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

Hobbs, Timothy tjhobbs at mail.smu.edu
Thu Aug 6 12:17:05 EDT 2020


Hi Jan and all,


Thanks for your detailed checks of the various calculations in the proposal and elsewhere.  I am glad that things seem to align generally (at least that's my reading --- e.g., that you recover what is shown in my arXiv posting), albeit with some outstanding ambiguities.


I have to confess that I've been too swamped to go through this in much detail lately, but, for the time being, it is certainly conceivable that a factor of 2 may have crept into the structure functions (but let me see what I think).  I know that these codes and their descendants passed through several hands at various points, so I am not sure all these years later what changes may have been implemented.  I do recall building structure functions from the LO quark-model expressions (a la the PDG) at the time, which seemed sufficient at least for the proposal and MC simulations.  You mentioned some comparisons with LHA sets (I am not sure which exactly), but it is not terribly surprising that lower-x distributions shift at least somewhat, depending upon the comparison you mean.


Anyway, I would be happy to discuss details with a smaller group at some point after I've had a chance to digest your messages and have performed a bit of archaeology with these bits of code.


Best,

Tim

________________________________
From: Jan C. Bernauer <jan.bernauer at t-online.de>
Sent: Thursday, August 6, 2020 10:43:41 AM
To: tdis at jlab.org; Hobbs, Timothy; Wally Melnitchouk
Subject: Inconsistencies in the TDIS proposal: Some things I found

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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