[Halld-tagger] [EXTERNAL] Fwd: Accidental subtraction
Susan Schadmand
s.schadmand at gmail.com
Mon Feb 20 06:06:51 EST 2023
If there will be significant events in the side peaks, then they have
to be subtracted?
On 20.02.23 11:59, Richard Jones wrote:
> 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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