Low energy atmospheric muon neutrinos in MACRO MACRO Collaboration

We present the measurement of two event samples induced by atmospheric n of average energy E ;4 GeV. In the first m n sample, a neutrino interacts inside the MACRO detector producing an upward-going muon leaving the apparatus. The ratio of the number of observed to expected events is 0.57"0.05 "0.06 "0.14 with an angular distribution similar to stat syst theor that expected from the Bartol atmospheric neutrino flux. The second is a mixed sample of internally produced downwardgoing muons and externally produced upward-going muons stopping inside the detector. These two subsamples are selected by topological criteria; the lack of timing information makes it impossible to distinguish stopping from downgoing muons. The ratio of the number of observed to expected events is 0.71"0.05 "0.07 "0.18 . The observed deficits in each stat syst theor subsample is in agreement with neutrino oscillations, although the significance is reduced by the large theoretical errors. However, the ratio of the two samples causes a large cancellation of theoretical and of some systematic errors. With the ratio, we rule out the no-oscillation hypothesis at 95% c.l. Furthermore, the ratio tests the pathlength dependence of possible oscillations. The data of both samples and their ratio favor maximal mixing and Dm ;10–10 eV . These parameters are in agreement with our results from upward throughgoing muons, induced by n of much higher energies. q 2000 m Elsevier Science B.V. All rights reserved. The results from several underground detectors which measure the flux of atmospheric neutrinos 1 Also Universita della Basilicata, 85100 Potenza, Italy. ` 2 Also INFN Milano, 20133 Milano, Italy. 3 Also Istituto TESRErCNR, 40129 Bologna, Italy. 4 Also Dept. of Phys., Pennsylvania St. Univ., Univ. Park, PA 16801, USA. 5 Also Universita di Trieste and INFN, 34100 Trieste, Italy. ` 6 Also Dip. di Energetica, Universita di Roma, 00185 Roma, ` Italy. 7 Ž . Also Dip. Fisica Univ. della Calabria, Rende Cs , Italy. 8 The Colorado Coll., Colorado Springs, CO 80903, USA . 9 Also INR, Russian Academy of Science, 117312 Moscow, Russia. 10 Also Dept. of Phys. J. Madison Univ., Harrisonburg, VA 22807, USA. 11 Also Scuola Normale Superiore di Pisa, 56010 Pisa, Italy. 12 Also Inst. Space Sciences, 76900 Bucharest, Romania. 13 Corresponding author. E-mail: SPURIO@BO.INFN.IT. give strong indication that n ’s oscillate into neutrim w x nos of another type 1–3 . Fully-contained and partially-contained neutrino-induced events observed in underground detectors come from neutrinos of energy ;1 GeV. The flux of atmospheric neutrinos of several tens of GeV can be inferred from the measurement of neutrino-induced upward-going muons Ž that traverse the entire detector up-throughgoing . muons . The hypothesis of neutrino oscillations, with best-fit parameters sin 2u ;1 and Dm in the mix range of a few times 10 eV , can explain the observed anomalies both in the ratio of contained nm Ž . to n events Super-K, Soudan 2 and in the zenith e angle distribution of up-throughgoing muons Ž . MACRO, Super-K . The MACRO detector measures atmospheric neuŽ . trinos of high energy median energy ;50 GeV w x 3,4 and few GeV energy. The interpretation of our high energy data in terms of n oscillations came m ( ) M. Ambrosio et al.rPhysics Letters B 478 2000 5–13 7 from a deficit and from an anomalous zenith angle distribution of the observed up-throughgoing muons originating from n interactions in the rock below m w x the detector. In an updated analysis 5 with 607 Ž . observed events 824 expected both the event reduction and the shape of the zenith angle distribution confirm our previous data and are in agreement with the hypothesis of n oscillations. Assuming a two m flavour oscillation: P n TMn s1ysin 2u Psin 1.27PDmLrE Ž . Ž . m m mix n