Abstract
We perform the fit of electroweak precision observables within the Standard Model with a 126 GeV Higgs boson, compare the results with the theoretical predictions and discuss the impact of recent experimental and theoretical improvements. We introduce New Physics contributions in a model-independent way and fit for the S , T and U parameters and for a modified Higgs coupling to vector bosons. We point out that compos- ite Higgs models are very strongly constrained. Finally, we compute the bounds on dimension-six operators relevant for the electroweak fit. Electroweak Precision Observables (EWPO) have played a key role in constraining New Physics (NP) for the past twenty years (1-11). The most striking exam- ples of the power of these indirect constraints are the pre- diction of the top and Higgs masses. The experimen- tal situation improved dramatically in the past year, with the precise measurement of the Higgs mass at the LHC (12, 13). In addition, the information on other key Stan- dard Model (SM) parameters such as the top and W boson masses has increased considerably, leading altogether to a sizable progress in the electroweak (EW) fit. It is therefore phenomenologically relevant to reassess the constraining power of the EW fit in the light of these recent experimen- tal improvements. To this aim, we perform the EW fit in the SM and update the constraints on oblique NP. On the theory side, the full two-loop fermionic EW contributions to the R 0 observable have been recently numerically cal- culated in ref. (14). The implementation of this result in the global fit has a large impact but represents a nontriv- ial problem, as we illustrate in detail below. A very in- teresting question that can be tackled with present data is whether the Higgs boson is elementary or composite. Us- ing a general e ective Lagrangian for Higgs boson inter- actions (15-18), we analyze the constraints on the Higgs coupling to vector bosons, and find out that this coupling can be determined from the fit with an uncertainty of 5% at 95% probability, while much larger departures from the SM value are expected in generic composite Higgs mod- els. Thus, the EW fit points to an elementary Higgs or to composite Higgs models in which additional contributions are present to restore the agreement with EWPO. Finally, we consider the most general e ective Lagrangian relevant
Highlights
The part of the Standard Model (SM) Lagrangian relevant for the computation of Electroweak Precision Observables (EWPO) can be defined in terms of the following free parameters: the fine structure constant α, the muon decay constant Gμ, the Z boson mass MZ, the strong coupling αs(MZ2), the top quark mass mt and the Higgs mass mh
For a recent discussion of the theoretical status of EWPO we refer the reader to ref. [28]; here we focus on the discussion of the calculation of fermionic twoloop corrections to the Zbbvertex
From this approximate formula alone we cannot extract the values of ρZf including fermionic two-loop corrections, that are necessary to compute other ρZf -dependent observables such as R0, R0c, ΓZ and the hadronic cross section
Summary
Silvestrini2,d 1INFN, Sezione di Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy 2INFN, Sezione di Roma, Piazzale A. Moro 2, I-00185 Roma, Italy 3Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A.
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