Light Mass Higgs meeting 6

42/3-032 (CERN)



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BSM round-table (with focus on couplings)
The meeting starts at 13.30.
Present (19): Chanon, David, Del Duca, Denner, Drieu la Rochelle, Duhrssen, Gascon-Shotkin, Grazzini, Grojean, Krauss, Ille, Mariotti, Nason, Oleari, Passarino,  Tanaka,  Vicini, Zanetti, Zeppenfeld.
On EVO (15): Carena, Contino, Deandrea, Degrassi, d'Enterria, Kiss, Mangano, Falkowski, Lethuiller, Rattazzi, Rauch, Rebuzzi, Sgandurra, Wagner, Zhang
Grazzini gives a short introduction and explains the purpose of the meeting, which is to move the first steps beyond the interim recommendation provided in arXiv:1209.0040 by finding an effective lagrangian parametrization of possible new physics effects that could be observed in the coupling structure of the new resonance discovered by ATLAS and CMS.
- Grojean gives his presentation on an effective lagrangian parametrizing the BSM effects based on the spin 0, CP-even hypotesis, assuming custodial symmetry and no Higgs FCNC. His talk is based on a written proposal presented to the working group together with Contino, Espinosa, Falkowski, Ghezzi, Muehlleitner, Rattazzi, Sundrum, Trott and Vicini.
Deandrea comments that this parametrization is very general and asks how many parameters can actually be constrained at present. Duhrssen replies that the precision will improve and fits with more parameters will be possible.
Rattazzi says that a linear combination of cW and cZ is equivalent to LEP S parameter and then it is constrained to be very small.
- Denner gives his presentation on the implementation of anomalous couplings in HAWK and Prophecy4f.
- The main discussion starts:
Grazzini asks Grojean how gauge invariance is implemented in the case in which the symmetry is non-linearly realised. Grojean replies that the theory is still gauge invariant but the gauge symmetry is realized non linearly.
Grazzini says that it would be important to have a critical comparison of the two proposed approaches: the chiral lagrangian by Grojean and collaborators, and the set of dim-6 operators added in Buchmuller-Wyler approach or similar.
Rattazzi states that the approach proposed by Grojean is more general, and that it reduces to the other one when the symmetry is linearly realized.
A discussion starts on the possible inclusion of form factors. Zeppenfeld claims that dim-8 operators could be important (if new particles with mass around O(100) GeV do exist). Rattazzi completely disagrees and says that one should avoid the use of form factors. Grojean states that one should start with dim-6, then move to dim-8. 
Zeppenfeld proposes to use VBF jet pT distributions to constrain the operators and then see what form factor would reproduce those distributions.
Rattazzi states that light (on-shell) small new physics effects cannot be taken into account by the effective lagrangian, but that does not mean that we should go to form factors.
Duehrssen comments that if the form factor is completely free, it will presumably fit to whatever the data are.
Zeppenfeld adds that VBFNLO can give the pT of jets as function of Wmunu^2.
Rattazzi adds that a form factor corresponds to an infinite sum, and thus to infinite parameters.
Contino states that their approach is more general the the one based on a basis of dim-6 operators, since for example, it gives the possibility to describe a Higgs "impostor" like the dilaton.
Passarino says that he is not completely convinced of the generality of the proposal by Grojean. To go beyond LO we need loops, which means to have the full SM lagrangian. How to work beyond the unitary gauge ? Where are the goldstone bosons going? How to compute Hgg or ggH at NLO ? If the Higgs field is not embedded in a doublet, how can the SM be recovered in the low energy limit?
Grojean states that divergences in O(p^2) terms are cancelled by adding O(p^4) terms. At any order in the low-energy expansion renormalisation can be done
with a finite number of counterterms.
Passarino states that in the effective theory UV divergences are introduced that are not present in the underlying "true" theory. UV divergences would, in the limit, lead to logs that might need to be resummed.
Rattazzi replies that effective lagrangians always works like that. 
Zeppenfeld states that as you go up in order, the number of parameters in the effective theory increases, while the true renormalizable theory has very few.
Passarino adds that the issue of how to treat effective operators in SM loops is not trivial and
needs to be clarified.
Contino stresses that their effective lagrangian helps to describe also non SM cases, still giving the possibility to go back to the linear realisation in which the Higgs is part of a doublet.
Carena states that we should keep options open, and not discard the possibility of light degrees of freedom.
Denner asks Grojean how the SM is recovered from their effective lagrangian.
Passarino says he would like to see the gauge fixing term and the ghost lagrangian.
Duehrssen asks how does measuring WW and ttH make a prediction to Hgg?
Passarino states that if differences from SM are at the 1% level, the theory is not ready to comment on that. If they are at the 50% level, then we should move to another theory and redefine our starting point. Zeppenfeld says that 20% uncertainty might not be enough to draw definite conclusions.
The meetings ends at 15.30.
There are minutes attached to this event. Show them.
    • 13:30 13:40
      Introduction 10m
    • 13:40 13:55
      Lagrangians 15m
      Speaker: Christophe Grojean (CERN)
    • 13:55 14:10
      Nonstandard Higgs couplings in HAWK and Prophecy4f 15m
      Speakers: Ansgar Denner (Paul Scherrer Institut (CH)) , Ansgar Denner (Paul Scherrer Institut)
    • 14:10 15:30
      Discussion 1h 20m