B. Aas

New precision measurements of the muonic 3d52−2p32 X-ray transition in 24Mg and 28Si: Vacuum polarisation test and search for muon-hadron interactions beyond QED☆

Abstract Motivated by the importance of scalar particles in the current work on gauge theories of fundamental interactions, we have performed an improved muonic-atom experiment to search for long-range muon-hadron interactions. We remeasured the wavelengths of the 3 d 5 2 −2 p 3 2 X-ray transitions in 24Mg and 28Si with the bent-crystal spectrometer at the SIN muon channel. The relative difference between the X-ray wavelength λexp and the theoretical value as obtained from QED calculations, averaged over the two elements, is λ exp -λ QUED /λ QUED = (-0.2 +-3.1) × 10 6 Assuming the validity of the QED calculations, we can put limits on an additional muon-nucleon interaction: If such an interaction were mediated, for example, by a scalar or vector (isoscalar) boson with a mass smaller than about 1 MeV, the corresponding coupling constant is gμgN ⩽ 0.8 × 10−6 × e2. Alternatively, if additional muon-hadron interactions are negligible, our result corresponds to a 950 ppm test of the vacuum polarisation effect in QED. The result can also be interpreted as a 3 ppm measurement of the negative muon mass.

Precision measurement of the 2p-1s transition in muonic 12C: Search for new muon-nucleon interactions or accurate determination of the rms nuclear charge radius☆

The wavelength of the 2p32-ls12 transition in muonic 12C was measured with a crystal spectrometer as 16.473765 (88) pm. With an improved model-independent analysis we deduce an rms charge radius for 12C of 2.4829 (19) fm. A 2.4 standard deviation difference between our rms charge radius and that deduced from recent elastic electron-scattering experiments is tentatively attributed to a short-range additional interaction between muon and nucleons. A comparison is made with other experiments yielding information on such interactions.

Vacuum polarization test and search for muonhadron interactions from muonic X-rays:: Crystal-spectrometer experiments

Abstract We have measured the wavelengths of the 3 d 5 2 −2 p 3 2 and the 3 d 3 2 -2 p 1 2 X-ray transitions in μ - 24 Mg, - 28 Si and - 31 P with the bent-crystal spectrometer at the SIN muon channel. The X-rays are measured relative to the wavelengths of the 84 keV and the 63 keV γ-rays of 170 Tm and 169 Yb which have recently been calibrated to about 1 ppm. The measured X-ray wavelengths λ exp are compared with theoretical values λ th , as obtained from QED calculations. The relative difference, averaged over all six measured transitions, is λ exp −λ th λ th = (2 ± 8) × 10 −6 This result corresponds to a test of the vacuum polarization effect in QED of (0.6 ± 2.4) × 10 −3 . Assuming the QED calculations to be correct, we can use the result to put limits on additional muon-nucleon interactions (as required by gauge theories). If such an interaction is mediated by a scalar, isoscalar boson with a mass smaller than 1 MeV, the coupling constant is found to be g N g μ 4π = (−4 ± 17) × 10 −9 Alternatively, we can deduce from our experiments the most accurate direct value to date for the negative muon mass, m μ - = 105.65906(91) MeV .

Observation of nuclear rotational spectra in pion capture on 165Ho and 175Lu

Abstract In X-ray spectra of pionic atoms of 165 Ho and 175 Lu we observed nuclear γ-rays of Dy and Yb isotopes, respectively, arising from the pion capture reactions 165 Ho(π − , x n 165−x Dy ∗ , and 175 Lu(π − x n 175− Yb ∗ , where x = 1, 3, 5, 7, 9, 11.

