Beyond brane-Higgs regularisation: clarifying the method and model

TL;DR

This paper clarifies the calculation of fermion masses and Yukawa couplings in higher-dimensional brane-Higgs models, emphasizing the role of Bilinear Brane Terms and resolving previous mathematical discrepancies.

Contribution

It introduces a rigorous method for fermion spectrum calculation that avoids Higgs regularisation, highlighting the importance of Bilinear Brane Terms and boundary conditions.

Findings

Proper treatment eliminates profile jumps at the Higgs boundary.

The method confirms that excited fermion masses are independent of 'wrong-chirality' Yukawa terms.

Results relax bounds on Kaluza-Klein masses from flavor-changing processes.

Abstract

The class of higher-dimensional scenarios, based on a brane-localised Higgs boson coupled to bulk fermions, can address both the flavour puzzle and gauge hierarchy problem. A key question arises due to the possibility of fermion profile discontinuities at the Higgs boundary: how to calculate rigorously the fermion mass spectrum and effective four-dimensional (4D) Yukawa couplings? We show that the proper treatment, leading to physically consistent solutions, does not rely on any Higgs peak regularisation but requires the presence of certain Bilinear Brane Terms (BBT). In particular, no profile jump should appear and the Higgs regularisations turn out to suffer from mathematical discrepancies reflected in two non-commutativities of calculation steps debated in the literature. The introduction of BBT can by replaced by vanishing conditions for probability currents at the considered flat interval boundaries. Both contribute to the definition of the field geometrical configuration of the model, even in the free case. The BBT could allow to elaborate an ultra-violet origin of the chiral nature of the Standard Model and of its chirality distribution among quarks/leptons. The current conditions are implemented via essential boundary conditions to be contrasted with the natural boundary conditions derived from the action variation. All these theoretical conclusions are confirmed in particular by the converging exact results of the 4D versus 5D approaches. The analysis is completed by a description of the appropriate energy cut-off procedure. The new calculation methods presented, implying the independence of excited fermion masses and 4D Yukawa couplings on the 'wrong-chirality' Yukawa terms, have impacts on phenomenological results like the relaxing of previously obtained strong bounds on Kaluza-Klein masses induced by flavour changing reactions generated through exchanges of the Higgs field.