arXiv subjects

Thorsten Renk

Publications and source records attributed to Thorsten Renk.

At least 91 records · Page 5Linked to original sources

arXiv2007

Jet-quenching in a 3D hydrodynamic medium

We study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering approximation. The soft medium is described by a 3D hydrodynamical model and we treat the averaging over all possible parton paths through the medium without approximation. While the nuclear suppression factor $R_{AA}$ does not reflect the high quality of the medium description (except in a reduced systematic uncertainty in extracting the quenching power of the medium), the hydrodynamical model also allows to study different centralities and in particular the angular variation of $R_{AA}$ with respect to the reaction plane, allowing for a controlled variation of the in-medium path-length. We study the angular dependence of $R_{AA}$ for different centralities, discuss the influence of hydrodynamical expansion and flow and comment on the comparison with preliminary data.

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arXiv2007

What to learn from dilepton transverse momentum spectra in heavy-ion collisions?

Recently the NA60 collaboration has presented high precision measurements of dimuon spectra double differential in invariant mass $M$ and transverse pair momentum $p_T$ in In-In collisions at $158 {\rm AGeV}$. While the $M$-dependence is important for an understanding of in-medium changes of light vector mesons and is $p_T$ integrated insensitive to collective expansion, the $p_T$-dependence arises from an interplay between emission temperature and collective transverse flow. This fact can be exploited to derive constraints on the evolution model and in particular on the contributions of different phases of the evolution to dimuon radiation into a given $M$ window. We present arguments that a thermalized evolution phase with $T > 170 {\rm MeV}$ leaves its imprint on the spectra.

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arXiv2007

The rapidity structure of Mach cones and other large angle correlations in heavy-ion collisions

The pattern of angular correlations of hadrons with a (semi-)hard trigger hadron in heavy-ion collisions has attracted considerable interest. In particular, unexpected large angle structures on the away side (opposite to the trigger) have been found. Several explanations have been brought forward, among them Mach shockwaves and Cherenkov radiation. Most of these scenarios are characterized by radial symmetry around the parton axis, thus angular correlations also determine the rapidity dependence of the correlation. If the observed correlations are remnants of an away side parton after interaction with the medium created in the collision, pQCD allows to calculate the distribution $P(y)$ of the away side partons in rapidity. The measured correlation then arises as a folding of $P(y)$ and the rapidity structure of the correlation taking into account the detector acceptance. This places non-trivial and rather stringent constraints on the underlying scenario. We investigate these dependences and demonstrate that Mach shockwaves survive this folding procedure well whereas Cherenkov radiation scenarios face new challenges.

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arXiv2007

Prospects of Medium Tomography using 2-,3- and 4-Particle Correlations for a (semi-)hard Trigger

Hard partons propagating through hot and dense matter lose energy, leading to the observed depletion of hard hadron spectra in nucleus nucleus collision as compared to scaled proton proton collisions. This lost energy has to be redistributed in the medium due to the conservation of energy, which is manifest in the p_T dependence of the angular correlation pattern of hadrons associate with a (semi-) hard trigger. While at low p_T a splitting of a broad peak is observed, at high p_T the structure shows vacuum width, albeit with reduced yield. This sugests a transfer of energy from hard partons to a collectively recoiling medium. We present a systematic study of these phenomena using a realistic medium evolution and a Monte-Carlo simulation of the experimental trigger and show what information about the medium can be derived from multiparticle correlations.

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arXiv2007

Angular hadron correlations probing the early medium evolution

Hard processes are a well calibrated probe to study heavy-ion collisions. However, the information to be gained from the nuclear suppression factor R_AA is limited, hene one has to study more differential observables to do medium tomography. The angular correlations of hadrons associated with a hard trigger appear suitable as they show a rich pattern when going from low p_T to high p_T. Of prime interest is the fate of away side partons with an in-medium pathlength O(several fm). At high p_T the correlations become dominated by the punchtrough of the away side parton with subsequent fragmentation. We discuss what information about the medium density can be gained from the data.

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arXiv2006

Dimuon transverse momentum spectra as a tool to characterize the emission region in heavy-ion collisions

Previous dilepton measurements in heavy-ion collisions have mainly focused on invariant mass spectra to clarify in-medium changes of vector meson properties. However, a dimuon is characterized by two scales -- invariant mass $M$ and transverse momentum $p_T$. Like transverse momentum spectra of hadrons, $p_T$ spectra of dileptons arise from an interplay between emission temperature and collective transverse flow, whereas the invariant mass is insensitive to flow. Having two control parameters of which only one is sensitive to flow allows at given $M$ to characterize the emission region in terms of average temperature and flow. Thus, one is able to study what phases of the fireball evolution radiate into a given mass window. We demonstrate this technique using the dimuon transverse momentum spectra measured by the NA60 collaboration and present strong arguments that a thermalized evolution phase with $T > 170$ MeV leaves an imprint in the spectra.

hep-ph↗

arXiv2006

Reply to 'Comment on: 'Non-perturbative finite T broadening of the rho meson and dilepton emission in heavy ion-collisions''

Nota bene: the numerical calculation underlying Version 1 of this reply and the original work Phys.Rev. C71:064903,2005 contains a basic numerical error (wrong factor in the self-energy formulas of Phys. Rev.C71:064903,2005). This renders the numerical results presented there and in Version 1 of the reply invalid and enforces a careful reinvestigation of Phys. Rev.C71:064903,2005 and the reply. Calculations for an erratum of Phys. Rev. C71:064903,2005 are in progress.

