[Tdis] [EXTERNAL] Inconsistencies in the TDIS proposal: Some things I found
Jan C. Bernauer
jan.bernauer at stonybrook.edu
Mon Aug 10 14:03:32 EDT 2020
Hi all,
An update from the trenches of the generator writing:
1) G4SBS seems to agree on the definition of F2. Would be nice if
somebody who knows what they are doing (Tim, Wally?) can double check
though. So if there is no other effect, I'm pretty sure we see ~5 times
more than we thought.
2) G4SBS also agrees with my estimate of the total DIS rate. It's
actually slightly higher, but I think that's because the simulated e-arm
acceptance is slightly bigger. I'll update my analysis with that. I'm
pretty sure it's very close to 2 times what the proposal has, so I'd
hazard a guess and a factor of 2 comes from the number of molecules to
the number of protons. It's also assuming that we run at 77K -- I
think we talked about running at room temperature, that makes it 4 times
worse of course. (2 times worse than what is in). Can somebody
(Carlos?) run g4sbs in DIS configuration and see the total rate between
x 0.05 and 0.2? I assume a particle is accepted if
Harm.SBSGem.Track.P[0]>1 (for very few it's not track 0).
So I think, if we run room temp, we'll get half the DIS rate we assumed,
but about 2.5 times the TDIS rate (but in a different distribution, as
far as I can tell). I'll make some updated reach plots, but I don't have
background numbers. Does anyone have those for all bins?
Best,
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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