Results of a search for free quarks with a large avalanche chamber in high-energy neutrino interactions

Summary

A search for free quarks liberated via neutrino-quark coupling using the highest possible available energy was performed at CERN (WA-44 experiment) in the neutrino wide-band beam. The neutrino interactions take place in a 23 ton lead target. A forward large avalanche chamber is triggered when one or more charged particles from the neutrino (antineutrino) interactions cross two sets of counter hodoscopes. Ionization measurements in the avalanche chamber have been performed using two measuring machines: HPD in Bologna and PERPR in Frascati. The 6% expected statistical fluctuations in the ionization measurements have been obtained on one metre long tracks. This allowed an accurate analysis of the data to search in the 1/3 and even in the 2/3 charge regions. About 3.7·10⁵ tracks were observed in the chamber, from about 10⁵ neutrino (antineutrino) interactions. The analysis of the digitized data has selected 20 candidates for fractional charges. After a detailed analysis of both pictures and measurements, and a complete investigation of all possible instrumental effects, one event remains as a fractional charge (+1/3) candidate. This candidate has a small probability to be an edge effect in the apparatus. However this event is at the expected level of candidate events from the rejection power of our detector. Upper limits (90% c.l.) on the flux of quarks produced in both neutrino and antineutrino interactions are at the level of a few 10⁻⁵.