arXiv subjects

Thorsten Renk

Publications and source records attributed to Thorsten Renk.

At least 19 recordsLinked to original sources

arXiv2014

A Monte Carlo study of jet fragmentation functions in PbPb and pp collisions at sqrt{s}=2.76 TeV

The parton-to-hadron fragmentation functions (FFs) obtained from the YaJEM and PYTHIA6 Monte Carlo event generators, are studied for jets produced in a strongly-interacting medium and in the QCD "vacuum" respectively. The medium modifications are studied with the YaJEM code in two different scenarios by (i) accounting for the medium induced virtuality DeltaQ^2 transferred to the leading parton from the medium, and (ii) by altering the infrared sector in the Borghini-Wiedemann approach. The results of our simulations are compared to experimental jet data measured by the CMS experiment in PbPb and pp collisions at a center-of-mass energy of 2.76 TeV. Though both scenarios qualitatively describe the shape and main physical features of the FFs, the ratios are in much better agreement with the first scenario. Results are presented for the Monte Carlo FFs obtained for different parton flavours (quark and gluon) and accounting exactly, or not, for the experimental jet reconstruction biases.

hep-ph↗

arXiv2014

A study of the constraining power of high P_T observables in heavy-ion collisions

Since the start of the LHC heavy ion program, a multitude of rather different high transverse momentum (P_T) observables has become available to study the physics of the interaction of hard partons with a QCD medium. Similarly, multiple theoretical models for this interaction exist and have been compared with available data, and regularly physics conclusions are drawn based on the agreement of a model with a particular data subset. However, such an agreement is only a necessary condition to identify a physics mechanism, not a sufficient one - it needs to be demonstrated that the agreement is not accidential or generic, in other words the observable actually needs to measure the physics in question. The aim of the study presented here is to illustrate this problem by computing various high P_T observables in three different models, two of which are known to be grossly wrong ab initio, and to study in which observables what amount of discrepancy with the data appears. The surprising outcome is that jet yield observables are not very sensitive to the physics mechanism of jet-medium interaction and hence do not constrain models well.

hep-ph↗

arXiv2014

Thermal photon $v_3$ at LHC from fluctuating initial conditions

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 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 value.

nucl-th↗

arXiv2014

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-linear dependence of their emission rates on temperature, thermal photons and their v_n are very sensitive to the initial state graininess. Finally, the v_2 and v_3 of high P_T hadrons coming from hard processes reflect the attenuation pattern of partons propagating through the inhomogeneous matter density after some fluid dynamical evolution. Combining information from all these channels does not yet lead to a fully consistent picture, however intriguing trends pointing towards non-trivial initial state dynamics emerge.

hep-ph↗

arXiv2014

The Rapidity Dependence of Jet Quenching

The suppression of high transverse momentum (P_T) jets and hadrons in ultrarelativistic heavy-ion collisions with respect to a p-p baseline in terms of the nuclear suppression factor R_AA is one of the key observables to gauge the density of a hot and dense QCD medium. However, the suppression measured by R_AA is not a straightforward measure of the medium properties, the value of the observable also depends on the ratio of quark to gluon jets and on the slope of the hard parton spectrum, which explains why R_AA is found to be fairly similar at RHIC and LHC despite the very different dynamics. Measuring high P_T jets and hadrons at forward rapidity offers the same possibility of varying medium density, parton mixture and spectral slope without the need to compare across different sqrt(s) and experiments. In this work, the well-tested jet quenching Monte-Carlo (MC) framework YaJEM is utilized to compute the rapidity dependence of R_AA for three test cases.

hep-ph↗

arXiv2014

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 ${\cal O}(α_s)$ 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 medium in both cases is implemented by altering the infrared sector using the Borghini-Wiedemann model. The energy collimation and jet shapes qualitatively describe a clear broadening of showers in the medium, which is further supported by YaJEM in the final comparison of the jet shape with CMS PbPb data at center-of-mass energy 2.76 TeV. The comparison of the biased versus unbiased YaJEM jet shape with the CMS data shows a more accurate agreement for biased showers and illustrates the importance of an accurate simulation of the experimental jet-finding strategy.

hep-ph↗

arXiv2014

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 presence of this bias makes the interpretation of observables non-intuitive, but at the same time represents an opportunity to design future measurements to selectively probe particular physics. In this work, an overview over the four types of biases occuring in triggered hard correlation observables is given, followed by a study of current jet correlation phenomenology in the light of the preceding discussion.

hep-ph↗

arXiv2014

On the sensitivity of jet quenching to near T_C enhancement of the medium opacity

