Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at sqrt(S(NN))= 2.76 TeV with the ATLAS Detector at the LHC (1011.6182v2)

Abstract: Using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally-segmented electromagnetic and hadronic calorimeters. The underlying event is measured and subtracted event-by-event, giving estimates of jet transverse energy above the ambient background. The transverse energies of dijets in opposite hemispheres is observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, and which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.

• The paper demonstrates a centrality-dependent dijet asymmetry with increased energy loss in central Pb-Pb collisions.
• The methodology uses advanced calorimetry to reconstruct jets with E_T thresholds of 100 GeV for leading and 25 GeV for recoiling jets.
• The findings imply substantial jet quenching due to medium interactions, advancing understanding of quark-gluon plasma dynamics.

Analysis of Dijet Asymmetry in Lead-Lead Collisions at $\sqrt{s_{NN}} = 2.76$ TeV

The paper presented by the ATLAS Collaboration describes a meticulous paper on the observation of centrality-dependent dijet asymmetry using data from lead-lead ($Pb-Pb$) collisions at a center-of-mass energy per nucleon pair of 2.76 TeV, facilitated by the Large Hadron Collider (LHC). It provides empirical data supporting significant energy imbalances in dijet pairs correlated with the centrality of collisions, which implicate substantial interactions between the jets and the medium.

Methodology

Jets were reconstructed from data collected through the ATLAS detector, utilizing sophisticated calorimetry for event-by-event subtraction of the underlying event energy. The researchers analyzed events having a leading jet energy of $E_{T1} > 100$ GeV and a second jet with $E_{T2} > 25$ GeV located in the opposite hemisphere. This analysis was facilitated by the comprehensive angular coverage and segmentation offered by the ATLAS calorimeter systems, encompassing both the electromagnetic and hadronic branches.

Results

The paper detects a noticeable increase in the asymmetry ($A_J$) between leading and second jets with enhanced event centrality. Such asymmetries were characterized by deviations that become more prominent as the overlap region of colliding nuclei grows, affecting jet energy distributions notably:

• Peripheral Collisions: Exhibit asymmetries similar to those observed in proton-proton collisions.
• Central Collisions: Indicate severe energy imbalances, with some events showing exceedingly high asymmetry values.

The distributions showed that in highly central events, leading jets were more frequently unmatched by a substantial recoiling jet, suggesting that significant energy is lost by the recoiling jet to the dense medium, which likely consists of quark-gluon plasma.

Implications

The findings imply the presence of extreme jet quenching in $Pb-Pb$ collisions, highlighting the major energy loss by jets traveling through and interacting with the hot and dense medium formed in such heavy-ion collisions. Notably, this observed asymmetry is absent in proton-proton interactions, underscoring the role of medium-induced modifications.

This paper contributes to a broader understanding of the dynamics involved in heavy-ion collisions and permits further exploration into the properties of quark-gluon plasma. It holds meaning for future theoretical advancements and experimental applications, such as better characterizing the energy loss mechanisms in quantum chromodynamics (QCD) media.

Potential Future Directions

Future research could focus on refining the understanding of energy dissipation processes at play within the medium, with potential development areas including:

• Enhanced precision in jet energy measurements.
• Analyzing the dependence of asymmetry on jet substructure and medium density.
• Comparative analyses across different collision energies and systems.

Moreover, expanding these insights with theoretical models may lead to improved predictions for medium-induced modifications within the standard model framework, potentially impacting the development of new physics beyond the standard model.

Overall, the observed centrality-dependent dijet asymmetry in $Pb-Pb$ collisions at the LHC distinctly enhances our comprehension of medium interactions in high-energy physics, setting the stage for ongoing exploration of the fundamental nature of matter under extreme conditions.