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

Mon Feb 20 05:59:57 EST 2023

Hi,

one thing that can be done is to drag along events gated on RF side peaks.
> Then you can see whether significant amounts of accidentals are left after
> full analysis.
>

This would mean opening up the primary coincidence peak window to include
side peaks, and then looking at the time spectrum for the final sample
after all cuts, right? No question, there will be significant events in the
side peaks from this investigation for any final state extracted from GlueX
phase II running. This is the reality for tagged experiments, otherwise we
could do event-by-event tagging. But the argument is that it doesn't matter
because the mistake you are making by selecting the wrong tag "doesn't get
it wrong by much". The challenge here is what to use for the flux that goes
with this analysis, because the flux is only defined for an
accidentals-subtracted yield.

-Richard Jones

On Mon, Feb 20, 2023 at 5:44 AM Susan Schadmand via Halld-tagger <
halld-tagger at jlab.org> wrote:

> *Message sent from a system outside of UConn.*
>
> Hi,
>
> one thing that can be done is to drag along events gated on RF side peaks.
> Then you can see whether significant amounts of accidentals are left after
> full analysis.
>
> Su.
>
> On 20.02.23 11:33, Richard Jones via Halld-tagger wrote:
>
> Hello Peter,
>
> Thanks for asking, I noticed this as well, but I thought it was only being
> used for PWA where the primary focus is on the angular distributions. I
> agree that it is a concern for differential cross sections. You are right
> that this is not properly taking into account the accidentals that are
> present. Essentially it amounts to a hybrid between a fully tagged and a
> fully untagged experiment. Here are those two extremes:
>
>    1. A(untagged) -- the photon energy is inferred from the reconstructed
>    final state, and used to compute all of the beam properties associated with
>    the event: the flux, the polarization, etc.
>    2. B(fully tagged) -- the photon energy is inferred from the
>    unambiguously associated hit in the tagger, which is used as input to the
>    kinematic fit and to lookup beam properties for the event
>
> At the rates of GlueX phase 2, we do not have the luxury of option 2 on an
> event-by-event basis, but we can achieve it by accidentals subtraction.
> Short of full accidentals subtraction there are several short-cuts you can
> use. All of these have uncontrolled systematics.
>
>    1. best chi square - put them all in a ring and take the last man
>    standing as the winner with weight 1
>    2. weighted average - count them all above some chi-square acceptance
>    cut and weight each event by 1/n where n is the number of surviving tags
>
> Both of these methods reduce to tagging strategy A(tagged) at low rate,
> while they reduce to strategy B(untagged) at high rate. At GlueX Phase II
> intensities we are some intermediate hybrid of the two with these
> shortcuts, certainly not approximating B(fully tagged).
>
> To see what these short-cuts entail, consider the high-rate limit in the
> tagger. At high rate, the extracted cross section goes to infinity for a
> realistic tagger and an ideal GlueX detector. In reality, the asymptote
> would be something greater than one, channel and final state dependent, and
> probably run period dependent as well. The reason for this is that the
> tagger detection efficiency per beam photon goes down at high rate, while
> the accidentals continue to grow and generate a valid result for any
> reconstructed final state, tagged or untagged. So the flux that you need to
> put into the denominator under the yield for extracting a cross section
> will be different depending on the final state. Using the same flux
> regardless of final state could be a leading cause for why we are seeing
> different cross sections for the charged and neutral decays of eta.
>
> Beyond that, the shape of the flux spectrum (and the polarization spectrum
> for polar observables) is different from the shape of the energy dependence
> of the reconstructed yield. Take for example the energy-dependent cross
> section around the coherent edge. The tagged flux has a sharp edge, whereas
> the reconstructed yield washes out the edge with a resolution that depends
> on everything in sight: the kinematic fit cut, conditions in the detector,
> the particular final state, etc. One way to reduce our dependence on the
> different beam photon energy resolutions in the flux and yields is to
> average over a wide range in beam energy. As long as we are not interested
> in the s-dependence of the cross section, this might be justified and would
> reduce the systematics from these short-cut approaches.
>
> In PWA, i understand that this avoids the pain of negative weights and so
> improves the statistical error from the fits (or at least it gives that
> feeling). In fact, it introduces a set of new systematic errors of its own
> that will probably drive us back to the more rigorous approach before we
> are done. For the moment I am not speaking up about this because we just
> need to get our first results out. But eventually this needs to be given a
> critical review. I hope to be part of that at some level, as soon as my
> work on photon beam systematics reaches a level where it can be used for
> publications.
>
> -Richard Jones
>
> On Sun, Feb 19, 2023 at 11:13 AM Peter Hurck <Peter.Hurck at glasgow.ac.uk>
> wrote:
>
>> *Message sent from a system outside of UConn.*
>>
>> Hi Richard,
>>
>> During the collaboration meeting a few people presented analyses which
>> used a chi^2 ranking with a +-2ns cut around the RF peak instead of tagger
>> accidental subtraction.
>>
>> My initial thought was that this is wrong and not recommended. Did the
>> guidance by the beam line group change regarding this issue? Given that the
>> current a2 cross-section analysis is using this method and there is a big
>> push to publish it asap I am concerned that this might not be resolved
>> properly and might set a bad precedent going forward.
>>
>> Given that you are the expert on this topic, what are your thoughts on
>> this issue? Is that a legitimate way to perform analyses?
>>
>> Cheers,
>> Peter
>>
>>
>> ----------------------------------------------------
>> Dr Peter Hurck (né Pauli)
>>
>> My new email address is
>> Peter.Hurck at glasgow.ac.uk
>>
>> Research Associate
>> Nuclear and Hadron Physics Research
>> School of Physics and Astronomy
>> University of Glasgow
>>
>>
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