[Clas12_sidis_working_group] Fwd: Fw: [EXTERNAL] J12-24-RunGroup A

Mon Jul 1 11:15:54 EDT 2024

FYI

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*From:* Latifa Elouadrhiri <latifa at jlab.org>
*Sent:* Sunday, June 30, 2024 10:02 PM
*To:* clas12_first_exp at jlab.org <clas12_first_exp at jlab.org>
*Cc:* Latifa Elouadrhiri <latifa at jlab.org>
*Subject:* Fw: [EXTERNAL] J12-24-RunGroup A

Dear All,

Below are the questions from Marco Radici, our PAC reader, along with the
attached TAC report and the theory report. We need to address all the
questions in a written document this week and update the presentation,
accordingly and taking into account the comments and questions made during
the CLAS collaboration meeting.

I will work with the deep exclusive process to address the related
questions. Harut, please coordinate with the SIDIS group to address the
questions in your section. The comments in other sections are minor.

Our CLAS12-first experiment meeting on Wednesday morning will focus on
these issues.

Best regards,

Latifa

------------------------------
*From:* Marco Radici <marco.radici0 at gmail.com>
*Sent:* Sunday, June 30, 2024 2:24 PM
*To:* Latifa Elouadrhiri <latifa at jlab.org>
*Cc:* Markus Diehl <markus.diehl at desy.de>; Ilieva, Yordanka <ILIEVA at sc.edu>
*Subject:* [EXTERNAL] J12-24-RunGroup A

Dear Latifa,
   I'm writing to you as the contact person for the Jeopardy RunGroup A
J12-24-RunGroupA, entitled
"11 GeV Polarized Electrons on Liquid Hydrogen Target to Study Proton
Structure, 3D Imaging, and Gluonic Excitations"
to be considered at the upcoming JLab PAC52 meeting.

You should have already received the Theory report, and I'm sorry of being
late with respect to that one.
But overall I must confirm some of the remarks contained in that report.
While the very broad physics program covered by the proposal is still
central to the physics agenda of JLab12, and its scientific motivation
remain strong since the last Jeopardy review (if I'm not mistaken, it is
PAC48, where the scientific rating was "A"), I share the opinion of Theory
report's authors that some parts of the proposal should be completed with a
more detailed analysis of the expected statistical impact from the
remaining beam time (65 PAC days of the total 139).

An explicit example of it is given in Sec. 3.1 on TCS: comparison of "blue
points" with "gray points" in Fig.10 gives a visual estimate of what will
be gained with the remaining beam time w.r.t. data collected in the 2018
and 2019 runs.

This kind of information is missing in most of the discussion on Deep
Exclusive Processes:

- in Sec. 1.3 (E12-06-108) where in Fig.2 it would be particularly useful
in comparison with the displayed theoretical calculations

- in Sec. 1.4 (E12-12-007) where there is no plot of the t-dependence of
the cross section nor of the estimate of its error with accumulated and
remaining statistics

- in Sec. 1.5 in hard exclusive rho electroproduction, where there is a
vague reference to SDMEs: are they measurable with expected total
statistics or not? Incidentally, it might be interesting to look also at
the sigma_L / sigma_T ratio, where other data exists and show a puzzling
deviation from the perturbative QCD prediction (see the Compass publication
E.P.J. C83 (23) 924)

- in Sec. 1.6 (DVMP with pions to study TDA), where an order of magnitude
increase in statistics is wished but there is no study of the statistical
impact of remaining PAC days on the errors indicated in Fig. 5

- in Sec. 1.7 (Sullivan process for pion GPDs), where there is no
indication of the statistical impact on errors in Fig. 6; incidentally,
where does the blue curve come from? Neither caption nor text mention it

- in Sec. 1.8 (N* DVCS): there is a statement (".. Based on the statistics
already collected with RG-A, a differential study is only possible in one
kinematic variable, in addition to the phi-dependence, with relatively
large uncertainties..") which is not supported by any figure. So that it's
hard first to convince of the mentioned agreement with theoretical
predictions, and secondly to estimate the impact on uncertainties from
accumulating the remaining statistics.

