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

[Susan Schadmand]

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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