arXiv subjects

Carsten Greiner

Publications and source records attributed to Carsten Greiner.

At least 91 records · Page 5Linked to original sources

arXiv2014

Momentum imbalance of D mesons in ultra-relativistic heavy-ion collisions at LHC

As a new observable for heavy flavor correlations the momentum imbalance $A_D$ of D mesons is proposed. It is defined analogously to the jet momentum imbalance $A_J$ of fully reconstructed jets. However, since D mesons are flavor tagged particles, no jet reconstruction is necessary. $A_D$ quantifies the influence of the medium created in heavy-ion collisions on correlated charm pairs. We present results with the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS), which describes well the nuclear modification factor and elliptic flow of all heavy flavor particles at RHIC and LHC. The $A_D$ distribution in heavy-ion collisions at LHC is shifted to larger values of $A_D$ compared to proton-proton collisions. We argue that this shift is due to medium effects and can be explained partially by a path length imbalance of charm pairs and partially by momentum fluctuations in the initial charm pair distribution.

hep-ph↗

arXiv2014

Suppression of the LHC $p/π$ ratio due to the QCD mass spectrum

Recent measurements of the proton to pion ratio at $\sqrt{s}_{NN}=2.7$ TeV by the ALICE collaboration at the LHC have found it to be lower than predictions from thermal fits. In this paper we investigate the role that the extended mass spectrum via Hagedorn states- massive resonances that follow an exponential mass spectrum and possess large decay widths- play in the determination of particle ratios at LHC through a scenario of multi-particle reactions and dynamical chemical equilibrium within the hadron gas phase. We show that it is possible to describe the lower $p/π$ ratio at LHC while still obtaining the experimental ratio of $K/π$ and $Λ/π^+$ in the Hagedorn state scenario if the protons are underpopulated at the switching temperature from hydrodynamics to the hadron gas phase.

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arXiv2014

Hadronic vs. partonic J/psi production in the Statistical Hadronization Model

Assuming the Statistical Hadronization Model and charm flavour conservation, one can quantify the deviation of charm and anti-charm quarks from chemical equilibrium both in the hadronic phase and in the quark-gluon plasma. By linking the hadronic fireball volume and the corresponding plasma source volume via entropy conservation as proposed by Grandchamp et al., a direct comparison between the $J/ψ$ yields in both pictures can be used to obtain limits for the charm quark mass in the medium.

hep-ph↗

arXiv2014

Elliptic Flow Suppression due to Hadron Mass Spectrum

Hadron resonance gas models provide a good description of the equation of state of quantum chromodynamics determined by lattice QCD calculations at temperatures $T \sim 100-155$ MeV. In this paper we investigate the effects of an exponentially increasing hadron mass spectrum (Hagedorn spectrum) on the azimuthal anisotropy of the rapidly expanding matter formed in ultrarelativistic heavy ion collisions. If the temperature at which the conversion from fluid degrees of freedom to hadrons is sufficiently close to the Hagedorn temperature, the production of Hagedorn resonances suppresses the differential elliptic flow of all hadron species.

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arXiv2014

Shear viscosity of an ultrarelativistic Boltzmann gas with isotropic inelastic scattering processes

We derive an analytic expression for the shear viscosity of an ultra-relativistic gas in presence of both elastic $2\to 2$ and inelastic $2\leftrightarrow 3$ processes with isotropic differential cross sections. The derivation is based on the entropy principle and Grad's approximation for the off-equilibrium distribution function. The obtained formula relates the shear viscosity coefficient $η$ to the total cross sections $σ_{22}$ and $σ_{23}$ of the elastic resp. inelastic processes. The values of shear viscosity extracted using the Green-Kubo formula from kinetic transport calculations are shown to be in excellent agreement with the analytic results which demonstrates the validity of the derived formula.

hep-th↗

arXiv2014

Spin Asymmetry of J/psi in Peripheral Pb+Pb Collisions at LHC

By generalizing the statistical model for particle production to the spin degree of freedom of initially produced J/psi, we study the spin projection J_y of J/psi perpendicular to the reaction plane in peripheral heavy ion collisions at the LHC energy that leads to a strong, albeit of short duration, magnetic field. We find that for J/psis produced directly from charm and anticharm quarks in the color singlet state, like that in the Color-Singlet Model, their yield in the presence of the magnetic field is larger for J_y=0 than for J_y=1 or -1. This leads to a spin asymmetry of finally produced J/ψeven after including their final-state scattering in the produced quark-gluon plasma.