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arXiv2006

What does the rho-meson do? In-medium mass shift scenarios versus hadronic model calculations

The NA60 experiment has studied low-mass muon pair production in In-In collisions at $158 {\rm AGeV}$ with unprecedented precision. With these results there is hope that the in-medium modifications of the vector meson spectral function can be constrained more thoroughly than before. We investigate in particular what can be learned about collisional broadening by a hot and dense medium and what constrains the experimental results put on in-medium mass shift scenarios. The data show a clear indication of considerable in-medium broadening effects but disfavor mass shift scenarios where the $ρ$-meson mass scales with the square root of the chiral condensate. Scaling scenarios which predict at finite density a dropping of the $ρ$-meson mass that is stronger than that of the quark condensate are clearly ruled out since they are also accompanied by a sharpening of the spectral function.

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arXiv2006

Mach cones and dijets - jet quenching and fireball expansion dynamics

The energy loss of a hard parton is regarded as a useful tool to probe properties of a soft medium. However, the information obtained by a measurement of the nuclear suppression R_AA is not very sensitive to medium properties. Thus, more differential observables are needed. Here we focus on the pattern of angular correlation of hadrons associated with a hard trigger. These show rich pattern when going from low p_T to high p_T associate momentum. At low p_T, the pattern of associate hadrons can be understood in terms of Mach shocks excited by the energy lost from the away side parton while the parton itself is absorbed by the medium. At high p_T, fluid dynamics is not applicable any more and the correlations become dominated by the punchtrough of the away side parton with subsequent fragmentation. We present an analysis of these phenomena taking into account the full expansion dynamics of soft matter and show how both transverse and longitudinal expansion play a crucial role in understanding the data.

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arXiv2006

Towards jet tomography: gamma-hadron correlations

Hard pQCD processes taking place in ultrarelativistic heavy-ion collisions are a well-calibrated probe. It is believed that the interaction with the surrounding medium of outgoing partons from a hard vertex is capable of revealing details of the medium. We demonstrate that correlation measurements of hard photon-hadron back-to-back coincidences are a tool suitable to extract such tomographic information. Introducing the concept of averaged evergy loss probability distributions, we first argue that almost no information about details of the medium evolution is reflected in the nuclear suppression factor R_AA. Thus, a wide variety of jet quenching scenarios and geometries are compatible with the measured data. This problem can be overcome by a gamma-hadron correlation measurement. We show that averaged probability distributions for quarks are accessible experimentally and sketch an analysis procedure capable of distinguishing different energy loss scenarios leading to the same nuclear suppression factor.

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arXiv2006

Through the blackness -- high p_T hadrons probing the central region of 200 AGeV Au-Au collisions

The energy loss of high p_T partons propagating through a hot and dense medium is regarded as a valuable tool to probe the medium created in ultrarelativistic heavy-ion collisions. The angular correlation pattern of hadrons associated with a hard trigger in the region of p_T ~1-2 GeV which exhibits a dip in the expected position of the away side jet has given rise to the idea that energy is lost predominantly to propagating collective modes ('Mach cones'). Recent measurements by the STAR collaboration have shown that for a high p_T > 8 GeV trigger the angular pattern of associate hadrons for p_T > 4 GeV shows the emergence of the expected away side peak. These di-jet events suggest that the away side parton may emerge occasionally without substantial energy loss. Since in such a back-to-back configuration one of the partons may travel through the central region of the fireball, the average in-medium pathlength is substantial and the expected energy loss is not only sensitive to the initial geometry of matter but also to the change of geometry due to expansion. We show that radiative energy loss is able to explain the dijet events provided that the expansion of the medium is taken into account.

