We propose a system of evolution equations that describe in-medium time-evolution of transverse-momentum-dependent quark and gluon fragmentation functions. Furthermore, we solve this system of ...equations using Monte Carlo methods. We then quantify the obtained solutions in terms of a few characteristic features, namely the average transverse momentum
⟨
|
k
|
⟩
and energy contained in a cone, which allow us to see different behaviour of quark and gluon initiated final-state radiation. In particular, the later allows us to conclude that in the gluon-initiated processes there is less energy in a cone, so that the quark jet is more collimated.
Charmonium production at heavy-ion colliders is considered within the comovers-interaction model. The formalism is extended by including possible secondary
J
/
ψ
production through recombination and ...an estimate of recombination effects is made without adjusting the model parameters. The comovers-interaction model also includes a comprehensive treatment of initial-state nuclear effects, which are discussed in the context of such high energies. With these tools, the model properly describes the centrality and the rapidity dependence of experimental data at RHIC energy,
GeV, for both Au+Au and Cu+Cu collisions. Predictions for LHC,
TeV, are presented and the assumptions and extrapolations involved are discussed.
Can the RHIC J/ψ puzzle(s) be settled at LHC? Bravina, L.; Capella, A.; Ferreiro, E. G. ...
The European physical journal. C, Particles and fields,
06/2009, Letnik:
61, Številka:
4
Journal Article
Recenzirano
Odprti dostop
One observes strong suppression effects for hard probes, e.g. the production of
J
/
ψ
or high-
p
T
particles, in nucleus–nucleus (
AA
) collisions at RHIC. Surprisingly, the magnitude of the ...suppression is quite similar to that at SPS. In order to establish whether these features arise due to the presence of a thermalized system of quarks and gluons formed in the course of the collision, one should investigate the impact of suppression mechanisms which do not explicitly involve such a state. We calculate shadowing for gluons in the Glauber–Gribov theory and propose a model invoking a rapidity-dependent absorptive mechanism motivated by energy-momentum conservation effects. Furthermore, final-state suppression due to interaction with comoving matter (hadronic or pre-hadronic) has been shown to describe the data at SPS. We extend this model by including the backward reaction channel, i.e. recombination of open charm, which is estimated directly from
pp
data at RHIC. Strong suppression of charmonium both in
pA
and
AA
collisions at LHC is predicted. This is in stark contrast with the predictions of models assuming QGP formation and thermalization of heavy quarks.
HYDJET++ is a Monte Carlo event generator for simulation of relativistic heavy ion AA collisions considered as a superposition of the soft, hydro-type state and the hard state resulting from ...multi-parton fragmentation. This model is the development and continuation of HYDJET event generator (Lokhtin and Snigirev, EPJC 45 (2006) 211). The main program is written in the object-oriented C++ language under the ROOT environment. The hard part of HYDJET++ is identical to the hard part of Fortran-written HYDJET and it is included in the generator structure as a separate directory. The soft part of HYDJET++ event is the “thermal” hadronic state generated on the chemical and thermal freeze-out hypersurfaces obtained from the parameterization of relativistic hydrodynamics with preset freeze-out conditions. It includes the longitudinal, radial and elliptic flow effects and the decays of hadronic resonances. The corresponding fast Monte Carlo simulation procedure, C++ code FAST MC (Amelin et al., PRC 74 (2006) 064901; PRC 77 (2008) 014903) is adapted to HYDJET++. It is designed for studying the multi-particle production in a wide energy range of heavy ion experimental facilities: from FAIR and NICA to RHIC and LHC.
Program title: HYDJET++, version 2
Catalogue identifier: AECR_v1_0
Program summary URL:
http://cpc.cs.qub.ac.uk/summaries/AECR_v1_0.html
Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland
Licensing provisions: Standard CPC licence,
http://cpc.cs.qub.ac.uk/licence/licence.html
No. of lines in distributed program, including test data, etc.: 100 387
No. of bytes in distributed program, including test data, etc.: 797 019
Distribution format: tar.gz
Programming language: C++ (however there is a Fortran-written part which is included in the generator structure as a separate directory)
Computer: Hardware independent (both C++ and Fortran compilers and ROOT environment 1 (
http://root.cern.ch/) should be installed)
Operating system: Linux (Scientific Linux, Red Hat Enterprise, FEDORA, etc.)
RAM: 50 MBytes (determined by ROOT requirements)
Classification: 11.2
External routines: ROOT 1 (
http://root.cern.ch/)
Nature of problem: The experimental and phenomenological study of multi-particle production in relativistic heavy ion collisions is expected to provide valuable information on the dynamical behavior of strongly-interacting matter in the form of quark–gluon plasma (QGP) 2–4, as predicted by lattice Quantum Chromodynamics (QCD) calculations. Ongoing and future experimental studies in a wide range of heavy ion beam energies require the development of new Monte Carlo (MC) event generators and improvement of existing ones. Especially for experiments at the CERN Large Hadron Collider (LHC), implying very high parton and hadron multiplicities, one needs fast (but realistic) MC tools for heavy ion event simulations 5–7. The main advantage of MC technique for the simulation of high-multiplicity hadroproduction is that it allows a visual comparison of theory and data, including if necessary the detailed detector acceptances, responses and resolutions. The realistic MC event generator has to include maximum possible number of observable physical effects, which are important to determine the event topology: from the bulk properties of soft hadroproduction (domain of low transverse momenta
p
T
≲
1
GeV
/
c
) such as collective flows, to hard multi-parton production in hot and dense QCD-matter, which reveals itself in the spectra of high-
p
T
particles and hadronic jets. Moreover, the role of hard and semi-hard particle production at LHC can be significant even for the bulk properties of created matter, and hard probes of QGP became clearly observable in various new channels 8–11. In the majority of the available MC heavy ion event generators, the simultaneous treatment of collective flow effects for soft hadroproduction and hard multi-parton in-medium production (medium-induced partonic rescattering and energy loss, so-called “jet quenching”) is lacking. Thus, in order to analyze existing data on low and high-
p
T
hadron production, test the sensitivity of physical observables at the upcoming LHC experiments (and other future heavy ion facilities) to the QGP formation, and study the experimental capabilities of constructed detectors, the development of adequate and fast MC models for simultaneous collective flow and jet quenching simulations is necessary. HYDJET++ event generator includes detailed treatment of soft hadroproduction as well as hard multi-parton production, and takes into account known medium effects.
