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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 class="moz-cite-prefix">On 20.02.23 11:33, Richard Jones via
Halld-tagger wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CABfxa3QpAYofmie_UmXv23dg6Gny08Spr9ebpdKxdbe5OJrq9Q@mail.gmail.com">
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<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>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<br>
</div>
</div>
</blockquote>
</div>
</div>
</div>
<br>
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