Search for lepton-number- and baryon-number-violating tau decays at Belle

We search for lepton-number- and baryon-number-violating decays ${\tau }^{-}\to \overline{p}{e}^{+}{e}^{-}$, $p{e}^{-}{e}^{-}$, $\overline{p}{e}^{+}{\mu }^{-}$, $\overline{p}{e}^{-}{\mu }^{+}$, $\overline{p}{\mu }^{+}{\mu }^{-}$, and $p{\mu }^{-}{\mu }^{-}$ using $921{\mathrm{fb}}^{-1}$ of data, equivalent to $(841\pm 12)\times {10}^6$ ${\tau }^{+}{\tau }^{-}$ events, recorded with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. In the absence of a signal, 90% confidence-level upper limits are set on the branching fractions of these decays in the range $(1.8-4.0)\times {10}^{-8}$. We set the world’s first limits on the first four channels and improve the existing limits by an order of magnitude for the last two channels.

Figure 1

A schematic of 3–1 topology defined in the CM frame. The blue dotted line divides the event into two hemispheres based on the thrust-axis direction.

Figure 2

$\mathrm{\Delta }E–M_{\mathrm{rec}}$ distribution for the ${\tau }^{-}\to \overline{p}{e}^{+}{e}^{-}$ signal MC sample. The red box denotes the signal region, the region outside it is the sideband, and the area between two green dashed lines excluding the red box is the $\mathrm{\Delta }E$ strip. The size of the blue filled box represents the number of events in a given bin. For other channels these three regions are similarly defined except that the red box position is changed owing to the difference in $\mathrm{\Delta }E$ and $M_{\mathrm{rec}}$ resolutions.

Figure 3

$M_{\mathrm{rec}}$ and $\mathrm{\Delta }E$ distributions in the sideband for ${\tau }^{-}\to \overline{p}{e}^{+}{e}^{-}$ before the photon conversion veto applied. Black arrows denote the signal region. Signal MC events are arbitrarily normalized while background MC events are scaled to the number of data events.

Figure 4

$M_{e^{+}{e}^{-}}$ and $M_{\overline{p}{e}^{+}}$ (electron hypothesis) distributions in the ${\tau }^{-}\to \overline{p}{e}^{+}{e}^{-}$ sideband. Black arrows show the conversion veto position. Signal MC events are arbitrarily normalized while background MC events are scaled to the number of data events.

Figure 5

$\mathrm{\Delta }E–M_{\mathrm{rec}}$ distributions where the red box denotes the signal region and the green $\mathrm{\Delta }E$ strip is used to calculate the expected background. Black dots represent the data.