The anomalous magnetic moment of positive and negative muons

The anomalous g-factor a ≡ (g?2)/2 has been measured for muons of both charges in the Muon Storage Ring at CERN. The two results, a_μ⁺ = 1165910(12) × 10^?9 and a_μ^? = 1165936(12) × 10^?9, are in good agreement with each other, and combine to give a mean a_μ = 1165922(9) × 10^?9, which is very close to the most recent theoretical prediction 1165921(10) × 10^?9. For the experimental results, the total statistical and systematic error is given. The measurements thus confirm the remarkable QED calculation plus hadronic contribution, and serve as a precise verification of the CPT theorem for muons.     

Transfer of negative muons from hydrogen to oxygen

Negative muons were trapped in gaseous mixtures of hydrogen and oxygen. The time distributions of the muonic oxygen X-rays showed a clear deviation from a single exponential structure. From these time distributions we evaluated the transfer rate from thermalized muonic hydrogen atoms to oxygen. In order to test the hypothesis of an energy-dependent transfer rate to oxygen we performed Monte Carlo simulations of the thermalization and the transfer process. We report on the present status of our investigations.     

Chemical effects in the capture of negative muons

Muonic Lyman-series X-ray spectra have been measured for several isoelectronic and isostructural molecular series and related pure elements. Seven muonic Lyman series lines were observed from liquid argon with relative intensities consistent with neighboring isoelectronic ions, but in stark contrast to the single line previously reported for a gaseous argon target. The measured relative capture rates and Lyman intensity correlations provide new evidence for chemical structure influences on the capture of negative muons.     

Fission of Th and U in the interaction with negative muons

Muon-induced fission of ²³²Th and ²³⁸U was studied, measuring the time correlation between stopping muons, fission events and electrons.

The measured muon lifetimes are τ_μ(²³²Th) = 77.3±0.3 ns and τ_μ(²³⁸U) = 77.1±0.2 ns. After prompt fission the muon was found to be attached predominantly to the heavy fragment with a lifetime for ²³²Th of 132±7 ns and for ²³⁸U of 134±4 ns. The probability for muon conversion was found to be a few percent per fission event.

Indications were found that the muonic shape isomer is populated in ²³⁸U when the nucleus is excited in a radiationless muonic transition.     

Zero residual polarization of negative muons implanted in C60

We report preliminary results on ^–-depolarization in C₆₀. In contrast to graphite and diamond no clear ^–SR signal could be detected in TF-measurements in C₆₀.     

On total capture rate of negative muons in4He

Total capture rate ofμ ^? in⁴He is calculated using a modified Irving wave function, whose parameters are obtained from a variational calculation of the binding energy of⁴He using a central velocity-dependent potential.     

Isotope effect in nuclear capture of negative muons in xenon

The lifetime of negative muons in the ¹²⁹Xe 1_s state was measured. The muon capture rate in ¹²⁹Xe is compared with that in the ^{132, 136}Xe isotopes. The capture rate was found to depend on the mass number of the cited isotopes. The experimental results are compared with the results of calculations by the semiempirical Goulard-Primakoff formula.     

Probing oxygen in high Tc superconductor LaSrCuO by negative muons

A novel microscopic method of muon spin rotation for negative muon (μ⁻) attached to oxygen is applied on highT _c materials LaSrCuO. The paramagnetic shift and relaxation rate are measured as a function of Sr concentration and temperature. Analysis shows the large difference in shift and relaxation between two different sites. A large anisotropy was also observed by preliminary measurements along different crystalline axes.     

Measurement of helium-3 and deuterium stopping power ratio for negative muons

The measurement method and results measuring of the stopping power ratio of helium-3 and deuterium atoms for muons slowed down in the D/³He mixture are presented. Measurements were performed at four values of pure ³He gas target densities, ϕ_He = 0.0337, 0.0355, 0.0359, 0.0363 (normalized to the liquid hydrogen density) and at a density 0.0585 of the D/³He mixture. The experiment was carried out at PSI muon beam μE4 with the momentum Pμ= 34.0 MeV/c. The measured value of the mean stopping ratio S$_{3^He/D}$ is 1.66±0.04.     

Influence of molecular properties on Coulomb capture of negative pions and muons

The meso-molecular model of pion or muon capture involves a parameter which represents the probability of de-excitation from the molecular orbital to an atomic orbital centred on a particular atom. This parameter depends on the fractional ionic character of the bonds in the molecule. An extension to the existing formalism is given so that the influence of molecular structure may be taken into account.     

Relativistic effect on magnetic moments of negative muons bound to high-Z nuclei

The g factors of the negative muons bound to Zn, Cd, and Pb nuclei have been determined. The observed values after the corrections for the internal fields have shown substantial deviations from the free-muon g factor: $\text{(g}{}_{\mathrm{<mtext>free</mtext>}}\text{-g)}\text{g}{}_{\mathrm{<mtext>free</mtext>}}$ is (1.3±0.6)% for Zn, (2.2±1.4) % for Cd, and (5.0±2.2)% for Pb. These shifts are in good agreement with the relativistic binding corrections for finite-size nuclei calculated by Ford et al.     

Separation of positive and negative muons in a giant shower by the geomagnetic field

The separation of positive and negative muons in a giant shower by the geomagnetic field is observed at a very high level of significance 0.03%. This result is obtained by interpreting in terms of the quark-gluon string model the indications of all detector stations that were triggered in this shower (which was detected at the Yakutsk array) on the basis of calculations of the responses of these stations both with and without taking the Earth’s magnetic field into account. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 177–182 (10 August 1998)     

Residual polarization of negative muons implanted in C60 and K3C60

Negative muons captured by atoms with zero nuclear spin usually retain about 1/6 of their initial spin polarization P_0 after cascading down to their lowest Bohr orbital. This is particularly observed for μ^- implanted into graphite and diamond. However, μ^- implanted into a powder of the fullerene C₆₀ are found to show essentially zero residual polarization. A residual polarization of \sim P_0/6 is again seen in metallic K₃C₆₀. We speculate that these results are to be explained on the basis of hyperfine effects after completion of the muonic cascade. This revised version was published online in August 2006 with corrections to the Cover Date.     

Measurement of the probability of atomic capture and depolarization of negative muons in high-temperature superconductors

The possibility of investigating high-temperature superconductors (HTSC) by the μSR method with negative muons is discussed in the paper. The results of measurement of the probability of negative muon capture by oxygen and residual polarization of muons in the 1s-state of oxygen in some HTSC at room temperature are reported.     

Measurement of the spectral flux density of negative muons at very low energies

The spectral flux density of negative muons has been measured down to 5 eV.     

Slowing-down of negative muons and formation of muonic hydrogen in hydrogen gas below 1 Torr

Muonic hydrogen atoms were produced at 1.0, 0.5 and 0.25 Torr H₂ gas pressure using the magnetic bottle technique. The energy-time correlation of the muonic K-X-rays was determined. The moderation time τ_m of the muon needed for the last stage of energy loss (from ≈ 2 keV until atomic capture) was measured and is compared with predictions.     

The Coulomb capture of negative muons by hydrogen atoms in the non-adiabatic close-coupling approximation

We present some results obtained within the non-adiabatic close-coupling approximation for Coulomb capture during collisions of slow negative muons with hydrogen atoms. The calculations are performed in momentum-space and a statistical distribution analysis is used to obtain the final cross sections. The present model results are in harmony with existing calculations. We conclude that further calculations within the present approach are justified.