Upper Bound on the Decay $\tau \to \mu \gamma$ from the Belle Detector

We have performed a search for the lepton-flavor-violating decay $\tau \to \mu \gamma$ using a data sample of $86.3{\mathrm{fb}}^{-1}$ accumulated by the Belle detector at KEK. No evidence for a signal is seen, and we set an upper limit for the branching fraction of $\mathcal{B}(\tau \to \mu \gamma )<3.1\times {10}^{-7}$ at the 90% confidence level.

Figure 1

Distributions of the opening angle between (a) the $\mu$ and $\gamma$ on the signal side, and (b) the tagging track and the missing momentum. MC distributions for signal and ${\tau }^{+}{\tau }^{-}$ events are indicated by shaded and open histograms, respectively, and data by closed circles. The arrows show the selected ranges.

Figure 2

Distribution of events in the $m_{\mathrm{miss}}^2$-$p_{\mathrm{miss}}$ plane. The selection boundary is indicated by two lines for (a) signal MC events and (b) ${\tau }^{+}{\tau }^{-}$ MC events.

Figure 3

Remaining events in data (circles) and the expected density for signal MC events (shaded) in the $\Delta E$-$M_{\mathrm{inv}}$ plane. The region between the dashed lines is kept excluded until the selection criteria are finalized and the expected background estimated. The signal box (defined in the text) is indicated by a dash-dotted box.

Figure 4

$\Delta E$ distributions in the signal region, $1.71\mathrm{GeV}/c^2<M_{\mathrm{inv}}<1.82\mathrm{GeV}/c^2$. The expected background is indicated by the solid (dashed) curve for $1+\Lambda =1.22(1.14)$ and $\kappa =0.11(0.14)$. The histogram is an average of both sideband spectra. The data are the points with error bars. The vertical dashed lines indicate the $\Delta E$ range of the signal box.