Solution method: A heavy ion event in HYDJET++ is a superposition of the soft, hydro-type state and the hard state resulting from multi-parton fragmentation. Both states are treated independently. HYDJET++ is the development and continuation of HYDJET MC model 12. The main program is written in the object-oriented C++ language under the ROOT environment 1. The hard part of HYDJET++ is identical to the hard part of Fortran-written HYDJET 13 (version 1.5) and is included in the generator structure as a separate directory. The routine for generation of single hard NN collision, generator PYQUEN 12,14, modifies the “standard” jet event obtained with the generator PYTHIA 6.4 15. The event-by-event simulation procedure in PYQUEN includes
1.
generation of initial parton spectra with PYTHIA and production vertexes at given impact parameter;
2.
rescattering-by-rescattering simulation of the parton path in a dense zone and its radiative and collisional energy loss;
3.
final hadronization according to the Lund string model for hard partons and in-medium emitted gluons.
Then the PYQUEN multi-jets generated according to the binomial distribution are included in the hard part of the event. The mean number of jets produced in an AA event is the product of the number of binary NN subcollisions at a given impact parameter and the integral cross section of the hard process in
NN collisions with the minimum transverse momentum transfer
p
T
min
. In order to take into account the effect of nuclear shadowing on parton distribution functions, the impact parameter dependent parameterization obtained in the framework of Glauber–Gribov theory 16 is used. The soft part of HYDJET++ event is the “thermal” hadronic state generated on the chemical and thermal freeze-out hypersurfaces obtained from the parameterization of relativistic hydrodynamics with preset freeze-out conditions (the adapted C++ code FAST MC 17,18). Hadron multiplicities are calculated using the effective thermal volume approximation and Poisson multiplicity distribution around its mean value, which is supposed to be proportional to the number of participating nucleons at a given impact parameter of AA collision. The fast soft hadron simulation procedure includes
1.
generation of the 4-momentum of a hadron in the rest frame of a liquid element in accordance with the equilibrium distribution function;
2.
generation of the spatial position of a liquid element and its local 4-velocity in accordance with phase space and the character of motion of the fluid;
3.
the standard von Neumann rejection/acceptance procedure to account for the difference between the true and generated probabilities;
4.
boost of the hadron 4-momentum in the center mass frame of the event;
5.
the two- and three-body decays of resonances with branching ratios taken from the SHARE particle decay table 19.
The high generation speed in HYDJET++ is achieved due to almost 100% generation efficiency of the “soft” part because of the nearly uniform residual invariant weights which appear in the freeze-out momentum and coordinate simulation. Although HYDJET++ is optimized for very high energies of RHIC and LHC colliders (c.m.s. energies of heavy ion beams
s
=
200
and 5500 GeV per nucleon pair, respectively), in practice it can also be used for studying the particle production in a wider energy range down to
s
∼
10
GeV
per nucleon pair at other heavy ion experimental facilities. As one moves from very high to moderately high energies, the contribution of the hard part of the event becomes smaller, while the soft part turns into just a multi-parameter fit to the data.
Restrictions: HYDJET++ is only applicable for symmetric AA collisions of heavy (
A
≳
40
) ions at high energies (c.m.s. energy
s
≳
10
GeV
per nucleon pair). The results obtained for very peripheral collisions (with the impact parameter of the order of two nucleus radii,
b
∼
2
R
A
) and very forward rapidities may be not adequate.
Additional comments: Accessibility
http://cern.ch/lokhtin/hydjet++
Running time: The generation of 100 central (0–5%) Au+Au events at
s
=
200
A
GeV
(Pb+Pb events at
s
=
5500
A
GeV
) with default input parameters takes about 7 (85) minutes on a PC 64 bit Intel Core Duo CPU @ 3 GHz with 8 GB of RAM memory under Red Hat Enterprise.
References:
1 I.P. Lokhtin, A.M. Snigirev, Eur. Phys. J. C 46 (2006) 211.
2 N.S. Amelin, R. Lednicky, T.A. Pocheptsov, I.P. Lokhtin, L.V. Malinina, A.M. Snigirev, Iu.A. Karpenko, Yu.M. Sinyukov, Phys. Rev. C 74 (2006) 064901.
3 N.S. Amelin, I. Arsene, L. Bravina, Iu.A. Karpenko, R. Lednicky, I.P. Lokhtin, L.V. Malinina, A.M. Snigirev, Yu.M. Sinyukov, Phys. Rev. C 77 (2008) 014903.
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