X iv : 0 90 8 . 15 54 v 1 [ he p - ph ] 1 1 A ug 2 00 9 Yukawa Alignment in the Two - Higgs - Doublet Model

In multi-Higgs-doublet models the alignment in flavour space of the relevant Yukawa matrices guarantees the absence of tree-level flavour-changing couplings of the neutral scalar fields. We analyze the consequences of this condition within the two-Higgs-doublet model and show that it leads to a generic Yukawa structure which contains as particular cases all known specific implementations of the model based on Z 2 symmetries. All possible freedom in the Yukawa sector gets parameterized in terms of three complex couplings ς f. In spite of having flavour conservation in the neutral scalar couplings, the phases of these three parameters represent potential new sources of CP violation. 1. The two-Higgs-doublet model (THDM) [1, 2] constitutes one of the simplest extensions of the Standard Model (SM) and incorporates new and interesting phenomenological features. Many new-physics scenarios, including supersymmetry, can lead to a low-energy spectrum containing the SM fields plus only one additional scalar doublet. Therefore, the THDM is also a very convenient effective field theory framework to investigate low-energy effects of more generic dynamical settings. The extended scalar sector accommodates two charged and three neutral scalar fields, in addition to the three Goldstones needed to generate the gauge boson masses. This rich scalar spectrum provides a broad range of dynamical possibilities, such as potential new sources of CP symmetry breaking, including spontaneous CP violation, axion phenomenology or dark matter candidates, just to mention a few.