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 <italic>positive</italic> elliptic flow. In this paper, we demonstrate that in smaller systems, such as proton&#8211;proton and peripheral ion&#8211;ion collisions, microscopic models for final state interactions, can produce anisotropies where the elliptic flow is <italic>negative</italic> &#8212; 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.

Sweden

Christian Bierlich

Peter Christiansen

Gösta Gustafson

Leif Lönnblad

Spain

Robin Törnkvist

Korinna Zapp

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

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 . 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.

Chiara Roux

Portugal

José Milhano

How many interactions does it take to modify a jet?

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 Alpaca, which faithfully encodes the AMY effective kinetic theory. Alpaca reproduces quantitatively previously known 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 difference 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.

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

In this manuscript, we illustrate how to use the newly proposed  $$\tau $$  <math> <mi>τ</mi> </math>   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  $$\tau $$  <math> <mi>τ</mi> </math>   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  $$p_{T}$$  <math> <msub> <mi>p</mi> <mi>T</mi> </msub> </math>  -,  $$\Delta R$$  <math> <mrow> <mi>Δ</mi> <mi>R</mi> </mrow> </math>  - or mass-based jet selection.

Liliana Apolinario

Pablo Guerrero-Rodríguez

Exploring the time axis within medium-modified jets

We introduce the parton cascade Alpaca, which evolves parton ensembles corresponding to single events according to the effective kinetic theory of QCD at high temperature formulated by Arnold, Moore and Yaffe by explicitly simulating elastic scattering, splitting and merging. By taking the ensemble average over many events the phase space density (as evolved by the Boltzmann equation) is recovered, but the parton cascade can go beyond the evolution of the mean because it can be turned into a complete event generator that produces fully exclusive final states including fluctuations and correlations. The parton cascade does not require the phase space density as input (except for the initial condition at the starting time). Rather, effective masses and temperature, which are functions of time and are defined as integrals over expressions involving the distribution function, are estimated in each event from just the parton ensemble of that event. We validate the framework by showing that ensembles sampled from a thermal distribution stay in thermal equilibrium even after running the simulation for a long time. This is a non-trivial result, because it requires all parts of the simulation to intertwine correctly.

Norway

Aleksi Kurkela

AMY Lorentz invariant parton cascade: the thermal equilibrium case

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.

The role of initial state radiation in quenched jets

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.

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

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.

Liliana Apolinário

André Cordeiro

Time reclustering for jet quenching studies

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.

United States

Raghav Kunnawalkam Elayavalli

CERN

Medium response in JEWEL and its impact on jet shape observables in heavy ion collisions

In this publication the first official release of the JewelÂ 2.0.0 code [The first version JewelÂ 1 (Zapp et al. in Eur Phys J C 60:617, 2009 ) could only treat elastic scattering explicitly and the code was never published, The code can be downloaded from the official Jewel homepage http://jewel.hepforge.org ] is presented. Jewel is a Monte Carlo event generator simulating QCD jet evolution in heavy-ion collisions. It treats the interplay of QCD radiation and re-scattering in a medium with fully microscopic dynamics in a consistent perturbative framework with minimal assumptions. After a qualitative introduction into the physics of Jewel detailed information about the practical aspects of using the code is given. The code is available from the official Jewel homepage http://jewel.hepforge.org .