Abstract
We derive the constraints that can be imposed on the dimension-6 effective theory extension of the Standard Model, using gluon fusion-initiated Higgs boson pair production at the LHC. We use a realistic analysis focussing on the $hh \rightarrow (b\bar{b}) ( \tau^+ \tau^- )$ final state, including initial-state radiation and non-perturbative effects. We include the statistical uncertainties on the signal rates as well as conservative estimates of the theoretical uncertainties. We first consider a theory containing only modifications of the trilinear coupling, through a $c_6 \lambda\, H^6/ v^2$ Lagrangian term, and then examine the full parameter space of the effective theory, incorporating current bounds obtained through single Higgs boson measurements. We also consider an alternative scenario, where we vary a smaller sub-set of parameters. Allowing, finally, the values of the other coefficients to vary within \textit{projected} experimental ranges, we find that the currently unbounded parameter, $c_6$, could be constrained to lie within $|c_6| \lesssim 0.6$ at 1$\sigma$ confidence, at the end of the high-luminosity run of the LHC (14~TeV) in the full model, and to $-0.6 \lesssim c_6 \lesssim 0.5$ in the alternative model. This study constitutes a first step towards the inclusion of multi-Higgs boson production in a full fit to the dimension-6 effective field theory framework.
Highlights
Where the self-couplings λ = λ = m2h/2v2 within the SM, with v 246 GeV the vacuum expectation value.1 only the first term of eq (1.1) has been probed, through the measurement of the Higgs boson mass, mh 125 GeV
It can be seen that the SM relation of λ = λis broken by the effective field theory (EFT) effects: an accurate measurement of both couplings is a powerful probe of new physics in the Higgs sector, as already mentioned, measurement of the quartic coupling does not seem to be possible in the foreseeable future
We have investigated the dimension-6 effective theory description of beyond-the-Standard Model modifications to Higgs boson pair production
Summary
New Physics associated to a new scale Λ v can be described in a model-independent way by augmenting the Lagrangian of the SM with all possible gauge-invariant operators of mass dimension D > 4, where the leading effects arise from D = 6 operators (neglecting lepton-number violating operators, irrelevant to our study) Working at this level, the extension of the SM that we consider for our analysis of Higgs boson pair production reads. Leads to the coefficients of the Yukawa-like terms in the second row of eq (2.1) being proportional to the (SM-like) Yukawa couplings yt,b,τ and in particular allows to neglect the corresponding contributions involving the light fermions, QL = (tL, bL) and LL = (ντ , τL) Note that the latter can be justified without assuming MFV [96]. We examine the terms relevant to Higgs physics
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