Study of the electromagnetic transition form-factors in \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0 decays with NA60

TL;DR

This study remeasured electromagnetic transition form factors in ta and irc; decays using NA60 data, confirming a significant deviation from vector meson dominance with improved precision and providing a new branching ratio estimate.

Contribution

The paper provides the first high-precision measurements of the ta and irc; electromagnetic transition form factors, confirming a strong enhancement beyond vector meson dominance.

Findings

Form factor ta: mbda^{-2} = 1.95 b1 0.17 (stat) b1 0.05 (syst) GeV^{-2}

Form factor irc;: mbda^{-2} = 2.24 b1 0.06 (stat) b1 0.02 (syst) GeV^{-2}

Improved measurement of BR(irc; bb b7 b5b1b0 bb b7 b5b1b0 b0) = (1.73 b1 0.25 (stat) b1 0.14 (syst)) imes 10^{-4}

Abstract

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A GeV In-In collisions. The mass and pT spectra associated with peripheral collisions can quantitatively be described by the known neutral meson decays. The high data quality has allowed to remeasure the electromagnetic transition form factors of the Dalitz decays \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0. Using the usual pole approximation F = (1-M_{\mu\mu}^{2}/\Lambda^{2})^{-1} for the form factors, we find \Lambda^{-2} (in GeV^{-2}) to be 1.95+-0.17(stat.)+-0.05(syst.) for the \eta and 2.24+-0.06(stat.)+-0.02(syst.) for the \omega. While the values agree with previous results from the Lepton-G experiment, the errors are greatly improved, confirming now on the level of 10\sigma the strong enhancement of the \omega form factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR(\omega -> \mu^+\mu^-\pi^0) = [1.73+-0.25(stat.)+-0.14(syst.)]*10^{-4} has been obtained as a byproduct.