arXiv subjects

Thorsten Renk

Publications and source records attributed to Thorsten Renk.

At least 73 records · Page 4Linked to original sources

arXiv2009

Medium-modified Jets in Heavy-Ion Collisions

The suppression of single inclusive hadron spectra in heavy-ion collision as compared to the scaled expectation from proton-proton collisions has long been regarded as an interesting tool to study properties of the bulk matter in heavy-ion collisions. However, the limitations of this class of observables has become increasingly obvious, and both experimental and theoretical efforts are now made to go beyond single hadrons to fully reconstructed jets. Monte-Carlo simulations of in-medium parton showers are currently considered the most promising tool to theoretically access jet physics in heavy-ion collisions. In this paper, I review some of the first results obtained with the MC code YaJEM (Yet another Jet Energy-loss Model) for both single hadron and jet observables.

hep-ph↗

arXiv2009

A comparison study of medium-modified QCD shower evolution scenarios

The computation of hard processes in hadronic collisions is a major success of perturbative Quantum Chromodynamics (pQCD). In such processes, pQCD not only predicts the hard reaction itself, but also the subsequent evolution in terms of parton branching and radiation, leading to a parton shower and ultimately to an observable jet of hadrons. If the hard process occurs in a heavy-ion collision, a large part of this evolution takes place in the soft medium created along with the hard reaction. An observation of jets in heavy-ion collision thus allows a study of medium-modified QCD shower evolution. In vacuum, Monte-Carlo (MC) simulations are well established tools to describe such showers. For jet studies in heavy-ion collisions, MC models for in-medium showers are currently being developed. However, the shower-medium interaction depends on the nature of the microscopic degrees of freedom of the medium created in a heavy-ion collision which is the very object one would like to investigate. This paper presents a study in comparison between three different possible implementations for the shower-medium interaction, two of them based on medium-induced pQCD radiation, one of them a medium-induced drag force, and shows for which observables differences between the three scenarios become visible. We find that while single hadron observables such as R_AA are incapable of differentiating between the scenarios, jet observables such as the longiudinal momentum spectrum of hadrons in the jet show the potential to do so.

hep-ph↗

arXiv2009

Systematic Comparison of Jet Energy-Loss Schemes in a realistic hydrodynamic medium

We perform a systematic comparison of three different jet energy-loss approaches. These include the Armesto-Salgado-Wiedemann scheme based on the approach of Baier-Dokshitzer-Mueller-Peigne-Schiff and Zakharov (BDMPS-Z/ASW), the Higher Twist approach (HT) and a scheme based on the approach of Arnold-Moore-Yaffe (AMY). In this comparison, an identical medium evolution will be utilized for all three approaches: not only does this entail the use of the same realistic three-dimensional relativistic fluid dynamics (RFD) simulation, but also includes the use of identical initial parton-distribution functions and final fragmentation functions. We are, thus, in a unique position, not only to isolate fundamental differences between the various approaches, but also to make rigorous calculations for different experimental measurements using "state of the art" components. All three approaches are reduced to a version which contains only one free tunable parameter, this is then related to the well known transport parameter $\hat{q}$. We find that the parameters of all three calculations can be adjusted to provide a good description of inclusive data on $R_{AA}$ versus transverse momentum. However, we do observe slight differences in their predictions for the centrality and azimuthal angular dependence of $R_{AA}$ vs. $p_T$. We also note that the value of the transport coefficient $\hat{q}$ in the three approaches to describe the data differs significantly.

nucl-th↗

arXiv2008

Jet modification in 200 AGeV Au-Au collisions

The computation of hard processes in hadronic collisions is a major success of perturbative Quantum Chromodynamics. The environment of heavy-ion collisions offers the opportunity to embed such hard processes into a soft medium which is created simultaneously and study the medium-induced modifications. On the level of single high transverse momentum hadrons, a reduction in yield, the so-called quenching is observed. However, on the level of jets, the energy-momentum flux carried by hadrons is conserved, i.e. the effect of the medium is a redistribution of energy and momentum, and statements about quenching of jets can only be made for specific cuts used to identify the jet. In this paper, we present a baseline computation for the expected modification of the longitudinal momentum distribution in jets by the medium created in 200 AGeV Au-Au collisions given a realistic set of experimental cuts used to identify jets in a heavy-iron environment. For this purpose, we use a Monte-Carlo code developed to simulate in-medium shower evolution following a hard process and a 3-d hydrodynamical evolution to simulate the soft medium.

