0000000000930807
AUTHOR
Thorsten Renk
Thermal photon v3 at LHC from fluctuating initial conditions
Abstract We calculate the triangular flow parameter v 3 of thermal photons for 0–40% central collisions of Pb nuclei at LHC using an event-by-event hydrodynamic model with fluctuating initial conditions. Thermal photon v 3 with respect to the participant plane angle is found to be positive and significant compared to the elliptic flow parameter v 2 of thermal photons. In addition, photon v 3 as a function of p T shows similar qualitative nature to photon v 2 in the region 1 p T 6 GeV / c . We argue that while v 3 originates from ϵ 3 deformations of the initial state density distribution, fast buildup of radial flow due to fluctuations is the main driving mechanism for the observed large val…
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.…
Monte Carlo Simulation for Elastic Energy Loss of Hard Partons in a Hydrodynamical Background
We have developed a Monte Carlo simulation describing the $2 \rightarrow 2$ scatterings of perturbatively produced, non-eikonally propagating high-energy partons with the quarks and gluons of the expanding QCD medium created in ultrarelativistic heavy ion collisions. The partonic scattering rates are computed in leading-order perturbative QCD (pQCD), while three different hydrodynamical scenarios are used to model the strongly interacting medium. We compare our results and tune the model with the neutral pion suppression observed in $\sqrt{s_{NN}}=200$ GeV Au+Au collisions at the BNL-RHIC. We find the incoherent nature of elastic energy loss incompatible with the measured angular dependence…
Energy loss in a fluctuating hydrodynamical background
Recently it has become apparent that event-by-event fluctuations in the initial state of hydrodynamical modelling of ultrarelativistic heavy-ion collisions are crucial in order to understand the full centrality dependence of the elliptic flow coefficient v_2. In particular, in central collisions the density fluctuations play a major role in generating the spatial eccentricity in the initial state. This raises the question to what degree high P_T physics, in particular leading-parton energy loss, which takes place in the background of an evolving medium, is sensitive to the presence of the event-by-event density fluctuations in the background. In this work, we report results for the effects …
In-medium jet shape from energy collimation in parton showers: Comparison with CMS PbPb data at 2.76 TeV
Abstract: We present the medium-modified energy collimation in the leading-logarithmic approximation (LLA) and next-to-leading-logarithmic approximation (NLLA) of QCD. As a consequence of more accurate kinematic considerations in the argument of the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) fragmentation functions (FFs) we find a new NLLA correction O ð α s Þ which accounts for the scaling violation of DGLAP FFs at small x . The jet shape is derived from the energy collimation within the same approximations and we also compare our calculations for the energy collimation with the event generators PYTHIA 6 and Y A JEM for the first time in this paper. The modification of jets by the …
Deflected Jets Can Not Explain the Double-Hump Structure in Triggered Correlations in Heavy-Ion Collisions
Jet deflection by a flowing medium is one of the ideas brought forward to explain the splitting of the correlation function of hadrons associated with a high transverse momentum trigger from a jet-like structure observed in p-p and d-Au collisions to a double-hump structure on the away side in Au-Au collisions. However, just considering the parton kinematics needed to explain the data shows that any attempt of detailed modelling deflected jets either cannot agree with the data or leads to internal contradictions and violations of basic physics principles. The idea that the deflection of jets by the medium can explain the experimentally observed structures should therefore be discarded.
