Helicity amplitudes in the ${\bar{B}} \rightarrow D^{*} {\bar{\nu }}_\tau \tau$ decay with V-A breaking in the quark sector

Abstract

In view of the recent measurement of the $F_L^{D^*}$ magnitude in the ${\bar{B}} \rightarrow D^{*} {\bar{\nu }}_\tau \tau$ reaction we evaluate this magnitude within the standard model and for a family of models with the $\gamma ^\mu -\alpha \gamma ^\mu \gamma _5$ current structure for the quarks for different values of $\alpha$. At the same time we evaluate also the transverse contributions, $M=-1$, $M=+1$, and find that the difference between the $M=-1$ and $M=+1$ contributions is far more sensitive to changes in $\alpha$ than the longitudinal component. These findings should be looked as an incentive to measure the transverse helicities which are bound to be a far more sensitive magnitude to possible new physics than $F_L^{D^*}$.