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 behave close to the phase transition exist. While often a behaviour with the energy density qhat ~ epsilon^3/4 is assumed which leads to a parametric decrease of the scaled qhat(T)/T^3 close to the critical temperature T_C, other scenarios expect instead a near T_C enhancement of jet quenching. In this work, the systematic response of both the extraction of qhat and v_2 at high P_T to modification of jet quenching close to T_C is systematically investigated within YaJEM, a well-tested in-medium shower evolution Monte-Carlo (MC) code.

hep-ph↗

arXiv2014

Triangular flow of thermal photons from an event-by-event hydrodynamic model for 2.76A TeV Pb+Pb collisions at LHC

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 global geometry of the produced matter in the event-by-event hydrodynamic framework are responsible for this substantial value of $v_3({\rm PP})$. However, as expected, the triangular flow parameter calculated with respect to the reaction plane $v_3$(RP) is found to be close to zero. We show that $v_3$(PP) strongly depends on the value of the fluctuation size scale $σ$ especially in the higher $p_T \, (\ge 3 {\rm GeV}/c)$ region where a larger value of $σ$ results in a smaller $v_3({\rm PP})$. In addition, the $v_3{\rm (PP)}$ is found to increase with the assumed formation time of the thermalized system.

hep-ph↗

arXiv2014

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 the event by event probability distribution of the normalized $v_n$ distribution as a function of centrality, which is to high accuracy independent on the details of the subsequent fluid dynamical evolution and hence directly reflects the primary distribution of spatial eccentricities. We present a study of this observable using a variety of Glauber-based models and argue that the experimental data place very tight constraints on the initial distribution of matter and rule out all simple Glauber-based models.

nucl-th↗

arXiv2013

Charm energy loss and D-D correlations from a shower picture

Measurements of mesons containing charm or bottom quarks at high transverse momentum (P_T) constitute an interesting set of probes for the nature of the interaction of hard partons with a QCD medium as created in ultrarelativistic heavy-ion (A-A) collisions. Not only can D and B mesons to a reasonable accuracy be assumed to represent the hadronization of a b or c quark after passage through the medium, i.e. they explicitly reflect quark-medium interaction, but also their interaction physics is expected to be different from light partons traversing a medium: The quark mass restricts radiation phase space, leading to a reduction of both vacuum and medium induced radiation, a phenomenon known as the 'dead cone effect'. Due to this difference in physics, heavy quark interaction with the medium is usually treated in the leading parton energy loss approximation, whereas the theory of light parton physics has moved to modeling the full medium-modified shower evolution. In this work, an attempt is made to create a more unified description of light parton and heavy quark physics at high P_T by computing charm-medium interactions in terms of the nuclear suppression factor and back-to-back correlations using an in-medium shower evolution Monte Carlo (MC) code that is well tested for light parton physics.

hep-ph↗

arXiv2013

Biased Showers - a common conceptual Framework for the Interpretation of High P_T Observables in Heavy-Ion Collisions

After the start of the LHC, a plethora of novel observables for jet tomography in heavy-ion collisions has appeared. Many of these studies have yielded counter-intuitive null results of apparently unmodified jets, which have sparked (sometimes exotic) theoretical efforts to explain these findings. However, it has to be realized that almost all current high P_T observables measure conditional probabilities of events, not probabilities. Thus, the correct starting point for their theoretical understanding is Bayes' formula, and the biases introduced by the conditioning are crucial to understanding the outcome. Once this is introduced properly into the modelling process, the counter-intuitive results are seen to find a natural explanation in terms of various biases and the puzzles largely disappear. In this work, a conceptual framework to classify the various observables according to the types of bias they are sensitive to is presented and illustrated with a large number of case studies ranging from simple jet finding to 2+1 dihadron triggered correlations.

hep-ph↗

arXiv2013

Jet quenching and heavy quarks

Jet quenching and more generally physics at high transverse momentum P_T scales is a cornerstone of the heavy-ion physics program at the LHC. In this work, the current understanding of jet quenching in terms of a QCD shower evolution being modified by the surrounding medium is reviewed along with the evidence for this picture from light parton high P_T observables. Conceptually, the same QCD shower description should also be relevant for heavy quarks, but with several important modifications introduced by the quark masses. Thus especially in the limit of small jet energy over quark mass E_jet/m_q, the relevant physics may be rather different from light quark jets, and several attempts to explain the observed phenomenology of heavy quarks at high P_T are discussed here.

hep-ph↗

arXiv2013

The space-time structure of the energy deposition into the bulk medium due to jet quenching

While jet quenching in ultrarelativistic heavy-ion collisions is firmly established as a phenomenon resulting from the interplay between hard perturbative and soft fluid-dynamical Quantum Chromodynamics (or equivalently the interaction between hard probes and bulk QCD matter), less is known with certainty about the reaction of bulk matter to the passage of a jet. On general grounds, a jet interacting while passing through a medium represents a source of energy and momentum for the bulk matter fluid. If the precise form of such a source term is known, the reaction of the medium can be computed using fluid dynamics. Recent advances in the understanding of hard probes due to the wealth of data from RHIC and LHC allow to constrain the source term better by determining the energy flow away from hard modes. The aim of this work is to discuss what can be learned from such constraints in the context of the in-medium shower evolution code YaJEM-DE and to illustrate the role of fluctuations in the energy deposition.

