S. Petrushanko

This report presents the capabilities of the CMS experiment to explore the rich heavy-ion physics programme offered by the CERN Large Hadron Collider (LHC). The collisions of lead nuclei at energies , will probe quark and gluon matter at unprecedented values of energy density. The prime goal of this research is to study the fundamental theory of the strong interaction — Quantum Chromodynamics (QCD) — in extreme conditions of temperature, density and parton momentum fraction (low-x).

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 pT≲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-pT 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-pT 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 generation of initial parton spectra with PYTHIA and production vertexes at given impact parameter; rescattering-by-rescattering simulation of the parton path in a dense zone and its radiative and collisional energy loss; final hadronization according to the Lund string model for hard partons and in-medium emitted gluons. generation of the 4-momentum of a hadron in the rest frame of a liquid element in accordance with the equilibrium distribution function; 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; the standard von Neumann rejection/acceptance procedure to account for the difference between the true and generated probabilities; boost of the hadron 4-momentum in the center mass frame of the event; the two- and three-body decays of resonances with branching ratios taken from the SHARE particle decay table [19]. 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∼2RA) 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=200A GeV (Pb+Pb events at s=5500A 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.

We estimate the amount of collective "elliptic flow" expected at mid-rapidity in proton-proton (p-p) collisions at the CERN Large Hadron Collider (LHC), assuming that any possible azimuthal anisotropy of the produced hadrons with respect to the plane of the reaction follows the same overlap-eccentricity and particle-density scalings as found in high-energy heavy ion collisions. Using a Glauber eikonal model, we compute the p-p eccentricities, transverse areas and particle-multiplicities for various phenomenological parametrisations of the proton spatial density. For realistic proton transverse profiles, we find integrated elliptic flow v2 parameters below 3% in p-p collisions at sqrt(s) = 14 TeV.

The LHC data on azimuthal anisotropy harmonics from PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and interpreted in the framework of the HYDJET++ model. The cross-talk of elliptic $$v_2$$ and triangular $$v_3$$ flow in the model generates both even and odd harmonics of higher order. Comparison with the experimental data shows that this mechanism is able to reproduce the $$p_\mathrm{T}$$ and centrality dependencies of quadrangular flow $$v_4$$ , and also the basic trends for pentagonal $$v_5$$ and hexagonal $$v_6$$ flows.

Partial contributions of elliptic v_2 and triangular v_3 flows to the hexagonal v_6 flow are studied within the HYDJET++ model for Pb+Pb collisions at sqrt{s}=2.76 TeV. Scaling of the ratio v_6^{1/6}{Psi_2} / v_2^{1/2}{Psi_2} in the elliptic flow plane, Psi_2, is predicted in the range 1 < p_T < 4 GeV/c for semicentral and semiperipheral collisions. Jets increase this ratio by about 10% and also cause its rise at p_T > 3.5 GeV/c. The part of v_6 coming from v_2 is instantly increasing as the reaction becomes more peripheral, whereas the contribution of v_3 to v_6 drops. This behavior explains the experimentally observed increase of correlations between second and sixth harmonics and decrease of correlations between third and sixth harmonics with rising impact parameter $b$. Our study favors the idea that basic features of the hexagonal flow can be understood in terms of the interplay of elliptic and triangular flows.

The started LHC heavy ion program makes it possible to probe new frontiers of the high temperature Quantum Chromodynamics. It is expected that the role of hard and semi-hard particle production processes may be significant at ultra-high energies even for bulk properties of the created matter. In this paper, the LHC data on multiplicity, hadron spectra, elliptic flow and femtoscopic correlations from PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed in the framework of the HYDJET++ model. The influence of the jet production mechanism on these observables is discussed.

The formation and evolution of the elliptic flow pattern in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s}=5.5A$ TeV and in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s}=200A$ GeV are analyzed for different hadron species within the framework of the HYDJET$++$ Monte Carlo model. The model contains both hydrodynamic state and jets, thus allowing for a study of the interplay between the soft and hard processes. It is found that jets terminate the rise of the elliptic flow with increasing transverse momentum. Since jets are more influential at the Large Hadron Collider (LHC) than at the Relativistic Heavy Ion Collider (RHIC), the elliptic flow at LHC should be weaker than that at RHIC. The influence of resonance decays on particle elliptic flow is also investigated. These final state interactions enhance the low-${p}_{T}$ part of the ${v}_{2}$ of pions and light baryons and work toward the fulfillment of idealized constituent quark scaling.