hep-ph↗

arXiv2008

Angular variation of hard back-to-back hadron suppression in heavy-ion collisions

The basic idea of jet tomography is to infer information about the density evolution of the medium created in heavy-ion (A-A) collisions by studying the suppression of hard probes in an A-A environment as compared to the baseline process known from p-p collisions. The suppression of back-to-back correlations in heavy-ion collisions allows, due to a different geometrical bias, a view into the medium which is qualitatively different from the one offered by single hadron suppression. A control parameter for the suppression corresponding to a systematic variation of in-medium pathlengths and density can be obtained by studying collisions at finite impact parameter b. A systematic variation of pathlength can then be introduced by studying the suppression pattern as a function of the angle phi with the reaction plane. Using a 3-d hydrodynamical evolution model for the medium and a Monte-Carlo model which has been shown to successfully reproduce the measured suppression of back-to-back correlations in central collisions of Au-Au at 200 AGeV, we compute the suppression as a function of phi for b of 2.4 fm. 4.5 fm, 6.3 fm and 7.5 fm. Given that this involves variations in both control parameters b and phi a comparison with data should eventually allow to place strong constraints on the combination of energy loss model and medium evolution model.

hep-ph↗

arXiv2008

Parton shower evolution in a 3-d hydrodynamical medium

We present a Monte Carlo simulation of the perturbative Quantum Chromodynamics (pQCD) shower developing after a hard process embedded in a heavy-ion collision. The main assumption is that the cascade of branching partons traverses a medium which (consistent with standard radiative energy loss pictures) is characterized by a local transport coefficient qhat which measures the virtuality per unit length transferred to a parton which propagates in this medium. This increase in parton virtuality alters the development of the shower and in essence leads to extra induced radiation and hence a softening of the momentum distribution in the shower. After hadronization, this leads to the concept of a medium-modified fragmentation function. On the level of observables, this is manifest as the suppression of high transverse momentum (PT) hadron spectra. We simulate the soft medium created in heavy-ion collisions by a 3-d hydrodynamical evolution and average the medium-modified fragmentation function over this evolution in order to compare with data on single inclusive hadron suppression and extract the qhat which characterizes the medium. Finally, we discuss possible uncertainties of the model formulation and argue that the data in a soft momentum show evidence of qualitatively different physics which presumably cannot be described by a medium-modified parton shower.

hep-ph↗

arXiv2008

Energy deposition in hard dihadron triggered events in heavy-ion collisions

The experimental observation of hadrons correlated back-to-back with a (semi-)hard trigger in heavy ion collisions has revealed a splitting of the away side correlation structure in a low to intermediate transverse momentum (P_T) regime. This is consistent with the assumption that energy deposited by the away side parton into the bulk medium produced in the collision excites a sonic shockwave (a Mach cone) which leads to away side correlation strength at large angles. A prediction following from assuming such a hydrodynamical origin of the correlation structure is that there is a sizeable elongation of the shockwave in rapidity due to the longitudinal expansion of the bulk medium. Using a single hadron trigger, this cannot be observed due to the unconstrained rapidity of the away side parton. Using a dihadron trigger, the rapidity of the away side parton can be substantially constrained and the longitudinal structure of the away side correlation becomes accessible. However, in such events several effects occur which change the correlation structure substantially: There is not only a sizeable contribution due to the fragmentation of the emerging away side parton, but also a systematic bias towards small energy deposition into the medium and hence a weak shockwave. In this paper, both effects are addressed.

