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BR Subgroup Meeting: focus on Dalitz decay
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May 7, 2013, 14:30 -16:00
https://indico.cern.ch/conferenceDisplay.py?confId=250520

Present: ~ 26 Vidyo connections
	Huang, Chaparro, Mueck, Passarino, Spira, Paganis, Scifo, Randle-Conde,
	Rebuzzi (chair), Velasco, Tanaka, Heinemeyer, Liu, Puljak, Denner, 
	Groth-Jensen, Stoynev, Marchiori, Peters, Saxon, Gillberg, Rustem, Ruan,
	Duehrssen, 2 x phone, Stroynowski, ...

1) H->llgamma at CMS		Stoyan Emilov Stoynev (Northwestern)

	Reported CMS analysis status on H->gamma+gamma, Zgamma and ll+gamma plausibility studies. 
	Higgs Dalitz decay is simulated with MCFM. Plan is to interface it to PS MC. 
	CMS H->gamma+gamma analysis requires no conversion events from the vertex thus there is no overlap 
	with H->llgamma
	On-going efforts to lower M*<50GeV in Higgs Dalitz decay study, where experimental 
	challenge is the reconstruction of low invariant mass di-lepton system. 
	Very recent work by G. Bodwin and F. Petriello was shown for H->Vgamma (V=J/Psi, Upsilon). 

2) H->llgamma at ATLAS		Aidan Sean Randle-Conde (Southern Methodist)

	Reported ATLAS analysis status on H->Z(ll)+gamma and H->mu+mu and H->gamma+gamma.
	Internal conversion gamma:->ffbar is included in Pythia 8 (but not in Pythia 6).
	Question from experimental side is that 1) need for MC tool, 2) discriminant variable 
	and 3) signal definition. 
	
- Questions to Stoyan and Aidan

	Daniela: estimate of BR's (slide 5)
	Answer: MCFM

	Someone: Looking for low di-lepton mass events?
	Answer: events are in... no special selection, not treated specially so far.

	Rei: Only available MCFM and Pythia8? Both agree with 5-6% Dalitz contamination in H->gamma+gamma?
	Answer: Correct, nothing for the full treatment over the full spectrum.

3) Higgs Dalitz decay		Giampiero Passarino (Torino)

	The name Dalitz Decay must be reserved for the full process H->ffbar+gamma !
	The most important is 
		a) the definition of visible photon to distinguish between ff and ff+gamma,
		b) cuts are on M(ff) to isolate pseudo-observables. 
	One has to distinguish:
		- H->ff+ soft(collinear) photon(s) which is part of the real corrections to H->ff at (N)NLO,
		- a visible photon and a soft ff-pair.
	H->Zgamma is not well defined being a gauge-variant part of H->ffbar+gamma(gamma visible).
	One can put cuts around on-shell Z with small mass window but but there is a contamination due to 
	many non-resonant backgrounds.
	Program: gg->ffbar+gamma calculation by Giampiero can be done but currently Higgs at rest frame.
	If program is urgently needed, contact Dicus and Repko (arXiv:1302.2159, PRD 87 (2013) 077301).

- Questions to Giampiero and Discussions	

	Rei: box-diagram is interesting. No calculation for qq->H+gamma via W-loop box diagram no MC ? 
	Giampiero: right...

	Michael S.: no interference between LO and NLO? Only for mf = 0, but mtau <> 0?
	Giampiero: correct, there is interference effect once put fermion mass.

	Michael S.: photon splitting into f \bar f, collinear singularity! large logs...
	Giampiero: right. low di-photon mass => large logs that survive, 

   	resummation unclear.

	Michael S.: correspond to running QED coupling
	Giampiero:  I think that I tend to agree with you, need to check

	Daniela: how to interface MCFM to PYTHIA etc. ?
	Styan: CMS wants to study MCFM as startup tool. 
	Rei: we had better contact K. Ellis et al. and make coherent efforts. 

	Daniela: how to generate MC ?
	all: had better for whole phase space without cuts.
	Daniela and Giampiero can exploit the possibility of using the Prophecy4f/POWHEG tool to merge
	POWHEG Higgs boson with Giampiero's calculations.

	Krisztian: but efficiencies are different and how to interpret the results ?
	Rei: physics in H->mumu and H->Zgamma->ffbar+gamma and one day we want to put BR in PDG. 
	We need signal definition.

Conclusion:
	- it was suggested that Higgs Dalitz decay analysis experts from ATLAS and CMS sits down together 
	  with BR theory experts
	- they should come to possible agreement on Higgs Dalitz decay signal definition
	- they also should make the clear milestones for tool developments for short term and longer time scale.


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Minutes taken by Daniela, Sven and Reisaburo.