The LHC data on event-by-event harmonic flow coefficients measured in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and interpreted within the HYDJET $$++$$ model. To compare the model results with the experimental data the unfolding procedure is employed. The essentially dynamical origin of the flow fluctuations in hydro-inspired freeze-out approach has been established. It is shown that the simple modification of the model via introducing the distribution over spatial anisotropy parameters permits HYDJET $$++$$ to reproduce both elliptic and triangular flow fluctuations and related to it eccentricity fluctuations of the initial state at the LHC energy.

The hybrid model HYDJET++, which combines soft and hard components, is employed for the analysis of dihadron angular correlations measured in PbPb collisions at center-of-mass energy sqrt(s_NN) = 2.76 TeV. The model allows for study both individual contributions and mutual influence of lower flow harmonics, v_2 and v_3, on higher harmonics and dihadron angular correlations. It is shown that the typical structure called ridge in dihadron angular correlations in a broad pseudorapidity range could appear just as interplay of v_2 and v_3. Central, semi-central and semi-peripheral collisions were investigated. Comparison of model results with the experimental data on dihadron angular correlations is presented for different centralities and transverse momenta p_T.

The Monte Carlo HYDJET++ model, that contains both hydrodynamic state and jets, is applied to study the influence of the interplay between soft and hard processes on the formation of the elliptic flow in heavy-ion collisions at RHIC and LHC energies. Jets are found to cease the hydro-like increase of the elliptic flow with rising p_T. Since jets are more influential at LHC than at RHIC, the v_2 at LHC should be weaker than that at RHIC. Violation of the number-of-constituent-quark (NCQ) scaling is predicted. The decays of resonances are found to enhance the low-p_T part of the elliptic flow of pions and light baryons, and work toward the fulfillment of the NCQ scaling.

Triangular flow $v_3$ of identified and inclusive particles in Pb+Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV is studied as a function of centrality and transverse momentum within the \textsc{hydjet++} model. The model enables one to investigate the influence of both hard processes and final-state interactions on the harmonics of particle anisotropic flow. Decays of resonances are found to increase the magnitude of the $v_3(p_{\rm T})$ distributions at $p_{\rm T} \geq 2$ GeV/$c$ and shift their maxima to higher transverse momenta. The $p_{\rm T}$-integrated triangular flow, however, becomes slightly weakened for all centralities studied. The resonance decays also modify the spectra towards the number-of-constituent-quark scaling fulfillment for the triangular flow, whereas jets are the main source of the scaling violation at the energies available at the CERN Large Hadron Collider (LHC). Comparison with the corresponding spectra of elliptic flow reveals that resonance decays and jets act in a similar manner on both $v_3(p_{\rm T})$ and $v_2(p_{\rm T})$ behavior. Obtained results are also confronted with the experimental data on differential triangular flow of identified hadrons, ratio $v_3^{1/3}(p_{\rm T}) / v_2^{1/2}(p_{\rm T})$ and $p_{\rm T}$-integrated triangular flow of charged hadrons.

Data from the Large Hadron Collider on elliptic flow correlations at low and high $p_T$ from Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$~TeV are analyzed and interpreted in the framework of the HYDJET++ model. This model allows us to describe simultaneously the region of both low and high transverse momenta and, therefore, to reproduce the experimentally observed nontrivial centrality dependence of elliptic flow correlations. The origin of the correlations between low and high-$p_T$ flow components in peripheral lead-lead collisions is traced to correlations of particles in jets.

Abstract Data from the Large Hadron Collider on the charge balance function in Pb+Pb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and interpreted within the framework of the HYDJET++ model. This model allows us to qualitatively reproduce the experimentally observed centrality dependence of the balance function widths at relatively low transverse momentum intervals due to the different charge creation mechanisms in soft and hard processes. However, a fully adequate description of the balance function in these intervals implies an essential modification of the model by including exact charge conservation via the canonical rather than the grand canonical ensemble. A procedure is proposed for introducing charge correlations into the thermal model without changing other model parameters. With increasing transverse momenta, the default model results describe the experimental data much better because the contribution of the soft component of the model is significantly reduced in these transverse momentum intervals. In practical terms, there is a transition to a single source of charge correlations, namely, charge correlations in jets in which exact charge conservation holds at each stage.

We discuss the importance of high-pT hadron and jet measurements in nucleus-nucleus collisions at the CERN Large Hadron Collider.