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 s…
From R_AA via correlations to jets - the long road to tomography
The main motivation to investigate hard probes in heavy ion collisions is to do tomography, i.e. to infer medium properties from the in-medium modification of hard processes. Yet while the suppression of high P_T hadrons has been measured for some time, solid tomographic information is slow to emerge. This can be traced back to theoretical uncertainties and ambiguities in modelling both medium evolution and parton-medium interaction. Ways to overcome these difficulties are to constrain models better and to focus on more differential observables. Correlations of high P_T hadrons offer non-trivial information beyond what can be deduced from single hadron suppression. They reflect not only the…
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 …
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, elasti…
Hard dihadron correlations in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider
High transverse momentum (${P}_{T}$) processes are considered to be an important tool to probe and understand the medium produced in ultrarelativistic heavy-ion collisions via the interaction of hard, perturbatively produced partons with the medium. In this context, triggered hard dihadron correlations constitute a class of observables set between hard single inclusive hadrons (dominated by the leading jet fragments) and fully reconstructed jets; while they probe some features of the perturbative QCD evolution of a parton shower in the medium, they do not suffer from the problem of finding a suitable separation between soft hadrons coming from perturbative jets and soft hadrons coming from …
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…
A comparison of various measures for the average transport coefficient qhat
Jet quenching, i.e. the suppression of high transverse momentum P_T hadron production in ultrarelativistic heavy-ion collisions is among the most striking experimental signatures of bulk medium formation. Efforts with the aim of extracting quantitative information about the bulk medium from the measured suppression mainly focus on the extraction of an averaged transport coefficient as a measure of the medium jet quenching power, with the underlying assumption that is a meaningful quantity to make comparisons both among different models and between models and data. In this note, the main uncertainties associated with the extraction of from model fits to data are briefly reviewed before the n…
Jet quenching in the strongly-interacting quark–gluon plasma
We propose a hybrid model for medium-induced parton energy loss, in which the hard scales in the process are treated perturbatively, while the soft scales which involve strong coupling dynamics are modeled by AdS/CFT calculations. After fitting a single parameter on R_AA for central Au+Au collisions, we are able to predict different observables like R_AA and I_AA as a function of centrality and reaction plane. We obtain a consistent picture of how jet quenching is modified if the quark-gluon plasma is strongly interacting, and we provide quantitative predictions.
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 $\rho$ and $\omega$-mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the $\rho$-meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons wi…
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 isolat…
In-medium jet shape from energy collimation in parton showers: Comparison with CMS PbPb data at 2.76 TeV
We present the medium-modified energy collimation in the leading-logarithmic approximation (LLA) and next-to-leading-logarithmic approximation (NLLA) of QCD. As a consequence of more accurate kinematic considerations in the argument of the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) fragmentation functions (FFs) we find a new NLLA correction ${\cal O}(\alpha_s)$ which accounts for the scaling violation of DGLAP FFs at small $x$. The jet shape is derived from the energy collimation within the same approximations and we also compare our calculations for the energy collimation with the event generators Pythia 6 and YaJEM for the first time in this paper. The modification of jets by the …
A systematic comparison of jet quenching in different fluid-dynamical models
Comparing four different (ideal and viscous) hydrodynamic models for the evolution of the medium created in 200 AGeV Au-Au collisions, combined with two different models for the path length dependence of parton energy loss, we study the effects of jet quenching on the emission-angle dependence of the nuclear suppression factor R_AA(phi) and the away-side per trigger yield I_AA(phi). Each hydrodynamic model was tuned to provide a reasonable description of the single-particle transverse momentum spectra for all collision centralities, and the energy loss models were adjusted to yield the same pion nuclear suppression factor in central Au-Au collisions. We find that the experimentally measured…
Constraints from $v_2$ fluctuations for the initial state geometry of heavy-ion collisions
The ability to accurately compute the series of coefficients $v_n$ characterizing the momentum space anisotropies of particle production in ultrarelativistic heavy ion collisions as a function of centrality is widely regarded as a triumph of fluid dynamics as description of the bulk matter evolution. A key ingredient to fluid dynamical modeling is however the initial spatial distribution of matter as created by a yet not completely understood equilibration process. A measurement directly sensitive to this initial state geometry is therefore of high value for constraining models of pre-equilibrium dynamics. Recently, it has been shown that such a measurement is indeed possible in terms of th…
gamma-hadron correlations as a tool to trace the flow of energy lost from hard partons in heavy-ion collisions
High transverse momentum (P_T) gamma-hadron correlations are currently being regarded as the 'golden channel' for the study of the medium produced in ultrarelativistic heavy ion collisions by means of hard probes. This is due to several reasons, all linked to the fact that because of the smallness of the electromagnetic coupling alpha, the photon does not substantially interact with the medium and is expected to escape unmodified. Thus, a high P_T photon indicates a hard process in the collision independent of the position of the hard vertex. In contrast, there may not be a clear signal for a hard process involving strongly interacting partons if the production vertex is deep in the medium …
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…
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 …
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.
