Higgs pairs

ISUB =

(141)	$\mathrm{f}_i \overline{\mathrm{f}}_i \to \gamma/\mathrm{Z}^0/\mathrm{Z}'^0$
297	$\mathrm{f}_i \overline{\mathrm{f}}_j \to \H ^{\pm} \mathrm{h}^0$
298	$\mathrm{f}_i \overline{\mathrm{f}}_j \to \H ^{\pm} \H ^0$
299	$\mathrm{f}_i \overline{\mathrm{f}}_i \to \mathrm{A}\mathrm{h}^0$
300	$\mathrm{f}_i \overline{\mathrm{f}}_i \to \mathrm{A}\H ^0$
301	$\mathrm{f}_i \overline{\mathrm{f}}_i \to \H ^+ \H ^-$

The subprocesses 297-301 give the production of a pair of Higgses via the $s$-channel exchange of a $\gamma^* / \mathrm{Z}^0$ or a $\mathrm{W}^{\pm}$ state.

Note that Higgs pair production is still possible through subprocess 141, as part of the decay of a generic combination of $\gamma^*/\mathrm{Z}^0/\mathrm{Z}'^0$. Thus it can be used to simulate $\mathrm{Z}^0 \to \mathrm{h}^0 \mathrm{A}^0$ and $\mathrm{Z}^0 \to \H ^0 \mathrm{A}^0$ for associated neutral Higgs production. The fact that we here make use of the $\mathrm{Z}'^0$ can easily be discounted, either by letting the relevant couplings vanish, or by the option MSTP(44)=4.

Similarly the decay $\gamma^*/\mathrm{Z}^0/\mathrm{Z}'^0 \to \H ^+ \H ^-$ allows the production of a pair of charged Higgs particles. This process is especially important in $\mathrm{e}^+\mathrm{e}^-$ colliders. The coupling of the $\gamma^*$ to $\H ^+ \H ^-$ is determined by the charge alone (neglecting loop effects), while the $\mathrm{Z}^0$ coupling is regulated by PARU(142), and that of the $\mathrm{Z}'^0$ by PARU(143). The $\mathrm{Z}'^0$ piece can be switched off, e.g. by MSTP(44)=4. An ordinary $\mathrm{Z}^0$, i.e. particle code 23, cannot be made to decay into a Higgs pair, however.

The advantage of the explicit pair production processes is the correct implementation of the pair threshold.