The jet reconstruction algorithms and calibration techniq ues implemented in the CMS reconstruction software are studied with high-statistics Monte Carlo samples of QCD dijet events. Generated events are passed through a full detector-level simulation of the CMS detector including readout digitization in the presence of pile-up at an instantaneous luminosity of L = 2×10 33 cm 2 s 1 . Effects of detector resolution and granularity on the jet resolutions, effic iencies and instrumental background rates are estimated. These measures of performance are compared for a set of jet algorithms, algorithm parameters, and calorimeter cell thresholds. The uniformity and linearity of the jet response are evaluated by comparing particle-level and reconstructed jets over a wide range of transverse momenta throughout the angular coverage of the calorimeters. Fits to the ratio o f reconstructed to generated jet transverse energy give a transverse energy resolution of 10-15% (8-10%) at 100 GeV (200 GeV) over the pseudorapidity range 0 < |�| < 5. The angular resolution for 100 GeV (200 GeV) jets is 0.02-0.035 (0.02) radians.

We study the influence of geometric and dynamical anisotropies on the development of flow harmonics and, simultaneously, on the second- and third-order oscillations of femtoscopy radii. The analysis is done within the Monte Carlo event generator HYDJET++, which was extended to dynamical triangular deformations. It is shown that the merely geometric anisotropy provides the results which anticorrelate with the experimental observations of either $v_{2}$ (or $v_{3}$ ) or second-order (or third-order) oscillations of the femtoscopy radii. Decays of resonances significantly increase the emitting areas but do not change the phases of the radii oscillations. In contrast to the spatial deformations, the dynamical anisotropy alone provides the correct qualitative description of the flow and the femtoscopy observables simultaneously. However, one needs both types of the anisotropy to match quantitatively the experimental data.

We analyze the azimuthal anisotropy of jet spectra due to energy loss of hard partons in quark-gluon plasma, created initially in nuclear overlap zone in collisions with non-zero impact parameter. The calculations are performed for semi-central Pb-Pb collisions at LHC energy.

Triangular flow of strange and non-strange hadrons in Pb+Pb collisions at LHC energies is studied within the HYDJET++ model, which combines hard processes in a hot and dense partonic medium with parametrized hydrodynamics. Together with an extensive table of resonances, this circumstance enables us to investigate (i) the interplay between hard and soft processes and (ii) between jets and final-state interactions. Jets are found to be the main reason for violation of the NCQ scaling for v3 at LHC, whereas decays of resonances improve the scaling fulfillment. Comparison with the experimental data is performed.

HYDJET++ model, which combines parametrized hydrodynamics with jets, is employed to study the dependence of the hexagonal flow, ν6, on the elliptic, ν2, and triangular, ν3, flows. Calculations are performed for lead-lead collisions at √s = 2.76 ATeV with centrality 10% ≤ σ/σgeo ≤ 50%. Hexagonal flow in the model emerges only due to the presence of ν2 and ν3. Position of its event plane, Ψ6, is found to be closer to Ψ3 for semi-central events, whereas in peripheral reactions it is closer to Ψ2, in line with the experimental analysis of the plane correlations. The amplitude of “induced" hexagonal flow is also comparable with that obtained in experiments. It means, in particular, that many features of the ν6 can be explained by interplay of elliptic and triangular flows.

Fluctuations of anisotropic flow in lead-lead collisions at LHC energies arising in HYDJET++model are studied. It is shown that intrinsic fluctuations of the flow which appear mainly because of the fluctuations of particle multiplicity, momenta and coordinates are insufficient to match the measured experimental data, provided the eccentricity of the freeze-out hypersurface is fixed at any given impact parameter b. However, when the variations of the eccentricity in HYDJET++ are taken into account, the agreement between the model results and the data is drastically improved. Both model calculations and the data are filtered through the unfolding procedure. This procedure eliminates the non-flow fluctuations to a higher degree, thus indicating a dynamical origin of the flow fluctuations in HYDJET++ event generator.

Recent results on heavy-ion physics at the LHC (CERN) by the Compact Muon Solenoid (CMS) collaboration are analyzed. The tests of quark-gluon plasma carried out with the CMS detector provide valuable information about the behavior of matter in the extreme regime of very high energy nucleus-nucleus collisions.

