Jet radius dependence of dijet momentum balance and suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

, , , , This paper presents a detailed analysis of the jet radius dependence of dijet momentum balance and suppression in Pb+Pb collisions at 5.02 TeV, utilizing data collected by the ATLAS detector at the LHC. The study focuses on measuring the momentum balance between leading back-to-back jets in high-energy collisions, comparing results from Pb+Pb collisions to those from $pp$ collisions. The data sets used for this analysis include 1.72 $nb^{-1}$ of Pb+Pb collision data collected in 2018 and 255 pb$^{-1}$ of $pp$ collision data collected in 2017, both at a center-of-mass energy of $\sqrt{s_{\rm NN}}=$ 5.02 TeV. Jets are reconstructed using the anti-$k_t$ algorithm with varying jet radius parameters ($R=$ 0.2, 0.3, 0.4, 0.5, and 0.6), allowing for a comprehensive investigation into the impact of jet size on dijet momentum balance. The study constructs dijet momentum balance distributions for leading jets with transverse momenta ranging from 100 to 562 GeV for smaller jet radii ($R=$ 0.2, 0.3, and 0.4) and from 158 to 562 GeV for larger jet radii ($R=$ 0.5 and 0.6). By normalizing these distributions, direct comparisons can be made between dijet yields in Pb+Pb collisions and dijet cross sections in $pp$ collisions. The results reveal a clear suppression of more balanced dijets in Pb+Pb collisions compared to $pp$ collisions, alongside an enhancement for more imbalanced dijets. The observed jet radius dependence indicates that this suppression is more pronounced for imbalanced dijets than balanced ones. Furthermore, the study measures pair nuclear modification factors for jet pairs, finding that subleading jet yields are more suppressed than leading jet yields within dijets. A notable observation is the decreasing level of suppression with increasing jet radius, highlighting a distinct trend in the modification factors based on jet size. Overall, these findings provide valuable insights into jet quenching scenarios within the quark-gluon plasma environment present in high-energy nuclear collisions. The detailed analysis presented in this paper contributes significantly to our understanding of particle interactions under extreme conditions and offers new constraints for theoretical models describing these phenomena.

Introduction

The study of high-energy collisions, such as those occurring at the Large Hadron Collider (LHC), provides valuable insights into the fundamental properties of matter. In particular, collisions between heavy ions, such as lead nuclei, can create a state of matter known as the quark-gluon plasma (QGP). This state is believed to have existed in the early universe and is characterized by its extremely high temperature and energy density. Studying particle interactions within this environment allows for a better understanding of the strong force that binds quarks together and how it behaves under extreme conditions.

One important aspect of these studies is measuring dijet momentum balance in Pb+Pb collisions compared to $pp$ collisions. Dijets are pairs of back-to-back jets created from energetic partons produced during a collision. The leading jet refers to the more energetic jet in a pair, while the subleading jet has less energy. By comparing dijet yields in different collision scenarios, we can gain insight into how particles interact with each other within different environments.

Data Collection

This research paper utilizes data collected by the ATLAS detector at the LHC during 2017 and 2018 runs. The data sets used include 1.72 $nb^{-1}$ of Pb+Pb collision data collected in 2018 and 255 pb$^{-1}$ of $pp$ collision data collected in 2017, both at a center-of-mass energy of $\sqrt{s_{\rm NN}}=$ 5.02 TeV.

Jets are reconstructed using an algorithm called anti-$k_t$, which groups particles based on their proximity to each other in space and their momenta. This algorithm also allows for varying jet radius parameters ($R=$ 0.2, 0.3, 0.4, 0.5, and 0.6), which determines the size of the jets being reconstructed. This range of jet radii allows for a comprehensive investigation into the impact of jet size on dijet momentum balance.

Analysis and Results

The study constructs dijet momentum balance distributions for leading jets with transverse momenta ranging from 100 to 562 GeV for smaller jet radii ($R=$ 0.2, 0.3, and 0.4) and from 158 to 562 GeV for larger jet radii ($R=$ 0.5 and 0.6). By normalizing these distributions, direct comparisons can be made between dijet yields in Pb+Pb collisions and dijet cross sections in $pp$ collisions.

The results reveal a clear suppression of more balanced dijets in Pb+Pb collisions compared to $pp$ collisions, alongside an enhancement for more imbalanced dijets. This indicates that there is a higher probability of producing imbalanced dijets in heavy ion collisions due to interactions within the QGP environment.

Furthermore, the study measures pair nuclear modification factors (NMFs) for jet pairs, which compare the yield of jets in Pb+Pb collisions to those in $pp$ collisions after accounting for differences in collision geometry and energy density effects. The NMFs show that subleading jet yields are more suppressed than leading jet yields within dijets, indicating that particles with less energy are more affected by interactions within the QGP.

A notable observation is the decreasing level of suppression with increasing jet radius, highlighting a distinct trend in the modification factors based on jet size. This suggests that larger jets may be less affected by interactions within the QGP compared to smaller ones.

Implications

These findings have important implications for our understanding of particle interactions under extreme conditions such as those found within the QGP created during high-energy nuclear collisions. The observed suppression and enhancement of dijet yields, as well as the jet radius dependence, provide valuable constraints for theoretical models describing these phenomena.

The results also have implications for future studies at the LHC and other high-energy colliders. By continuing to investigate dijet momentum balance in different collision scenarios, we can further refine our understanding of the QGP and its properties.

Conclusion

In conclusion, this research paper presents a detailed analysis of the jet radius dependence of dijet momentum balance and suppression in Pb+Pb collisions at 5.02 TeV using data collected by the ATLAS detector at the LHC. The results reveal a clear suppression of more balanced dijets in heavy ion collisions compared to $pp$ collisions, with subleading jets being more affected than leading jets within dijets. The decreasing level of suppression with increasing jet radius suggests that larger jets may be less impacted by interactions within the QGP environment. These findings contribute significantly to our understanding of particle interactions under extreme conditions and offer new constraints for theoretical models describing these phenomena.