Pathlength dependence of energy loss within in-medium showers
Studying the pathlength dependence of high P_T hadron suppression in heavy-ion collisions by measuring the dependence of hard hadron production on the angle phi with the reaction plane in non-central collisions has so far been one of the most successful tools in constraining the microscopical picture of leading parton energy loss. With the imminent start of the LHC heavy-ion program and the possibility of full jet reconstruction, the focus has shifted to models which are capable of simulating full in-medium parton showers rather than tracing the leading parton only. Yet, on the level of single inclusive hadron observables, such shower models need to reproduce the findings of leading parton …
Medium-modified Jet Shapes and other Jet Observables from in-medium Parton Shower Evolution
The suppression of large transverse momentum hadrons in heavy-ion (A-A) collisions as compared to their scaled expectation from proton-proton collisions due to the interaction of hard partons with the hot and dense QCD medium in A-A collisions is experimentally a well established phenomenon. Focusing on leading hadrons produced in hard processes, the medium effect appears as energy loss. Beyond that, the question is how the lost energy is redistributed in the medium. With increased experimental statistics and most importantly the kinematic range of the LHC, studying the properties of full jets rather than leading hadrons is becoming feasible. On the theory side, analytic models and Monte-Ca…
Triangular flow of thermal photons from an event-by-event hydrodynamic model for 2.76A TeV Pb + Pb collisions at the CERN Large Hadron Collider
We calculate the triangular flow parameter v3 of thermal photons from an event-by-event ideal hydrodynamic model for 0–40% central collisions of Pb nuclei at √sNN = 2.76 TeV at the CERN Large Hadron Collider. v3 determined with respect to the participant plane (PP) is found to be nonzero and positive, and its pT dependence is qualitatively similar to the elliptic flow parameter v2(PP) of thermal photons in the range 1 pT 6 GeV/c. In the range pT 3 GeV/c, v3(PP) is found to be about 50–75% of v2(PP) and for pT > 3 GeV/c the two anisotropy parameters become comparable. The value of v3 is driven by local density fluctuations both directly via the creation of triangular geometry and indirectly …
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.
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 p…
Sensitivity of jet quenching to enhancement of the medium opacity near TC
One of the main goals of the study of high transverse momentum (P_T) observables in the context of ultrarelativisic heavy-ion collisions is the determination of properties of the produced QCD matter. In particular, the transport coefficients qhat and ehat, characterizing the interaction of the medium with a high p_T parton, are accessible via high P_T probes. However, a precision extraction of their temperature dependence from current data faces the problem that neither the spacetime geometry of the evolving matter droplet nor the link between thermodynamics and transport coefficients is unambiguously known, and various conjectured scenarios how thermodynamics and transport coefficients beh…
Hard and soft probe - medium interactions in a 3D hydro+micro approach at RHIC
We utilize a 3D hybrid hydro+micro model for a comprehensive and consistent description of soft and hard particle production in ultra-relativistic heavy-ion collisions at RHIC. In the soft sector we focus on the dynamics of (multi-)strange baryons, where a clear strangeness dependence of their collision rates and freeze-out is observed. In the hard sector we study the radiative energy loss of hard partons in a soft medium in the multiple soft scattering 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 a…
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.
