[Tdis] [EXTERNAL] Inconsistencies in the TDIS proposal: Some things I found

Tue Aug 11 16:05:08 EDT 2020

Yes, it was a recommendation of TR to use the room temperature target. We still can vary the pressure if needed.
________________________________
From: Tdis <tdis-bounces at jlab.org> on behalf of Jan C. Bernauer <jan.bernauer at stonybrook.edu>
Sent: Tuesday, August 11, 2020 4:03 PM
To: tdis at jlab.org <tdis at jlab.org>
Subject: Re: [Tdis] [EXTERNAL] Inconsistencies in the TDIS proposal: Some things I found

Hi all,

Could somebody quickly confirm that we'll indeed run at room temp, so with roughly 0.7e36 instead of 3e36 /cm^2/s?

Thanks,

Jan

On 8/6/2020 11:43 AM, Jan C. Bernauer wrote:
Hi Tim, Hi Wally, Hi TDIS,

Sorry for the lengthy email.

Some updates and questions to my talk: https://hallaweb.jlab.org/wiki/images/2/2f/TDIS_sim.pdf where I found what I believe are internal inconsistencies, and differences to my own implementation of F_2, f_\pi, and a full MC.

Wally, Tim, I also send this to you, because I believe you might have additional insight to the source of some of these figures/values.

0) Does anybody have the exact bins used for the projected result figures? Especially the one as a function of t. That would be very helpful!

1) I do see about a factor of 2 more inclusive CS than in the proposal. I think it's likely that this error is on my end, I have to verify with g4SBS. In any case, the code version I got from Carlos (Thanks!), authored from Tim and Wally, do not calculate that as far as I can tell. It's not helping us in any case, we would just need less current I'd assume.

2) It would be great to figure out in which configuration the code was for the generation of table 6 and 7. I assume it was this code I attach here?  Is that the code version after a factor 2 has already been found? I believe so, because I can get the F_2^{\pi p} plots out that are in the new proposal, which are higher than the old proposal.

Some things which I found in the code which do not match the experiment:

Theta_e is ~12, not 35 degrees. That only affects the calculation of Q^2, which is slightly affecting the proton PDFs. Can't explain a big difference.

The code was set up for pi^+, I'm looking at pi^0, so I changed the isospin factor  to 1. I changed some other integration ranges (ymax=1, xmax=0, km1, km2 ), and also implemented a cosph cut (proton theta<70)

With these changes, I match exactly (on a log scale :) )  my own implementation  and the plots in the new proposal.

3) BUT the ratio was still off. I traced it down to  F_2^p. The relevant lines are here:

         CALL SETCTQ6(1)  ! CTEQ 'MS-bar' SCHEME.
         u_pro = CTQ6PDF (1, REAL(x), SQRT(REAL(Q2)))
         ubar_pro = CTQ6PDF (-1, REAL(x), SQRT(REAL(Q2)))
         d_pro = CTQ6PDF (2, REAL(x), SQRT(REAL(Q2)))
         dbar_pro = CTQ6PDF (-2, REAL(x), SQRT(REAL(Q2)))

         F2neu = 2.*x * ((4./9.)*(d_pro + dbar_pro)
     &             + (1./9.)*(u_pro + ubar_pro))

 a) LHAPDF 6 and this code has a different definition for what PID=1 and 2 are. That stumped me for a while, but I'm pretty sure u_pro is indeed the proton u PDF.

b) For the neutron, d_neu=u_pro and vice versa, so the line actually reads 2 *x * ( 4/9 ( u_neu +ubar_neu) + 2/9 ( d_neu +dbar_neu)). I changed it back to be correct for the proton.

c) BUT: I do not have the 2 there. Where does that come from? I'm not super versed in PDFs, but it is my understanding that that shouldn't be there.  If not, can anybody please explain?

( d) There is also a small difference in the code here and LHAPDF for x<0.1.  10% or so. That must be in the underlying PDF or Q^2 evolution)

In any case, without the 2, my code and this code essentially agree on F_2^p (with Q^2=1, very close to the plots in the proposal. With Q^2 changing, slightly different from the proposal, maybe 30% at most, but both codes the same way. Makes me think that the proposal line comes from a different program, which might explain the discrepancy with the 2).

4) Going back to table 6. We already know that it was not updated from the first proposal, so it likely already has a factor of 2 missing in the F/F ratio. With this additional factor 2, we are getting very close to what my program has, 4 is close enough to 5 that I would believe the rest is acceptance, slightly different cuts, etc, or the first factor 2 was actually 2.5 or something.

With these changes, both my code as well as the code I got from Carlos, modified as described above, gives a ratio F/F of 550 for the first line in the text. This assumes we accept k between 60 and 500 Mev, x between 0.05 and 0.2, at around 12 degrees, with 30 to 70 deg proton angle.   This is also the number my MC gets, and roughly what I would get looking at the plots.  Or is there some other cut that should be applied? Cut on z (y in the code)?

TLDR: If all my assumptions are correct, we see indeed 5.5 times more TDIS events (per DIS event) than we thought!

Let me know what you think!

Best,

Jan

Attached: TDIS_orig.f, code I got from Carlos. TDIS.f: Code with my modifications.

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--
Dr. Jan C. Bernauer
Assistant Professor
Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
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