Commissioning and running of sPHENIX

[Jamie Nagle <jamie.nagle AT colorado.edu>]

Hello sPHENIXians,

sPHENIX will be doing a vernier scan where the beams are steered across each other by C-AD in order to measure the cross section of the ZDC and MBD(coincidence), such that one can relate them to our integrated luminosity and hence have cross section measurements of photon, jet, pizero, Upsilon production in pp 200 GeV.    For PHENIX, detailed in the reference below, the result was that the BBC (aka the MBD) sees a cross section of 23 mb (about half the 42 mb inelastic cross section) with an uncertainty from the vernier scan of +/- 9.7%.    This uncertainty translates directly into the, for example, jet cross section.

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.76.051106

Note that the MBD is moved further away from the interaction point compared to the PHENIX BBC, so it will see a different pp 200 GeV cross section.    There are some useful recent studies from JaeBeom on this point linked here:

https://indico.bnl.gov/event/22533/contributions/88321/attachments/53076/90764/BulkTG_20240305.pdf

The sPHENIX plan had been to do 2-3 such vernier scans and have a resulting uncertainty of similar order.   Recently C-AD noted a potential  problem with such  scans.

sPHENIX will be running with the DX magnets in the so-called "- polarity" configuration for a negative crossing angle of 2 mrad, which is nicely shown in the drawing from Kiel Hock (see attached).     This enables a larger clearance consistent with the beta* we will be running at.     However, in this configuration, one can only tune the crossing angle to a minimum of 0.85 mrad (i.e., not all the way to zero).     It  is very challenging to extract the right information from the vernier scan if not done at zero crossing angle -- hence the problem.

After a meeting with C-AD, the nominal proposal (with further inputs) will be to change the setting to "+ polarity", which takes C-AD about two hours, do a zero crossing vernier scan, and then change it back to "- polarity", which takes another two hours.    We imagine doing the first such vernier scan about one month into the run when we have fully understood the MBD / ZDC and timing, etc.

We would then want to  have C-AD and our experts examine these results  in detail before any decision about doing additional vernier scans later in the run.     

One concern is that the ZDC and MBD cross sections will be different when the crossing angle is 0 versus 2 mrad, i.e., because the acceptance of the detectors will be different.    This is particularly acute in the ZDC where the beam angle moves the neutrons over by 1.8 cm at the face of the ZDC (about the width of one SMD slat).     The current thinking is to physically move the ZDC over by 1.8 cm so it has the best acceptance for the 2 mrad running.    The question would then be whether we move it back for the vernier scan at 0 mrad and then back again, or just leave it in  the same place.      Another option would be to move it by 0.9 cm, that way the acceptance of  the ZDC is the same for 0 and 2 mrad just with the neutrons offset by 0.9 cm in both cases, but in opposite directions.  Useful information regarding the expected energy and angular distribution of neutrons (and other particles) hitting the ZDC in pp 200 GeV is given in  the PHENIX reference below.   Sasha B. also mentioned that in  PHENIX, only the BBC (aka the MBD) was actually used for cross section  measurements - he can add more details as useful.   In that case, one might just leave the ZDC at 1.8 cm offset and not worry about the vernier scan issue with the ZDC.

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.88.032006

Inputs, thoughts, discussion - all welcome.

Sincerely,

Jamie, Caroline, Ralf, Sasha, Kin, John H

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|| James L. Nagle   
|| Professor of Physics, University of Colorado Boulder

|| EMAIL:   jamie.nagle AT colorado.edu
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