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studying the charmed hadron production is a particularly useful tool to probe properties of hot and dense matter created in relativistic heavy ion collisions . the modern pattern of multi - particle production in ( most central ) heavy ion collisions at rhic and lhc agrees with the formation of hot strongly - interacting matter with hydrodynamical properties ( `` quark - gluon fluid '' ) , which absorbs energetic quarks and gluons due to their multiple scattering and medium - induced energy loss ( see , e.g. ,  @xcite ) . within such paradigm , a number of questions on heavy flavor production arises .
are heavy quarks thermalized in quark - gluon plasma ? what is the mass dependence of in - medium quark energy loss ?
are charmed hadrons in a kinetic equilibrium with the medium ? how the specific pattern of quarkonium suppression is related to the interplay between thermal and non - thermal mechanisms of @xmath1-meson production ? in this paper , some results of the phenomenological analysis of lhc data on momentum spectra and elliptic flow of charmed hadrons ( d , @xmath1 ) in pbpb collisions at @xmath2 tev within two - component model hydjet++ are presented . the comparison with rhic results is also discussed .
hydjet++ is the model of relativistic heavy ion collisions , which includes two independent components : the soft hydro - type state ( `` thermal '' component ) and the hard state resulting from the medium - modified multi - parton fragmentation ( `` non - thermal '' component )  @xcite .
the soft component represents the hadronic state generated on the chemical and thermal freeze - out hypersurfaces obtained from the parametrization of relativistic hydrodynamics with preset freeze - out conditions ( the adapted event generator fast mc  @xcite ) .
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 a number of participating nucleons for a given impact parameter of an aa collision . to simulate the elliptic flow effect ,
the hydro - inspired parametrization is implemented for the momentum and spatial anisotropy of a thermal hadron emission source .
the approach used for the hard component is based on the pyquen partonic energy loss model  @xcite .
the simulation of single hard nn sub - collision by pyquen is constructed as a modification of the jet event obtained with pythia@xmath36.4 generator  @xcite .
it takes into account medium - induced rescattering , collisional and radiative energy loss of hard partons in the expanding quark - gluon fluid , realistic nuclear geometry and nuclear shadowing effect .
the mean number of jets produced in an aa event is calculated as a product of the number of binary nn sub - collisions at a given impact parameter per the integral cross section of the hard process with the minimum transverse momentum transfer @xmath4 .
the partons produced in hard processes with the momentum transfer lower than @xmath4 , are considered as being `` thermalized '' .
so , their hadronization products are included `` automatically '' in the soft component of the event .
the input parameters of the model for soft and hard components have been tuned from fitting to heavy ion data on various observables for inclusive hadrons at rhic  @xcite and lhc  @xcite .
charmed meson production in hydjet++ includes soft and hard components as well .
thermal production of @xmath5 , @xmath1 and @xmath6 hadrons is treated within the statistical hadronization approach  @xcite .
momentum spectra of charm hadrons are computed according to the thermal distribution , and the multiplicities @xmath7 ( @xmath8 ) are calculated through the corresponding thermal numbers @xmath9 as @xmath10 , where @xmath11 is the charm enhancement factor ( or charm fugacity ) , and @xmath12 is the number of charm quarks in a hadron @xmath13 .
the fugacity @xmath11 can be treated as a free parameter of the model , or calculated through the number of charm quark pairs obtained from pythia and multiplied by the number of nn sub - collisions .
non - thermal charmed hadrons are generated by pyquen taking into account in - medium energy loss of heavy ( b , c ) quarks within `` dead - cone '' generalization  @xcite of bdmps model  @xcite for radiative loss and high - momentum transfer limit  @xcite for collisional loss .
some time ago it was demonstrated in  @xcite that if @xmath1-mesons are produced in hydjet++ at the same freeze - out parameters as for inclusive ( light ) hadrons , then simulated @xmath0- and @xmath14-spectra in 20% of most central auau collisions at rhic energy @xmath15 gev are much wider than ones measured by phenix  @xcite .
however the assumption that the thermal freeze - out for @xmath1-mesons happens at the same temperature as chemical freeze - out ( with reduced collective velocities ) allows hydjet++ to reproduce the data .
it turned out that the similar situation holds at the lhc .
figure  [ jpsi_temp ] shows the comparison of hydjet++ simulations with the alice data  @xcite for @xmath0-spectrum of @xmath1-mesons in 20% of most central pbpb collisions at @xmath2 tev .
one can see that if thermal freeze - out for @xmath1 s happens at the same temperature as chemical freeze - out ( with reduced collective velocities ) , then simulated spectra match the data up to @xmath16 gev/@xmath17 .
the discrepancy at high @xmath0 may indicate on the necessity to tune pythia for charmonium production in this kinematic domain .
the @xmath0-dependence of the elliptic flow coefficient @xmath18 measured by alice  @xcite is also reproduced by hydjet++ simulations in such a case ( figure  [ jpsi_v2 ] ) .
the contribution of non - thermal component is important at high @xmath0 .