The jet shape modification due to partonic energy loss in the dense QCD matter is investigated with the help of the special transverse energy-energy correlator in the vicinity of maximum energy deposition of every event. In the accepted scenario with scattering of jet hard partons off comoving medium constituents this correlator is independent of the pseudorapidity position of a jet axis and becomes considerably broader (symmetrically over the pseudorapidity and the azimuthal angle) in comparison with $\mathit{pp}$-collisions. At scattering off ``slow'' medium constituents the broadening of correlation functions is dependent on the pseudorapidity position of a jet axis and increases noticeably in comparison with the previous scenario for jets with large enough pseudorapidities. These two considered scenarios result also in the different dependence of jet quenching on the pseudorapidity.

The CMS detector at the LHC is well suited for the study of quarkonium states J/ψ, ψ', ϒ, ϒ' and ϒ'', as well as Z0 boson production in heavy-ion collisions through their dimuon decays. The CMS detector has a large muon system with a trigger acceptance of ≈20% for ϒ and ≈1% for J/ψ. The precise tracker enables dimuon reconstruction with efficiencies above 90% in region |η| < 1.5 for dimuons from quarkonia decays, a dimuon mass resolution of 0.5% at the ϒ mass and background rejection. Results of quarkonium simulations are presented for several ion species with high and low background hypotheses. The expected dimuon invariant mass distributions are shown and the signal-to-background ratios, significance and the estimated statistics are also given. The CMS capability for detecting high-mass μ+μ− pairs from semileptonic decays and secondary J/ψ from single B decays (useful channels for studying medium-induced bottom-quark energy loss) is also discussed.

The recent developments on PYQUEN, HYDJET and HYDJET++ event generators are presented. The partonic energy loss model PYQUEN is implemented as the modification of the standard jet event obtained with the generator of hadron-hadron interactions PYTHIA. HYDJET and HYDJET++ are the Monte-Carlo event generators for the simulation of relativistic heavy ion AA collisions considered as a superposition of the soft, hydro-type state and the hard, multi-parton fragmentation. HYDJET++ model is the development and continuation of HYDJET generator, and it includes more detailed treatment of the hadronic state generated on the chemical and thermal freeze-out hypersurfaces (represented by the parameterization of relativistic hydrodynamics with preset freeze-out conditions), collective flow effects and decays of hadronic resonances. Some applications of above models to novel jet quenching observables are discussed.

Abstract It has been dozen years since HYDJET++ Monte-Carlo event generator for the simulation of relativistic heavy ion collisions was developed. Now the generator is widely used for the simulation of nucleus-nucleus interactions from NICA to LHC energies. The model calculations on soft and hard probes of quark-gluon plasma (including collective flow, different kinds of particle correlations, jets, D and B mesons etc.) agree well with the experimental data. In this paper the selected main results and the very new ones are presented.

The Monte Carlo event generator HYDJET++ is one of the few generators, designed for the calculations of heavy-ion collisions at ultrarelativistic energies, which combine treatment of soft hydro-like processes with the description of jets traversing the hot and dense partonic medium. The model is employed to study the azimuthal anisotropy phenomena, dihadron angular correlations and event-by-event (EbyE) fluctuations of the anisotropic flow in Pb+Pb collisions at ^/snn = 2.76 TeV. The interplay of soft and hard processes describes the violation of the mass hierarchy of meson and baryon elliptic and triangular flows at pT > 2 GeV/c, the fall-off of the flow harmonics at intermediate transverse momenta, and the worsening of the number-of-constituent-quark (NCQ) scaling of elliptic/triangular flow at LHC compared to RHIC energies. The cross-talk of v2 and v3 leads to emergence of higher order harmonics in the model and to appearance of the ridge structure in dihadron angular correlations in a broad pseudorapidity range. HYDJET++ possesses also the dynamical EbyE fluctuations of the anisotropic flow. The model results agree well with the experimental data.

Ultra-relativistic heavy-ion collisions at energies of RHIC and LHC are considered. For comparison with data the HYDJET++ model, which contains the treatment of both soft and hard processes, is employed. The study focuses mainly on the interplay of ideal hydrodynamics, final state interactions and jets, and its influence on the development of harmonics of the anisotropic flow. It is shown that jets are responsible for violation of the number-of-constituent-quark (NCQ) scaling at LHC energies. The interplay between elliptic and triangular flows and their contribution to higher flow harmonics and dihadron angular correlations, including ridge, is also discussed.

