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<p>Hi Andy,</p>
<p>There is study showing that in the low pt range, it's not always
the ME photon having the largest pT. So it might not work.<br>
But turnning off QED in Pythia is definitely a good idea!<br>
Thanks a lot! I'll try that.<br>
<br>
Best,<br>
Yichen<br>
</p>
<br>
<div class="moz-cite-prefix">On 07.03.2018 14:54, Andy Buckley
wrote:<br>
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cite="mid:local-5344fa7c-f8b5-v1.1.4-22d9f20d@unity">
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<div>Here you are hitting the problem that the processes you are
comparing are not actually the same, and you need to find a
mapping between them.</div>
<br>
<div>I don't know what is most sensible in this case, but choosing
the highest-pT photon (or the one with largest kT-distance from
the lepton?) might be a reasonable approach. In general you
cannot know which one was "first", since both the shower photons
and the ME one are perturbative.</div>
<br>
<div>Alternatively, or to validate a heuristic for leading-photon
identification, you could turn off the QED shower in Pythia!</div>
<br>
<div>Andy</div>
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<div><strong>Dr Andy Buckley, Lecturer / Royal
Society University Research Fellow</strong></div>
<div>Particle Physics Experiment Group, University
of Glasgow</div>
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<div class="gmail_quote_attribution">On Mar 7 2018, at 1:03 pm,
Yichen <a class="moz-txt-link-rfc2396E" href="mailto:yichen.li@cern.ch"><yichen.li@cern.ch></a> wrote:</div>
<blockquote><br>
<div><br>
<div>Hi Andy,</div>
<br>
<div>
<div>Thanks for the suggestion.</div>
<div>Yes, I've considered using PromptFinalState.</div>
<div>But the problem is there will be additional QED added
by Pythia8, in addition to the one in the ME, which will
be considered as PromptFinalState.</div>
<div>And I don't want these additional QED photons, since
the high order calculation, which will be used for
deriving the kfactor, consider only 1 photon radiation.</div>
<br>
<div>But I didn't study this effect quantitatively. Maybe it
can be neglected or suppressed with cuts. I'll take a
look.</div>
<br>
<div>Best,</div>
<div>Yichen</div>
</div>
<div>On 07.03.2018 10:56, Andy Buckley wrote:</div>
<blockquote><br>
<div>Hi Yichen,</div>
<br>
<div>The accepted way to identify hard-process leptons and
photons is by working backward from them in the final
state, and verifying that they do not have any hadronic
ancestors. This is specified in the ATLAS truth-object
definition PUB note, for example, and is most conveniently
achieved in Rivet by using the PromptFinalState.
Regardless of intentions, starting from looking for "ME"
particles with particular status codes will be on the
verge of unphysicality unless you are <em>very</em>
careful.</div>
<br>
<div>Hope that helps,</div>
<div>Andy</div>
<br>
<div>
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<div><strong>Dr Andy Buckley, Lecturer /
Royal Society University Research Fellow</strong></div>
<div>Particle Physics Experiment Group,
University of Glasgow</div>
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<div>On Mar 7 2018, at 9:26 am, Yichen <a
href="mailto:yichen.li@cern.ch"
title="mailto:yichen.li@cern.ch" moz-do-not-send="true"><yichen.li@cern.ch></a>
wrote:</div>
<blockquote><br>
<div><br>
<div>Dear Andy and Torbjör,</div>
<br>
<div>
<div>Thanks a lot for your replies!</div>
<br>
<div>By looking for these ME particles, we are not
trying to do unphysical measurement. Our motivation
is the following:</div>
<div>1) We unfolded our measurement to a particular
decay channel of the ttbar (e.g. to e/mu+jets
channel). So we need the decay information of the
ttbar to remove other channels so that we can
compare the MC sample, which includes all
non-hadronic decay channels, with the unfolded
result.</div>
<div>2) We are trying to calculate kfactor for our MC
sample in a phase space where there is jet
multiplicity cut (as well as lepton/photon pt/eta
