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<p>Dear Andy and Torbjör,
</p>
<p>Thanks a lot for your replies!<br>
<br>
By looking for these ME particles, we are not trying to do
unphysical measurement. Our motivation is the following:<br>
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.<br>
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.<br>
</p>
Here is the piece of code in Rivet (for analyzing e+jets channel):<br>
""<br>
bool foundMEel = false;<br>
bool foundMEnuel = false;<br>
bool foundMEph = false;<br>
Particles all = event.allParticles();<br>
foreach (Particle p, all) {<br>
if (p.genParticle()->status() == 23) {<br>
if (fabs(p.pid()) == 11) foundMEel = true;<br>
if (fabs(p.pid()) == 12) foundMEnuel = true;<br>
if (fabs(p.pid()) == 22) foundMEph = true;<br>
}<br>
}<br>
if (!(foundMEel && foundMEnuel && foundMEph))
return;<br>
""<br>
I put an example in cern lxplus068 /tmp/yili/example/<br>
If you can't use lxplus, I also put a copy in dropbox:
<a class="moz-txt-link-freetext" href="https://www.dropbox.com/sh/5tdl2slccr3dc9u/AACHY9hlB6pvdrpcSql_2M_na?dl=0">https://www.dropbox.com/sh/5tdl2slccr3dc9u/AACHY9hlB6pvdrpcSql_2M_na?dl=0</a><br>
<br>
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<br>
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.<br>
<br>
Best,<br>
Yichen<br>
<br>
<div class="moz-cite-prefix">On 06.03.2018 21:09, Andy Buckley
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:local-4c561572-d92e-v1.1.4-22d9f20d@unity">
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<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>
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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 6 2018, at 6:13 pm,
Yichen <a class="moz-txt-link-rfc2396E" href="mailto:yichen.li@cern.ch"><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>
<div>_______________________________________________</div>
<div>Rivet mailing list</div>
<div><a class="moz-txt-link-abbreviated" href="mailto:Rivet@projects.hepforge.org">Rivet@projects.hepforge.org</a></div>
<div><a class="moz-txt-link-freetext" href="https://www.hepforge.org/lists/listinfo/rivet">https://www.hepforge.org/lists/listinfo/rivet</a></div>
</div>
</blockquote>
</blockquote>
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