The event topology in relativistic heavy ion collisions is determined by various multi-particle production mechanisms. The simultaneous model treatment of different collective nuclear effects at high energies (such as a hard multi-parton fragmentation in hot QCD-matter, thermal resonance production, hydrodynamical flows, etc.) is actual but rather complicated task. We discuss the simulation of the above effects by means of Monte-Carlo model HYDJET++.

The heavy ion event generator HYDJET++ is presented. HYDJET++ simulates relativistic heavy ion AA collisions 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. The hard parts of HYDJET and HYDJET++ are identical. The soft part of HYDJET++ contains the following important additional features as compared with HYDJET: resonance decays and more detailed treatment of thermal and chemical freeze-out hypersurfaces. HYDJET++ is capable of reproducing the bulk properties of heavy ion collisions at RHIC (hadron spectra and ratios, radial and elliptic flow, femtoscopic momentum correlations), as well as high-pT hadron spectra. Some applications of HYDJET++ at LHC are discussed.

The model to simulate rescattering and partonic energy loss in ultrarelativistic heavy ion collisions is presented. The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets. This model is capable of reproducing main features of the jet quenching pattern at RHIC, and is applied to probe jet quenching in various novel channels at LHC.

Triangular flow v_3 of identified and inclusive particles in lead-lead collisions at sqrt{s_{NN}} = 2.76 TeV is studied as a function of centrality and transverse momentum within the HYDJET++ model. The model enables to investigate the influence of both hard processes and final-state interactions on the harmonics of particle anisotropic flow. Decays of resonances are found to increase the magnitude of the v_3(p_T)-distributions at p_T > 2 GeV/c and shift their maxima to higher transverse momenta. The p_T-integrated triangular flow, however, becomes slightly weakened for all centralities studied. The resonance decays also modify the spectra towards the number-of-constituent-quark (NCQ) scaling fulfillment for the triangular flow, whereas jets are the main source of the scaling violation at energies of Large Hadron Collider. Comparison with the corresponding spectra of elliptic flow reveals several similarities in v_3(p_T) and v_2(p_T) behavior. Obtained results are also confronted to the available experimental data.

An ultra-relativistic heavy-ion collision at LHC energies is a mixture of soft and hard processes. For comparison with data we employ the HYDJET++ model, which combines the description of soft processes with the treatment of hard partons propagating hot and dense nuclear medium. Importance of the interplay of ideal hydrodynamics, final state interactions and jets for the description of harmonics of the anisotropic flow is discussed. Jets are found to be the main source of violation of the number-of-constituent-quark (NCQ) scaling at LHC energies. Many features of higher flow harmonics and dihadron angular correlations, including ridge, can be described by the interference of elliptic and triangular flows.

We apply HYDJET++ model, which contains the treatment of both soft and hard processes, to study the heavy-ion collisions at LHC energies. The interplay of parametrised hydrodynamics and jets describes many features of the development of particle anisotropic flow including the break-up of mass hierarchy of elliptic and triangular flow, the falloff of the flow at certain transverse momentum and violation of the number-ofconstituent- quark (NCQ) scaling at LHC energies compared to the lower ones. Other signals, such as long-range dihadron correlations (ridge) and event-by-event (EbyE) fluctuations of the flow are also discussed. Model calculations demonstrate a good agreement with the available experimental data.

Ultra-relativistic heavy-ion collisions at energies of RHIC and LHC are considered. For comparison with data the HYDJET++ model, which contains the treatment of both soft and hard processes, is employed. The study focuses mainly on the interplay of ideal hydrodynamics, final state interactions and jets, and its influence on the development of harmonics of the anisotropic flow. It is shown that jets are responsible for violation of the number-of-constituent-quark (NCQ) scaling at LHC energies. The interplay between elliptic and triangular flows and their contribution to higher flow harmonics and dihadron angular correlations, including ridge, is also discussed.

The azimuthal anisotropy of charged particles in heavy ion collisions is an important probe of quark-gluon plasma evolution at early stages. The nuclear reaction plane can be determined independently by different detector subsystems and using different analysis methods. This paper reports the capability of the CMS detector at the LHC to reconstruct the reaction plane of the collision and to me asure elliptic flow with calorimetry and a tracking system. The analysis is based on a full CMS detector simulation of Pb + Pb events with the HYDJET event generator.

