A precise microwave spectroscopy measurement on the muonium ground state: hyperfine structure interval and muon magnetic moment

Abstract

An experiment is in progress at the Los Alamos Meson Physics Facility (LAMPF) to determine with high precision the hyperfine structure interval AV (to -10 ppb) and the muon to proton magnetic moment ratio &,/pp (to -60 ppb) in the ground state of muonium @+e-). These precision gods correspond to increases in precision of AV and of &/pp by about a factor of 5 compared to present knowledge. The ExperimentThe general method of the experiment [1,2] is microwave magnetic resonance spectroscopy as applied to muonium, which relies on parity nonconservation in the n: + 4 pLf v decay to produce polarized p+ and in the p+ -+ e+V,,v, decay to indicate the spin direction of p ' . An earlier LAMPF experiment provides the present experimental values [3]: AveXp = 4 463 302.88(16) kHz (36 ppb) pp I pp = 3.183 346 1(11)(360 ppb) and The theoretical expression for AV can be Av(theory) = Av(binding,rad) + Av(recoi1) + Av(rad-recoil) + Av(weak) written as: Except for the small term Av(weak) coming from the weak interaction, this expression arises solely from the electromagnetic interaction of two pointlike leptons of different masses in their bound state. The present theoretical value is [4]: Avth=4,463,3O2.63( 1.34)(0.06)(0.17) kHz (0.3 ppm) The principal error of 1.34 kHz arises from the uticertainty in &,/pp. The second uncertainty arises from that in a based on the electron 8-2 experiment, and the third is the estimate of the error in the latest QED calculation [4]. Weak neutral current effects associated with Z exchange in the e-p interaction contribute 0.065 kHz or 15 ppb and are included in Avth. The experimental and theoretical values for AV agree well: Avth -AveXp = -0.23 1.4) l c H~ "th -= -(0.06 k 0.3)ppm. A V exp In the experiment in progress microwave induced Zeeman transitions are measured in a static magnetic field of 1.7 T. Resonance lines have been obtained by sweeping the magnetic field with a fixed microwave frequency. The new experiment improves on the earlier one [3] in several ways.First, the p+ beam intensity is now 1 x lo' p+/s, with a duty factor of 6 to 9% and with greater purity, achieved principally by use of an E x g separator to reduce the e+ background in the beam. Second, the magnetic field from a commercial Magnetic Resonance Imaging superconducting magnet system operating in persistent mode provides a field with a homogeneity of better than 1 ppm over the active region of the microwave cavity and with a stability of 0.01 ppm to 0.1 ppm/hr. Third, an electrostatic chopper in the muon beam line provides a muon beam with an on-period of 4 ps and an off-period of 10 ps, which allowed the observation of a resonance line from muonium atoms which had lived longer than the 2.2 vs mean lifetime T~ of muons [5]. Such resonance lines can be narrower than the natural linewidth determined by T~. Indeed linewidths narrower than the natural linewidth by factors of up to 3 were obtained.During runs in 1994 and 1995 with a total beam time of about 3 mos., data were taken with stopping Kr gas pressures of 0.5, 1.0 an...

Precision Muonium Spectroscopy

“…This results in a resonance lineshape with a central feature that is narrower than the natural linewidth δν HFS ,nat = 1/(2π • τ µ + ) = 144kHz. 85 This feature provides for a better determination of the line center. In order to obtain a time structure suited for an old muonium experiment the cw surface µ + beam at LAMPF was chopped using an electrostatic deflector.…”