sPHENIX discussion of physics

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

Dear sPHENIX colleagues,

Since we are less than a mere 18 months from the start of first data-taking, I thought it would be fun to try to drive more “physics-focused” discussions within the Collaboration. To this end, I’d like to start a tradition, around the beginning of every month, of sending around a Physics Roundup of a few selected, recent papers along with brief considerations for sPHENIX.

Please feel free to reply to the thread with your thoughts - we could also set up a Mattermost channel if there is enough interest! 

I'll start with just two papers from earlier this year, from our friends at the LHC.

Dennis

— 

CMS, High precision measurements of Z boson production in PbPb collisions at sqrt(sNN) = 5.02 TeV, INSPIRE link, pub. PRL

Summary: CMS analyzes the 1/TAA-scaled yields of Z bosons as a function of centrality, and report a systematic dip in peripheral events, which they argue indicates the presence of geometric & selection effects. This would have implications for interpreting the measured charged particle RAA (for example) in very peripheral events.

Considerations for sPHENIX: The HI community is quite interested in validating the collision geometry & centrality selection using EW boson probes. Should we add “prompt photon RAA, out to as peripheral as we can go” to our list of high-priority, early analyses? That would be a nice way to crisply demonstrate that sPHENIX has control over centrality selection & modeling. (And it will be easier to calibrate photons than jets early in the program!)

— 

ALICE, Measurement of the groomed jet radius and momentum splitting fraction in pp and Pb–Pb collisions at sqrt(sNN) = 5.02 TeV, INSPIRE link  sub. PRL

Summary: ALICE analyzes ~60 GeV track-jets, R=0.2 in central events, R=0.4 in semi-central ones, with a particular choice of SoftDrop parameters. They report that z_g is not modified in Pb+Pb collisions, but that theta_g peaks at lower values, and thus claim this demonstrates that the QGP medium modifies the angular structure of the parton shower.

Considerations for sPHENIX: We should strive to be doing this kind of physics, but with sufficient systematic control to push to significantly lower jet pT and/or larger R. I personally think it would be quite interesting to go so low that typical sub-jet pT scales begin to be similar to QGP medium scales. For the sPHENIX scientific case, we have argued that jet evolution at RHIC is significantly more medium-dominated than at the LHC - should we thus expect to see strong modifications in the parton shower structure?