Abstract We present the capabilities of the CMS experiment to explore the heavy-ion physics program of the CERN Large Hadron Collider (LHC). Collisions of lead nuclei at energies up to s N N = 5.5 TeV will probe quark and gluon matter at unprecedented values of energy density. The prime goal of this research is to study the fundamental theory of the strong interaction (QCD) in extreme conditions of temperature, density and parton momentum fraction. We give an overview of the potential of CMS to carry out a full set of representative Pb-Pb measurements both in “soft” and “hard” regimes. Measurements include “bulk” observables — charged hadron multiplicity, low- p T inclusive identified hadron spectra and elliptic flow — which provide information on the collective properties of the system; as well as perturbative processes — such as quarkonia, heavy quarks, jets, γ–jet and high p T hadrons — which yield “tomographic” information on the hottest and densest phases of the reaction. In addition, reference measurements that have been performed on early p+p collision data will be reviewed.

The azimuthal anisotropy of charged particle emission in heavy ion collisions is a sensitive probe of the properties and the dynamical evolution of the produced matter. The systematic study of elliptic flow (v2), and higher order harmonics can yield information about the equilibration timescale, the nuclear equation of state and the viscosity in the different stages of the system evolution, as well as the initial state conditions of the heavy ion collisions. Experimentally, it is important to be able to reconstruct the flow using different methods, since they have different sensitivity to non-flow correlations and fluctuations which affect the extracted signal. In CMS, the nuclear reaction plane can be determined independently using several different detector subsystems. The reconstruction of elliptic flow using calorimetry and the tracking system is presented.

HYDJET++ model combines the description of soft processes with the treatment of hard partons propagating hot and dense nuclear medium. The model is employed for the analysis of PbPb collisions at LHC energies. The cross-talk of elliptic v2 and triangular v3 flow in the model generates both even and odd harmonics of higher order, as well as long-range azimuthal dihadron correlations (“ridge” structure). The simple modification of the model via introducing the distribution over spatial anisotropy parameters permits HYDJET++ to reproduce both v2 and v3 flow fluctuations and related to it eccentricity fluctuations of the initial state. The data on momentum spectra and elliptic flow of charmed mesons are reproduced by HYDJET++ including thermal and non-thermal charm production mechanisms. A tolerable agreement of HYDJET++ predictions with the data for hard probes (including such observables as high-pT hadron and jet nuclear modification factors, and specific modification of longitudinal and radial jet profiles) testifies in favour of successful treatment of the hard component based on PYQUEN event generator.

A model to simulate rescattering and partonic energy loss in ultrarelativistic heavy ion collisions is presented.The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets.This model is capable of reproducing main features of the jet quenching pattern at RHIC, and is applied to probe the effect of jet quenching in various novel channels at LHC.

One of the important perturbative ("hard") probes of hot and dense QCD matter is the medium-induced energy loss of energetic partons, so called "jet quenching", which is predicted to be very different in cold nuclear matter and in QGP, and leads to a number of phenomena which are already seen in the RHIC data on the qualitative level. The inclusion of jet quenching and other important collective effects, such as radial and elliptic flows, in the existing Monte-Carlo models of relativistic heavy ion collisions is discussed. Some issues on the corresponding physical observables at RHIC and LHC energies obtained with HYDJET++ model are presented.

The measurement of the charmonium and bottomonium resonances in nucleus-nucleus collisions provides crucial information on high-density QCD matter. First, the suppression of quarkonia production is generally agreed to be one of the most direct probes of quark-gluon plasma formation. The observation of anomalous J/ψ suppression at the CERN-SPS and at RHIC is well established but the clarification of some important remaining questions requires equivalent studies of the ϒ family, will be possible at the LHC. Second, the production of heavy-quarks proceeds mainly via gluon-gluon fusion processes and, as such, is sensitive to saturation of the gluon density at low-x in the nucleus. Measured departures from the expected vacuum quarkonia cross-sections in Pb+Pb collisions at the LHC will thus provide valuable information not only on the thermodynamical state of the produced partonic medium, but also on the initial-state modifications of the nuclear parton distribution functions. The capabilities of the LHC detectors (ALICE, ATLAS and CMS) to study quarkonia production in Pb+Pb collisions at 5.5 TeV per nucleon pair are discussed.

The azimuthal anisotropy of charged particles in heavy ion collisions is an important probe of quark-gluon plasma evolution at early stages. In the present paper the elliptic flow pattern in Pb+Pb collisions at sqrt{s}=5.5 TeV is analyzed for different hadron species in the frameworks of HYDJET++ Monte-Carlo model. The influence of resonance decays on particle flow is investigated. The different methods of elliptic flow reconstruction are compared under LHC conditions.

