Documentation

JEWEL manual

The current JEWEL manual is

• K. C. Zapp: "JEWEL 2.0.0 - Directions for Use", Eur.Phys.J. C74 (2014) 2762 [arXiv:1311.0048].

Other relevant publications

• José Guilherme Milhano and Korinna Zapp: "Improved background subtraction and a fresh look at jet sub-structure in JEWEL", arXiv:2207.14814.
  Abstract: Interactions of hard partons in the Quark Gluon Plasma (QGP) created with relativistic heavy ion collisions lead to characteristic modifications of the internal structure of reconstructed jets. A large part of the observed jet sub-structure modifications stem from the QGP's response to energy and momentum deposited by hard partons. Good control over medium response in theoretical calculations is thus instrumental to a quantitative understanding of medium modified (quenched) jets in heavy ion collisions. We present an improved way of handling the medium response in the jet quenching model JEWEL and present results for a variety of jet sub-structure observables. The new recoil handling is more versatile and robust than the old scheme, giving a better control over many observables and, in particular, greatly improves the description of the jet mass.
• Raghav Kunnawalkam Elayavalli and Korinna Christine Zapp: "Medium response in JEWEL and its impact on jet shape observables in heavy ion collisions", arXiv:1707.01539.
  Abstract: Realistic modeling of medium-jet interactions in heavy ion collisions is becoming increasingly important to successfully predict jet structure and shape observables. In JEWEL, all partons belonging to the parton showers initiated by hard scattered partons undergo collisions with thermal partons from the medium, leading to both elastic and radiative energy loss. The recoiling medium partons carry away energy and momentum from the jet. Since the thermal component of these recoils' momenta is part of the soft background activity, comparison with data requires the implementation of a subtraction procedure. We present two independent procedures through which background subtraction can be performed and discuss the impact of the medium recoil on jet shape observables. Keeping track of the medium response significantly improves the JEWEL description of jet shape measurements.
• Raghav Kunnawalkam Elayavalli and Korinna Christine Zapp: "Simulating V+jet processes in heavy ion collisions with JEWEL", Eur. Phys. J. C 76 (2016) no.12, 695 [arXiv:1608.03099].
  Abstract: Processes in which a jet recoils against an electroweak boson complement studies of jet quenching in heavy ion collisions at the LHC. As the boson does not interact strongly it escapes the dense medium unmodified and thus provides a more direct access to the hard scattering kinematics than can be obtained in di-jet events. First measurements of jet modification in these processes are now available from the LHC experiments and will improve greatly with better statistics in the future. We present an extension of JEWEL to boson-jet processes. JEWEL is a dynamical framework for jet evolution in a dense background based on perturbative QCD, that is in agreement with a large variety of jet observables. We also obtain a good description of the CMS and ATLAS data for y+jet and Z+jet processes at 2.76 TeV and 5.02 TeV.
• Korinna C. Zapp: "Geometrical aspects of jet quenching in JEWEL", Phys. Lett. B735 (2014) 157 [arXiv:1312.5536].
  Abstract: In this publication the performance of the Monte Carlo event generator JEWEL in non-central heavy-ion collisions is investigated. JEWEL is a consistent perturbative framework for jet evolution in the presence of a dense medium. It yields a satisfactory description of a variety of jet observables in central collisions at the LHC, although so far with a simplistic model of the medium. Here, it is demonstrated that also jet measurements in non-central collisions, and in particular the dependence of the jet suppression on the angle relative to the reaction plane, are reproduced by the same model.
• K. C. Zapp, F. Krauss and U. A. Wiedemann: "A perturbative framework for jet quenching", JHEP 1303 (2013) 080 [arXiv:1212.1599].
  Abstract: We present a conceptually new framework for describing jet evolution in the dense medium produced in ultra-relativistic nucleus-nucleus collisions using perturbative QCD and its implementation into the Monte Carlo event generator JEWEL. The rescattering of hard partons in the medium is modelled by infrared continued pQCD matrix elements supplemented with parton showers. The latter approximate higher order real-emission matrix elements and thus generate medium-induced gluon emissions. The interplay between different emissions is governed by their formation times. The destructive interference between subsequent scattering processes, the non-Abelian version of the Landau-Pomeranchuk-Migdal effect, is also taken into account. In this way the complete radiation pattern is consistently treated in a uniform way. Results obtained within this minimal and theoretically well constrained framework are compared with a variety of experimental data susceptible to jet-quenching effects at both RHIC and the LHC. Overall, a good agreement between data and simulation is found. This new framework also allows to identify and quantify the dominant uncertainties in the simulation, and we show some relevant examples for this.
