arXiv subjects

Korinna Zapp

Publications and source records attributed to Korinna Zapp.

At least 19 recordsLinked to original sources

arXiv2026

A dynamical implementation of colour coherence for quenched jets in JEWEL

Colour coherence affects the radiation pattern of hard partons both in vacuum and in a dense coloured background formed in heavy ion collisions. In vacuum evolution it leads to the well-known phenomenon of angular ordering, and in heavy ion collisions the appearance of a medium resolution scale strongly affects the way in which a fragmenting hard parton interacts with the background medium. In this paper I present the implementation of colour coherence in the JEWEL event generator for jet evolution in a dense medium. In each interaction between a hard parton and the medium it is checked whether the momentum transfer of the scattering is sufficient to resolve the colour dipole. In this way it is dynamically decided which structures stay coherent. Importantly, scatterings that resolve an individual parton disrupt the colour coherence, which affects the next splitting via the loss of angular ordering. This leads to a suppression of hard radiation, and consequently a reduction in overall scattering rate, which is the dominant source of effects of colour coherence observable in reconstructed jets. I discuss these modifications using the examples of nuclear modification factor, jet fragmentation function and jet-hadron correlations.

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arXiv2025

Colour coherence in small collision systems

The observation of collectivity in collisions of small systems has constituted a challenge for the heavy-ion community for over a decade now. The absence of jet quenching in those systems presents an apparent contradiction to the presence of an azimuthal anisotropy of high-$p_\perp$ particles. In the present work, we investigate the role of colour coherence in this puzzle. For that, we use the \textsc{Jewel} Monte Carlo model in its latest version, which includes effects of colour coherence in the jet-medium interactions. We then compare the two scenarios, with and without colour coherence, and quantify the effect on hadron and jet $R_{AA}$ as well as on high-$p_\perp$ $v_2$. The results indicate that, although coherence effects do account for an increase in $R_{AA}$, they do not affect $v_2$ to the same extent. Using hydrodynamic profiles generated with \textit{Trajectum} we compare O+O and Pb+Pb collisions at the same charged particle multiplicity and find that the nuclear modification factors are the same in both systems despite their different shapes.

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arXiv2025

Jet quenching without energy loss

The onset of jet quenching, i.e. the suppression of high transverse momentum particles and jets, is an important question in the context of understanding the onset of collective behaviour and small collision systems. We investigate a minimal scenario where a hard parton experiences a single soft re-scattering that leaves the kinematics unmodified, but the colour exchange leads to a loss of colour coherence that is observable in the final distribution of fragments. In particular, the formation time of the first splitting as reconstructed at hadron level from jets using the formation time clustering algorithm is sensitive the loss of colour coherence. Moreover, it can distinguish the coherence loss scenario from a corresponding energy loss scenario, since small energy loss effects leave the formation time distribution unchanged.

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arXiv2025

How many interactions does it take to modify a jet?

It is a continued open question how there can be an azimuthal anisotropy of high $p_\perp$ particles quantified by a sizable $v_2$ in p+Pb collisions when, at the same time, the nuclear modification factor $R_\text{AA}$ is consistent with unity. We address this puzzle within the framework of the jet quenching model \textsc{Jewel}. In the absence of reliable medium models for small collision systems we use the number of scatterings per parton times the squared Debye mass to characterise the strength of medium modifications. Working with a simple brick medium model we show that, for small systems and not too strong modifications, $R_\text{AA}$ and $v_2$ approximately scale with this quantity. We find that a comparatively large number of scatterings is needed to generate measurable jet quenching. Our results indicate that the $R_\text{AA}$ corresponding to the observed $v_2$ could fall within the experimental uncertainty. Thus, while there is currently no contradiction with the measurements, our results indicate that $v_2$ and $R_\text{AA}$ go hand-in-hand. We also discuss departures from scaling, in particular, due to sizable inelastic energy loss.

