First results from the HERMES experiment

Abstract

The Hermes experiment at the HERA proton positron collider has been designed to study the spin structure of the nucleon. Hermes measures the inclusive and semi-inclusive spin dependent deep inelastic scattering of polarized positrons off polarized nucleons with a large acceptance forward spectrometer. Two novel techniques are employed, the usage of an internal polarized gas target and the spin rotation of the transversely polarized positrons in the HERA positron storage ring. In 1995, its first year of data taking, Hermes measured the spin structure function g 1 n( x) of the neutron and its first moment Γ 1 n = −0.037 ± 0.013( stat) ± 0.005( sys) ± 0.006( extrapol) [1], in good agreement with existing data. As a first semi-inclusive result, the hadron asymmetries have been extracted from the 1995 data. They are slightly negative and have small statistical errors. Additionally, the angular distribution of π + mesons from the ϱ 0 → π +π − decay could be extracted with high accuracy. this yields the ratio of longitudinal to transversely polarized ϱ 0s, assuming the validity of s-channel helicity conservation. The comparison of ϱ 0 production on 3He and H targets can be used to investigate possible effects of nuclear transparency. The data on hydrogen measured in 1996 are a factor of 4 better in figure of merit. The conversion of the existing threshold Čerenkov counter into a RICH detector, which will take place during the 1997 winter shutdown, will significantly improve the particle identification capabilities of Hermes.