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If there will be significant events in the side peaks, then they
have to be subtracted?<br>
<br>
<div class="moz-cite-prefix">On 20.02.23 11:59, Richard Jones wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CABfxa3QSFcJUvoFmMyj+cSiZUrxTFSdYmdJmz5=6jvnMsfELUA@mail.gmail.com">
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<div dir="ltr">Hi,
<div><br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">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.<br>
</blockquote>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>-Richard Jones</div>
<br class="gmail-Apple-interchange-newline">
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Mon, Feb 20, 2023 at 5:44
AM Susan Schadmand via Halld-tagger <<a
href="mailto:halld-tagger@jlab.org" moz-do-not-send="true"
class="moz-txt-link-freetext">halld-tagger@jlab.org</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<div
style="background-color:rgb(255,235,156);width:100%;border-style:none;border-color:rgb(250,235,204);border-width:1pt;padding:10pt;font-size:11pt;line-height:12pt;font-family:Calibri;color:rgb(0,0,0);text-align:left"><span
style="color:rgb(156,101,0)"></span>*Message sent from a
system outside of UConn.*</div>
<br>
<div>Hi,<br>
<br>
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.<br>
<br>
Su.<br>
<br>
<div>On 20.02.23 11:33, Richard Jones via Halld-tagger
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div class="gmail_quote">
<div dir="ltr">Hello Peter,
<div><br>
</div>
<div>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:</div>
<div>
<ol>
<li>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.</li>
<li>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</li>
</ol>
<div>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.</div>
</div>
<div>
<ol>
<li>best chi square - put them all in a ring
and take the last man standing as the winner
with weight 1</li>
<li>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</li>
</ol>
<div>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).</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>-Richard Jones</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sun, Feb 19,
2023 at 11:13 AM Peter Hurck <<a
href="mailto:Peter.Hurck@glasgow.ac.uk"
target="_blank" moz-do-not-send="true"
class="moz-txt-link-freetext">Peter.Hurck@glasgow.ac.uk</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px
0px 0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div>
<div
style="background-color:rgb(255,235,156);width:100%;border-style:none;border-color:rgb(250,235,204);border-width:1pt;padding:10pt;font-size:11pt;line-height:12pt;font-family:Calibri;color:rgb(0,0,0);text-align:left"><span
style="color:rgb(156,101,0)"></span>*Message
sent from a system outside of UConn.*</div>
<br>
<div>
<div>Hi Richard,</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>Given that you are the expert on this
topic, what are your thoughts on this
issue? Is that a legitimate way to perform
analyses?</div>
<div><br>
</div>
<div>Cheers,</div>
<div>Peter</div>
<div><br>
</div>
<br>
<div>
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<div><font color="#7b7b7b">----------------------------------------------------<br>
Dr Peter Hurck (né
Pauli)<br>
<br>
My new email address is</font></div>
<div><font color="#7b7b7b"><a
href="mailto:Peter.Hurck@glasgow.ac.uk" target="_blank"
moz-do-not-send="true"
class="moz-txt-link-freetext">Peter.Hurck@glasgow.ac.uk</a></font></div>
<div><font color="#7b7b7b"><br>
Research Associate<br>
Nuclear and Hadron
Physics Research<br>
School of Physics and
Astronomy<br>
University of Glasgow</font></div>
</div>
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</div>
</div>
</div>
</div>
</div>
</div>
<br>
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</blockquote>
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