Double Higgs boson production at TeV e plus e - colliders with effective field theories: Sensitivity to BSM Higgs couplings

In this work we study the production of two Higgs bosons at the two planned electron positron colliders with energies at the TeV domain, CLIC and ILC, within the context of effective field theories (EFTs) to describe beyond the Standard Model Higgs physics. We focus first on the case of the Higgs effective field theory (HEFT) and next we compare with the case of the Standard Model effective field theory (SMEFT). The predictions for double Higgs production in both EFTs are first presented for the most relevant subprocess participating in the total process of our interest, e+e- -HH nu nu over bar , which is the scattering of two gauge bosons, WW -HH, also called WW fusion. The predictions for the cross section sigma(WXWY -HH) as a function of the subprocess energy are analyzed in full detail for the two EFTs, for all the polarization channels with longitudinal and transverse modes XY = LL; TT; LT; TL, and for the most relevant effective operators in both cases. We will demonstrate that in the HEFT case, the total cross section can be fully understood in terms of the LL contribution and this in turn is dominated at these TeV energies mainly by two HEFT coefficients. By doing the matching between the two EFTs at the level of the scattering amplitude for the subprocess, we will be able to find the correspondence of the leading coefficients in the HEFT and the SMEFT. In the final part of this work we will then explore the sensitivity to these two most relevant HEFT coefficients, at CLIC (3 TeV, 5 ab-1) and ILC (1 TeV, 8 ab-1). We will then conclude on the accessible region of these two parameters by studying the predicted rates at these two e+e- colliders for the final state bb over bar bb over bar nu nu over bar leading to characteristic signals with four bottom jets and missing energy.