Also, about DVCS (E12-06-119) and Fig. 1 in Sec. 1.2, I think it would be
useful to refine the comparison with more updated theoretical calculations,
as suggested also in the Theory report.
KM15 has been now superseeded by KM20 (P.R.L. 125 (20) 232005, with flavor
separation of CFFs); it could be useful to have a look also at calculations
with NLO corrections (JHEP 12 (23) 192), as well as to the results in the
PARTON framework (Moutarde et al., E.P.J. C79 (19) 614). Both groups use
modern analysis techniques, based on Neural Networks.
P.S. In Fig. 1 both labels VGG and GK are not explained in the caption.

As for Sec. 2 on pion SIDIS (E12-06-112 and E12-06-112A), I share the
general comment of the Theory report.
In SIDIS at low Q2, there are important questions that need to be
addressed, like the validity of TMD factorization, the separation between
current and target fragmentation, the role of vector meson decays, etc..
I'm convinced that the best observable to attack these problems is the
differential cross section. Azimuthal/spin asymmetries, as ratios of cross
sections, can indeed hide the answer to above questions. Even
multiplicities, as ratios to the collinear inclusive cross section, can
show deviations from theoretical expectations that are not directly related
to the elementary mechanisms to be explored (see, for example, the "SIDIS
normalization problem" in recent global extractions of TMDs).
Given the potential of exploring the 5-dim. phase space in
(x,z,Q2,PhT,phi), the RunGroup A should concentrate in extracting the cross
section differential in these variables at the best accuracy provided by
the attainable total statistics.

I have also a couple of questions on the text of Sec.2:

- Eq.(1) is anticipated by a paragraph warning about the lack of
information related to the part of the unpolarized structure function F_UU
related to the longitudinal polarization of the exchanged virtual photon:
F_UU,L.
However, Eq.(1) shows an ambiguous F_UU notation and its expression
includes twist-3 contributions (A_UUcosphi and A_LUsinphi). Does this mean
that F_UU includes only the photon-transverse contribution F_UU,T, since
F_UU,L is at twist 4? The text should better clarify the point

- when building the multiplicities, NLO collinear PDFs and FFs are used,
while the comparison with theory calculations is claimed to be done at LO.
Why?

- in the discussion of Fig.8, there is no mention of the x variable; the
plots are for a specified Q-y bin...

- about Dihadron Fragmentation Functions (DiFFs), there is a misleading
statement in the text. The claim about G1perp is made that ".. Extending
this analysis with a partial wave expansion will give access to the
correlation between the fragmenting quark angular momentum with the angular
momentum of the dihadron...".
This is not correct. G1perp means that the fragmenting quark is always
longitudinally polarized. And the partial-wave expansion gives information
on the relative orbital momentum inside the hadron pair.

- DiFFs are mentioned as tools to access the chiral-odd PDF e(x). But the
A_LU asymmetry at twist 3 contains several other contributions that are not
mentioned. Are they small and negligible?

I find Secs. 4,5,6 clearly written and self-comprehensive.
I have only these small remarks:

- Fig.14: no color code is mentioned to identify the different
contributions to the total curve

- Fig. 15: the caption says that the Partial-Wave Amplitudes are plotted as
functions of the four-momentum transfer squared. Is this the meaning of the
label "r" in the plots? It is an unusual notation...

Best regards,
Marco Radici

-- 
============================================

Marco Radici

INFN - Sezione di Pavia
via Bassi 6
I-27100 Pavia
Italy

tel. +39 0382 987451
fax  +39 0382 423241

email  marco.radici at pv.infn.it
web    https://urldefense.proofpoint.com/v2/url?u=http-3A__www.hadronicphysics.it_hasqcd_&d=DwIFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=DY5bvdgdiVAuV4VN9BUWoHOeTrDboZpgTlUO3E73MLYiH0KpQi_Cig1v0M8doABE&m=ELSG635ISqjs4rJoiety69UsFmhhT8cogNi2zKQn2ZQWdbos1VUvAjCEjxua6lUH&s=GeVf-sELQ11orf9vG2Stl2JVxaUYw5VJEQj5dk4nLns&e= 
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