle production, statistical models, decays of resonances.
PACS: 24.10.Pa, 24.60.-k, 25.75.Dw, 24.60.Ky, 25.75.Nq, 12.38.Mh.
Classification: 11.2, 11.3.
External routines: CERN libraries, mathlib, kernlib, packlib
Does the new version supersede the previous version?: Yes
Nature of problem: Event-by-event fluctuations have been recognized to be the physical observable capable to constrain particle production models. Therefore, consideration of event-by-event fluctuations is required for a decisive falsification or constraining of (variants of) particle production models based on (grand-, micro-) canonical statistical mechanics phase space, the so called statistical hadronization models (SHM). As in the case of particle yields, to properly compare model calculations to data it is necessary to consistently take into account resonance decays. However, event-by-event fluctuations are more sensitive than particle yields to experimental acceptance issues, and a range of techniques needs to be implemented to extract `physical' fluctuations from an experimental event-by-event measurement.
Solution method: The techniques used within the SHARE suite of programs [1] are updated and extended to fluctuations. A full particle data-table, decay tree, and set of experimental feed-down coefficients are provided. Unlike SHAREv1.x, experimental acceptance feed-down coefficients can be entered for any resonance decay. SHAREv2 can calculate yields, fluctuations, and bulk properties of the fireball from provided thermal parameters; alternatively, parameters can be obtained from fits to experimental data, via the MINUIT fitting algorithm [2]. Fits can also be analyzed for significance, parameter and data point sensitivity. Averages and fluctuations at freeze-out of both the stable particles and the hadronic resonances are set according to a statistical prescription, calculated via a series of Bessel functions, using CERN library programs. We also have the option of including finite particle widths of the resonances. A χ² minimization algorithm, also from the CERN library programs, is used to perform and analyze the fit. Please see [1] for more details on these.
Reasons for new version: The vast amount of high quality soft hadron production data, from experiments running at the SPS, RHIC, in past at the AGS, and in the near future at the LHC, offers the opportunity for statistical particle production model falsification. This task has turned out to be difficult when considering solely particle yields addressed in the context of SHAREv1.x [1]. For this reason physical conditions at freeze-out remain contested. Inclusion in the analysis of event-by-event fluctuations appears to resolve this issue. Similarly, a thorough analysis including both fluctuations and average multiplicities gives a way to explore the presence and strength of interactions following hadronization (when hadrons form), ending with thermal freeze-out (when all interactions cease). SHAREv2 with fluctuations will also help determine which statistical ensemble (if any), e.g., canonical or grand-canonical, is more physically appropriate for analyzing a given system. Together with resonances, fluctuations can also be used for a direct estimate of the extent the system re-interacts between chemical and thermal freeze-out. We hope and expect that SHAREv2 will contribute to decide if any of the statistical hadronization model variants has a genuine physical connection to hadron particle production.
Summary of revisions: Fluctuations: In addition to particle yields, ratios and bulk quantities SHAREv2 can calculate, fit and analyze statistical fluctuations of particles and particle ratios; Decays: SHAREv2 has the flexibility to account for any experimental method of allowing for decay feed-downs to the particle yields; Charm flavor: Charmed particles have been added to the decay tree, allowing as an option study of statistical hadronization of J/ψ, χ_c, D_c, etc.; Quark chemistry: Chemical non-equilibrium yields for both u and d flavors, as opposed to generically light quarks q are considered; η - η′ mixing, etc., are properly dealt with, and chemical non-equilibrium can be studied for each flavor separately; Misc: Many new commands and features have been introduced and added to the basic user interface. For example, it is possible to study combinations of particles and their ratios. It is also possible to combine all the input files into one file.
Additional comments: The user should consult [1] regarding the principles of user interface and for all particle yield related physics and program instructions, other than the parameter additions and minor changes described here. SHAREv2 is downward compatible for the changes of the user interface, offering the user of SHAREv1 a computer generated revised input files compatible with SHAREv2.
Running time: We encounter, in the FORTRAN version computation, times up to seconds for evaluation of particle yields. These rise by up to a factor of 300 in the process of minimization and a further factor of a few when χ²/N_DoF profiles and contours with chemical non-equilibrium are requested.
References:
[1]	G. Torrieri, S. Steinke, W. Broniowski, W. Florkowski, J. Letessier and J. Rafelski, SHARE: Statistical hadronization with resonances, Comput. Phys. Commun. 167, 229 (2005).
[2]	F. James and M. Roos, Minuit A System For Function Minimization And Analysis Of The Parameter Errors And Correlations, Comput. Phys. Commun. 10, 343 (1975).