Search for relativistic quarks in cosmic rays at sea level and at mountain altitude

Using a telescope composed of 6 scintillation counters and 16 units of wire spark chambers with an aperture of 0.38 m2 sr, fractionally charged particles (quarks) in cosmic rays have been looked for at sea level for 2050 hours and then at mountain altitude for 5500 hours and the upper limits for the fluxes of quarks are set to beI ⩽ 1.30 · 10−10 cm−2 sr−1 s−1 at sea level andI ⩽ 5.7 · 10−11 cm−2 sr−1 s−1 at 2770 m altitude with 90% confidence level for relativistic particles with charge \(\frac{1}{3}e\). The upper limits to the production cross-sections have been estimated as a function of the masses of quarks and of the assumed values for absorption mean free paths. If one assumes that the quark production cross-sections are 1 µb and that the absorption mean free paths are 120 g cm−2, then the lower limit of the mass of quarks isM(1/3e) ⩾ 9.5 GeV · c−2 from the mountain altitude data.