Impact of Jet Size on Momentum Balance in Heavy Ion Collisions

This article discusses how the size of jets affects the way they balance each other when colliding heavy ions at high energy levels. The study is based on experiments conducted using the ATLAS detector at the Large Hadron Collider (LHC), focusing on lead-lead (Pb+Pb) collisions.

#What are Jets?

In particle physics, jets are streams of particles that emerge when high-energy quarks and gluons, the building blocks of matter, collide. When a heavy ion collision occurs, these jets can form in pairs, known as Dijets. This occurs because of the way particles scatter off each other.

#The Setting: Heavy Ion Collisions

Heavy ion collisions involve smashing together large nuclei at incredibly high speeds. This creates extreme conditions that mimic the early universe, allowing scientists to study states of matter that existed shortly after the Big Bang. One such state is called the Quark-gluon Plasma (QGP), where quarks and gluons are not confined in particles like protons and neutrons.

#The Study's Goal

The main goal of this study is to measure how the size of jets in these collisions affects how much energy they lose. The research specifically looks at two key measurements: the momentum balance between leading (the most energetic) and subleading (the less energetic) jets, and how these values change as the size of the jet's radius varies.

#Methodology

#Data Collection

The study uses data collected from two separate years during Pb+Pb collisions at an energy of 5.02 TeV. The jets were reconstructed using a method called the anti-kT algorithm with different jet sizes ranging from 0.2 to 0.6.

#Jet Sizes

Various jet sizes were examined to determine how they impacted the dijet momentum balance. Smaller jets (like those with a radius of 0.2) tend to be more sensitive to initial conditions and energy loss, while larger jets (up to 0.6) may capture more energy but spread it across a wider area.

#Key Findings

#Dijet Momentum Balance

The study found that balanced dijets (where both jets have similar energies) are less common in Pb+Pb collisions compared to proton-proton (pp) collisions. Meanwhile, imbalanced dijets (where one jet has much more energy than the other) are more likely to appear.

#Jet Radius Dependence

The jet size strongly affects the dijet yields. More imbalanced jets show a greater increase in yields as the size increases compared to the balanced ones. This means that as jets grow larger, they seem to retain energy better, especially when they are not well-balanced.

#Nuclear Modification Factors

The research also calculated factors that show how the energy loss of these jets changes in heavy ion collisions compared to what is expected in simpler collisions like pp. It was found that the suppression of subleading jets is more significant than that of leading jets, further showing that jet size plays a critical role in these interactions.

#Understanding Quark-Gluon Plasma

When jets travel through QGP, they lose energy due to interactions with the medium. This energy loss can occur through radiation and collisions with other particles.

#Physics Concepts Explained

#Centrality

Centrality refers to how close the two colliding nuclei are to each other during a collision. More central collisions occur when the nuclei overlap significantly, leading to a more intense environment.

#Jet Quenching

Jet quenching describes the phenomenon where jets lose energy as they move through the QGP. This energy loss changes how we interpret the results from high-energy collisions since we expect jets to behave differently in dense matter.

#The Role of Geometry

The structure of the colliding nuclei affects the jets' paths through the QGP. The longer a jet travels through the plasma, the more energy it is likely to lose. This is known as path-length dependence, where the distance a jet travels through the medium influences its energy loss.

#Implications for Future Research

The findings of this study provide crucial insights into how jets behave in the QGP. They can help refine theoretical models that predict jet behavior in extreme conditions.

#The Need for Further Studies

The current results also highlight the complexity of jet interactions and the need for further exploration into how different factors, including jet size and collision geometry, affect their energy loss.

#Conclusion

Understanding how jet radius impacts momentum balance sheds light on the nature of the quark-gluon plasma and the fundamental processes governing particle interactions in high-energy physics. These results not only enhance our comprehension of jet behavior in heavy ion collisions but also contribute to the bigger picture of particle physics and the early universe.

#Acknowledgments

This research was made possible through the contributions of numerous institutions and individuals dedicated to advancing high-energy physics and understanding the fundamental forces of nature.