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

Dear sPHENIX Calibration and Jet Structure TGs,

Here I follow up on two points raised during the BUR discussion this morning ( https://indico.bnl.gov/event/9139/ ).

1. Equivalent pp luminosity in peripheral Au+Au events for Year-1 calibrations

In the current version of the BUR, we will record 39B in Year-1 Au+Au data-taking. If we consider >60% peripheral events (for example) and ignore any MB trigger inefficiency, those will be 40% of the events. The <Ncoll> for this selection is 12.4 according to MCGlauber v3.2. Thus the equivalent pp luminosity is: 

39B * 0.40 * 12.36 / 42 mb = 4.6e+9 mb^-1 = 4.6 pb^-1 .

For triggerable events, we could sample 47B in Year-1 running, which in >60% peripheral events is an equivalent p+p luminosity of 5.5 pb^-1. 

Either number seems very small for in situ calibration studies, except perhaps for modest pT hadron track->calo (?). 

2. Numbers for the Direct Photon / pi0 ratio at RHIC

I attach a plot of the expected direct photon / pi0 yield ratio made using our pQCD calculations. Note that there is no isolation or ID requirement, this is just the ratio of expected pQCD rates. You can see that the ratio is >1 starting at perhaps ~20 GeV in Au+Au, but only at ~35 GeV in p+p.

However, I also attach Fig 1.50 from the MIE proposal document, which shows that the measured gamma_dir / pi0 ratio by PHENIX is actually somewhat higher than this.

Dennis 

Dennis V. Perepelitsa

Assistant Professor, Physics Department

University of Colorado Boulder

Attachment: RHIC_pi0gamma_ratio_pp.pdf
Description: RHIC_pi0gamma_ratio_pp.pdf

Attachment: sPHENIX_MIE_gammapi0ratio.pdf
Description: sPHENIX_MIE_gammapi0ratio.pdf