sPHENIX discussion of physics

["Perepelitsa, Dennis" <dvp AT bnl.gov>]

Dear sPHENIX colleagues,

Although we have had some significant physics discussion in last week’s Collaboration Meeting and then Summer School, I wanted to keep up the tradition of this monthly mailing.

For this month, I’d like to highlight the recent ATLAS dijet asymmetry measurement:

ATLAS, Measurements of the suppression and correlations of dijets in Pb+Pb collisions at sqrt(s_NN) = 5.02 TeV, INSPIRE link, sub. PRC 

The paper reports distributions of xJ = subleading jet pT / leading jet pT in AA events compared to that in p+p. This observable plays a special role in the history of heavy ion physics at the LHC, given the first observation of a dijet asymmetry in Pb+Pb collisions way back in 2010. 

Since these measurement are sensitive to the energy loss of both jets, they are often framed as a useful way to probe the interplay between the (1) path-length dependence of energy loss (in general, the path lengths seen by the two jets are anti-correlated, perhaps resulting in a higher rate of unbalanced configurations), and (2) the relative magnitude of the jet-by-jet fluctuations in the energy loss process (which would affect both jets incoherently, potentially favoring a flatter distribution). As a technical note, the measurement is fully corrected for the pT bin migration of both jets, requiring a challenging, so-called “2-D”, unfolding. 

As a specific advance over previous measurements, this paper reports not just per-pair-normalized xJ distributions, but really the absolute two-jet cross-section in p+p collisions, and its analogue in Pb+Pb collisions. Doing this reveals that the change in the xJ distribution is driven by the suppression of balanced configurations, with the rate of unbalanced configurations staying very similar in Pb+Pb as in p+p. I would be curious if somebody has a picture of what this implies about the physics… 

Dennis