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arXiv2014

Elliptic flow and nuclear modification factor in ultra-relativistic heavy-ion collisions within a partonic transport model

The quark gluon plasma produced in ultra-relativistic heavy-ion collisions exhibits remarkable features. It behaves like a nearly perfect liquid with a small shear viscosity to entropy density ratio and leads to the quenching of highly energetic particles. We show that both effects can be understood for the first time within one common framework. Employing the parton cascade Boltzmann Approach to Multi-Parton Scatterings (BAMPS), the microscopic interactions and the space-time evolution of the quark gluon plasma are calculated by solving the relativistic Boltzmann equation. Based on cross sections obtained from perturbative QCD with explicitly taking the running coupling into account, we calculate the nuclear modification factor and elliptic flow in ultra-relativistic heavy-ion collisions. With only one single parameter associated with coherence effects of medium-induced gluon radiation, the experimental data of both observables can be understood on a microscopic level. Furthermore, we show that perturbative QCD interactions with a running coupling lead to a sufficiently small shear viscosity to entropy density ratio of the quark gluon plasma, which provides a microscopic explanation for the observations stated by hydrodynamic calculations.

hep-ph↗

arXiv2013

Nonequilibrium dynamics and transport near the chiral phase transition of a quark-meson model

Based on the 2PI quantum effective action of the linear sigma model with constituent quarks, we develop a transport approach to study systems out of equilibrium. In particular, we focus on the chiral phase transition as well as the critical point, where nonequilibrium effects near the phase transition give rise to critical behavior such as the fluctuation of the baryon number density. Predictions for long-range correlations and fluctuations of observables in our model could be used to study fundamental properties of the QCD phase transition. In the previous version of our transport model the chiral fields are implemented as mean fields, whereas quarks are treated as on-shell particles in the Vlasov equation with a dynamical force term. The current update includes also the distribution functions of sigma mesons and pions in a self-consistent way. On this basis a dissipation kernel between the mean fields and particle modes can be implemented.

hep-ph↗

arXiv2013

Asymptotic description of finite lifetime effects on the photon emission from a quark-gluon plasma

Direct photons play an important role as electromagnetic probes from the quark-gluon plasma (QGP) which occurs during ultrarelativistic heavy-ion collisions. In this context, it is of particular interest how the finite lifetime of the QGP affects the resulting photon production. Earlier investigations on this question were accompanied by a divergent contribution from the vacuum polarization and by the remaining contributions not being integrable in the ultraviolet (UV) domain. In this work, we provide a different approach in which we do not consider the photon number density at finite times, but for free asymptotic states obtained by switching the electromagnetic interaction according to the Gell-Mann and Low theorem. This procedure eliminates a possible unphysical contribution from the vacuum polarization and, moreover, renders the photon number density UV integrable. It is emphasized that the consideration of free asymptotic states is, indeed, crucial to obtain such physically reasonable results.

hep-ph↗

arXiv2013

Heavy vs. light flavor energy loss within a partonic transport model

The full space-time evolution of gluons, light and heavy quarks in ultra-relativistic heavy-ion collisions is studied within the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). We discuss for light and heavy quarks the elastic and radiative energy loss with a running coupling. Radiative processes, in particular, are implemented through an improved version of the Gunion-Bertsch matrix element, which is derived from comparisons to the exact result, explicitly taking finite heavy quark masses into account. Subsequently, we present results with and without radiative processes and compare them to experimental data at LHC. A focus is put on the nuclear modification factor of charged hadrons and D mesons.

hep-ph↗

arXiv2013

Radiative parton processes in perturbative QCD - an improved version of the Gunion and Bertsch cross section from comparisons to the exact result

In this work we compare the Gunion-Bertsch approximation of the leading order perturbative QCD radiation matrix element to the exact result. To this end, we revisit the derivation of the Gunion-Bertsch approximation as well as perform extensive numerical comparisons of the Gunion-Bertsch and the exact result. We find that when employing the matrix elements to obtain rates or cross sections from phase space integration, the amplitude by Gunion and Bertsch deviates from the correct result in characteristic regions of the phase space. We propose an improved version of the Gunion-Bertsch matrix element which agrees very well with the exact result in all phase space regions.

hep-ph↗

arXiv2013

Non-equilibrium photon production arising from the chiral mass shift

We investigate the photon emission arising from the shift of the quark/antiquark masses during the chiral phase transition in the early stage of ultrarelativistic heavy-ion collisions. As this mass shift leads to spontaneous creation of quark-antiquark pairs and thus contributes to the formation of the quark-gluon plasma, our investigations are relevant in the context of finite-lifetime effects on the photon emission from the latter. Earlier investigations on this topic by Boyanovsky et al. were plagued by a UV divergent contribution from the vacuum polarization and the non-integrability of the remaining contributions in the ultraviolet domain. In contrast to these investigations, we do not consider the photon number-density at finite times but for free asymptotic states obtained by an adiabatic switching of the electromagnetic interaction according to the Gell-Mann and Low theorem. This approach eliminates possible unphysical contributions from the vacuum polarization and renders the resulting photon spectra integrable in the ultraviolet domain. It is emphasized that the consideration of free asymptotic states is indeed crucial to obtain such physically reasonable results.