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 (pT ) 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 pT processes can be dominantly ascribed to interactions with the medium during the propagation of partons. Various high pT observables have been suggested for tomography, among them hard single hadron suppression, dihadron correlations and γ-hadron correlations. In th…
The Mach cone signal and energy deposition scenarios in linearized hydrodynamics
Particle correlation measurements associated with a hard or semi-hard trigger in heavy-ion collisions may reflect Mach cone shockwaves excited in the bulk medium by partonic energy loss. This is of great interest because, when compared with theory, such measurements can provide information on the transport properties of the medium. Specifically, the formation of Mach cone shockwaves is sensitive to the viscosity and speed of sound, as well as the detailed nature of the jet medium interaction. However, modeling the physics of shockwave excitation to obtain a meaningful comparison with the measured correlations is very challenging since the correlations arise from an interplay of perturbative…
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.
Understanding LHC jets in the light of RHIC data
Hard probes are a cornerstone in the ongoing program to determine the properties of hot and dense QCD matter as created in ultrarelativistic heavy ion collisions. LHC measurements have so far resulted in a wealth of high P_T data, opening new kinematic windows with high statistics. Yet on first glance, several observations are counter-intuitive and seem to contradict results from the RHIC high P_T program. This calls for a combined analysis of high P_T hadrons and reconstructed jets at RHIC and LHC in a unified framework testing a large number of theoretical models for both medium evolution and shower medium interactions against the systematics of the data. A consistent picture of shower-me…
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…
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 t…
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 Nuclear Suppression Factor for P_T increasing towards the Kinematic Limit
The suppression of high transverse momentum P_T hadron production in ultrarelativistic heavy-ion (A-A) collisions as compared to the scaled expectation from proton-proton (p-p) collisions expressed as the nuclear modification factor R_AA is experimentally well established and can be traced back to interactions between the hard parton shower and the soft bulk matter. Physical intuition suggests that the medium modification should cease to be important when the hard scale of parton production is much larger than the typical momentum scale in the medium (e.g. the temperature T) and that consequently R_AA(P_T) -> 1 for P_T >> T. However, R_AA is not a 'simple' observable, but rather re…
YaJEM — A MONTE CARLO CODE FOR IN-MEDIUM SHOWER EVOLUTION
High transverse momentum (P_T) QCD scattering processes are regarded as a valuable tool to study the medium produced in heavy-ion collisions, as due to uncertainty arguments their cross section should be calculable independent of medium properties whereas the medium then modifies only the final state partons emerging from a hard vertex. With the heavy-ion physics program at the CERN LHC imminent, the attention of high P_T physics in heavy ion collisions is shifting from the observation of hard single hadrons to fully reconstructed jets. However, the presence of a background medium at low P_T complicates jet-finding as compared to p-p collisions. Monte-Carlo (MC) codes designed to simulate t…
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…
Modelling jet quenching
Abstract High P T measurements of hard hadrons or jets at RHIC and LHC appear contradictory and in some cases counterintuitive, but upon closer investigation they represent a coherent picture of jet-medium interaction physics which can be established with systematic comparisons of models against a large body of data. This picture is consistent with a perturbative QCD mechanism and does not require exotic assumptions. This overview outlines how several key measurements each partially constrain shower-medium interaction physics and how from the sum of those the outlines of the mechanism of jet quenching can be deduced. Most current jet results from LHC can be naturally understood in this pict…
Centrality and initial formation time dependence of the emission of thermal photons from fluctuating initial conditions at RHIC and LHC
Abstract Event-by-event fluctuating initial conditions (IC) in the ideal hydrodynamic calculation are known to enhance the production of thermal photons significantly compared to a smooth initial state averaged profile in the range p T > 1 GeV / c for 200A GeV Au+Au collisions at RHIC and 2.76A TeV Pb + Pb collisions at LHC. The ‘hotspots’ or the over-dense regions in the fluctuating IC produce more high p T photons compared to the smooth IC due to the strong temperature dependent emission of the thermal radiation. This enhancement is expected to be more pronounced for peripheral collisions, for lower beam energies, and for larger values of plasma formation time. A suitably normalized ratio…
