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Probing the neutrino mass through semileptonic meson decays

Damir Bečirević, Claire Chevallier, Svjetlana Faifer, Nejc Košnik, Lovre Pavičić

Abstract

We argue that a detailed analysis of semileptonic decays can test the possibility of a massive neutrino. The key observable, related to the forward-backward asymmetry, is exactly zero for a massless neutrino but becomes non-zero if the neutral lepton is heavy and interacts with Standard Model fields via left-handed operators. For right-handed interactions, this quantity differs significantly from zero even for a massless right-handed neutrino. We demonstrate this explicitly using the example of a pseudoscalar meson decaying into another pseudoscalar meson. A similar discussion applies to decays into a vector meson, with an additional subtlety addressed in this work.

Probing the neutrino mass through semileptonic meson decays

Abstract

We argue that a detailed analysis of semileptonic decays can test the possibility of a massive neutrino. The key observable, related to the forward-backward asymmetry, is exactly zero for a massless neutrino but becomes non-zero if the neutral lepton is heavy and interacts with Standard Model fields via left-handed operators. For right-handed interactions, this quantity differs significantly from zero even for a massless right-handed neutrino. We demonstrate this explicitly using the example of a pseudoscalar meson decaying into another pseudoscalar meson. A similar discussion applies to decays into a vector meson, with an additional subtlety addressed in this work.
Paper Structure (9 sections, 52 equations, 5 figures)

This paper contains 9 sections, 52 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Effective Theory and Observables
  3. Decay to a pseudoscalar meson
  4. LH neutral lepton
  5. RH neutral lepton
  6. Illustrative examples
  7. Decay to a vector meson
  8. More possibilities
  9. Summary

Figures (5)

  • Figure 1: Kinematics of the semileptonic decay of a pseudoscalar to a vector meson, $M\to V (\to P \pi) \ell N$. For a decay to a pseudoscalar meson, the angles $\phi$ and $\theta_P$ are not present.
  • Figure 2: Forward--backward asymmetry of $\overline K^0 \to \pi^+ \mu \text{'inv'}$ integrated over the entire phase space, $\langle A_{FB}\rangle$, and its components corresponding to two helicities of the lepton pair, $\langle A_{FB}^\pm\rangle$, all plotted as functions of $m_{N_L}$ ($m_{N_R}$). The dashed lines correspond to the SM values.
  • Figure 3: Two components of the forward--backward asymmetry of $B \to D \tau \text{'inv'}$ corresponding to two helicities of the lepton pair. In the left (right) plots we show the dependence of the fully integrated $\langle A_{FB}^\pm\rangle$ as a function of the mass of $N_L$ ($N_R$). Plotted are the cases for two different values of $C_V^{LL}$ ($C_V^{LR}$).
  • Figure 4: Two components of the forward--backward asymmetry of $B \to D^\ast \tau \text{'inv'}$ corresponding to the helicity of the outgoing leptons. In the left (right) plots we show the dependence of the fully integrated $\langle A_{FB,L}^\pm\rangle$ as a function of the mass of $N_L$ ($N_R$). Plotted are the cases for two different values of $C_V^{LL}$ ($C_V^{LR}$).
  • Figure 5: Same as in Fig. \ref{['fig:3']} but for $\langle A_{FB,T}^\pm\rangle$.