sPHENIX is a new detector at RHIC.

[David Morrison <morrison AT bnl.gov>]

Dear sPHENIX collaborators,

The annual PAC meeting concluded Friday afternoon, and the 
recommendations read out at the closeout were well aligned with the 
goals detailed in our beam use proposal.  Both privately and publicly, 
members of the PAC made very positive comments about the material we 
prepared for them.  The BUP was succinct and persuasive, and the 
presentations by Jamie Nagle about the accomplishments and experience of 
Run-24 and by Dennis Perepelitsa of the BUP aims and supporting 
arguments, were very well received.  The whole sPHENIX collaboration 
deserves to be proud of the high quality material marshaled in short 
order to be ready for this PAC meeting.  A big thanks go out to all for 
the hard and smart work!

The main PAC recommendation was that Run-25 should aim to collect a 
large 200 GeV AuAu data set.  Both STAR and sPHENIX want this, so the 
PAC's endorsement of that goal wasn't surprising. They noted the merits 
of our integrated luminositygoal of 7/nb.  Recall that that goal lies 
beyond the upper end of C-AD's projected reach in a 28 cryo-week run.

Assuming the AuAu goals are met, the further aims of STAR and sPHENIX 
diverge somewhat.  There was discussion with the PAC to explore the 
decision tree in our BUP and, to a lesser extent, some of the exotic 
options in the STAR BUP.  The PAC was clearly weighing the merits of 
buttressing the statistics of AuAu and pp for the core sPHENIX mission 
against the opportunity cost of failing to use highly capable 
instrumentation to study pAu and OO.  They recommended scheduling a PAC 
meeting mid-way through Run-25 to take stock of progress and prospects 
in order to optimize the achievable science for the rest of the run.

Ultimately, the length of Run-25 will be constrained by available 
funding, but the PAC was strongly focused on how to make room for as 
long a run as possible in CY'25.  We were asked how soon sPHENIX could 
be ready for Run-25 to start, and whether we could practically make use 
of a very long (say, 40 week) Run-25.  We were reminded by Wolfram 
Fischer that RHIC can go into a low power "pause" in which the cryo 
plant is kept operating, but other demands are dialed back as much as 
possible.  In this mode, a cryo-week costs about 1/7 of the usual cost.  
However, no beams are circulated and no progress is made toward any 
integrated luminosity goals.  One could imagine starting Run-25 in March 
2025, running for some months, going into a low energy pause during the 
hottest part of the summer, and then resuming later. We'll need to 
discuss whether that could be an operational approach that would 
advantage us.

There was a focus on getting the most out of CY'25 in part because the 
Accelerator Safety Envelope (ASE) expires at the end of 2025. The ASE is 
a document which is required for RHIC to be allowed to operate, and it 
has to be approved by DOE.  Getting a new ASE approved is never a sure 
thing, and anyway, it would require a solid year of work from a number 
of people across NPP.  Despite the effort needed, the PAC recommended 
that an extension of the ASE be pursued and urged BNL to take steps to 
ensure that running in CY'26 was not precluded.  This is good advice.

All in all, the PAC meeting was a big success for sPHENIX.  Our top 
priority was endorsed, and the focus was on achieving the RHIC physics 
mission -- exactly what we want.

Regards,
Dave and Gunther

--
David Morrison  Brookhaven National Laboratory  phone: 631-344-5840
                Physics Department, Bldg 510 C    fax: 631-344-3253
                          Upton, NY 11973-5000  email:dave AT bnl.gov