[Halld-tagger] [EXTERNAL] Fwd: Accidental subtraction

[Richard Jones]

Jon,

   - *Is the tagged flux is universal, if you adopt the typical accidental
   subtraction scheme?*

Yes, that is my understanding.

   - *Do we gain meaningful statistical precision by doing something that
   sidesteps accidental subtraction?*

Yes, the statistical precision will be better using a hybrid scheme. A
given data set will yield smaller statistical error bars, splitting up a
run into sub-bits and comparing the results from each bit will have smaller
scatter between the results from different bits, confirming the statistical
errors, etc. But one is trading off statistical precision here for
systematics on the flux.

Here I add another question I thought of that you might be wondering about:

   - *Doesn't the simulation take everything into account in the hybrid
   scheme anyway? So the acceptance becomes rate dependent, which it is
   already anyway due to pile-up in the detector, so what's the problem with
   letting the simulation deal with pile-up effects in the tagger as well?*

One could have designed the simulation that way, but we (I) didn't. My
experience told me that systematics from rate-dependent effects start
accumulating in the tagger at MUCH lower rates than they show up in the
detector subsystems, ie. extra tracks in the FDC, accidental association in
the start counter, TOF, FCAL, etc. The job of trying to describe the
rate-dependent behavior of the individual tagging counters reliably in the
simulation is much more difficult, and with limited time and manpower to
devote to this task, it could be nonconvergent. That is why there is no
tagger microscope or hodoscope in the hdgeant simulation. The downside of
that is that accidentals subtraction is the only model-independent method
we have with the current toolset to produce a differential cross section.

-Richard Jones

On Wed, Feb 22, 2023 at 4:30 PM Jonathan Zarling <jzarling at jlab.org> wrote:

> *Message sent from a system outside of UConn.*
>
> Hi all,
>
> I appreciate the deep thinking on this, I too had been wondering about
> implications of the various strategies. I guess I'd like to ask/clarify two
> things:
>
>    -
> *Is the tagged flux is universal, if you adopt the typical accidental
>    subtraction scheme? *I.e. if properly calibrated, it should
>    incorporate any rate dependence, tagger inefficiency, dependence on final
>    state, etc. Assuming this strategy fits well with the downstream analysis.
>    Then on the other hand, if you do something else, say picking our
>    "pick the best in-time chi^2 photon" cut, the tagged flux doesn't have any
>    easy correspondence.
>    - *Do we gain meaningful statistical precision by doing something that
>    sidesteps accidental subtraction?*
>    If the tagger efficiency becomes large enough in a stats. limited
>    analysis, this "pick the best in-time chi^2 photon" would also pick up
>    events where the true photon is lost, but some accidental comes along with
>    similar-ish energy. This feels hacky to me. Could we just go whole-hog and
>    not skip using beam photons altogether? At least in principle. I'm
>    wondering about the b1 pi cross sections and charmonia measurements. Just
>    curious if the charge to the beamline group becomes the same here.
>
> I hope I'm not retreading anything above, there was a lot to go through
> here. I guess I'm particularly interested to make sure the eta p cross
> section results (which DON'T do any of this best combo picking, typical
> accidental subtraction only) shouldn't be affected by the discussion in
> this chain.
>
> Cheers,
> Jon
> ------------------------------
> *From:* Halld-tagger <halld-tagger-bounces at jlab.org> on behalf of Richard
> Jones via Halld-tagger <halld-tagger at jlab.org>
> *Sent:* Tuesday, February 21, 2023 9:49 AM
> *To:* Shepherd, Matthew <mashephe at indiana.edu>
> *Cc:* Hall D beam working group <halld-tagger at jlab.org>
> *Subject:* Re: [Halld-tagger] [EXTERNAL] Fwd: Accidental subtraction
>
> Matt,
>
> You cannot use the tagged flux unless you use an accidentals subtraction
> algorithm. Here are the rules I am claiming.
>
>    1. if you do accidentals subtraction then you have complicated PWA
>    fits, but at least you know what your flux should be.
>    2. if you do hybrid tagging without full accidentals subtraction then
>    you have simple PWA fits, but then you have problems knowing what your flux
>    should be.
>
> This is something of a no free lunch theorem that applies here. See my
> first response to Peter for more details on how the flux is problematic in
> a hybrid tagging scheme.
> -Richard
>
> On Tue, Feb 21, 2023 at 9:02 AM Shepherd, Matthew <mashephe at indiana.edu>
> wrote:
>
> *Message sent from a system outside of UConn.*
>
>
>
>
> ---------- Forwarded message ----------
> From: "Shepherd, Matthew" <mashephe at indiana.edu>
> To: Richard Jones <richard.t.jones at uconn.edu>
> Cc: Hall D beam working group <halld-tagger at jlab.org>
> Bcc:
> Date: Tue, 21 Feb 2023 14:01:31 +0000
> Subject: Re: [Halld-tagger] [EXTERNAL] Fwd: Accidental subtraction
>
> Hi Richard,
>
> > On Feb 20, 2023, at 11:08 AM, Richard Jones <richard.t.jones at uconn.edu>
> wrote:
>
> >  Likewise with trying to measure an absolute differential cross section
> for the a2 over a continuum of rho,pi using amplitude analysis to extract
> the a2 part. The problem I am pointing to here is this: what to use for the
> flux is no longer model-independent if you are not doing proper accidentals
> subtraction.
>
> Not sure I understand the details here... "model-independent"?
>
> When doing amplitude analysis the output of the analysis is a tagged,
> acceptance-corrected yield over a range of beam energy.  We then used a
> tagged flux to turn this number into a cross section.  When obtaining the
> tagged acceptance corrected yield, we can use two methods of handling
> pileup of beam photons in the signal RF bin and they produce the same
> result.
>
> Matt
>
>
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