Discussion of sPHENIX note sPH-cQCD-2017-001

[John Lajoie <lajoie AT iastate.edu>]

Hi Jamie,

    Thanks for the thorough and thoughtful comments.  As for the "money plot" for DY I am working on this with Hannu Paukkunen. He's agreed to see if we can do a reweighing to estimate the effect on EPPS16 if we can provide him with some pseudodata - I'm working on getting him the pseudodata.

    Figure 1.7 assume a Q^2 range from 25-64 GeV^2, consistent with the invariant mass region 5-8 GeV, as described in Section 4.2. We have not done a serious study of forward photons with the FEMC as merged pi0 showers will be an issue at the highest rapidities, which access the lowest x in the nucleus.  In PHENIX we had to build the MPC-EX to do this  from 3 < eta < 4, and the FEMC tower size is larger - 5.5 cm vs. 2.2 cm for the MPC.  I would suspect you could do something at the lower rapidities, but the details would need to be studied.

    However, DY is far superior to prompt photons because in DY you get full reconstruction of the event kinematics at LO. The ability to reconstruct the kinematics at LO should have a much stronger constraint on the nuclear PDF's as you don't just constrain the integral, but the shape as well.  Prompt photons + jet could do this, but again you wouldn't be able to get the high rapidity prompt photons that reach the lowest x in the nucleus. 

    The kinematic coverage is the "big deal" - not only do you reach low x where there is a dearth of data to constrain the nPDF's but in a limited Q^2 region you reach lower x than the EIC will reach. This makes the measurement complementary to the EIC, not directly competing with it.

    Hope this helps.

John

On 5/25/2017 5:14 PM, Jamie Nagle wrote:

(2) how antiquarks move around in nuclei?    antiquarks only come from quantum fluctuations in the proton and in nuclei and can tell us a lot about QCD.    Antiquark distributions in protons have already shown some big surprises.   Measure antiquarks via DY in p+A.    

       Where is the money plot?  Apologies here, but I do not find any such plot.   Something with the x-distribution of                  anti-up or anti-down in nuclei at a given Q^2 with experimental uncertainties ...  overlay with EPPS16 fit results.

       Figure 1.7 is okay, but not a delivery/money plot.    

       A couple other questions on DY.   For Figure 1.7, what is the DY mass range assumed to be accessible?   

       I am also interested in how one compared the deliverable from forward direct, real photons to these DY virtual

       photons.    I think that needs a crisp answer/discussion.

       Anti-quarks are unique (?) to hadron-hadron, in other experiments by running to separate proton+neutron and

       proton+proton one can get at anti-up versus anti-down.   That is not the case here...   One gets at the sea via

       DIS, so the real argument is in the kinematic coverage?

--

John Lajoie

Professor of Physics

Iowa State University

 

(515) 294-6952

lajoie AT iastate.edu

Contact me: john.lajoie