[Primexd] Questions on the PrimEx analysis strategy

[Igal Jaegle]

Thank you, Ashot, for finding the time to give your answers in your busy schedule.

Questions #1, I see that we are converging toward the same answer.

Questions #2, my understanding is that Ilya used different elementary amplitude scenario/model for the Nuclear Coherent but these changes were not applied to the Nuclear Incoherent or what you call background. I do not agree with this approach. If the elementary amplitude used in the Nuclear Coherent is changed then the exact same changed must be applied to the Nuclear Incoherent part. This cannot be neglected and looking at the variation of the chi^2 will not capture correctly the systematic error linked to the model dependence.

Here by different models, I am also referring  to the inclusion yes or no of the quasi-free Primakoff/Coulomb and more importantly the corresponding interference between the quasi-free Primakoff/Coulomb and rho+omega terms.

Questions #3, we do not agree. Any changes to the elementary amplitudes contributing to the Nuclear Coherent term must also be applied to the Nuclear Incoherent term because any changes to the elementary amplitudes will change automatically the shape of the Nuclear Incoherent particularly at small angle particularly for the eta case.

tks ig.
________________________________
From: gasparan <gasparan at jlab.org>
Sent: Thursday, October 31, 2024 2:10 PM
To: Igal Jaegle <ijaegle at jlab.org>
Cc: primexd at jlab.org <primexd at jlab.org>; Malte Albrecht <malte at jlab.org>; Justin Stevens <jrsteven at jlab.org>; Eugene Chudakov <gen at jlab.org>; Shepherd, Matthew <mashephe at indiana.edu>
Subject: Re: [Primexd] Questions on the PrimEx analysis strategy


  Hi Igal,

Here are my initial answers to your three basic questions outlined in
your recent text. The text is also attached to this email
and it is not edited for a wider use.

Question #1
Which process(es) can we measure?
My answer:
If one includes the BCAL in coincidence with the eta detected in the
FCAL then the list of measurable processes is rather large, including
some elastic and very many inelastic reactions.
However, we cannot measure the gamma + He4 -> eta + He4 elastic process
at these small angles since recoil He4 (or alpha) can not reach to the
BCAL sensitive area due to its small recoil energy. With that in mind,
all Primakoff type experiments so far are designed to not attempt
measuring the recoil nuclei since it is not possible at ~~1-degree
angles. In particular, our current eta-proposal is designed/approved
without measuring the recoil alpha particles in the elastic reaction.
Rather we assume that at these angles (0-4 degrees) the events without
the recoil alpha detection can be: (a) Primakoff coherent; (b) nuclear
coherent; (c) interference between them; (d) all kind of incoherent and
inelastic processes having an eta as a one of final states. We call the
(d) as a background in our experiment(s).
The remaining question is how to identify them and fit out underneath of
the  Primakoff part but, this is a separate question.

Question #2
Can we rely on only one model and/or only one scenario for the radiative
decay width extraction?
My answer:
It is always better to have more than one model to use in the extraction
process (since any model is not a fundamental theory, also includes some
limited information only). If we succeed to have at least two models, we
need to use them both. If we do not have the second model, then we will
rely on it and state it in the publication.
About the “…only one scenario…”. Obviously, it is better to have two or
more ways to extract the decay width. In pi0 PrimEx in Hall B we had 3.
Here we are trying hard to have 2 at least.

Question #3
  Can the built-in model interdependence between the Nuclear Incoherent
term and the Primakoff and Nuclear Coherent terms be neglected?
My answer:
This question needs more clear wording to try answering it. However, I
will guess the question and try to answer it. Two processes (mentioned
above) (a) Primakoff (that is electromagnetic coherent exchange
reaction) and (b) the “Nuclear coherent” (that is the same only with a
hadronic exchange) kinematically (and quantum mechanically) are the same
(if you want, similar) elastic and coherent processes. They can be
simulated by theory very accurately. Small exception is some constants
and parameters, mostly for the nuclear coherent part. In addition, since
they are quantum mechanically similar, they have an interference term as
a process, this is also a predictable thing with the phase angle.
The so-called “incoherent part” in reality is all processes other than
first two. We call them as a “background” in the Proposal.
The short answer: If we have all these 4 processes simulated (predicted)
by model(s) then we should not “… neglect…” or ignore any process, we
simply include them in our extracted cross section for all forward
angles. This is related to the good case when we are hoping to 1-3%
extraction. Since (unfortunately) we are not there yet there are several
variations in my previous statement. That is a separate subject, and we
had discussed them in our meeting many-many times.

Best,
Ashot


On 2024-10-18 12:08, Igal Jaegle via Primexd wrote:
> Hi, Ashot,
>
>  I put into a note my questions,
> https://halldweb.jlab.org/doc-private/DocDB/ShowDocument?docid=6668
> [1].
>
>  A dedicated PrimEx discussion at the GlueX Physics Working Group
> Meeting would be highly beneficial, especially if there are differing
> perspectives on how to address these questions. We would greatly
> welcome such a discussion.
>
>  tks ig.
>
> Links:
> ------
> [1] https://halldweb.jlab.org/doc-private/DocDB/ShowDocument?docid=6668
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