hep-ph↗

arXiv2013

Elliptic flow of thermal photons from event-by-event hydrodynamic model

Elliptic flow of direct photons in relativistic heavy ion collisions is believed to be dominated by contribution from thermal radiation of quark gluon plasma up to $p_T$ $\sim 5$ GeV/$c$, although other sources start outshining the thermal contribution at already smaller values of $p_T$ in the direct photon spectrum. The elliptic flow of thermal photons from ideal hydrodynamics considering a smooth initial density distribution under-predicts the PHENIX direct photon data from 200A GeV Au+Au collisions at RHIC by a large margin in the range $1 < p_T < 5$ GeV/$c$. However, a significant enhancement of thermal photon production due to fluctuations in the initial QCD matter density distributions is expected. We show that such fluctuations result in substantially larger photon elliptic flow for $p_T > 2.5 $ GeV/$c$ compared to a smooth initial-state-averaged density profile. The results from event-by-event hydrodynamics are found to be sensitive to the fluctuation size parameter. However, the effects of initial state fluctuations are insufficient to account for the discrepancy to the PHENIX data for direct photon elliptic flow. Furthermore, the photon $v_2$ is reduced even more when we include the NLO pQCD prompt photon component. We also calculate the spectra and elliptic flow of thermal photons for 2.76A TeV Pb+Pb collisions at LHC and for the 0--40% centrality bin. Thermal photons from event-by-event hydrodynamics along with prompt photons from NLO pQCD calculations explain the ALICE preliminary direct photon data well in the region $p_T \ge 2.5$ GeV/c. Similar to RHIC, the elliptic flow results at LHC are again found to be much smaller than the ALICE preliminary $v_2$ data.

hep-ph↗

arXiv2013

Photon Emission from a Medium-Modified Shower Evolution

Photons from the interaction of a highly energetic jet with a thermal medium are an important contribution to the total photon yield measured in ultrarelativistic heavy-ion collisions and also an important probe to study the medium degrees of freedom. Previously this contribution has often been computed in the context of a leading parton energy loss approximation. In this work, jet-medium interaction photons are instead estimated using a medium-modified shower evolution model, where the energy degradation due to vacuum radiation prior to medium formation, the virtuality evolution of intermediate states and the photon emission from subleading shower partons is taken into account consistently. The results indicate that the leading parton energy loss approximation does not appear to work well for photon emission from jet-medium interaction.

hep-ph↗

arXiv2013

The Physics probed by the P_T Dependence of the Nuclear Suppression Factor

The nuclear suppression factor R_AA of single inclusive hadrons measured in ultrarelativistic heavy-ion collisions was the first observable to study jet quenching, i.e. the final state interaction of hard parton showers with the surrounding bulk matter. While its transverse momentum (P_T) dependence of R_AA observed at RHIC was weak and hence never decisive in constraining models, there is now a strong and non-trivial P_T dependence observed at the LHC. This has been a challenge for several models which worked well at RHIC kinematics. However, in more general terms it is also of importance to understand what physical properties of the hard process and the parton-medium interaction are reflected in R_AA(P_T). The results of the work presented here suggest that the two main effects underlying the P_T dependence are the Quantum-Chromodynamics scale evolution of the fragmentation function and the limited distance (set by the typical medium length scale) for which a shower evolves in the medium.

hep-ph↗

arXiv2012

Jet-mass Dependence of the in-Medium Shower Modification

While the modification of jets by a medium as created in ultrarelativistic heavy-ion collisions is often thought of as a phenomenon affecting a hard near on-shell parton and hence treated as partonic energy loss, a more realistic view is to think of the medium effect as a relatively small correction to the Quantum Chromodynamics (QCD) vacuum radiation pattern which reduces the initial high virtuality of hard partons and leads to the development of a shower. The uncertainty relation can be used to argue that a significant part of this shower is developed even before a medium can be formed. The initial virtuality of a shower-initiating parton is reflected in the final state in the measured invariant mass of a jet - high mass jets are characterized by more branchings, higher multiplicity and a wider angular structure. Since most of this structure is determined before the medium affects the jet, selecting a particular jet mass range strongly biases the partonic configuration that enters the medium, and thus the medium is expected to modify high mass jets more strongly than low mass jets. In this work, this scenario is explored using the in-medium shower evolution code YaJEM and a possible strategy for a measurement is suggested.

hep-ph↗