sPHENIX discussion of physics

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

Hi sPHENIX colleagues,

The focus of this month’s mailing is on measurements of Upsilon production & dynamics in small systems, featuring two recent papers from CMS.

1. CMS, Nuclear modification of Upsilon states in pPb collisions at 5.02 TeV, INSPIRE link, sub. PLB

This paper measures a differential suppression of the 1S, 2S, and 3S states in p+Pb collisions compared to the p+p baseline. The effects are particularly dramatic for the 3S (RpA ~ 0.5), which the paper argues can be described within a comover interaction model, i.e. as the result of final state rescattering. Small wonder the 3S is so hard to detect at all in AA collisions! 

2. CMS, Investigation into the event-activity dependence of Upsilon(nS) relative production in proton-proton collisions at 7 TeV, INSPIRE link, pub. JHEP

This paper measures how the nS/1S ratios decrease with increasing multiplicity just in p+p collisions, but then performs some additional diagnosis of the origin of this effect. For example, whether drop in the nS/1S ratio is different for Upsilons at different pT - yes, it’s larger for lower-pT Upsilons. As another example, whether it is stronger in the presence of multiple tracks immediately surrounding the Upsilon - in fact, it’s actually a somewhat weaker effect in this case, which argues against a comover picture. There are other studies - it is a detailed and interesting paper.

What do Upsilon experts think about these?

Following our nominal beam use plan, we expect to have large statistics p+Au and p+p datasets in 2024, and we will have a large tracking acceptance around the Upsilons we reconstruct. This could be a nice opportunity for sPHENIX to address questions of Upsilon production and dynamics in small systems at RHIC!

Dennis