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arXiv2024

Going against the flow: Revealing the QCD degrees of freedom in hadronic collisions

In collisions between heavy nuclei, such as those at the Large Hadron Collider (LHC) at CERN, hydrodynamic models have successfully related measured azimuthal momentum anisotropies to the transverse shape of the collision region. For an elliptically shaped interaction area, the hydrodynamic pressure gradient is greater along the minor axis, resulting in increased particle momentum in that direction - a phenomenon known as positive elliptic flow. In this paper, we demonstrate that in smaller systems, such as proton-proton and peripheral ion-ion collisions, microscopic models for final state interactions, can produce anisotropies where the elliptic flow is negative - that is, the momentum is largest along the major axis, contrary to hydrodynamic predictions. We present results from two distinct microscopic models: one based on repulsion between string-like fields and another based on effective kinetic theory. Negative elliptic flow is a solid prediction of the string interaction model while in the model based on kinetic theory it is linked to a finite interaction range. Consequently, an experimental determination of the sign of elliptic flow, will provide novel insights into the degrees of freedom governing strong nuclear interactions in high-energy collisions and the way in which they interact.

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arXiv2024

Exploring the time axis within medium-modified jets

In this manuscript, we illustrate how to use the newly proposed $τ$ re-clustering algorithm to select jets with different degrees of quenching without biasing their initial transverse momentum spectrum. Our study is based on Z+jet simulated events using the JEWEL Monte Carlo event generator to account for jet quenching effects. We apply the $τ$ re-clustering algorithm to extract a proxy for a time axis (formation time) within the evolving medium. This information allows us to label jets according to their fragmentation pattern and select populations with enhanced sensitivity to quenching effects. Our results illustrate the potential of jets as precision tools for QGP tomography. Further, we show that the discussed method minimizes the biases stemming from $p_{T}$-, $dR$- or mass-based jet selection.

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arXiv2024

Small systems and the single-hit approximation in the AMY parton cascade ALPACA

Understanding how momentum anisotropies arise in small collision systems is important for a quantitative understanding of collectivity in terms of QCD dynamics in small and large collision systems. In this letter we present results for small collision systems from the newly developed parton cascade \textsc{Alpaca}, which faithfully encodes the AMY effective kinetic theory. \textsc{Alpaca} reproduces quantitatively previously know results from a calculation in the single-hit approximation for small values of the coupling. We discuss in detail how such a comparison is to be carried out. Particularly at larger coupling a generic differences between the two approaches becomes apparent, namely that in parton cascades particles interact over a finite distance while in direct integrations of the Boltzmann equation the interactions are local. This leads to quantitative differences in the extracted values for the elliptic flow coefficient. These discrepancies appear in situations where the mean free path is not large compared to the interaction time and the applicability of kinetic theory is thus questionable.

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arXiv2024

Effects of Multi-Parton Interactions in Jet Quenching in Heavy-Ion Collisions

We perform the first systematic study of the effects of multi-parton interactions (MPI's) in the context of jet quenching in heavy-ion collisions with the jet quenching model JEWEL. We use the simple MPI model of PYTHIA 6, on which JEWEL is based. We find negligible effects on all observables except jet-hadron and Z-hadron correlations, which show a moderate enhancement at large distances. More detailed analysis at parton level reveals that, in heavy-ion collisions, the MPI contribution to jets is suppressed by quenching effects.

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arXiv2024

AMY Lorentz invariant parton cascade -- the thermal equilibrium case

We introduce ALPACA, a Lorentz invariant parton cascade encoding the AMY effective kinetic theory of QCD at high temperatures. It solves the Boltzmann equation by explicitly simulating the evolution of parton ensembles corresponding to single events. We discuss how the effective masses and temperature entering the elastic collision and splitting/merging rates can be estimated from just a single event. We perform an extensive validation of the framework by showing that it reproduces the expected behaviour in thermal equilibrium.

