Observation of the Muon Inner Bremsstrahlung at LEP1

TL;DR

This study measured muon bremsstrahlung photons at LEP1, confirming QED predictions and observing the bremsstrahlung dead cone, contrasting with anomalous soft photon excesses seen in hadronic Z^0 decays.

Contribution

First observation of the bremsstrahlung dead cone in direct photon production at LEP and validation of QED predictions for muon bremsstrahlung rates.

Findings

Photon rates agree with QED predictions within uncertainties.

Bremsstrahlung dead cone observed for the first time at LEP.

No anomalous soft photon excess in muon events.

Abstract

Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2 < E_gamma <= 1 GeV and transverse momentum with respect to the parent muon p_T < 40 MeV/c, and 1 < E_gamma <= 10 GeV and p_T < 80 MeV/c . A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z^0 decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06 +/- 0.12 +/- 0.07 in the photon energy range 0.2 < E_gamma <= 1 GeV and 1.04 +/- 0.09 +/- 0.12 in the photon energy range 1 < E_gamma <= 10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP.