Neutral Current Induced Reactions in the Gargamelle Experiment

Abstract

From several exposures of Gargamelle filled with heavy freon (CF₃Br) to the CERN ν and ν̄ beams, three candidates were found for the purely leptonic neutral current process:

$$ {\overline \nu _{\mu }} + {e^{ - }} \to {\overline v _{\mu }} + {e^{ - }} $$

The total background to this reaction was estimated to be 0.44 ± 0.13 events. The observed cross section ist σ_obs = 0.05 · 10–⁴¹ E _{ν ̄} cm². Within the framework of the Weinberg-Salam model the following 90 % confidence limits can be set on ${\sin ^{2}}{\theta _{w}}$:

$$ 0.1 < {\sin ^{2}}{\theta _{w}} < 0.4 $$

For events with an energy of the hadron system greater than 1 GeV, the ratios of the semi-leptonic neutral current to the charged current cross sections were found to be R_ν = 0.25 ± 0.04 and R _ν ̄ = 0.56 ± 0.09. Assuming the validity of scaling and a V and/or A type structure for the neutral current, our results differ from the predictions of parity conserving models by more than 3 standard deviations. Using the Weinberg-Salam model and the calculations of Sehgal, sin²θ_w is found tobe 0.31 ^+0.07_-0.07 from R_ν and 0.33 ^+0.07_-0.10 from R _ν ̄.