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arXiv2023

Thermalization and isotropization in the AMY parton cascade ALPACA

We look at thermalization and isotropization processes in the newly introduced AMY QCD kinetic theory parton cascade ALPACA. For thermalization, we consider the case of overoccupied initial conditions, and study the time evolution of the distribution as it relaxes to thermal equilibrium. We find that the system thermalizes as expected compared to known analytical results. For anisotropic systems, we take a first look at the qualitative behaviour of isotropization for Color Glass Condensate-like initial conditions in a homogeneous box with periodic boundary conditions.

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arXiv2022

The role of initial state radiation in quenched jets

Jet quenching in heavy ion collisions and in particular the sub-structure of quenched jets are promising tools for investigating the microscopic processes underlying jet quenching and the background medium's response to energy and momentum depositions. A quantitative understanding of the data can, however, be complicated by the presence of initial state radiation in reconstructed jets. Using an extended version of JEWEL the effect of initial state radiation on different jet observables is studied in proton-proton and heavy ion collisions. It is shown that, depending on the observable and the jet radius, the initial state contributions can be sizable. Some general insights into when sizable effects can be expected also emerges.

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arXiv2022

Improved background subtraction and a fresh look at jet sub-structure in JEWEL

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.

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arXiv2021

Time reclustering for jet quenching studies

The physics program of ultra-relativistic heavy-ion collisions at the Large Hadron Collider (LHC) and Relativistic Heavy-Ion Collider (RHIC) has brought a unique insight into the hot and dense QCD matter created in such collisions, the Quark-Gluon Plasma (QGP). Jet quenching, a collection of medium-induced modifications of the jets' internal structure that occur through their development in dense QCD matter, has a unique potential to assess the time structure of the produced medium. In this work, we perform an exploratory study to identify jet reclustering tools that can potentiate future QGP tomographic measurements with jets at current energies. Our results show that by using the inverse of formation time to obtain the jet clustering history, one can identify more accurately the time structure of QCD emissions inside jets, even in the presence of jet quenching.

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arXiv2012

Systematic improvement of QCD parton showers

In this contribution, we will give a brief overview of the progress that has been achieved in the field of combining matrix elements and parton showers. We exemplify this by focusing on the case of electron--positron collisions and by reporting on recent developments as accomplished within the Sherpa event generation framework.

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arXiv2011

Monte Carlo Tools for Jet Quenching

A thorough understanding of jet quenching on the basis of multi-particle final states and jet observables requires new theoretical tools. This talk summarises the status and propects of the theoretical description of jet quenching in terms of Monte Carlo generators.

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arXiv2009

A local Monte Carlo implementation of the non-abelian Landau-Pomerantschuk-Migdal effect

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 L^2-dependence of average parton energy loss and the characteristic $\sqrtω$-modification of the gluon energy distribution. Additional kinematic constraints are found to modify these L^2- and $ω$-dependencies characteristically in accordance with analytical estimates.

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arXiv2009

JEWEL - a Monte Carlo Model for Jet Quenching

The Monte Carlo model JEWEL 1.0 (Jet Evolution With Energy Loss) simulates parton shower evolution in the presence of a dense QCD medium. In its current form medium interactions are modelled as elastic scattering based on perturbative matrix elements and a simple prescription for medium induced gluon radiation. The parton shower is interfaced with a hadronisation model. In the absence of medium effects JEWEL is shown to reproduce jet measurements at LEP. The collisional energy loss is consistent with analytic calculations, but with JEWEL we can go a step further and characterise also jet-induced modifications of the medium. Elastic and inelastic medium interactions are shown to lead to distinctive modifications of the jet fragmentation pattern, which should allow to experimentally distinguish between collisional and radiative energy loss mechanisms. In these proceedings the main JEWEL results are summarised and a Monte Carlo algorithm is outlined that allows to include the Landau-Pomerantschuk-Migdal effect in probabilistic frameworks.

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arXiv2009

A Monte Carlo Model for 'Jet Quenching'

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.

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