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

Tue Aug 11 16:03:08 EDT 2020

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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