• K. C. Zapp, J. Stachel and U. A. Wiedemann: "A local Monte Carlo framework for coherent QCD parton energy loss", JHEP 1107 (2011) 118 [arXiv:1103.6252].
  Abstract: Monte Carlo (MC) simulations are the standard tool for describing jet-like multi-particle final states. To apply them to the simulation of medium-modified jets in heavy ion collisions, a probabilistic implementation of medium-induced quantum interference effects is needed. Here, we analyze in detail how the quantum interference effects included in the BDMPS-Z formalism of medium-induced gluon radiation can be implemented in a quantitatively controlled, local probabilistic parton cascade. The resulting MC algorithm is formulated in terms of elastic and inelastic mean free paths, and it is by construction insensitive to the IR and UV divergences of the total elastic and inelastic cross sections that serve as its basic building blocks in the incoherent limit. Interference effects are implemented by reweighting gluon production histories as a function of the number of scattering centers that act within the gluon formation time. Unlike existing implementations based on gluon formation time, we find generic arguments for why a quantitative implementation of quantum interference cannot amount to a mere dead-time requirement for subsequent gluon production. We validate the proposed MC algorithm by comparing MC simulations with parametric dependencies and analytical results of the BDMPS-Z formalism. In particular, we show that the MC algorithm interpolates correctly between analytically known limiting cases for totally coherent and incoherent gluon production, and that it accounts quantitatively for the medium-induced gluon energy distribution and the resulting average parton energy loss. We also verify that the MC algorithm implements the transverse momentum broadening of the BDMPS-Z formalism. We finally discuss why the proposed MC algorithm provides a suitable starting point for going beyond the approximations of the BDMPS-Z formalism.
• K. Zapp, J. Stachel and U. A. Wiedemann: "A Local Monte Carlo implementation of the non-abelian Landau-Pomerantschuk-Migdal effect", Phys. Rev. Lett. 103 (2009) 152302 [arXiv:0812.3888].
  Abstract: The non-abelian Landau-Pomeranschuk-Migdal (LPM) effect arises from the quantum interference between spatially separated, inelastic radiation processes in matter. A consistent probabilistic implementation of this LPM effect is a prerequisite for extending the use of Monte Carlo (MC) event generators to the simulation of jet-like multi-particle final states in nuclear collisions. Here, we propose a local MC algorithm, which is based solely on relating the LPM effect to the probabilistic concept of formation time for virtual quanta. We demonstrate that this implementation of formation time physics alone accounts probabilistically for all analytically known features of the non-abelian LPM-effect, including the characteristic L²-dependence of average parton energy loss and the characteristic √ω- modification of the gluon energy distribution. Additional kinematic constraints are found to modify these L²- and ω-dependencies characteristically in accordance with analytical estimates.
• K. Zapp, G. Ingelman, J. Rathsman, J. Stachel and U. A. Wiedemann: "A Monte Carlo Model for 'Jet Quenching'", Eur. Phys. J. C 60 (2009) 617 [arXiv:0804.3568].
  Abstract: We have developed the Monte Carlo simulation program JEWEL 1.0 (Jet Evolution With Energy Loss), which interfaces a perturbative final state parton shower with medium effects occurring in ultra-relativistic heavy ion collisions. This is done by comparing for each jet fragment the probability of further perturbative splitting with the density-dependent probability of scattering with the medium. A simple hadronisation mechanism is included. In the absence of medium effects, we validate JEWEL against a set of benchmark jet measurements. For elastic interactions with the medium, we characterise not only the medium-induced modification of the jet, but also the jet-induced modification of the medium. Our main physics result is the observation that collisional and radiative medium modifications lead to characteristic differences in the jet fragmentation pattern, which persist above a soft background cut. We argue that this should allow to disentangle collisional and radiative parton energy loss mechanisms by measuring the n-jet fraction or a class of jet shape observables.

