How to decay HNL particle of mass just below the W mass into off-shell W
Dear Experts,
I am having a problem with making sense of unexpectedly high raw (number of generated events) MC efficiencies when I generate the process with the mass of BSM particle I am using close to W boson mass which it decays into.
I am working with HeavyN model [1] and attempting at generating NLO tt̄ events and then decay them with Madspin, where we probe the regime with mHNL < mW. We allow W of SM top to decay hadronically or leptonically whereas we strictly ask W from HNL from BSM top to decay only hadronically.
SM Top: t -> Wb -> jj or lv
BSM top: t -> Wb -> Nl where N -> Wl and W -> jj
To translate this into MG5 syntax:
1) Generate tt̄ events with Proc_card, I do:
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generate p p > t t~ [QCD]
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2) In a second step I want to decay these top quarks using MadSpin:
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set spinmode none # 3body decay
decay t > b w+, (w+ > n ssl+ , n > ssl+ j j) # since mN < mW 2-body decay syntax not possible
decay t~ > b~ w-, w- > all_sm all_sm NP=0 #all_sm is defined as l, j and v to avoid Ns here
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Now, we probe the range 15-80GeV for HNL and expecting to have a peak sensitivity of about 50GeV and lower efficiencies towards the end points. We see this shape between 15-75GeV, however at 80.0GeV the raw MC number of events passing selections start to increase again. Making a finer check we also studied 77 and 79GeV and noticed the unexpected rise of efficiency happens at 79GeV as well, but not at 77GeV. To give some feeling it goes like that for some cut level (100K events):
65Gev: 27.8%
70Gev: 21.7%
75Gev: 14.3%
77Gev: 12.6%
79Gev: 14.3%
80Gev: 15.3%
We also note that cross-section results are reasonable at all mass points. Therefore, we suspect this is to do with the BW_cut settings. Checking posts on this forum my understanding is that there are two different BWcuts:
Run_card "bwcutoff" whose default is 15 and regulates |m-m0| < bwcutoff x Gamma
MadSpin "BW_cut" whose default is 1.5 and regulates |m-m0|*|m-m0| < BW_cut x Gamma
We have made some checks with various bwcutoff values(5,15,40) and have not seen any differences.
When one plugs in the Gamma_W=2.085 and BW_cut=1.5 and assuming m0=mW=80.4 one comes to the value of m=78.7GeV for cut-off. This seems to fit inbetween the 77GeV (no efficiency problem) and 79GeV (efficiency problem). However, setting BW_cut to 0 does not change the outcome either.
We would like to then consult you on the following question:
1) Is it possible to generate HNL=80.0GeV mass point in a valid way?
2) Is there a hierarchy between two BW cuts. Do they affect the same particles? Does one overwrite the other if stricter?
3) is the default value for BW_cut still 1.5. Because when I want to enter non-integer values (0.5 for example) program crashes and says BW_cut must be an integer? I am using MG.2.7.3
Thank you in advance!
Cheers,
Ogul
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