in . One can then define four invariant $\tan\beta$-like parameters,[^12] \_U&& -e\^[-i]{} =-e\^[-i]{}, \_De\^[-i]{} =e\^[-i]{},\[tb1\]\
\^\_U&& -e\^[-i]{}= -e\^[-i]{}, \^\_De\^[-i]{}=e\^[-i]{}.\[tb2\] Without the factors of $e^{i\eta}$, the corresponding $\tan\beta$-like parameters are complex pseudo-invariants, whose magnitudes are basis-independent. The interpretation of these $\tan\beta$-like parameters is simplest in the Higgs basis, which is defined by $\hat v=(1\,,\,0)$. In the Higgs basis, the parameters in are (up to an overall phase) simply the ratios of gaugino-higgsino-Higgs couplings. One can now investigate the limit in which all dimension-four couplings respect supersymmetry. In this case, the invariant and pseudo-invariant Higgs fields are constrained by , in which case, \[susytb\] =\_U=(\_D)\^=\_U\^=(\_D\^)\^\*,. Corrections to the above relations at the few percent level are expected at the one-loop level due to supersymmetry-breaking effects that enter into the loops. In principle, one could extract the $\tan\beta$ parameters of from collider data (assuming gaugino-higgsino-Higgs couplings could be measured with sufficient accuracy), and check to see whether holds.

\[sec:conclusions\]Conclusions
==============================

The THDM includes the Higgs sector of the MSSM as a particular case. In the THDM only quantities that are invariant under a unitary basis change, $\Phi_a\to U_{ab}\Phi_b$ (where $a,b=1,2$ and $U$ is an arbitrary unitary matrix) can have physical meaning. Thus, it 