cuts). The higher order calculation is only at ME
level. So to get the kfactor, we want to identify
the ME leptons/photons in the MC sample and trace
their history until they reach their final states
and then use these final states to define the phase
space.</div>
</div>
<div>Here is the piece of code in Rivet (for analyzing
e+jets channel):</div>
<div>""</div>
<div>bool foundMEel = false;</div>
<div>bool foundMEnuel = false;</div>
<div>bool foundMEph = false;</div>
<div>Particles all = event.allParticles();</div>
<div>foreach (Particle p, all) {</div>
<div> if (p.genParticle()->status() == 23) {</div>
<div> if (fabs(p.pid()) == 11) foundMEel = true;</div>
<div> if (fabs(p.pid()) == 12) foundMEnuel = true;</div>
<div> if (fabs(p.pid()) == 22) foundMEph = true;</div>
<div> }</div>
<div>}</div>
<div>if (!(foundMEel && foundMEnuel &&
foundMEph)) return;</div>
<div>""</div>
<div>I put an example in cern lxplus068
/tmp/yili/example/</div>
<div>If you can't use lxplus, I also put a copy in
dropbox: <a
href="https://www.dropbox.com/sh/5tdl2slccr3dc9u/AACHY9hlB6pvdrpcSql_2M_na?dl=0"
title="https://www.dropbox.com/sh/5tdl2slccr3dc9u/AACHY9hlB6pvdrpcSql_2M_na?dl=0"
moz-do-not-send="true">https://www.dropbox.com/sh/5tdl2slccr3dc9u/AACHY9hlB6pvdrpcSql_2M_na?dl=0</a></div>
<br>
<div>In this example code, the evnt/ dir hosts the HEPMC
format of the MC sample. Its corresponding LHE can be
found with the name events.lhe</div>
<div>From the events.lhe, you can find that this is a
pure e+jets channel sample. So with the above code,
every event should pass the cut. But only 24% passed.</div>
<br>
<div>Best,</div>
<div>Yichen</div>
<br>
<div>
<div>On 06.03.2018 21:09, Andy Buckley wrote:</div>
</div>
<blockquote><br>
<div>Hi Yichen,</div>
<br>
<div>One of the major design features of Rivet is to
avoid unphysical search criteria like this.
Different generators write hard processes with
different status codes and different topologies, so
there is no portable way to look for their
particles. More importantly, there is nothing
physically distinct about "the matrix element" from
the rest of the perturbative simulation: you are in
danger of "measuring" an artefact of the
calculation, rather than something physical.</div>
<br>
<div>On your specific question, I don't know where the
problem is, since you've not supplied any example
code. But if you are using the Rivet FinalState, for
example, then the name gives a hint as to why there
are no matrix element particles in it! I guess that
you are doing something else, though, if you were
able to get what you wanted with Pythia6. Rivet
doesn't do any special treatment of generator
internal status codes.</div>
<br>
<div>Andy</div>
<br>
<div>
<div>
<div>
<div>
<table cellspacing="0" cellpadding="0">
<tbody>
<tr>
<td colspan="2">
<div style="padding-bottom:15px;">
<div><strong>Dr Andy Buckley,
Lecturer / Royal Society
University Research Fellow</strong></div>
<div>Particle Physics Experiment
Group, University of Glasgow</div>
</div>
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<div>On Mar 6 2018, at 6:13 pm, Yichen <a
href="mailto:yichen.li@cern.ch"
title="mailto:yichen.li@cern.ch"
moz-do-not-send="true"><yichen.li@cern.ch></a>
wrote:</div>
<blockquote><br>
<div>
<div>Dear Pythia8/Rivet experts,</div>
<br>
<div>I'm using Rivet to analyze a MG5+Pythia8
ttbar+photon sample.</div>
<div>And I'm trying to get the matrix element
particles using status==23 cut.</div>
<div>But it turns out that in many cases,
"status==23" can't capture all the</div>
<div>matrix element particles!</div>
<div>(I did similar study for Pythia6, where all
matrix element particles can</div>
<div>be identified with status==3)</div>
<br>
<div>Do you know what could be the source of the
problem?</div>
<div>I'm not sure if it's due to Pythia8 or Rivet.
So I send this email to</div>
<div>both mailing lists.</div>
<div>And please let me know if you need more
information!</div>
<br>
<div>Thanks a lot!</div>
<br>
<div>Yichen Li</div>
<br>
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