Changelog

2.4.0

• The only difference to version 2.3.0 is that the final state radiation off initial state emissions now also receives medium modifications. Please note that previously these were generated by the Pythia parton shower, now they are handled by the Jewel parton shower. This leads to small differences also in the p+p baseline.

• *** ATTENTION *** To cure unintuitive behaviour of the simple medium model (medium-simple.f) when running for small nuclei the meaning of the parameter TI specifying the initial temperature changes: it is now the initial temperature in the center (x=y=0) of a central (b=0) collision (previously is was the initial temperature averaged over the transverse plane of a b=0 collision). THIS IS NOT BACKWARD COMPATIBLE! The conversion factor going from the old to the new definition is roughly 1.35 for Pb. If you don't want to change your parameter settings, you can compile jewel-2.3.0 with the medium-simple.f file from the 2.2.0 release.
• JEWEL has moved to LHAPDF6. Nuclear pdf sets can now be accessed directly through LHAPDF6. The parameter NSET has therefore dissappeared and the parameter MASS is now an integer. For some pdf sets the communication does not work properly and they don't report their alpha_s(M_Z) value to PYTHIA. In this case alpha_s(M_Z) has to be set by hand via the parameter PDFALPHAS (default value is 0.118 used by CT14nlo and EPPS16NLO).
• Re-scattering of hard recoiling partons is now possible. It is disabled by default and can be enabled by setting SCATRECOIL to 'true'. The parameter RECHARDCUT specifies which part of the recoil population is allowed to re-scatter: recoils with momentum larger than RECHARDCUT*3*T in the local fluid rest frame can re-scatter. The default value of RECHARDCUT is 5. Enabling re-scattering of recoils dramatically increases the number of particles treated in an event and can lead to an overflow of the event record. The run time also increases considerably.
• The dummy particles needed for subtraction of thermal momenta are now massive and placed at the same rapditiy and pseudo-rapidity as the thermal momentum.
• There are new options the treatment of recoiling partons, which can be chosen with the parameter KINMODE. The options are
  • KINMODE = 0: recoiling partons are massless
  • KINMODE = 1 (default): recoiling partons keep their thermal masses
  • KINMODE = 2: recoiling partons can go off-shell and radiate provided the momentum transfer is large enough, otherwise they keep their thermal mass
• There are different options for how the subtraction of thermal momenta can be done, regulated by RECMODE (note that the actual subtraction has to be performed externally, RECMODE only affects the information written out by Jewel). Recoils that have momentum smaller than RECSOFTCUT*3*T in the local fluid rest frame are classified as soft and are removed from the event record. The default value of RECSOFTCUT is 0.
  • RECMODE = 0 (default): for recoils not classified as soft the incoming thermal momenta are written out for subtraction, the dummy particles point in the direction of the incoming thermal momenta
  • RECMODE = 1: for recoils not classified as soft the incoming thermal momenta are written out for subtraction, but the dummy particles point in the direction of the scattered (i.e. recoiling) thermal momentum
  • RECMODE = 2: the difference q = p_out - p_in between thermal (four)momenta before and after scattering is written out (these have to be added to the event), the dummy particles point in the direction of the q-vectors (note that re-scattering of recoils is not possible in this mode)
  • RECMODE = 3: like option 2 for soft recoils and like option 0 for the others

• added additional output needed for subtraction of thermal momentum components when including recoil effects
• repaired old distance based string building routine

• added V+jet processes
• event generation in all isospin channels
• fixed rapidity asymmetry in recoil distribution (thanks to Michael Oliver)

• a bug in the parametrised background (simple medium) affecting the rapidity dependence was fixed

• a bug leading to a crash when keeping the recoiling scattering centres in the event was fixed