Search for rare multi-pion decays of the tau lepton using the BABAR detector

A search for the decay of the τ lepton to rare multi-pion final states is performed using the BABAR detector at the PEP-II asymmetric-energy e⁺e^- collider. The analysis uses 232 fb^-1 of data at center-of-mass energies on or near the Υ(4S) resonance.

In the search for the τ→5pi.2pi0.nu decay, we observe 10 events with an expected background of 6.5{+2.0}_{-1.4} events. In the absence of a signal, we calculate the decay branching ratio upper limit 𝓑(τ→5pi.2pi0.nu) < 3.4x10^-6 at 90% confidence level. This is more than a factor of 30 improvement over the previously established limit. In addition, we search for the exclusive decay mode τ→2omega.pi.nu with the further decay of omega→pi+.pi-.pi0. We observe 1 event, expecting 0.4^{+1.0}_{-0.4} background events, and calculate the upper limit 𝓑(τ→2omega.pi.nu) < 5.4x10^-7 at 90% confidence level. This is the first upper limit for this mode. In the search for the τ→7pi.(pi0).nu decay, we observe 24 events with an expected background of 21.6+/-1.3 events. In the absence of a signal, we calculate the upper limit 𝓑(τ→7pi.(pi0).nu) < 3.0x10^-7 at 90% confidence level. This is an improvement by nearly an order of magnitude over the previously established limit. In addition, we set the first upper limits for the exclusive seven-pion decays B(τ→7pi.nu) < 4.3x10^-7 and B(τ→7pi.pi0.nu) < 2.5x10^-7.

We also study the ττ→5pi.pi0.nu mode and measure its branching ratio B(τ→5pi.pi0.nu) = (1.79+/-0.04+/-0.14)x10^-4, which is consistent with and more precise than the measurements from other experiments.