sPHENIX discussion of physics

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

Dear sPHENIX colleagues,

As promised, I continue with a monthly mailing intended to stimulate discussion about physics within the Collaboration.

This month, I have a followup on direct photon-based checks of our centrality calibration, and a summary of an experimental paper from earlier this year.

Please feel free to reply with your thoughts about either one - or start a discussion on another topic!  

Dennis

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Last month, Anne asked about a statistical projection of the photon RAA as a function of centrality using early data - with the motivation that we would want an early “in situ” check on our centrality calibration, especially in peripheral events.

I attach a plot of this projection using Year 1 Au+Au data and Year 2 p+p data, with different minimum photon pT values (depending on where you think we can efficiently separate them from decay photons using isolation and ID methods). I also assume we can use a light triggering scheme to record all pT > 10 GeV photon events.

Under these assumptions, it appears we will have reasonable statistics even in peripheral events - so we should be able to make this important check on our understanding of the collision geometry early on! For comparison, the most recent PHENIX measurement of this quantity (https://inspirehep.net/literature/1116179) has ~20% statistical uncertainties for the RAA of ~10 GeV photons in 60-92% Au+Au events - whereas in this simple sPHENIX projection, it might be ~1-2% in 70-80% collisions for pT = 10 GeV photons.

I would be happy to repeat this exercise for our expected Year 2 p+Au collision data if folks are interested.

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ATLAS, Measurement of the nuclear modification factor for muons from charm and bottom hadrons in Pb+Pb collisions at 5.02 TeV with the ATLAS detector, INSPIRE link, sub. PLB

Summary: ATLAS reports the yield of muons from heavy-flavor decays, statistically separating them into those from c- vs. b-hadron decays via their DCA. They report the RAA for the c- and b-hadron decay muons separately, over a broad enough muon pT range that one can see a clear “mass effect” (difference in RAA) at low muon pT which then systematically disappears at higher muon pT.

Considerations for sPHENIX: One strength of this particular measurement is that ATLAS can make a RAA(charm)/RAA(bottom) double ratio, which cancels some systematic uncertainties and helps to better isolate the pT-dependence of the mass effect. This seems like a very effective way to probe how the relative contribution of different energy loss processes for heavy flavor quarks changes with pT. sPHENIX has produced some nice statistical projections for yields of reconstructed prompt and non-prompt D mesons in our HF TG note (see Fig 27 here: https://indico.bnl.gov/event/3960/attachments/31206/49247/sPH-HF-2017-002-v1.pdf) . Can we take advantage of this same idea here, i.e. cancelling uncertainties in the double ratio, since this is a separation of D meson yields along their flight times? And what would it take to extend that particular projection to higher pT, where we expect light-, charm- and bottom-quark energy loss to become similar?

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Attachment: h1_photon_RAA_cent.pdf
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Attachment: h1_photon_yields_CENT.pdf
Description: h1_photon_yields_CENT.pdf