Results of the Torino search for quarks

After a brief review of previous experiments which have indicated the presence of ''quark candidates'' among the shower particles detected at 40 m w.e. underground, the results of a further search with two spark chamber telescopes are reported. Small pulses in the top scintillators of the triggering arrangement and delayed arrivals with respect to the shower front were used as initial ''quark signature''. Interactions originated by the traversing particles in five layers of 5 cm Pb plus 1.8 cm Fe could serve as an additional criterion to establish the nature of the particles. The visualizing technique allowed rigorous elimination of all spurious effects resulting from soft accompaniment which could have contributed in the earlier measurements. Although also the final runs showed an unexplained excess of delayed small-pulse events corresponding to a ''flux'' of about 1.10-9 particles/cm2 s sr, the observations are not considered as convincing evidence for the existence of quarks. The interaction characteristics of the delayed particles are indistinguishable, within the statistical uncertainties, from those of the shower muons