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arXiv2013

Collisional energy loss of heavy quarks

We develop a transport approach for heavy quarks in a quark-gluon plasma, which is based on improved binary collision rates taking into account quantum statistics, the running of the QCD coupling and an effective screening mass adjusted to hard-thermal loop calculations. We quantify the effects of in-medium collisions by calculating the heavy flavor nuclear modification factor and the elliptic flow for RHIC energies, which are comparable to radiative effects. We also derive an analytic formula for the mean collisional energy loss of an energetic heavy quark in a streaming quark gluon plasma.

hep-ph↗

arXiv2013

B_c meson enhancement and the momentum dependence in Pb+Pb collisions at LHC energy

B_c meson production in Pb+Pb collisions at sqrt{s}=2.76 A TeV is surveyed in both a statistical coalescence model and a transport model. The nuclear modification factor R_{AA} is predicted to be between 2 and 18 in the most central collisions, which can help to confirm the regeneration mechanism. In addition, the momentum dependence is also investigated as given by the transport model. A strong suppression of the transverse momentum is found in central collisions accompanying the enhancement in yield. The spectrum and elliptic flow of B_c are also discussed.

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arXiv2012

Open heavy flavor and J/psi at RHIC and LHC within a transport model

The production and space-time evolution of heavy quarks and J/psi in the quark gluon plasma is studied within the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). This framework allows interactions among all partons: gluons, light quarks and heavy quarks. Heavy quarks, in particular, interact with the rest of the medium via binary scatterings with a running coupling and a more precise Debye screening which is derived from hard thermal loop calculations. We compare our results of the elliptic flow and nuclear modification factor not only to experimental data of heavy flavor electrons at RHIC, but also to LHC data of heavy flavor electrons, muons, D mesons, and non-prompt J/psi. Where no data is available yet, we make predictions for those observables. Furthermore, results on prompt J/psi elliptic flow are reported for RHIC energy within the same framework, taking the dissociation as well as regeneration of J/psi in the quark-gluon plasma into account.

hep-ph↗

arXiv2012

Open heavy flavor in Pb+Pb collisions at sqrt(s)=2.76 TeV within a transport model

The space-time evolution of open heavy flavor is studied in Pb+Pb collisions at sqrt(s)=2.76 TeV using the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). An updated version of BAMPS is presented which allows interactions among all partons: gluons, light quarks, and heavy quarks. Heavy quarks, in particular, interact with the rest of the medium via binary scatterings with a running coupling and a Debye screening which is matched by comparing to hard thermal loop calculations. The lack of radiative processes in the heavy flavor sector is accounted for by scaling the binary cross section with a phenomenological factor K=3.5, which describes well the elliptic flow v_2 and nuclear modification factor R_AA at RHIC. Within this framework we calculate in a comprehensive study the v_2 and R_AA of all interesting open heavy flavor particles at the LHC: electrons, muons, D mesons, and non-prompt J/psi from B mesons. We compare to experimental data, where it is already available, or make predictions. To do this accurately next-to-leading order initial heavy quark distributions are employed which agree well with proton-proton data of heavy flavor at sqrt(s)=7 TeV.

hep-ph↗

arXiv2012

Open heavy flavor and J/psi at RHIC and LHC

The suppression and collective flow of open and hidden heavy flavor in ultra-relativistic heavy ion collisions is studied with the partonic transport model Boltzmann Approach to MultiParton Scatterings (BAMPS). Charm and bottom quarks interact with the rest of the medium via elastic scatterings with an improved Debye screening and a running coupling. Heavy flavor electron data from RHIC can only be described if the cross section is multiplied with K=3.5 to mimic the contribution from radiative processes. With the same value for K we compare to elliptic flow data of D mesons at the LHC and find a good agreement. However, the nuclear modification factor of D mesons, non-prompt J/psi, muons, and electrons at the LHC is underestimated in our calculations. In addition, we present first BAMPS results on the suppression and elliptic flow of J/psi mesons, which are treated in the same framework.

hep-ph↗

arXiv2012

Off-equilibrium photon production during the chiral phase transition

In the early stage of ultrarelativistic heavy-ion collisions chiral symmetry is restored temporarily. During this so-called chiral phase transition, the quark masses change from their constituent to their bare values. This mass shift leads to the spontaneous non-perturbative creation of quark-antiquark pairs, which effectively contributes to the formation of the quark-gluon plasma. We investigate the photon production induced by this creation process. We provide an approach that eliminates possible unphysical contributions from the vacuum polarization and renders the resulting photon spectra integrable inthe ultraviolet domain. The off-equilibrium photon numbers are of quadratic order in the perturbative coupling constants while a thermal production is only of quartic order. Quantitatively, we find, however, that for the most physical mass-shift scenarios and for photon momenta larger than 1 GeV the off-equilibrium processes contribute less photons than the thermal processes.

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