Andrei Rabusov "Partial-wave analysis of tau- -> pi- pi- pi+ nu_tau at Belle"
by
HEPHY Library
The parameters of the axial-vector form-factor, relevant for Beyond-Standard-Model searches, are not well known. The tau-lepton, a heavier analog of the electron which can decay into hadrons, provides a clean laboratory to study these parameters for a low energy range where the $\mathrm{a}_1(1260)$ meson dominates the $\tau^-\to\pi^-\pi^-\pi^+\nu_\tau$ decay. To know how $\mathrm{a}_1(1260)$ and its excited states affect the decay, we disentangle possible individual quantum states in an unbinned maximum-likelihood fit using data collected with the Belle detector at the KEKB $\mathrm{e}^+\mathrm{e}^-$ collider. We observe an $\mathrm{a}_1(1420)$-like signal in the $1^+[\mathrm{f}_0(980)\pi]_\mathrm{P}$~wave for the first time in tau-lepton decays. Also, we find a significant signal of the $1^-[\omega(782)\pi]_\mathrm{P}$~wave driven by the rare decay of $\omega(782)\to\pi^+\pi^-$, which violates $G$-parity. We validated these findings using a model-independent approach. These results will improve modelling in simulation studies necessary for measuring the tauonic electric and magnetic dipole moments and Michel parameters.