Lepton-mass effects in the decays $H \to ZZ^{\ast} \to \ell^{+} \ell^{-} \tau^{+} \tau^{-}$ and $H \to WW^{\ast} \to \ell \nu \tau \nu_{\tau}$

Abstract

We consider lepton-mass effects in the cascade decays $H\to Z(\to \ell^{+} \ell^{-})+Z^{\ast}(\to \tau^{+}\tau^{-})$ and $H\to W^{-}(\to \ell^{-}\bar \nu_{\ell})+W^{+\ast}(\to \tau^{+}\nu_{\tau})$. Since the scale of the problem is set by the off-shellness $q^{2}$ of the respective gauge bosons in the limits $(m_{\ell}+m_{\ell'})^{2} \le q^{2} \le (m_{H}-m_{W,Z})^{2}$ and not by $m_{W,Z}^{2}$, lepton-mass effects are non-negligible for the $\tau$ modes in particular close to the threshold of the off-shell decays. Lepton-mass effects show up e.g.\ in the three-fold joint angular decay distribution for the decays. Nonzero lepton masses lead to leptonic helicity-flip contributions which in turn can generate novel angular dependencies in the respective three-fold angular decay distributions. Lepton-mass effects are more pronounced in the $H \to Z(\to \ell\ell)Z^{\ast}(\to\tau\tau)$ mode which, in part, is due to the fact that the ratio of lepton helicity flip/nonflip contributions in the decay $Z^{\ast} \to \ell^{+}\ell^{-}$ is four times larger than in the decay $W^{+\ast}\to \ell^{+}\nu$. We also briefly consider the corresponding off-shell -- off-shell decays $H\to Z^{\ast}(\to \ell^{+}\ell^{-})+Z^{\ast}(\to \tau^{+}\tau^{-})$ and $H\to W^{-\ast}(\to \ell^{-}\bar \nu_{\ell})+W^{+\ast}(\to \tau^{+}\nu_{\tau})$.