LHC_HZZ_POWHEG example

[Generic User (don't modify these fields)]

Hi,

I am trying to produce some Higgs events using LHC_HZZ_POWHEG as base. I output events in HepMC format then process them. In my processing I use HepMC::GenEvent.signal_process_vertex() to find vertex which contain leptons from higgs. Then I add up 4 vectors of the leptons to reconstruct Higgs. However pretty often pt comes up as 0. It doesn't make much sense to me. Am I using wrong approach? I am using Sherpa1.3.1 with MCFM.

Thanks, Sami

[Frank Krauss]

My suspicion is that you look at the vertex at Matrix Element level. There, typically, the pt of the four leptons sums up to 0 - two incoming gluons parallel to the beam produce the Higgs, which decays, so nothing suspicious.

To cure the problem, you may want to consider a more physical way of analysing your events; just look for 4 leptons, sufficiently hard and isolated in your final state and reconstruct the pt from them.

I hope this helps.

[Generic User (don't modify these fields)]

Thanks for the answer,

Other than selecting final state leptons, which it self brings several complications especially when leptons are of the same flavour, is there a way to associate leptons to parent Z particles? I thought of using a cone around the lepton in signal vertex and its decay vertex but this might not work all the time. Do you have any other idea? Is Z boson visible in blob format event record?

Thanks, Sami

[Frank Krauss]

No you lost me. Why is it now complicated to have leptons of the same flavour - I thought this is what you wanted to study?

And, well, of course not, no you cannot associate leptons with Z's in our event record. I thought you wanted to see the effect of a quantum interference, so associating leptons to Z's would make them distinguishable. It's like you try to tell in a double-slid, which slid the particle went through - clearly no idea that works like quantum physics. And because we're nice people we do not allow you to do stupid things with our code, so no Z's - unless you do not want interference, which is identical to different lepton flavours. So you'll have to ask yourself: how would you actually measure this in the real world. I hope this clarifies the situation.

[Generic User (don't modify these fields)]

Hi Krauss,

What I want to is to look at the properties of higgs and compare these distrubutions with other generators. As you know better different generators have different approaches to same problem and might have slightly different results. In other words in your double slit example I want to look at the properties of source rather than interference pattern and the slits. Of course for any physical analysis only final state particles matter, in which case I wouldn't care about the event record. A good physical analysis should be blind to input and shouldn't care whether it is coming from MC or data. However for the time being I want to see the effect of varying several input parameters on higgs particle properties for several generators. Of course whether these variations can be visible in rather realistic analyses is a different matter. Or maybe I misunderstood something completely.

Cheers, Sami