sPHENIX is a new detector at RHIC.

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

Hello All,

I realize the collaboration meeting has a rather packed agenda, and wanted to raise an issue that touches on many topics - jet/photon physics, simulations, triggers, data rates.    

At the CD-1 review, we mostly emphasized that the experiment in Au+Au will take ~ 15 kHz of minimum bias triggers and over the 5-year run plan that collects 239 billion events.    Most of our performance plots are based on this data sample - and this is necessary because it is very difficult to trigger on Upsilons in Au+Au or low to mid energy jets (particularly in an unbiased way).   However, for the sPHENIX R_AA compilation figure, the inclusive jets, hadrons, and photons are projections from statistics of nearly 1 trillion events following the concept that these are calorimeter only measurements so that all z-vertex collisions have good acceptance.   We have initial simulations indicating that we can effectively trigger on the highest energies for these events (triggering on photons > 20 GeV even in Au+Au is very easy and jets above ~ 45 GeV is likely to also work).   

Attached is a plot I think we were all a bit worried to show at the CD-1 review.   It shows the luminosity always exceeding the 15 kHz within |z| < 10 cm (which is good and a necessary requirement to achieve the 239 billion minimum bias sample) and the much larger (up to 200 kHz) interaction rate over all z-vertices (which is both good and bad).   Note that in the current set of performance plots this higher rate is utilized for the highest energy photons, hadrons, and jets.

So this leads to a series of questions...

1.   Outside of the |z| < 10 cm, will there be so much support material and services that even calorimeter only measurements will be compromised?    It would be great to hear from the simulations team a potential plan and timeline for implementing at least an approximate material budget in the GEANT-4 geometry outside the inner tracking detectors along the beam axis.    Then one can quantitatively address this question.

2.   Outside of the |z| < 10 cm, one may have the calorimeter only measurements, but only tracks going through part of the TPC and not the inner silicon detectors (particularly the MAPS).    Is this enough to provide any reliable tracking, or is it not useful?   Again, simulations including the support material is needed.   

3.  If there is no tracking, how interesting are the higher statistics for photons and jets without being able to make more differential measurements such as fragmentation functions etc. with this sample?   I note that at QM the photon-jet xJ distributions were quite interesting as a function of photon energy (a purely calorimetric measurement).

4.  Of course, if the TPC or other components end up not working with 200 kHz rates, there is only one decision and that is to turn down the luminosity.   From the attached figure, one could already saturate the 15 kHz rate and only have 100 kHz total rate.    In following up discussions John Haggerty has started with C-AD about optimizing beam parameters, having some simulation input on the above items would be very helpful.    

The Colorado group is glad to help as these issues touch on much of our physics and triggering interest.

Sincerely,

Jamie

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

|| On Sabbatical at CEA (Commissariat à l'énergie atomique) / Saclay

|| EMAIL:   jamie.nagle AT colorado.edu
|| SKYPE:  jamie-nagle        
|| WEB:      http://spot.colorado.edu/~naglej 
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