VOLUME 46, NUMBER 24 PHYSICAL REVIEW LETTERS 15 JvNs 1981 Observation of Prompt Like-Sign Dimuon Production in Neutrino Reactions K. Nishikawa'" and D. Buehholz Northseestern University, Evanston, Illinois 60201 B. C. Barish, J. F. Bartlett, ' ' H. Blair, Y. Chu, J. Lee, '" P. Linsay, ' ' J. Ludwig, H. Messner, P. Mine, '" F. J. Sciulli, M. Shaevitz, and E. Siskind' ' California Institute of Technology, I'asadena, California 91185 D. Edwards, H. Edwards, H. E. Fisk, Y. Fukushima, G. Krafczyk, and D. Nease'~' Fermi National Accelerator Laboratory, Batavia, Illinois 60510 A. Bodek and W. Marsh University of Rochester, Rochester, Nese York 14627 Q. Fackler Rockefeller University, Nese York, New York 10021 (Received 24 November 1980) We report on the observation of twelve like-sign Q p ) neutrino-induced dimuon events with muon momenta greater than 9 GeV. The background from Tt and & decay is 1.3 events so that we conclude that this prompt signal is real with a significance greater than 1 in 10 . Although the overall rate is higher than present theoretical estimates, the kinematic distri- butions of these events are qualitatively consistent with a picture of charm-anticharm pro- duction. The ratio of p p, /p shows a strong energy dependence and rises to (2.5+1.0) &&10 3 at E, =250 GeV. PACS numbers: 13.15.+g A unique tool to investigate new flavor produc- tion by the weak interactions has been multimuon production by neutrinos and antineutrinos. For example, the large rate for opposite-sign dimuon events and the increase of this rate with energy above 10 GeV was the first indication of a new heavy quark, charm. ' Like-sign dimuon events, on the other hand, may be very significant and have been observed, ' but are less well estab- lished experimentally because of large and uncer- tain backgrounds. This experiment establishes the existence of prompt like-sign dimuon events from neutrino interactions. The background is substantially lower than previous measurements' because of the high density of the detector, the higher neutrino-energy spectrum, and the kine- matic restrictions on the observed muons. For like-sign dimuon events, the second p cannot come from the decay of any known singly pro- duced quark (i. e. , u, c, 2, s, or 5'). Some calcu- lations of like-sign dimuon events have been made with use of heavy leptons, ' associated charm-anti- charm production, ' ' or cascades in which the second muon comes from second-generation de- cays of charm quarks. These calculations pre- dict small cross sections with large energy de- pendences due to threshold effects. We report here a measurement of like-sign di- muon production by neutrinos at energies up to 300 GeV. The experiment was performed at Fer- milab with use of the quadrupole triplet beam with 400-GeV incident protons. The detector con- sisted of a dual-density steel target (160 tons), a steel hadron absorber (224 tons), and magnet- ized solid steel toroids (440 tons) for muon mo- mentum analysis. The target was composed of eight 1.5 mx 1.5 mx 10 cm-thick steel plates in- terspersed with liquid scintillation counters. It was arranged in two densities (differing by a fac- tor of 2) to check our empirical estimate of non- prompt sources of multimuon production at low muon energy. A spark chamber was placed every 20 cm of steel (40 cm of steel) for the low- (high-) density target to allow tracking of the muons from neutrino interactions. The low-density target was larger in the transverse dimension (1.5x 3 1981 The American Physical Society 1555