The recent developments on PYQUEN, HYDJET and HYDJET++ event generators are presented. The partonic energy loss model PYQUEN is implemented as the modification of the "standard" jet event obtained with the generator of hadron-hadron interactions PYTHIA. HYDJET and HYDJET++ are the Monte-Carlo event generators for the simulation of relativistic heavy ion AA collisions considered as a superposition of the soft, hydro-type state and the hard, multi-parton fragmentation. HYDJET++ model is the development and continuation of HYDJET generator, and it includes more detailed treatment of the "thermal" hadronic state generated on the chemical and thermal freeze-out hypersurfaces (represented by the parameterization of relativistic hydrodynamics with preset freeze-out conditions), collective flow effects and decays of hadronic resonances. Some applications of above models to novel jet quenching observables are discussed.

Abstract LHC data on the correlations of the elliptic flow v 2 of particles at low and high transverse momenta p T from Pb+Pb collisions at center-of-mass energy per nucleon pair <?CDATA $\sqrt{{s}_{{NN}}}=5.02$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msqrt> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="italic">NN</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>5.02</mml:mn> </mml:math> TeV are analyzed in the framework of the HYDJET++ model. This model includes soft and hard components which allows to describe the region of both low and high transverse momenta. The origin of v 2 values in different p T regions is investigated at different centralities. It is shown that the experimentally observed correlations between v 2 at low and high p T in peripheral lead-lead collisions is due to correlation of particles in jets.

We present a selection of recent results by the CMS collaboration on heavy- ion physics.

Data from the Large Hadron Collider on the charge balance function in Pb+Pb collisions at center-of-mass energy 2.76~TeV per nucleon pair are analyzed and interpreted within the framework of the \hydjet++ model. This model allows us to qualitatively reproduce the experimentally observed centrality dependence of the balance function widths at relatively low transverse momentum intervals due to the different charge creation mechanisms in soft and hard processes. However, a fully adequate description of the balance function in these intervals implies an essential modification of the model by including exact charge conservation via the canonical rather than the grand canonical ensemble. A procedure is proposed for introducing charge correlations into the thermal model without changing other model parameters. With increasing transverse momenta, the default model results describe the experimental data much better because the contribution of the soft component of the model is significantly reduced in these transverse momentum intervals. In practical terms, there is a transition to a single source of charge correlations, namely, charge correlations in jets in which exact charge conservation holds at each stage.

We study the influence of spatial and dynamical anisotropies in relativistic heavy-ion collisions on the differential elliptic and triangular flows of charged hadrons and, simultaneously, on the second- and third-order oscillations of the femtoscopic radii. Calculations of Pb + Pb collisions at s = 2.76 TeV were done within the HYDrodynamics with JETs (HYDJET++) event generator, which allows one to investigate the role of each of the anisotropy kinds separately. It is found that the bare geometric anisotropy provides either the wrong sign of elliptic and triangular flows or out-of-phase oscillations of R2out and R2side, respectively. Dynamical anisotropy is able to describe qualitatively both characteristics correctly. For the correct quantitative description of the data, one has to employ both spatial and dynamical anisotropies.

HYDJET++ model combines the description of soft processes with the treatment of hard partons propagating medium.The model is employed for the analysis of PbPb collisions at LHC energies, particularly, the azimuthal anisotropy phenomena, flow, femtoscopy, hard probes.The influence of geometric and dynamical anisotropies on the development of flow harmonics and, simultaneously, on the second-and third-order oscillations of femtoscopy radii were studied.The interplay of soft hydro-like processes and jets is able to describe the violation of the mass hierarchy of meson and baryon elliptic and triangular flows at transverse momentum p T > 2 GeV/c, the falloff of the anisotropic flow harmonics at intermediate transverse momenta, and the worsening of the number-of-constituent-quark (NCQ) scaling of elliptic/triangular flow at LHC compared to RHIC energies.The cross-talk of elliptic and triangular flows leads to emergence of higher order harmonics in the model and to appearance of ridge structure in dihadron angular correlations in a broad pseudorapidity range.Recently, the model was further extended to describe quantitatively the event-by-event fluctuations of the anisotropic flow.The model calculations agree well with the experimental data.

The present article gives a brief account of some interesting results obtained most recently in studying heavy-ion physics—in particular, quark–gluon matter—with the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC).