Search for the decay ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}({\pi }^0){\nu }_{\tau }$

A search for the decay of the $\tau$ lepton to seven charged pions and one or zero ${\pi }^0$ mesons was performed using the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider. The analysis uses $232.2{\mathrm{fb}}^{-1}$ of data at center-of-mass energies on or near the ${\rm Y}(4S)$ resonance. We observe 24 events with an expected background of $21.6\pm 1.3$ events. Without evidence for a signal, we calculate an upper limit of $\mathcal{B}$$({\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}({\pi }^0){\nu }_{\tau })<3.0\times {10}^{-7}$ at 90% confidence level. This is an improvement by nearly an order of magnitude over the previously established limit. In addition, we set upper limits for the exclusive decays ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}{\nu }_{\tau }$ and ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}{\pi }^0{\nu }_{\tau }$.

Figure 1

Pseudo mass distributions of the seven charged tracks after the 1–7 topology selection for signal (dashed-dotted: ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}{\pi }^0{\nu }_{\tau }$; dotted: ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}{\nu }_{\tau }$), generic $\tau$ (including 5-prong decays) (dashed), and $e^{+}{e}^{-}\to q\overline{q}$ Monte Carlo events (solid). Entries are normalized to the data luminosity. To display the two signal modes we assumed a branching fraction of $2\times {10}^{-5}$.

Figure 2

Pseudo mass distributions for 1–7 topology data. (a) after 1–7 topology selection; (b) after particle identification; (c) after conversion veto; (d) after 1-prong tag.

Figure 3

Details of the data pseudo mass spectra. (a) ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}({\pi }^0){\nu }_{\tau }$, (b) ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}{\nu }_{\tau }$, (c) ${\tau }^{-}\to 4{\pi }^{-}3{\pi }^{+}{\pi }^0{\nu }_{\tau }$ event candidates.