Lack of low-density cosmic-ray tracks

A diligent search for low-density (quark) tracks in cloud-chamber pictures of cosmic rays has been negative. The flux of fractionally charged particles in extensive air showers (EAS) is estimated, at a 90% confidence level, to be less than 2 × 10⁻¹¹ cm⁻² sr⁻¹ sec⁻¹ for particles having a charge of 2/3e, less than 8 × 10⁻¹¹ for 1/3e, less than 7 × 10⁻¹⁰ for 1/4e, and less than 1 × 10⁻⁷ for 1/6e, where e is the charge on an electron. The chambers were triggered by EAS about once an hour; the ions were allowed to diffuse so that individual droplets could be counted; the positive- and negative-ion columns were separated in an electric field so that the timing of the tracks was measured and the proper operation of the cloud chamber monitored. Individual droplets were counted "by hand" (looking in microscopes) and by automated methods. Artificial low-density tracks were superimposed on regular pictures to train the scanners and to measure their efficiency. 200 000 stereo picture pairs containing 10⁶ cosmic-ray tracks have been scanned for quark tracks; none has been found.