In the analysis of background events from a counter-technique experiment to study cosmic-ray nuclei above 8-GV rigidity, two events were observed which are consistent with the assumption of Z--14, ,4-350, and 450 MeV/nucleon and which cannot be accounted for by more conventional background. Such events may be explained by the hypothesis of strange-quark matter (SQM). It is concluded that the existence of SQM has not been excluded by experiment at a flux level of ~6 x 10 ~9 cm ~2s ~' sr ~'. 12.38.Mh, 98.70.Sa There have been studies' of properties of strongly interacting matter in bulk. Witten 2 has proposed the possibility that matter consisting of a roughly equal number of up, down, and strange quarks may be stable at zero temperature and pressure. Stable strange matter has been studied by several authors 3 on the basis of the MIT bag model. The strange matter created in the early Universe would have evaporated and could not survive to the present time. 4 Possible sources of the strange matter would be in the collision of neutron stars, or in neutron stars with a superdense quark surface and in quark stars with thin nucleon envelopes. 5 If strange-quark matter is really stable, it must be an abundant component in cosmic radiation. On the other hand, Bjorken and McLerran 6 have sketched a hypothesis that the "Centauro" events 7 were caused by the explosion of a glob of highly dense matter. If the Centauro events are of a cosmic origin and they are accelerated with a mechanism similar to that for cosmic rays, they should be detected at the top of the atmosphere.Have existing direct experiments ruled out the scenario for the existence of cosmic quark matter or placed a certain restriction on the scenario? Along this line, we analyzed again the cosmic-ray data which were obtained with a counter telescope in 1981, and found a new type of event in galactic cosmic rays. Figure 1 shows a schematic view of an instrument, consisting of an acrylic Cherenkov counter (C, two channels C\ and Ci) and a scintillation counter (5, two channels S\ and S2) for measuring the primary charges with an accuracy of 0.35 unit of charges, a liquid Cherenkov counter of fluorocarbon (L, four channels L\, Li, £3, and L4) for measuring the particle energies around cutoff rigidities (10 GV) and two pairs of X-Y crossed multitube proportional counters 8 (MTPCs) for determining particle trajectories within an accuracy of 0.5 cm for single-particle as well as multiple tracks. For vertically incident carbon nuclei, about 195 photoelectrons were collected with the four combined 5-in. photomultiplier tubes (per channel) from the acrylate Cherenkov counter and about 35 photoelectrons with the three combined photomultiplier tubes (per channel) from the liquid Cherenkov counter. The geometric factor of this instrument is 6049 cm 2 sr and the total payload weighs 291 kg. It was flown in September 1981 from Sanriku Ballon Center, Japan. The instrument was rotating at 1 rpm in order to measure arrival directions of particles. Triggering by a coincidenc...
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