Spectral functions from hadronic tau decays and QCD

TL;DR

This paper analyzes hadronic tau decays to extract spectral functions, compares them with e+e- data, and uses them for QCD parameter determination, highlighting discrepancies and their implications.

Contribution

It provides new spectral function measurements from ALEPH, compares tau and e+e- data, and applies these to refine QCD parameters and strange quark mass estimates.

Findings

Discrepancy in the rho resonance line shape between tau and e+e- data.

Normalization difference of 3.3% after corrections.

Accurate determination of the strong coupling constant and strange quark mass.

Abstract

Hadronic decays of the $\tau$ lepton provide a clean environment to study hadron dynamics in an energy regime dominated by resonances. The interesting information is captured in the spectral functions. Recent results from ALEPH on exclusive channels are presented, with emphasis on the $\pi \piz$ final state which plays a crucial role for the determination of the hadronic contribution to the muon anomalous magnetic moment. A comparison between $2\pi$ spectral functions obtained in $\tau$ decays (after corrections for isospin-breaking) and $e^+e^-$ annihilation reveals some discrepancy in the line shape of the $\rho$ resonance which can be attributed to different pole mass values for the charged and neutral $\rho$'s, which are determined through a robust fitting procedure. However, after applying this correction, the normalization of the two spectral functions differ by 3.3%. Inclusive spectral functions are the basis for QCD analyses, which deliver an accurate determination of the strong coupling constant, quantitative information on nonperturbative contributions and a measurement of the mass of the strange quark.