Enhanced tau neutrino appearance through invisible decay

The decay of neutrino mass eigenstates leads to a change of the conversion and survival probability of neutrino flavor eigenstates. Exploiting the recent results released by the long-baseline OPERA experiment we perform the statistical investigation of the neutrino invisible decay hypothesis in the ${\nu }_{\mu }\to {\nu }_{\tau }$ appearance channel. We find that the neutrino decay provides an enhancement of the expected tau appearance signal with respect to the standard oscillation scenario for the long-baseline OPERA experiment. The increase of the ${\nu }_{\mu }\to {\nu }_{\tau }$ conversion probability by the decay of one of the mass eigenstates is due to a reduction of the “destructive interference” among the different massive neutrino components. Despite data showing a very mild preference for invisible decays with respect to the oscillations only hypothesis, we provide an upper limit for the neutrino decay lifetime in this channel of ${\tau }_3/m_3\gtrsim 1.3\times {10}^{-13}\mathrm{s}/\mathrm{eV}$ at the 90% confidence level.

Figure 1

Survival probability (top panel) and conversion probability (bottom panel), as a function of the ratio $L/E$ in the two-flavor approximation. The dotted blue lines correspond to the oscillation-only scenario. The red dashed lines refer to the oscillation plus decay hypothesis for ${\alpha }_3={10}^{11}\mathrm{eV}/\mathrm{s}$. The black solid lines correspond to the oscillation plus decay hypothesis for ${\alpha }_3={10}^{13}\mathrm{eV}/\mathrm{s}$. The green solid lines represent the pure decay case with ${\alpha }_3={10}^{11}\mathrm{eV}/\mathrm{s}$.

Figure 2

Conversion probability in the region of $L/E$ investigated by OPERA, Minos and T2K experiments. The dotted blue line corresponds to the oscillation-only scenario. The red dashed line refers to the oscillation plus decay hypothesis for ${\alpha }_3={10}^{11}\mathrm{eV}/\mathrm{s}$. The black solid line refers to the oscillation plus decay hypothesis for ${\alpha }_3={10}^{13}\mathrm{eV}/\mathrm{s}$. The arrows indicate the characteristic $L/E$ of OPERA, Minos, and T2K.

Figure 3

Expected number of ${\nu }_{\tau }$ events (solid black line) from Eq. (9) as a function of the decay parameter ${\alpha }_3$ and for $m_{23}^{\mathrm{BF}}$, ${\theta }_{23}^{\mathrm{BF}}$ [24]. The blue dashed line shows, in arbitrary units, the normalized likelihood function. The dotted red line is the number of observed ${\nu }_{\tau }$ events in OPERA.

Figure 4

Value of $\mathrm{\Delta }{\chi }^2$ as a function of ${\alpha }_3$ considering the other two oscillation parameters fixed to their best-fit values, i.e., $m_{23}^{BF}$, ${\theta }_{23}^{BF}$ (blue dashed line) and when the likelihood is marginalized with respect to these two (black solid line). The horizontal lines correspond to the $1\sigma$ (solid red line) and 90% confidence level (dotted red line).