Evidence for an Excess of Soft Photons in Hadronic Decays of Z^0

TL;DR

This study finds a significant excess of soft photons in hadronic Z^0 decays compared to Monte Carlo predictions, suggesting an unexplained phenomenon beyond standard bremsstrahlung.

Contribution

It provides the first evidence of an excess of soft photons in Z^0 hadronic decays, similar to anomalies seen in fixed target experiments.

Findings

Observed photon excess is about 3.4 times the expected bremsstrahlung

Excess photons are within the energy range 0.2-1 GeV and p_T < 80 MeV/c

Results suggest potential new physics or unaccounted processes in hadronic decays.

Abstract

Soft photons inside hadronic jets converted in front of the DELPHI main tracker (TPC) in events of qqbar disintegrations of the Z^0 were studied in the kinematic range 0.2 < E_gamma < 1 GeV and transverse momentum with respect to the closest jet direction p_T < 80 MeV/c. A clear excess of photons in the experimental data as compared to the Monte Carlo predictions is observed. This excess (uncorrected for the photon detection efficiency) is (1.17 +/- 0.06 +/- 0.27) x 10^{-3} gamma/jet in the specified kinematic region, while the expected level of the inner hadronic bremsstrahlung (which is not included in the Monte Carlo) is (0.340 +/- 0.001 +/- 0.038) x 10^{-3} gamma/jet. The ratio of the excess to the predicted bremsstrahlung rate is then (3.4 +/- 0.2 +/- 0.8), which is similar in strength to the anomalous soft photon signal observed in fixed target experiments with hadronic beams.