Nuclear Physics A433 (1985) 634-648 @ North-Holland Publishing Company MEASUREMENT OF THE I-NPERFINE SPLITTING OF THE 1s STATE IN MUONIC ‘Li AS A SEARCH FOR AXIAL-VECTOR MUON-NUCLEON INTERACTIONS+ W. RUCKSTUHL”, B. AASb, W. BEER, I: BELTRAMI’, K. BOSd, G. DE CHAMBRIER, P.F.A. GOUDSMIT, H.J. LEISI, G. STRASSNER, A. VACCHI” and J.M. VAN DER VELDEN’ Institute for Intermediate Energy Physics, ETHZ, c/o SZN, CH-5234 Villigen, Switzerland and B. JECKELMANN, U. KIEBELE and R. WEBER Physics Departement, University of Fribourg, CH-I 700 Fribourg, Switzerland Received 20 July 1984 Abstract: The magnetic hyperfine splitting (h.f.s.) in the 1s state of ‘Li has been measured with a precision of 1% using the bent-crystal spectrometer facility at the SIN muon channel. With an instrumental resolution of 2.5 eV FWHM the h.f.s. was completely resolved in the 2p- 1 s transition. The result is AE(Ml)=4684*49meV. A comparison of this result with the electromagnetic value yields a new upper limit for the strength of a possible muon-nucleon interaction mediated by an axial-vector boson, which is much lower than those that can be derived from other present data. 1. Introduction Nuclear magnetic moments can be measured precisely from the energy splitting in external magnetic fields. Since the muonic atom is a simple two-particle system the electromagnetic (Ml) h.f.s. can be readily calculated from the nuclear magnetic moment. As mentioned by Okun and Zakharov in their review article ‘), deviations of the experimentally observed hyperfine splitting can be ascribed to a new (non- electromagnetic) interaction between the muon and the nucleus. Measurements of ’ This work is based on the thesis of W. Ruckstuhl (ETH Zurich, No. 7061, 1982); it was supported in part by SIN, the Schweizerischer Nationalfonds and the Netherlands Organisation for the Advancement of Pure Scientific Research (ZWO). a Presknt address: Cal Tech, Pasadena, CA 91125, USA. b Present address: LAMPF, Los Alamos, NM 87545, USA. ’ Present address: SLAC, Bin 37, PO Box 4349, Stanford, CA 94305, USA. d Present address: NIKHEF, sectie H, postbus 41882, Amsterdam, NL. e Present address: Rockefeller Univ., c/o CERN, Geneva, Switzerland. f NIKHEF, sectie K, Postbus 41882, Amsterdam, NL. 634