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How many interactions does it take to modify a jet?

Chiara Le Roux, José Guilherme Milhano, Korinna Zapp

TL;DR

The paper investigates why high-$p_ ext{T}$ azimuthal anisotropy $v_2$ is observed in small collision systems despite near-unity $R_\text{AA}$. Using the JEWEL Monte Carlo with a brick medium, it quantifies jet–medium interaction strength via the proxy $\langle n_\text{int} \rangle \cdot (\text{DM})^2$ and demonstrates an approximate linear scaling of both $R_\text{AA}$ and high-$p_T$ $v_2$ with this quantity. A key finding is that achieving a measurable $v_2$ in small systems tends to accompany a non-negligible, though potentially modest, $R_\text{AA}$ suppression, with about 100–150 jet–medium interactions per di-jet event needed for a ~10% $R_\text{AA}$ decrease at certain $\text{DM}$ values. Departures from scaling arise from inelastic energy loss and system-size effects, while expanding-media calculations show enhanced suppression for the same scaling variable. Overall, the work links jet quenching observables in small systems to a simple interaction-counting framework and informs the interpretation of experimental data.

Abstract

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.

How many interactions does it take to modify a jet?

TL;DR

The paper investigates why high- azimuthal anisotropy is observed in small collision systems despite near-unity . Using the JEWEL Monte Carlo with a brick medium, it quantifies jet–medium interaction strength via the proxy and demonstrates an approximate linear scaling of both and high- with this quantity. A key finding is that achieving a measurable in small systems tends to accompany a non-negligible, though potentially modest, suppression, with about 100–150 jet–medium interactions per di-jet event needed for a ~10% decrease at certain values. Departures from scaling arise from inelastic energy loss and system-size effects, while expanding-media calculations show enhanced suppression for the same scaling variable. Overall, the work links jet quenching observables in small systems to a simple interaction-counting framework and informs the interpretation of experimental data.

Abstract

It is a continued open question how there can be an azimuthal anisotropy of high particles quantified by a sizable in p+Pb collisions when, at the same time, the nuclear modification factor 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, and 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 corresponding to the observed could fall within the experimental uncertainty. Thus, while there is currently no contradiction with the measurements, our results indicate that and go hand-in-hand. We also discuss departures from scaling, in particular, due to sizable inelastic energy loss.

Paper Structure

This paper contains 5 sections, 1 equation, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. JEWEL Monte Carlo Model
  3. The small systems set-up
  4. Results
  5. Conclusions

Figures (4)

  • Figure 1: Distributions of \ref{['distribution-NB-mean']} mean number of interactions per hard parton (see text for definition) in an event; and \ref{['distribution-NB-tot']} of total number of jet-medium interactions per d-jet for two different medium densities.
  • Figure 2: \ref{['RAANB']} R$_{\text{AA}}$ and \ref{['v2NB']}$v_2$ results also as a function of $\left< \text{n}_{\text{int}} \right> \cdot (\text{DM})^2$.
  • Figure 3: \ref{['RAAvsL']} R$_{\text{AA}}$ as a function of brick radius for points with similar $\left< \text{n}_{\text{int}} \right> \cdot (\text{DM})^2$. Since the points have slightly different $\left< \text{n}_{\text{int}} \right> \cdot (\text{DM})^2$ we have rescaled R$_{\text{AA}}$ to a common $\left< \text{n}_{\text{int}} \right> \cdot (\text{DM})^2$=$\unit[3.5]{GeV^2}$ assuming a linear dependence of R$_{\text{AA}}$ on $\left< \text{n}_{\text{int}} \right> \cdot (\text{DM})^2$. \ref{['numsplits']} Difference in number of splittings between the medium simulations and the corresponding vacuum corresponding to the points in \ref{['RAANB']}.
  • Figure 4: R$_{\text{AA}}$ as a function of $\left< \sum \text{DM}^2 \right>$ for the expanding medium, where DM is allowed to vary and $\left< \sum \text{DM}^2 \right>$ stands for the average sum of DM$^2$ for each interaction per parton.