Natural Alignment in Multi-Higgs Doublet Models

We present the complete set of continuous maximal symmetries that the potential of an $n$-Higgs Doublet Model ($n$HDM) should satisfy for natural Standard Model (SM) alignment. As a result, no large mass scales or fine-tuning is required for such alignment, which still persists even if these symmetries were broken softly by bilinear mass terms. In particular, the Maximal Symmetric $n$HDM (MS-$n$HDM) can provide both natural SM alignment and quartic coupling unification up to the Planck scale. Most remarkably, we show that the MS-2HDM is a very predictive extension of the SM governed by two only additional parameters: (i) the charged Higgs mass $M_{h^{\pm}}$ (or $m^2_{12}$) and (ii) $\tan\beta$, whilst the quartic coupling unification scale $\mu_X$ is predicted to assume two discrete values. With these two input parameters, the entire Higgs-mass spectrum of the model can be determined. Moreover, we obtain definite predictions of misalignment for the SM-like Higgs-boson couplings to the gauge bosons and to the quarks, which might be testable at future precision high-energy colliders.