Observation and studies of jet quenching in PbPb collisions at nucleon-nucleon center-of-mass energy = 2.76 TeV (1102.1957v2)

Abstract: Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the CMS detector at the LHC, using a data sample corresponding to an integrated luminosity of 6.7 inverse microbarns. Jets are reconstructed using the energy deposited in the CMS calorimeters and studied as a function of collision centrality. With increasing collision centrality, a striking imbalance in dijet transverse momentum is observed, consistent with jet quenching. The observed effect extends from the lower cut-off used in this study (jet transverse momentum = 120 GeV/c) up to the statistical limit of the available data sample (jet transverse momentum approximately 210 GeV/c). Correlations of charged particle tracks with jets indicate that the momentum imbalance is accompanied by a softening of the fragmentation pattern of the second most energetic, away-side jet. The dijet momentum balance is recovered when integrating low transverse momentum particles distributed over a wide angular range relative to the direction of the away-side jet.

• The paper reveals a significant centrality-dependent jet quenching effect in PbPb collisions, evidenced by increased dijet momentum imbalance.
• The study employs iterative cone and anti-kt algorithms, validated with Pythia simulations, to accurately reconstruct jets in a complex heavy-ion environment.
• The findings indicate energy loss redistribution to low-momentum particles and broader jet fragmentation, enhancing our understanding of QGP transport properties.

Observation and Studies of Jet Quenching in PbPb Collisions at 2.76 TeV

This paper presents an analysis of jet production in lead-lead (PbPb) collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV, utilizing data collected with the CMS detector at the LHC. The paper focuses on the phenomenon of jet quenching, a signature of the quark-gluon plasma (QGP), predicted by Quantum Chromodynamics (QCD) under extreme conditions.

Methodology

The analysis employs a data sample corresponding to an integrated luminosity of 6.7 $\mu$b$^{-1}$, reconstructing jets from energy deposits in the CMS calorimeters. The paper investigates how jet characteristics are modified as a function of collision centrality, with dijet events serving as the primary subjects due to their sensitivity to energy balance alterations caused by the medium.

Jets are reconstructed using both the iterative cone algorithm and anti-$k_{\rm T}$ algorithm, tailored for the highly complex environment of PbPb collisions. The performance of these algorithms is validated against simulations using the Pythia generator, both with and without embedding into heavy ion events.

Results

The analysis reveals a centrality-dependent asymmetry in dijet transverse momentum distributions. A significant jet quenching effect is observed, particularly in the most central collisions, where the measured dijet momentum imbalance is much larger than that predicted by Pythia simulations. Notably, the fraction of balanced jets declines steadily with increasing centrality. This imbalance persists over the full range of leading jet transverse momenta studied (120–210 GeV/c), indicating that quenching effects are not merely threshold artifacts.

Additionally, charged particle correlations with jets indicate a broader spatial distribution and softer fragmentation pattern for the subleading jet, especially in central collisions. The paper also explores the overall momentum balance by examining the missing transverse momentum, which demonstrates that the momentum balance of events can be restored when accounting for low transverse momentum particles spread over a wider angular range.

Implications

These results significantly enhance understanding of the QGP's transport properties. They indicate that energy lost by high-momentum partons traversing the medium is redistributed among low-momentum particles and extended angular regions, supporting a picture of the QGP as a strongly-interacting medium. This understanding is crucial for refining theoretical models describing heavy ion collisions and QGP properties.

Future Prospects

Further experimental and theoretical investigations are warranted to explore the detailed mechanisms of jet energy loss and modification in heavy ion collisions. Enhancements in detector technology and increased luminosity of LHC experiments may provide more precise measurements and facilitate studies over a broader energy range. These advancements will be instrumental in refining QCD-based models of the QGP and in exploring its characteristics with greater accuracy.

This paper represents a critical step in elucidating the behavior of partons in dense nuclear matter, providing a foundation for ongoing and future explorations in the field of high-energy nuclear physics.