Discussion of sPHENIX note sPH-cQCD-2017-001

[Ralf Seidl <rseidl AT rcf.rhic.bnl.gov>]

Hi Jamie and Colorado group,

thanks for producing these useful plots. We discussed adding the di-jet 
and photon-jet measurements to the LOI but did not have explicit 
calculations/simulations for it but the extended coverage should 
definitely help the x ranges probed, particularly in pA. While high x1 
is interesting, depending on the orientation of the A beam also the low 
x region can be probed very nicely by including forward jets. From your 
text I take that you require one central jet or photon but as John 
Koster's studies half a decade ago (though for pions) showed, two 
forward jets would push your kinematics even further although the 
statistics might be limiting you there.

The fact about the high x reach in forward jets is also exactly the 
point of the Transversity measurements which you criticized in your 
earlier comments (about the Collins plot) in the LOI. Maybe we can 
adjust the discussion a little but the point is that there are no 
transversity measurements at x>0.3 and as such the deviations between 
the two curves - one from positivity limits (the Soffer bound), the 
other from the functional form of the SIDIS+e+e- fits is a reasonable 
measure of the level of uncertainty.  Since the scale is given by the 
jet transverse momentum for these measurements, it is larger than what 
JLAB11 can reach although I did write it less explicitly in the LOI.

Cheers,

Ralf


On 27.05.2017 02:25, Jamie Nagle wrote:
> Hello All,
>
> Following on useful discussions with John Lajoie and Christine Aidala, 
> the Colorado group discussed a bit more what the additional Forward 
> Calorimeter could add to the sPHENIX jet program.    Attached are a few 
> slides with calculations from Dennis Perepelitsa at the PYTHIA level.   
> These just reflect the basic kinematics of p+p @ 200 GeV and the correct 
> number of equivalent N+N collisions sampled in the Au+Au data taking.
>
> We assume that sPHENIX can measure in the barrel trigger photons > 15 
> GeV and trigger jets > 20 GeV, and then correlated jets at forward down 
> to > 10 GeV.   Note that for correlations, the underlying event 
> background fluctuation contribution is substantially reduced (aka fake 
> jets) and one can subtract remaining pieces statistically.
>
> If one thought this was worth pursuing, we believe the case focuses on 
> the ability to measure dijets and gamma-jet where one pushes to high x1. 
>    This is interesting in p+Au where it tests thinking like the 
> shrinking proton picture (high x fluctuations of the proton result in a 
> smaller gluon cloud) and also cold nuclear matter energy loss (which is 
> predicted by Ivan Vitev to be a dominant effect at very high x).   In 
> Au+Au, one combines this with jets passing through a different 
> pseudorapidity range and thus a slightly different medium.    One can 
> then check if results across rapidities scale with x (for example as 
> they do in p+Pb for ATLAS) or other kinematics or medium variables.
>
> Aside from these direct measurements, the additional coverage is sure to 
> help with characterizing the underlying event in general for even 
> sPHENIX barrel jet measurements and provides pseudorapidity separated 
> flow characterizations (also useful by itself and for the underlying 
> event).   Lastly, as searches for medium response probe to larger R 
> values (R ~ 1), having an event characterization over a larger 
> pseudorapidity range is helpful.
>
> Sincerely,
>
> Jamie and Dennis
>
> ||--------------------------------------------------------------------------------
> || James L. Nagle
> || Professor of Physics, University of Colorado, Boulder
> || EMAIL: jamie.nagle AT colorado.edu <mailto:jamie.nagle AT colorado.edu>
> || SKYPE:  jamie-nagle
> || WEB: http://spot.colorado.edu/~naglej 
> <http://spot.colorado.edu/%7Enaglej>
> ||---------------------------------------------------------------------------------
>
>
> _______________________________________________
> sPH-cQCD-2017-001-l mailing list
> sPH-cQCD-2017-001-l AT lists.bnl.gov
> https://lists.bnl.gov/mailman/listinfo/sph-cqcd-2017-001-l