Measurements of azimuthal anisotropies of jet production in Pb+Pb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV with the ATLAS detector

, , , , In heavy-ion collisions, the azimuthal variation of jet yields provides crucial insights into the energy loss experienced by partons as they traverse the quark-gluon plasma. This study focuses on measuring the azimuthal anisotropy coefficients $v_2$, $v_3$, and $v_4$ for jets in Pb+Pb collisions at a center-of-mass energy of 5.02 TeV using data collected by the ATLAS detector at the LHC in 2015 and 2018, totaling an integrated luminosity of 2.2 nb$^{-1}$. The analysis explores how these coefficients vary with jet transverse momentum (ranging from 71 GeV to 398 GeV) and event centrality. The results reveal that a nonzero value of $v_2$ is observed in all but the most central collisions, with the highest values occurring for jets with lower transverse momentum in mid-central collisions, reaching up to 0.05. On the other hand, a smaller yet discernible nonzero value of approximately 0.01 is measured for $v_3$, showing no significant dependence on either jet transverse momentum or centrality. This suggests that fluctuations in the initial state play a minor but distinguishable role in jet energy loss. Interestingly, no significant deviation from zero is observed for $v_4$ within the studied kinematic range. These findings shed light on how partons lose energy while traversing through hot and dense nuclear matter, providing valuable information about the path-length dependence of this phenomenon within the quark-gluon plasma environment created during heavy-ion collisions at ultra-relativistic energies at the LHC.

Introduction

The study of heavy-ion collisions has been a crucial tool in understanding the properties of nuclear matter at extreme temperatures and densities. In particular, the azimuthal anisotropy coefficients $v_2$, $v_3$, and $v_4$ provide valuable insights into the energy loss experienced by partons as they traverse through the quark-gluon plasma (QGP) created in these collisions. The QGP is a state of matter where quarks and gluons are no longer confined within hadrons, but instead form a deconfined medium with strong interactions. This research paper focuses on measuring these anisotropy coefficients for jets in Pb+Pb collisions at a center-of-mass energy of 5.02 TeV using data collected by the ATLAS detector at the Large Hadron Collider (LHC).

Data Collection and Analysis Methodology

The data used in this analysis was collected by the ATLAS detector during two periods: one in 2015 and another in 2018, totaling an integrated luminosity of 2.2 nb$^{-1}$. The events were selected based on their centrality, which is determined by the amount of overlap between colliding nuclei. The more central collisions have higher multiplicities and therefore higher initial energy densities.

Jets were reconstructed using anti-$k_t$ algorithm with radius parameter R=0.4 from charged particles measured by inner tracking detectors combined with calorimeter information to improve jet resolution and reduce background contamination from underlying event activity. To measure jet yields as a function of azimuthal angle $\phi$, jets were divided into six bins equally spaced between -$\pi$/3 to $\pi$/3.

To extract anisotropy coefficients, each jet yield distribution was fitted with a Fourier expansion:

$$\frac{dN}{d\phi} = \frac{N_0}{2\pi}(1 + \sum_{n=1}^{\infty} 2v_n cos(n(\phi - \Psi_n)))$$

where $N_0$ is the average jet yield, $\Psi_n$ is the event-plane angle, and $v_n$ are the anisotropy coefficients. The values of these coefficients were then extracted by fitting this equation to the measured jet yield distributions.

Results

The results show that a nonzero value of $v_2$ is observed in all but the most central collisions, with the highest values occurring for jets with lower transverse momentum (pT) in mid-central collisions. This indicates that partons lose more energy when traversing through a longer path-length within the QGP medium. The maximum value of $v_2$ reaches up to 0.05 for pT < 100 GeV in mid-central collisions.

On the other hand, a smaller yet discernible nonzero value of approximately 0.01 is measured for $v_3$, showing no significant dependence on either jet pT or centrality. This suggests that fluctuations in the initial state play a minor but distinguishable role in jet energy loss compared to path-length dependence.

Interestingly, no significant deviation from zero is observed for $v_4$ within the studied kinematic range. This could be due to higher-order effects such as hydrodynamic flow or non-linearities in parton energy loss not accounted for in this analysis.

Implications and Future Prospects

These findings provide valuable insights into how partons lose energy while traversing through hot and dense nuclear matter created during heavy-ion collisions at ultra-relativistic energies at LHC. They also shed light on how different factors such as initial state fluctuations and path-length affect this phenomenon.

Future studies can expand on these results by including more data from different collision energies and systems, as well as incorporating other observables such as jet shape and fragmentation functions to further constrain the underlying mechanisms of parton energy loss in QGP. Additionally, studying the dependence of anisotropy coefficients on jet flavor and comparing with theoretical predictions can provide a deeper understanding of this phenomenon.

Conclusion

In conclusion, this research paper presents measurements of azimuthal anisotropy coefficients $v_2$, $v_3$, and $v_4$ for jets in Pb+Pb collisions at a center-of-mass energy of 5.02 TeV using data collected by the ATLAS detector at LHC. The results show that a nonzero value of $v_2$ is observed in all but the most central collisions, with higher values for lower pT jets in mid-central collisions. A smaller yet distinguishable nonzero value is measured for $v_3$, showing no significant dependence on either jet pT or centrality. No significant deviation from zero is observed for $v_4$. These findings provide valuable insights into parton energy loss within the QGP medium and pave the way for future studies to further understand this phenomenon.