Visible energy and angular distributions of the charged particle from the τ −decay in $$ b\to c\tau \left(\mu {\overline{\nu}}_{\mu }{\nu}_{\tau },{\pi \nu}_{\tau },{\rho \nu}_{\tau}\right){\overline{\nu}}_{\tau } $$ reactions

Abstract

We study the d2Γd/(dωd cos θd), dΓd/d cos θd and dΓd/dEd distributions, which are defined in terms of the visible energy and polar angle of the charged particle from the τ −decay in $$ b\to c\tau \left(\mu {\overline{\nu}}_{\mu }{\nu}_{\tau },{\pi \nu}_{\tau },{\rho \nu}_{\tau}\right){\overline{\nu}}_{\tau } $$ b → cτ μ ν ¯ μ ν τ πν τ ρν τ ν ¯ τ reactions. These differential decay widths could be measured in the near future with certain precision. The first two contain information on the transverse tau-spin, tau-angular and tau-angular-spin asymmetries of the $$ {H}_b\to {H}_c\tau {\overline{\nu}}_{\tau } $$ H b → H c τ ν ¯ τ parent decay and, from a dynamical point of view, they are richer than the commonly used one, d2Γd/(dωdEd), since the latter only depends on the tau longitudinal polarization. We pay attention to the deviations with respect to the predictions of the standard model (SM) for these new observables, considering new physics (NP) operators constructed using both right- and left-handed neutrino fields, within an effective field-theory approach. We present results for $$ {\Lambda}_b\to {\Lambda}_c\tau \left(\mu {\overline{\nu}}_{\mu }{\nu}_{\tau },{\pi \nu}_{\tau },{\rho \nu}_{\tau}\right){\overline{\nu}}_{\tau } $$ Λ b → Λ c τ μ ν ¯ μ ν τ πν τ ρν τ ν ¯ τ and $$ \overline{B}\to {D}^{\left(\ast \right)}\tau \left(\mu {\overline{\nu}}_{\mu }{\nu}_{\tau },{\pi \nu}_{\tau },{\rho \nu}_{\tau}\right){\overline{\nu}}_{\tau } $$ B ¯ → D ∗ τ μ ν ¯ μ ν τ πν τ ρν τ ν ¯ τ sequential decays and discuss their use to disentangle between different NP models. In this respect, we show that dΓd/d cos θd, which should be measured with sufficiently good statistics, becomes quite useful, especially in the τ → πντ mode. The study carried out in this work could be of special relevance due to the recent LHCb measurement of the lepton flavor universality ratio $$ {\mathrm{\mathcal{R}}}_{\Lambda_c} $$ ℛ Λ c in agreement with the SM. The experiment identified the τ using its hadron decay into π−π+π−ντ, and this result for $$ {\mathrm{\mathcal{R}}}_{\Lambda_c} $$ ℛ Λ c , which is in conflict with the phenomenology from the b-meson sector, needs confirmation from other tau reconstruction channels.

New physics effects on $Λ_b\to Λ^*_cτ\barν_τ$ decays

“…As already discussed in Ref. [68], the different deviation of the present R Λc and R D ( * ) ratios with respect to their SM values, suppression for R Λc versus enhancement for R D ( * ) , puts a very stringent test on NP extensions of the SM, since scenarios leading to different deviations from SM expectations seem to be required. In this respect, in the very recent work of Ref.…”

“…1 of Ref. [68]). The azimuthal angular (φ d ) distribution of the tau decay charged product is sensitive to possible CP odd effects (N H 3 term in Eq.…”

“…A further integration in ω additionally enhances the statistics. The obtained angular distribution [68]…”

“…In Refs. [68,70] the Λ b → Λ c τ − ντ and the B → D ( * ) τ − ντ decays were analyzed by employing the τ − → π − ν τ , ρ − ν τ and τ − → µ − νµ ν τ reconstruction channels. There, an special attention is paid to different quantities that can be measured by looking just at the visible kinematics of the charged particle produced in the τ decay [70].…”

“…Since the full visible-kinematics differential decay width may suffer from low statistics, possible statistically-enhanced distributions, which can be obtained by integrating in one or more of the related visible-kinematics variables, are analyzed in Ref. [68] in the search for NP.…”