The Higgs mechanism, which explains the masses of the elementary particles in the Standard Model, requires a doublet of scalar fields that interact with the fermions of the theory through Yukawa couplings. The current data, however, do not rule out the existence of more than one scalar doublet. In fact, some extensions of the Standard Model need more than one such doublets to work properly. That is the case of the Minimal Supersymmetric Standard Model, an extension considered to be austere which includes supersymmetric companions for all the known particles and requires a minimum of two doublets to be consistent, but it is not the only one: a wide variety of models incorporate a second doublet of scalar fields, seeking to resolve issues that range from matter-antimatter asymmetry in the universe to the existence of dark matter.

A group of theory researchers at IFIC have explored one of those families of models: Aligned Two-Higgs-Doublet Models (A2HDM). These theories seek to avoid one common pathology to models with two scalar doublets: flavour-changing neutral currents, which are severely constrained by experimental data. For that aim A2HDM 'align' the Yukawa matrices of both doublets, that is, they assume that the interactions of the second doublet and the first one are related, so that if the Higgs field that we know does not produce flavour-changing neutral currents neither will do the second doublet, at least to first order.

The phenomenology of these models is very rich: the second doublet provides three new scalar particles, one charged and two neutral, and their interactions may in some cases violate CP symmetry, that relates particles with their antiparticles. The authors examine these models and study the values of the parameters for which they are compatible with the measurements carried out at Fermilab's Tevatron and CERN's LHC.

“Towards a general analysis of LHC data within two-Higgs-doublet models”, Alejandro Celis, Victor Ilisie and Antonio Pich. JHEP 1312 (2013) 09, arXiv:1310.7941 

“LHC constraints on two-Higgs doublet models”, Alejandro Celis, Victor Ilisie and Antonio Pich. JHEP 07 (2013) 053, arXiv:1302.4022