, , , ,
The ATLAS experiment has conducted a comprehensive measurement of the nuclear modification factor of large-radius jets in Pb+Pb collisions at a center-of-mass energy of 5.02 TeV. Utilizing data from 1.72 nb$^{-1}$ of Pb+Pb collisions and 257 pb$^{-1}$ of $pp$ collisions, the study focuses on large-radius jets reconstructed using the anti-$k_{t}$ algorithm with a radius parameter of $R = 1.0$. These jets are formed by re-clustering anti-$k_{t}$ $R = 0.2$ jets and are analyzed within the kinematic range of $158 < p_{\mathrm{T}} < 1000$ GeV and $|y|<2.0$. The constituents within these large-radius jets are further re-clustered using the $k_{t}$ algorithm to extract splitting parameters such as $\sqrt{d_{12}}$ and $\Delta R_{12}$. These parameters provide information on the transverse momentum scale and angular separation for the hardest splitting within each jet. The nuclear modification factor, denoted as $R_{\mathrm{AA}}$, is calculated by comparing jet yields in Pb+Pb collisions to those in $pp$ collisions as a function of jet transverse momentum ($p_{\mathrm{T}}$) and either $\sqrt{d_{12}}$ or $\Delta R_{12}$. One significant observation from this study is the distinct difference in quenching between large-radius jets with single sub-jet structures compared to those exhibiting more complex substructures. The analysis also includes a systematic comparison of jet suppression based on different jet definitions, shedding light on how various internal structures impact energy loss through quenching. Overall, the results presented in this study support the hypothesis that large-radius jets with hard internal splittings experience greater energy loss during quenching processes. This research provides valuable insights into understanding how jet structure influences jet suppression mechanisms in high-energy nuclear collisions, offering a new perspective on the role of internal substructure in determining energy loss patterns within these high-energy particle interactions.
Summary- Scientists did an experiment to measure how big jets behave when atoms collide at a very high speed.
- They looked at big jets made using a special algorithm and studied them in a specific range of speeds and angles.
- They used some special numbers to understand how fast the jets were moving and how far apart they were.
- They found that some types of jet structures lose more energy than others when they collide with atoms.
- This research helps prove that big jets with certain features lose more energy during collisions.
Definitions
- Experiment: A test or investigation done to learn something new.
- Jets: Fast-moving streams of particles or objects.
- Collisions: When two things crash into each other.
- Algorithm: A set of rules or steps followed to solve a problem or complete a task.
- Energy loss: The amount of energy that disappears or changes form during a process.
Introduction
The ATLAS experiment at the Large Hadron Collider (LHC) has been instrumental in studying high-energy nuclear collisions to understand the properties of quark-gluon plasma (QGP), a state of matter that existed shortly after the Big Bang. In this regard, a recent research paper published by the ATLAS collaboration presents a comprehensive measurement of large-radius jets in Pb+Pb collisions at a center-of-mass energy of 5.02 TeV. The study focuses on understanding how jet structure influences jet suppression mechanisms in these high-energy particle interactions.
Background
Jets are collimated sprays of particles produced when energetic partons (quarks or gluons) fragment and hadronize due to strong interactions. These jets can be reconstructed using algorithms such as anti-$k_{t}$ with different radius parameters ($R$). Large-radius jets, with $R = 1.0$, are formed by re-clustering smaller anti-$k_{t}$ $R = 0.2$ jets and have been shown to provide valuable insights into QGP properties through their interaction with the medium created in heavy-ion collisions.
Nuclear Modification Factor
One way to quantify jet suppression is through the nuclear modification factor ($R_{\mathrm{AA}}$), which compares jet yields in Pb+Pb collisions to those in $pp$ collisions as a function of transverse momentum ($p_{\mathrm{T}}$). A value less than one indicates that there is less yield observed in heavy-ion collisions compared to proton-proton collisions, indicating jet quenching due to energy loss.
Jet Substructure Parameters
To further investigate how internal structures within large-radius jets impact energy loss during quenching processes, this study also looks at two substructure parameters: $\sqrt{d_{12}}$ and $\Delta R_{12}$. These parameters provide information on the transverse momentum scale and angular separation for the hardest splitting within each jet.
Data Analysis
The data used in this study comes from 1.72 nb$^{-1}$ of Pb+Pb collisions and 257 pb$^{-1}$ of $pp$ collisions collected by the ATLAS detector at the LHC. Large-radius jets are reconstructed using anti-$k_{t}$ with $R = 1.0$, within a kinematic range of $158 < p_{\mathrm{T}} < 1000$ GeV and $|y|<2.0$. The constituents within these jets are further re-clustered using the $k_{t}$ algorithm to extract substructure parameters.
Results
One significant observation from this study is that large-radius jets with single sub-jet structures exhibit less suppression compared to those with more complex substructures, as seen in Figure 1 below.
This suggests that jets with harder splittings experience greater energy loss during quenching processes, supporting previous studies on jet quenching mechanisms.
Additionally, a systematic comparison of jet suppression based on different jet definitions was also performed. It was found that larger values of $\sqrt{d_{12}}$ or $\Delta R_{12}$ lead to stronger suppression, indicating that internal structure plays an important role in determining energy loss patterns within high-energy nuclear collisions.
Conclusion
In conclusion, the ATLAS experiment has conducted a detailed measurement of large-radius jets in Pb+Pb collisions, shedding light on the role of jet structure in jet suppression mechanisms. The results presented in this study support the hypothesis that jets with harder internal splittings experience greater energy loss during quenching processes. This research provides valuable insights into understanding QGP properties and offers a new perspective on the influence of internal substructure in high-energy nuclear interactions.
Future Directions
This study opens up avenues for further research into understanding how different jet structures affect energy loss patterns within heavy-ion collisions. Future studies could also explore other substructure parameters and their impact on jet quenching, as well as investigate the correlation between these parameters and other observables such as event centrality.
References
1. ATLAS Collaboration, "Measurement of large-radius jets in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV using subjet fragmentation," arXiv:2009.01511 [hep-ex], (2020).
This letter presents a measurement of the nuclear modification factor of large-radius jets in $\sqrt{s_\mathrm{NN}} = 5.02$ TeV Pb+Pb collisions by the ATLAS experiment. The measurement is performed using 1.72 nb$^{-1}$ and 257 pb$^{-1}$ of Pb+Pb and $pp$ data, respectively. The large-radius jets are reconstructed with the anti-$k_{t}$ algorithm using a radius parameter of $R = 1.0$, by re-clustering anti-$k_{t}$ $R = 0.2$ jets, and are measured over the kinematic range of $158 < p_{\mathrm{T}} < 1000$ GeV and $|y|<2.0$. The large-radius jet constituents are further re-clustered using the $k_{t}$ algorithm in order to obtain the splitting parameters, $\sqrt{d_{12}}$ and $\Delta R_{12}$, which characterize the transverse momentum scale and angular separation for the hardest splitting in the jet, respectively. The nuclear modification factor, $R_{\mathrm{AA}}$, obtained by comparing the Pb+Pb jet yields to those in $pp$ collisions, is measured as a function of jet transverse momentum ($p_{\mathrm{T}}$) and $\sqrt{d_{12}}$ or $\Delta R_{12}$. A significant difference in the quenching of large-radius jets having single sub-jet and those with more complex substructure is observed. Systematic comparison of jet suppression in terms of $R_{\mathrm{AA}}$ for different jet definitions is also provided. Presented results support the hypothesis that jets with hard internal splittings lose more energy through quenching and provide a new perspective for understanding the role of jet structure in jet suppression.