Jets in medium - what RHIC and LHC measurements of R_AA and I_AA can teach about the parton-medium interaction
High P_T observables associated with hard pQCD processes are regarded as tomographic probes of the medium created in ultrarelativistic heavy-ion collisions. Unfortunately, tomography is unexpectedly complicated in practice due to model uncertainties and the highly averaged nature of many observables. However, a systematic comparison study demanding a simultaneous description of many observables within the same framework is sufficiently powerful to rule out a large fraction of conjectured models about the physics mechanism of parton-medium interaction. From this study, it can be inferred that the relevant mechanism is likely to be perturbatively tractable medium-induced radiation with a smal…
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 realist…
Sensitivity of jet quenching to enhancement of the medium opacity near TC
[Introduction] One of the main goals of the study of high transverse momentum ( P T ) observables in the context of ultrarelativistic heavy-ion collisions is the determination of properties of QCD matter. In particular, the transport coefficients ˆ q and ˆ e characterizing the interaction of the medium with a high p T parton are accessible via hard probes. However, a precision extraction of their temperature dependence from current data faces the problem that neither the space-time geometry of the evolving matter nor the link between thermodynamics and transport coefficients is unambiguously known. Thus, various conjectured scenarios exist for how thermodynamics and transport coefficients b…
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.
Collision centrality andτ0dependence of the emission of thermal photons from a fluctuating initial state in an ideal hydrodynamic calculation
Fluctuations in the initial QCD matter density distribution are found to enhance the production of thermal photons significantly in the range $2\ensuremath{\leqslant}{p}_{T}\ensuremath{\leqslant}4$ GeV/$c$ compared to a smooth initial state averaged profile in ideal hydrodynamic calculation for $200A$ GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and $2.76A$ TeV Pb+Pb collisions at the Large Hadron Collider (LHC). The thermal emission of photons is strongly dependent on the initial temperature of the system where the presence of ``hot spots'' in the initial state translates into enhanced production of photons compared to a smooth profile. The effect of fluctuations in t…
Triangular flow of thermal photons from an event-by-event hydrodynamic model for2.76ATeV Pb + Pb collisions at the CERN Large Hadron Collider
We calculate the triangular flow parameter $v_3$ of thermal photons from an event-by-event ideal hydrodynamic model for $0--40\%$ central collisions of Pb nuclei at $\sqrt{s_{NN}}$=2.76 TeV at LHC. $v_3$ determined with respect to the participant plane (PP) is found to be non-zero, positive and its $p_T$ dependence is qualitatively similar to the elliptic flow parameter $v_2$(PP) of thermal photons in the range $1 \le p_T \le 6$ GeV/$c$. In the range $p_T \, \le $ 3 GeV/$c$, $v_3$(PP) is found to be about $50--75\%$ of $v_2$(PP) and for $p_T \, >$ 3 GeV/$c$ the two anisotropy parameters become comparable. The local fluctuations in the initial density distribution as well as the initial glob…
Hard Probes in Heavy-Ion Physics
The aim of ultrarelativistic heavy ion physics is to study collectivity and thermodynamics of Quantum Chromodynamics (QCD) by creating a transient small volume of matter with extreme density and temperature. There is experimental evidence that most of the particles created in such a collision form indeed a thermalized system characterized by collective response to pressure gradients. However, a numerically small subset of high transverse momentum ($P_T$) processes takes place independent of the bulk, with the outgoing partons subsequently propagating through the bulk medium. Understanding the modification of such 'hard probes' by the bulk medium is an important part of the efforts to determ…
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 th…
Medium-modified jets and initial state fluctuations as sources of charge correlations measured at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)
We investigate the contribution of medium-modified jets and initial state fluctuations to the asymmetry in charged-particle production with respect to the reaction plane. This asymmetry has been suggested as a compelling signature for the chiral magnetic effect in QCD and makes the study of conventional scenarios for the creation of such charged-particle multiplicity fluctuations a timely endeavor. The different path-length combinations of jets through the medium in noncentral heavy ion collisions result in finite correlations of like and different charged particles emitted in the different hemispheres. Our calculation is based on the combination of jet events from Yet another Jet Energy-Lo…
