Using 5.33 × 106 single muons collected in 1.46 × 104 live hours by MACRO during the period 1991-1994, we have searched for a correlation between variations in the underground muon rate, Νμ, and seasonal temperature variations in the atmosphere. These correlations are found to be present with high statistical significance. Analysis of the relatively complete December 1992-December 1994 subset of the data yields a value for the temperature coefficient, αΤ = (Τ/Νμ)(∂Nu;μ/∂Tau;) = 0.83 ± 0.13. Analysis of the total data set gives consistent results. We have compared this result with the hypothesis that the muons observed in MACRO come from pion decays alone. Although our result is consistent with the 'pion only' hypothesis, a discussion of the sensitivity of our data sample to the kaon component of the cascades leading to observed muons underground will also be presented.
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We gratefully acknowledge the support of the director and of the staff of the Laboratori Nazionali de1G ran Sass0 and the invaluable assistanceo f the technical staff of the Institutions participating in the experiment.W e thank the Istituto Nazionale di Fisica Nucleare (INFN), the U.S. Departmento f Energy and the U.S. National Science Foundation for their generous support of the MACRO experiment. We thank INFN for providing fellowships and grants (FAI) for non Italian citizens. We also wish to thank the IspettoratoT elecomunicazioni ed Assistenza Volo dell’Aeronautica Italiana for kindly providing the temperatured ata.