Probing nonunitary mixing and $CP$ violation at a neutrino factory

A low-energy nonunitary leptonic mixing matrix is a generic feature of many extensions of the standard model. In such a case, the task of future precision neutrino oscillation experiments is more ambitious than measuring the three mixing angles and the leptonic (Dirac) $CP$ phase, i.e., the accessible parameters of a unitary leptonic mixing matrix. A nonunitary mixing matrix has 13 parameters that affect neutrino oscillations, out of which four are $CP$ violating. In the scheme of minimal unitarity violation we analyze the potential of a neutrino factory for determining or constraining the parameters of the nonunitary leptonic mixing matrix, thereby testing the origin of $CP$ violation in the lepton sector.

Figure 1

The 90% confidence level sensitivity of the IDS neutrino factory to the unitarity violating parameters ${\epsilon }_{\tau \mu }$ (left) and ${\epsilon }_{\tau e}$ (right). The different curves correspond to different sizes of the near $\tau$ detector, from left to right, 10 kton, 1 kton, 100 ton, no near detector.

Figure 2

The sensitivity of the IDS neutrino factory to the unitarity violating parameter ${\epsilon }_{\tau \mu }$, assuming that it takes the value ${\epsilon }_{\tau \mu }=3.2·{10}^{-3}\exp (i\pi /4)$ (left) and ${\epsilon }_{\tau \mu }=-i3.2·{10}^{-3}$ (right). The different curves correspond to different sizes of the near $\tau$ detector, from inner to outer curves, 10 kton, 1 kton, 100 ton, no near detector.

Figure 3

The 90% confidence level sensitivity of the IDS neutrino factory to the unitarity violating parameter ${\epsilon }_{\tau e}$ with ${\theta }_{13}=5°$ as well as $\delta =\pi /4$ (left) and $\delta =0$ (right). The different curves correspond to different sizes of the near $\tau$ detector, from left to right, 10 kton, 1 kton, 100 ton, no near detector.

Figure 4

The sensitivity of the IDS neutrino factory to the unitarity violating parameter ${\epsilon }_{\tau e}$, assuming that it takes the value ${\epsilon }_{\tau e}=5.0·{10}^{-3}\exp (i\pi /4)$ (left) and ${\epsilon }_{\tau \mu }=-i5.0·{10}^{-3}$ (right). The different curves correspond to different sizes of the near $\tau$ detector, from inner to outer curves, 10 kton, 1 kton, 100 ton, no near detector.