thus we conclude that @xmath1-meson production in heavy ion collisions at rhic and lhc may be reproduced by hydjet++ model under the assumption that thermal freeze - out of charmonia happens appreciably before thermal freeze - out of light hadrons  @xcite , presumably at chemical freeze - out ( with reduced radial and longitudinal collective velocities ) .
our interpretation of this is that the significant part of @xmath1-mesons ( up to @xmath16 gev ) is thermalized , but without being in a kinetic equilibrium with the medium .
elliptic flow coefficient @xmath19 of @xmath1-mesons at rapidity @xmath20 for 2040% centrality of pbpb collisions at @xmath21 tev .
the points are alice data  @xcite , histograms are simulated hydjet++ events ( blue  soft component , red  hard component , black - both components ) . ]
elliptic flow coefficient @xmath19 of @xmath1-mesons at rapidity @xmath20 for 2040% centrality of pbpb collisions at @xmath21 tev .
the points are alice data  @xcite , histograms are simulated hydjet++ events ( blue  soft component , red  hard component , black - both components ) . ]
elliptic flow coefficient @xmath19 of d@xmath22-mesons at rapidity @xmath23 for 1030% centrality of pbpb collisions at @xmath21 tev .
the points are alice data  @xcite , histograms are simulated hydjet++ events ( blue  soft component , red  hard component , black - both components ) . ]
elliptic flow coefficient @xmath19 of d@xmath22-mesons at rapidity @xmath23 for 1030% centrality of pbpb collisions at @xmath21 tev .
the points are alice data  @xcite , histograms are simulated hydjet++ events ( blue  soft component , red  hard component , black - both components ) . ]
at first we have checked that @xmath0-spectrum of d - mesons measured by star  @xcite in 10% of most central auau collisions at @xmath15 gev is reproduced by hydjet++ simulation with fugacity @xmath24 and the same freeze - out parameters as for @xmath1-mesons ( but not for inclusive hadrons ) .
it means that d - mesons like @xmath1-mesons are not in a kinetic equilibrium with the medium at rhic .
however the situation gets changed dramatically at the lhc .
figure  [ d_pt ] shows the comparison of hydjet++ simulations with the alice data  @xcite for @xmath0-spectra of d@xmath22 , d@xmath25 and d@xmath26 mesons in 20% of most central pbpb collisions at @xmath2 tev .
the freeze - out parameters as for inclusive hadrons have been used , the fugacity value @xmath27 being fixed from @xmath1 yield .
the model results are close to the data within the experimental uncertainties .
note that @xmath0-spectra and nuclear modification factors @xmath28 ( the ratio of particle yields in aa and pp collisions normalized on the mean number of nn sub - collisions ) of d - mesons are reproduced by hydjet++ up to very high @xmath0 .
so the treatment of heavy quark energy loss in hard component of the model ( pyquen ) seems quite successful .
in addition , hydjet++ is able to reproduce the alice data  @xcite on @xmath0-dependence of the elliptic flow coefficient @xmath18 ( figure  [ d_v2 ] ) .
thus in contrast to rhic , thermal freeze - out of d - mesons at the lhc happens simultaneously with thermal freeze - out of light hadrons .
therefore the significant part of d - mesons ( up to @xmath29 gev/@xmath17 ) seems to be in a kinetic equilibrium with the medium .
momentum spectra and elliptic flow of d and @xmath1 mesons in pbpb collisions at the lhc are reproduced by two - component model hydjet++ including thermal ( soft ) and non - thermal ( hard ) components .
thermal freeze - out of d - mesons happens simultaneously with thermal freeze - out of light hadrons .
thermal freeze - out of @xmath1-mesons happens appreciably before freeze - out of light hadrons , presumably at chemical freeze - out ( with reduced radial and longitudinal collective velocities ) .
thus the significant part of d - mesons ( up to @xmath29 gev / c ) seems to be in a kinetic equilibrium with the medium , while @xmath1 mesons are not .
the production of charmed mesons at high transverse momenta is determined by non - thermal charm production mechanism and medium - induced heavy quark energy loss .
discussions with l.v .
bravina , v.l .
korotkikh , l.v .
malinina , a.m. snigirev , e.e .
zabrodin and s.v .
petrushanko are gratefully acknowledged .
we thank our colleagues from alice and cms collaborations for fruitful cooperation .
this work was supported by the russian scientific fund under grant no .
14 - 12 - 00110 in a part of computer simulation of @xmath0-spectrum and elliptic flow of charmed mesons in pbpb collisions
. g.e . acknowledges the european social fund within the framework of realizing the project support of inter - sectoral mobility and quality enhancement of research teams at czech technical university in prague , cz.1.07/2.3.00/30.0034 .
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the phenomenological analysis of the lhc data on @xmath0-spectrum and elliptic flow of @xmath1 and d mesons in pbpb collisions at @xmath2 tev is presented . 
 the charmed meson production pattern in pbpb collisions may be reproduced by two - component model hydjet++ including thermal and non - thermal components . 
 the significant part of d - mesons is found to be in a kinetic equilibrium with the created medium , while @xmath1-mesons are not .