hep-ph↗

arXiv2008

Expectations for dihadron correlation measurements at the LHC

The suppression of high transverse momentum (P_T) inclusive hadron spectra in heavy-ion collisions as compared to the scaled expectation of high P_T hadron production in p-p collisions is the most direct manifestation of the interaction of hard partons with the soft bulk medium produced in heavy-ion collisions which is absent in p-p collisions. Yet the measured nuclear suppression factor R_AA is a very averaged quantity and hence only a limited amount of information about the medium evolution and the nature of the interaction with the medium can be deduced from R_AA. Measurements of hard back-to-back hadron correlations in 200 AGeV Au-Au collisions at RHIC have opened a new window to study the energy loss of partons in a medium in a more differential way and for a different distribution of in-medium pathlengths than in the case of R_AA. In this work, we present an extrapolation of our results for back-to-back yields at RHIC energies to 5.5 TeV Pb-Pb collisions at the CERN LHC. We also discuss differences and similarities between the measurement at RHIC.

hep-ph↗

arXiv2007

Can one extract energy loss probability distributions from R_AA?

The nuclear suppression of high transverse momentum P_T hadrons is one of the most striking findings in heavy-ion collision experiments. It has long been recognized that the suppression can be theoretically described by folding the primary parton spectrum with an energy loss probability distribution which is suitably averaged over the collision geometry. However, an interesting problem is to what degree the procedure can be inverted, i.e. given a measurement of the suppression factor R_AA with arbitrary precision, can the probability distribution of energy loss be extracted in a model-independent way? In this note, we present a conceptual study of the inversion problem for LHC energies and demonstrate that a measurement of R_AA alone is insufficient to determine the distribution, other observables such as gamma-hadron correlations must be taken into account.

hep-ph↗

arXiv2007

Mach cone shock waves at RHIC

Energy and momentum lost by hard jets propagating through hot and dense nuclear matter have to be redistributed in the medium. It has been conjectured that collective sound modes are excited. Those lead to Mach cone nuclear shock waves in the nuclear medium that are shown to account for three and four particle angular correlation structures of hadrons with a (semi-)hard trigger hadron in heavy-ion collisions at RHIC.

hep-ph↗

arXiv2007

The Phenomenology of Elastic Energy Loss

The unexpectedly strong suppression of high p_T heavy-quarks in heavy-ion collisions has given rise to the idea that partons propagating through a medium in addition to energy loss by induced radiation also undergo substantial energy loss due to elastic collisions. However, the precise magnitude of this elastic energy loss component is highly controversial. While it is for a parton inside a medium surprisingly difficult to define the difference between elastic and radiative processes rigorously, the main phenomenological difference is in the dependence of energy loss on in-medium pathlength: in a constant medium radiative energy loss is expected to grow quadratically with pathlength, elastic energy loss linearly. In this paper, we investigate a class of energy loss models with such a linear pathlength dependence and demonstrate that they are incompatible with measured data on hard hadronic back-to-back correlations where a substantial variation of pathlength is probed. This indicates that any elastic energy loss component has to be small.

hep-ph↗

arXiv2007

Low Mass Dimuons Produced in Relativistic Nuclear Collisions

The NA60 experiment has measured low-mass muon pair production in In-In collisions at 158 A GeV with unprecedented precision. We show that this data is reproduced very well by a dynamical model with parameters scaled from fits to measurements of hadronic transverse mass spectra and Hanbury-Brown and Twiss correlations in Pb-Pb and Pb-Au collisions at the same energy. The data is consistent with in-medium properties of $ρ$ and $ω$-mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the $ρ$-meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons with $M \apprle 0.9 {\rm GeV}/c^2$.

hep-ph↗

arXiv2007

Prospects of Jet Tomography Using Hard Processes inside a Soft Medium

The term 'tomography' is commonly applied to the idea of studying properties of a medium by the modifications this medium induces to a known probe propagating through it. In the context of ultrarelativistic heavy-ion collisions, rare high transverse momentum (p_T) processes taking place alongside soft bulk-matter production can be viewed as a tomographic probe as long as the energy scales are such that the modification of high p_T processes can be dominantly ascribed to interactions with the medium during the propagation of partons. Various high p_T observables have been suggested for tomography, among them hard single hadron suppression, dihadron correlations and gamma-hadron correlations. In this paper, we present a model study of a number of different observables within the same calculational framework to assess the sensitivity of the observables to different properties of the medium and discuss the prospects of obtaining tomographic information.

