[Tdis] [EXTERNAL] Inconsistencies in the TDIS proposal: Some things I found
Jan C. Bernauer
jan.bernauer at stonybrook.edu
Tue Aug 11 12:07:10 EDT 2020
Hi Wally,
Thanks! I think it's settled now that the factor 2 gain in the ratio is
indeed there, I'm just glad it goes in the right direction :)
I'll have a look at the NLO calculations.
Best,
Jan
On 8/11/2020 9:44 AM, Wally Melnitchouk wrote:
> Hi Jan,
>
> The leading order (in α_s) formula for F2 is F2(x,Q) = Σ_qe_q^2 * x
> *[q(x,Q) + qbar(x,Q)], without a factor 2 in front. Definitions like
> this you can check with the PDG, which I try to use as much as
> possible to standardize notations/conventions [see e.g. Eq. (18.18) of
> the online PDG].
>
> If you are interested in quantitive, or even semi-quantitative,
> calculations of F2, I would however use at least NLO PDFs with the
> corresponding NLO formula, which just involves a convolution of the
> (NLO) PDF with the hard scattering function [see e.g. Eq. (18.21) of
> PDG]. Also for standardization, you should probably use LHAPDF for any
> PDF set (which sounds like you have implemented already).
>
> As far as cuts implemented in the code, I cannot tell you anything
> reliable because I didn't work on the latest version of the code, so I
> can't really help with 2).
>
> It's a good confirmation at least that the two independent codes give
> similar results, with the factor 2 correction you found.
>
> Let me know if you have further questions.
>
> Wally
>
>
> ------------------------------------------------------------------------
> *From:* Jan C. Bernauer <jan.bernauer at stonybrook.edu>
> *Sent:* Tuesday, August 11, 2020 9:19 AM
> *To:* Wally Melnitchouk <wmelnitc at jlab.org>
> *Cc:* tdis at jlab.org <tdis at jlab.org>; Hobbs, Timothy <TJHobbs at mail.smu.edu>
> *Subject:* Re: [Tdis] [EXTERNAL] Inconsistencies in the TDIS proposal:
> Some things I found
>
> Hi Wally,
>
>
> Thank you for looking into this.
>
>
> Your suggestion to write my own implementation is a very good one, and
> exactly what I did. This is where I found the discrepancies to the
> proposal (except the ones in the proposal itself). I think there are
> two questions left for that:
>
> 1) Is my formula for F2p correct, or the one in the code I attached.
> I.e, is F2 = x * Sum quarkcharge_i^2
> quarkdistributionfunction_i(x,Q^2) (what I think) or is there an
> additional factor 2 (what the old code says).
>
> 2) Are there some other cuts I didn't realize
>
>
> I think all other points I raised are simple mistakes in the original
> proposal or otherwise understood artifacts.
>
>
> Other than that, your code + my code produce the same results. Q2
> can probably only be answered by the person who originally came up
> with the table in the proposal. But if you could write down what you
> think the formula for F2p is, it would be a good cross check that I
> didn't just misunderstand the textbooks.
>
>
> Best,
>
> Jan
>
>
> On 8/11/2020 12:30 AM, Wally Melnitchouk wrote:
>> Hi Jan,
>>
>> Thanks for your emails, and apologies for the delay.
>>
>> The programs you appended look familiar -- I think I wrote the core
>> of them back in the 1990s -- and Tim developed them some half dozen
>> years ago (and possibly other people before and since) when we were
>> working on some related applications.
>>
>> The best suggestion I would give anyone interested in implementing
>> the meson exchange contributions to the DIS structure functions would
>> be to write some script yourself from scratch. The formulas are
>> actually very simple, if you look at the analytic expressions. The
>> original ancient Fortran could probably now be written in a few lines
>> of python code and done much more efficiently, and writing it
>> yourself would give you a much clearer idea of what is being computed.
>>
>> I am happy to discuss the formulas with you or anyone else if you
>> have any questions -- maybe more efficiently over bluejeans/zoom.
>>
>> Best regards,
>> Wally
>>
>>
>> ------------------------------------------------------------------------
>> *From:* Jan C. Bernauer <jan.bernauer at stonybrook.edu>
>> <mailto:jan.bernauer at stonybrook.edu>
>> *Sent:* Monday, August 10, 2020 2:03 PM
>> *To:* tdis at jlab.org <mailto:tdis at jlab.org> <tdis at jlab.org>
>> <mailto:tdis at jlab.org>; Hobbs, Timothy <TJHobbs at mail.smu.edu>
>> <mailto:TJHobbs at mail.smu.edu>; Wally Melnitchouk <wmelnitc at jlab.org>
>> <mailto:wmelnitc at jlab.org>
>> *Subject:* Re: [Tdis] [EXTERNAL] Inconsistencies in the TDIS
>> proposal: Some things I found
>>
>> 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
>>> <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.
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> Tdis mailing list
>>> Tdis at jlab.org <mailto:Tdis at jlab.org>
>>> https://mailman.jlab.org/mailman/listinfo/tdis
>> --
>> Dr. Jan C. Bernauer
>> Assistant Professor
>> Department of Physics and Astronomy
>> Stony Brook University
>> Stony Brook, NY 11794-3800
> --
> Dr. Jan C. Bernauer
> Assistant Professor
> Department of Physics and Astronomy
> Stony Brook University
> Stony Brook, NY 11794-3800
--
Dr. Jan C. Bernauer
Assistant Professor
Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
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