HighpThadrons as probes of the central region of Au-Au collisions atSNN=200GeV
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 ${p}_{T}~1\text{\ensuremath{-}}2$ 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 ${p}_{T}g8$ GeV trigger the angular pattern of associate hadrons for ${p}_{T}g4$ GeV shows the emergence of the expected away side pea…
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. Fo…
Enhancement of thermal photon production in event-by-event hydrodynamics
Thermal photon emission is widely believed to reflect properties of the earliest, hottest evolution stage of the medium created in ultra-relativistic heavy-ion collisions. Previous computations of photon emission have been carried out using a hydrodynamical medium description with smooth, averaged initial conditions. Recently, more sophisticated hydrodynamical models which calculate observables by averaging over many evolutions with event-by-event fluctuating initial conditions (IC) have been developed. Given their direct connection to the early time dynamics, thermal photon emission appears an ideal observable to probe fluctuations in the medium initial state. In this work, we demonstrate …
Thermal photons from fluctuating initial conditions
Event-by-event fluctuations of initial QCD-matter density produced in heavy-ion collisions at RHIC enhance the production of thermal photons significantly in the region $2 \le p_T \le 4$ GeV/$c$ compared to a smooth initial-state averaged profile in the ideal hydrodynamic calculation. This enhancement is a an early time effect due to the presence of hotspots or over-dense regions in the fluctuating initial state. The effect of fluctuations is found to be stronger in peripheral than in central collisions.
Influence of initial state fluctuations on the production of thermal photons
Inhomogeneities in the initial QCD matter density distribution increase the production of thermal photons significantly compared to a smooth initial-state-averaged profile in the region $p_T > 1$ GeV/$c$ in an ideal hydrodynamic calculation. This relative enhancement is more pronounced for peripheral collisions, for smaller size systems as well as for lower beam energies. A suitably normalized ratio of central-to-peripheral yield of thermal photons reduce the uncertainties in the hydrodynamical initial conditions and can be a useful parameter to study the density fluctuations and their size. The fluctuations in the initial density distribution also lead to a larger elliptic flow of therm…
Systematics of parton-medium interaction from RHIC to LHC
Despite a wealth of experimental data for high-P_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. A key reason is that the pQCD parton spectrum at RHIC is falling so steeply that distinguishing even a moderate shift in parton energy from complete parton absorption is essentially impossible. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and, as a result, at RHIC energies only the pathlength dependence of energy loss offers some discriminating power. At LHC however, this is no longer the case: Due to the much flatter shape of the parton p_T spectra originating fr…
Jet correlations: opportunities and pitfalls
The simplest observables used to probe the interaction of hard partons with a QCD medium in ultrarelativistic heavy ion collisions measure disappearance, such as the nuclear modification factor R_AA. The information content of such observables is however limited. More differential information is obtained from triggered correlation observables where a trigger condition ensures that a hard event has taken place and the correlation of other objects in the event with the trigger contains information about the nature of parton-medium interaction. By construction, triggered correlation observables are conditional probabilities, i.e. they measure events biased by the trigger condition. The presenc…
Zeroing in on the initial state — tomography using bulk, jets and photons
One of the unsolved problems in the current 'standard model' of heavy ion physics is the apparent rapid thermalization of QCD matter in the pre-equilibrium stage. While it is challenging to probe this mechanism directly, there are now several observables available which allow tomographic imaging of the initial state geometry, which is expected to carry remnant information of the equilibration mechanism. On the fluid dynamics side, scaled fluctuations in the momentum space anisotropy parameters v_n image the initial eccentricity fluctuations epsilon_n almost directly with only a weak dependence on the details of the fluid dynamical evolution. From a different direction, due to the strong non…