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arXiv2006

Signatures of Mach Shocks at RHIC

Hard partons propagating through hot and dense matter loose energy, leading to the observed depletion of hard hadron spectra in nucleus nucleus collision as compared to scaled proton proton collisions. This lost energy has to reappear in the medium due to the conservation of energy. Apart from heating the medium, there is the possibility that a propagating collective mode is excited. We outline a formalism that can be used to track the propagation of such a mode through the evolving medium if its dispersion relation is known and to calculate the resulting distortion of hadronic spectra at freeze-out. Using this formalism, we demonstrate within a detailed picture of the evolution of the expanding system and the experimental trigger conditions that the assumption of a sound mode being excited is in line with 2-particle correlation measurements by PHENIX and STAR for a semi-hard trigger. In this case, the measurement is sensitive to the averaged speed of sound in the hot matter. We also make suggestions how this sensitivity can be improved.

hep-ph↗

arXiv2006

From In-In to Pb-Pb collisions: Lessons from high-precision Dilepton Measurements

The NA60 experiment has studied low mass muon pair production in In-In collisions at 158 AGeV with unprecedented precision. With these measurements one can constrain the in-medium modifications of hadrons embedded in nuclear matter more thoroughly than from previous experiments. In the present paper we argue that these constraints could be significantly further tightened and theoretical uncertainties in modeling of the medium evolution reduced by an additional experiment measuring low mass di-muon production in Pb-Pb collisions at 158 AGeV with the same resolution as provided by NA60 for the In-In system.

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arXiv2006

What the NA60 dilepton data can tell

The NA60 experiment has studied low-mass muon pair production in In-In collisions at 158 AGeV with unprecedented precision. Thus, there is hope that the in-medium modifications of the vector meson spectral function can be constrained more thoroughly than from previous experiments. Towards this goal, we present a study comparing different models for the in-medium modification of vector mesons. In particular, we investigate what can be learned about collisional broadening by a medium at finite temperature and baryon density and what model independent constraints can be obtained from the data. Of special relevance is a comparison to $p_T$-cuts of the dilepton spectra. Those not only provide independent tests of the evolution model and $p_T$-dependence of the spectral function but also unambiguously decide if the observed yield in the higher mass spectrum $M > 1 {\rm GeV}$ is mainly due to the hot phase of the evolution or four pion annihilation processes.

hep-ph↗

arXiv2005

Mass and Width of the Rho Meson in a Nuclear Medium from Brown-Rho Scaling and QCD Sum Rules

We explore the range of values of the in-medium width of a $ρ$-meson at rest which is compatibale with the QCD sum rule approach in a nuclear medium assuming vector meson dominance and a Brown-Rho scaling law of the $ρ$-meson mass with the chiral condensate. The lower and upper bounds for the in-medium width are found to be strongly increasing with the decreasing mass of the $ρ$-meson (increasing nuclear density). We also study the bounds for the in-medium width in models not satisfying the Brown-Rho scaling law. It is shown that the in-medium width depends on how rapidly the mass decreases in comparison to the change of the quark condensate. The bounds for the in-medium width increase with density only if the relative change of the quark condensate is stronger than the relative decrease in mass. This is important for experimental tests of the Brown-Rho scaling paradigm and other dropping $ρ$-mass scenarios.

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arXiv2005

Mach cones in an evolving medium

The energy and momentum lost by a hard parton propagating through hot and dense matter has to be redistributed in the nuclear medium. Apart from heating the medium, there is the possibility that collective modes are excited. We outline a formalism that can be used to track the propagation of such a mode through the evolving medium if its dispersion relation is known. Under the assumption that a sound wave is created, we track the jet energy loss as a function of spacetime and follow the resulting mach cone throughout the fireball evolution.We compare with the angular correlation pattern of hard hadrons as obtained by the PHENIX collaboration and find good agreement with the data provided that a substantial fraction of jet energy ($\sim$ 90%) is deposited into a propagating mode and that the hot matter can be characterized by an EOS with a soft point (not necessarily a mixed phase).

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arXiv2005

Dynamics of the Landau-Pomeranchuk-Migdal Effect in Au+Au Collisions at 200 AGeV

We study the role played by the Landau-Pomeranchuk-Midgal (LPM) effect in relativistic collisions of hadrons and heavy nuclei, within a parton cascade model. We find that the LPM effect strongly affects the gluon multiplication due to radiation and considerably alters the space-time evolution of the dynamics of the collision. It ensures a multiplicity distribution of hadrons in aggreement with the experimental proton-proton data. We study the production of single photons in relativistic heavy ion collisions and find that the inclusion of LPM suppression leads to a reduction in the single photon yield at small and intermediate transverse momenta. The parton cascade calculation of the single photon yield including the LPM effect is shown to be in good agreement with the recent PHENIX data taken at the Relativistic Heavy-Ion Collider.

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arXiv2005

The influence of flow on the jet quenching power in heavy-ion collisions

The flow pattern and evolution of the medium created in ultrarelativistic heavy ion collisions can have significant influence on the energy loss of hard partons traversing the medium. We demonstrate that within a range of assumptions for longitudinal and transverse flow which are all compatible with the measured hadronic single particle distributions, the quenching power of the medium can vary within almost a factor five. Thus, the choice of the medium evolution is one of the biggest uncertainties in jet quenching calculations and needs to be addressed with some care.

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