hep-ph↗

arXiv2007

Charged hadron R_AA as a function of p_T at the LHC

We compute the nuclear suppression factor R_AA for charged hadrons within a radiative energy loss picture using a hydrodynamical evolution to describe the soft medium inducing energy loss. A minijet + saturation picture provides initial conditions for LHC energies and leading order perturbative QCD (LO pQCD) is used to compute the parton spectrum before distortion by energy loss.

hep-ph↗

arXiv2007

Note on proton-antiproton suppression in 200 AGeV Au-Au collisions

We discuss the measured nuclear suppression of p + pbar production in 200 AGeV Au-Au collisions at RHIC within radiative energy loss. For the AKK set of fragmentation functions, proton production is dominated by gluons, giving rise to the expectation that the nuclear suppression for p + pbar should be stronger than for pions due to the stronger coupling of gluons to the quenching medium. Using a hydrodynamical description for the soft matter evolution, we show that this is indeed seen in the calculation. However, the expected suppression factors for pions and protons are sufficiently similar that a discrimination with present data is not possible. In the high p_T region above 6 GeV where the contributions of hydrodynamics and recombination to hadron production are negligible, the model calculation is in good agreement with the data on p + pbar suppression.

hep-ph↗

arXiv2007

Prospects of medium tomography using hard processes inside a soft medium

Hard processes leading to high transverse momentum hadron production are calculable in perturbative Quantum Chromodynamics (pQCD) for proton-proton collisions. In heavy-ion collisions, such processes occur as well, and due to a separation of scales are calculable on the level of the production of high p_T partons. These subsequently interact with the medium which originates from semi-hard and soft processes in the same collision before they hadronize. The presence of the medium thus modifies the momentum spectrum of outgoing partons and thus hard hadrons. The idea of jet tomography is to study this modification in order to learn about the medium density distribution. While this is a compelling idea, its practical application encounters some difficulties. In this paper, the capability of various observables to provide tomographic information about the medium is discussed and compared.

hep-ph↗

arXiv2007

Prospects of medium tomography using back-to-back hadron correlations

We discuss the prospects of extracting information about the bulk QCD matter distribution and evolution on the basis of hard hadronic back-to-back correlations in ultrarelativistic heavy-ion collisions. Using both hydrodynamical and parametrized evolution models for the spacetime evolution of the produced matter, which have been tested against RHIC data, we study six different setups for the spacetime dependence of hard-parton energy losses. Assuming that the energy loss of hard partons traversing the medium is radiative and calculable in the BDMPS formalism, we adjust one parameter, the quenching power scale, to the measured R_AA in each of the setups and study the systematic variations of the back-to-back yield as a function of p_T. We show which spacetime regions are probed by one-particle and two-particle observables and study the role of longitudinal and transverse expansion in some detail. We also comment on the importance of considering fluctuations around the average energy loss. We conclude that while current data are too limited in momentum coverage, future data for higher trigger energy might provide the lever arm in away side hadron momentum necessary to perform medium tomography, provided that sufficient precision can be achieved.

hep-ph↗

arXiv2007

Three-particle azimuthal correlations and Mach shocks

Measurements of angular correlations of hadrons with a (semi-)hard trigger hadron in heavy-ion collisions at RHIC show large angular structures opposite to the trigger which were a priori unexpected. These away side large angle correlations were first observed in two-particle correlations and have recently also been investigated in three-particle correlation measurements by the PHENIX and STAR collaborations. We show that the correlation signal can be understood in terms of sonic shockwaves ('Mach cones') excited by hard partons supersonically traversing the medium. The propagation of such shocks through the medium evolution is treated in a Monte Carlo (MC) framework. We demonstrate that two- and especially three-particle correlations offer non-trivial insight into the medium-averaged speed of sound and the evolution of flow. Our findings imply that the assumption of "deflected jets" is not necessary to account for the observed correlations.

hep-ph↗