Measurements of jet substructure in Pb+Pb collisions offer crucial insights into the phenomenon of jet quenching within the hot and dense QCD medium generated during these collisions. This study focuses on the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its correlation with jet substructure. Utilizing 1.72 nb$^{-1}$ of Pb+Pb data and 255 pb$^{-1}$ of $pp$ data at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, collected by the ATLAS detector at the Large Hadron Collider, large-radius jets are reconstructed through reclustering $R = 0.2$ calorimetric jets with transverse momentum exceeding $200$ GeV. The evaluation of jet substructure involves charged-particle tracks, while the overall level of jet suppression is quantified using the jet nuclear modification factor ($R_\mathrm{AA}$). The measurement explores how jet $R_\mathrm{AA}$ varies with jet $p_{\mathrm{T}}$, the charged $k_t$ splitting scale ($\sqrt{d_{12}}), and angular separation ($dR_{12}$) between two leading sub-jets. The results reveal that as $\sqrt{d_{12}}$ increases, there is a gradual decrease in jet $R_\mathrm{AA$, indicating more pronounced suppression for large-radius jets with higher $k_t$ splitting scales. Moreover, for values of $dR_{12}$ ranging from 0.01 to 0.2, there is a gradual decrease in jet $R_\mathrm{AA$, followed by stabilization at a constant level for $dR_{12} \gtrsim 0.2$. This observed dependence of jet suppression on jet substructure provides valuable insights into its role in the quenching process. Furthermore, systematic uncertainties related to Jet Energy Scale (JES), Jet Energy Resolution (JER), unfolding sensitivity to prior assumptions, tracking-related factors, and overall normalization uncertainties are thoroughly considered in this analysis. Variations accounting for these sources of uncertainty are evaluated by re-assessing all unfolded distributions and their corresponding 𝑅AA ratios to estimate the impact on the results. In conclusion, this comprehensive study not only sheds light on the intricate mechanisms underlying jet quenching but also underscores the significance of understanding how different aspects of jet substructure contribute to this phenomenon in high-energy heavy-ion collisions conducted at CERN's Large Hadron Collider.
SummaryMeasurements of how jets behave in collisions involving lead atoms give us clues about how jets are affected by the environment inside a particle collider. Scientists are studying how big jets are being blocked (suppressed) and looking at their internal structure. They used data from a specific detector at the Large Hadron Collider to analyze collisions between lead atoms and protons. By examining the tracks left by charged particles, scientists can measure how much these large jets are being suppressed. The study also looks at how different factors like momentum, distance between sub-jets, and their angles affect jet suppression.
Definitions
- Measurements: Finding out how big or small something is.
- Jet: A stream of particles moving very fast in a specific direction.
- Quenching: Cooling down or suppressing something's activity.
- QCD medium: The environment inside a particle collider where particles interact according to quantum chromodynamics theory.
- Detector: A device that detects and records information about particles produced in collisions.
- Pb+Pb collisions: Collisions involving lead atoms.
- Substructure: The internal arrangement or organization of something.
- Radius: The distance from the center to the outer edge of a circle or sphere.
- ATLAS detector: A specific detector used at the Large Hadron Collider for experiments.
- LHC (Large Hadron Collider): A large scientific instrument used to study particle physics.
- GeV/TeV: Units used to measure energy levels in particle physics experiments.
Introduction:
The study of high-energy heavy-ion collisions at the Large Hadron Collider (LHC) has provided crucial insights into the behavior of matter under extreme conditions. One phenomenon that has been extensively studied in these collisions is jet quenching, which refers to the suppression of energetic jets produced by hard scattering processes within the hot and dense medium created during these collisions. Jet quenching is a manifestation of strong interactions between partons and the QCD medium, providing valuable information about its properties.
In this context, a recent research paper titled "Measurements of jet substructure in Pb+Pb collisions offer crucial insights into the phenomenon of jet quenching" presents an in-depth analysis of large-radius jets with a radius parameter ($R = 1.0$) and their correlation with jet substructure. The study utilizes data collected by the ATLAS detector at LHC from lead-lead (Pb+Pb) and proton-proton ($pp$) collisions at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, providing new insights into how different aspects of jet substructure contribute to jet quenching.
Methodology:
To investigate jet substructure in Pb+Pb collisions, large-radius jets are reconstructed through reclustering $R = 0.2$ calorimetric jets with transverse momentum exceeding $200$ GeV. This approach allows for a more precise measurement of large-radius jets compared to traditional methods that rely on smaller radius parameters.
The evaluation of jet substructure involves charged-particle tracks, which are used to determine various observables such as charged $k_t$ splitting scale ($\sqrt{d_{12}})$ and angular separation ($dR_{12}$) between two leading sub-jets. These observables provide information about how energy is distributed within a jet and can be used to quantify its structure.
Results:
The main focus of this study is to explore how jet suppression, quantified using the jet nuclear modification factor ($R_\mathrm{AA}$), varies with different aspects of jet substructure. The results reveal that as $\sqrt{d_{12}}$ increases, there is a gradual decrease in jet $R_\mathrm{AA}$, indicating more pronounced suppression for large-radius jets with higher $k_t$ splitting scales. This suggests that jets with more complex substructure are more susceptible to interactions with the QCD medium.
Furthermore, the study also investigates the dependence of jet suppression on angular separation between two leading sub-jets ($dR_{12}$). It is observed that for values of $dR_{12}$ ranging from 0.01 to 0.2, there is a gradual decrease in jet $R_\mathrm{AA}$, followed by stabilization at a constant level for $dR_{12} \gtrsim 0.2$. This indicates that while small-angle correlations within a jet contribute significantly to its quenching, larger angles play a less significant role.
Systematic Uncertainties:
To ensure the robustness of their results, the authors thoroughly consider various sources of systematic uncertainties such as Jet Energy Scale (JES), Jet Energy Resolution (JER), unfolding sensitivity to prior assumptions, tracking-related factors and overall normalization uncertainties. These uncertainties are evaluated by re-assessing all unfolded distributions and corresponding 𝑅𝐴𝐴 ratios to estimate their impact on the results.
Conclusion:
In conclusion, this comprehensive study not only provides valuable insights into the intricate mechanisms underlying jet quenching but also highlights the significance of understanding how different aspects of jet substructure contribute to this phenomenon in high-energy heavy-ion collisions conducted at LHC. The results presented in this research paper pave the way for further studies on QCD medium properties and shed light on important questions regarding energy loss mechanisms in heavy-ion collisions.
Overall, this study showcases the importance of jet substructure measurements in understanding jet quenching and provides a valuable contribution to the field of high-energy nuclear physics. With ongoing advancements in experimental techniques and data analysis methods, we can expect further progress in our understanding of jet quenching and its role in probing the properties of QCD medium.
Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$^{-1}$ of Pb+Pb data and 255 pb$^{-1}$ of $pp$ data, both at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentum above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($R_\mathrm{AA}$). The jet $R_\mathrm{AA}$ is measured as a function of jet $p_{\mathrm{T}}$, the charged $k_t$ splitting scale ($\sqrt{d_{12}}$), and the angular separation ($dR_{12}$) of two leading sub-jets. The jet $R_\mathrm{AA}$ gradually decreases with increasing $\sqrt{d_{12}}$, implying significantly stronger suppression of large-radius jets with larger $k_t$ splitting scale. The jet $R_\mathrm{AA}$ gradually decreases for $dR_{12}$ in the range $0.01{-}0.2$ and then remains consistent with a constant for $dR_{12} \gtrsim 0.2$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.
