The rapid fire succession of results from the Pb+Pb run at the LHC continues. This morning the ATLAS Collaboration submitted a letter on jet quenching entitled:
- Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at √sNN = 2.76 TeV with the ATLAS Detector at the LHC
The manuscript reports a growing asymmetry between the triggered jet and the back-jet in dijet events as a function of increasing collision centrality. The trigger condition was a jet with total ET > 100 GeV accompanied by a backward jet (relative azimuthal angle larger than 90 degrees) with ET > 25 GeV. The average underlying event was subtracted in the determination of ET. 1693 di-jet events satisfying this condition were recorded in a sample of Pb+Pb events with integrated luminosity of 1.7 μb-1.
The di-jet asymmetry AJ is defined as the ratio of the difference between the ET of the trigger jet and the back-jet and their sum. Given the threshold condition on the back-jet, the largest possible asymmetry for a 100 GeV trigger jet is 75 GeV / 125 GeV = 0.6. For p+p collisions and peripheral Pb+Pb collisions the asymmetry distribution peaks at or near zero; for the 10% most central events, however, the distribution peaks at AJ ≈ 0.4. This means that in the most likely case, the back-jet carries 57% less total transverse energy in a cone of R = 0.4 than the trigger jet!
The data also provide some hints about where the energy missing from the back-jet goes. First, a global missing energy analysis does indicates that the energy was not lost into particles twhich escape detection by the calorimeter. Rather, the energy lost from the back-jet seems to be redistributed over a wide azimuthal range as shown for one example in Figure 1 of the ATLAS Letter.
Although it is too early to draw a quantitative comparison of the strength of jet quenching at RHIC and the LHC from these data, it seems clear that back-jets in central Pb+Pb collisions are drastically modified by the presence of the medium. The message is that jet quenching as a probe for hot QCD matter is alive and well at the LHC. The numbers provided by the ATLAS analysis should make it possible to extract a value for the Jet RAA for the trigger jet. Will the ATLAS Collaboration leave this analysis to theorists?