Monopole and quadrupole strong interaction effects in pionic atoms of 175Lu and 165Ho

Abstract We have measured the strong-interaction monopole energy shift e0 and width Γ0, and the quadrupole splitting of the 4f level in pionic 175Lu and 165Ho. The monopole results are: e0(Lu) = 597 ± 70eV, e0(Ho) = 295 ± 80 eV, Γ0(Lu) = 200 ± 70eV and Γ0(Ho) = 190 ± 40eV. The monopole shift in the case of lutecium deviates from the value obtained with a standard optical potential description of the pion-nucleus interaction. Using the (electromagnetic) quadrupole moment as measured recently in an experiment on muonic 175Lu, we find for the strong-interaction quadrupole constant of the 4f state in 175Lu e2 = − 89 ± 29 eV. The ratio e 2 e 0 is in good agreement with theoretical predictions. We relax the usual assumption in the optical potential of a homogeneous mixture of neutrons and protons and show that e0 and e2 are mainly given by the generalized moment 〈rκe−αr〉 (κ = 3.93, α = 0.21 fm−1) of the monopole and the quadrupole density of the neutrons, respectively. We deduce from the measured monopole shifts the ratio R between the generalized monopole moment above and the value of the same moment if neutrons and protons are homogeneously mixed (the latter is taken from muonic atom data). The results are R(Lu) = 1.34 ± 0.16 and R(Ho) = 1.14 ± 0.26. The deviation of R(Lu) from unity may be interpreted in two ways: the optical potential in the present form is not sufficient for the analysis of the 4f level shift in π-175Lu, or there is a difference between the neutron and proton monopole density distributions in this nucleus.

Spectroscopic quadrupole moments of 25Mg and 27Al from muonic X-rays

Abstract With the crystal spectrometer at Muon Channel I at SIN we measured the 3d 5 2 -2p 3 2 X-ray transitions in muonic 25Mg and 27Al. The h.f.s. of the 2p 3 2 was resolved and the spectroscopic quadrupole moment of the nuclear ground states determined: Q( 25 Mg ) = +0.201 ± 0.003 b , Q( 27 Al ) = +0.150 ± 0.006 b . We find agreement between the intrinsic quadrupole moment value derived from Q( 25 Mg ) with those calculated from measured B(E2) values. This indicates that for 25Mg the strong-coupling limit of the rotational model with an axially symmetric charge distribution is realized in the lower part of the ground-state rotational band. We combine the quadrupole moments Q with the quadrupole coupling constants measured in electronic atoms of 25Mg and 27Al and derive the electric field gradients at the nucleus (in units of e · a0−3, where a0 is the Bohr radius): 〈 d E d z 〉 = 0.339± 0.005 for the 3P2 state of 25Mg, 〈 d E d z 〉 = 0.54± 0.02 for the 2P 3 2 state of 27Al. From Hartree-Fock calculations of the 2P 3 2 state of Al we derive values for the Sternheimer correction factor R and compare them with calculated values.

Spectroscopic quadrupole moment of holmium from pionic X-ray measurement☆

Abstract From the observed quadrupole splitting of the 5g-4f X-ray transition in π-165Ho we determine the spectroscopic quadrupole moment of 165Ho to be Q = 3.47±0.11 b. The strong interaction shift and the width of the 4f level are found to be ϵ0 = 0.35±0.08 keV and Γ0 = 0.21±0.04 keV, respectively.

Energy of a muonic X-ray transition measured with a crystal spectrometer☆

Abstract The wavelength of the 3 d 5 2 −2 p 3 2 muonic X-ray transition in 28 Si has been measured relative to the wavelength of the 84 keV gamma ray from a 170 Tm source. The result is λ Si / λ Tm = 1.099675 (39). Using the reported value for λ Tm we find ( λ exp Si - λ th Si )/ λ th Si = (−9±35) × 10 −6 .

Crystal-spectrometer measurement of the 3d-2p X-ray transition in muonic 31P

Abstract We have measured the wavelength of the 3 d 5 2 −2 p 3 2 muonic X-ray transition in 31P relative to the wavelength of the 84keV γ-ray from a 170Tm source. The result is λ p λ Tm = 0.957247(31) . Using the reported value for λTm we find (λ exp p − λ th p ) λ th p = (−28±32) × 10 −6 , where λthp is the wavelength calculated according to the prescriptions of QED. Combining this result with the corresponding result reported earlier for μ- 28 Si , we obtain a limit for an additional muon-nucleon interaction. We compare this limit to the prediction of the Weinberg-Salam model, i.e., the interaction mediated by the Higgs boson.

Spectroscopic quadrupole moment of 27Al from muonic X-rays☆

Abstract We have observed, with the bent-crystal spectrometer at SIN, the quadrupole splitting of the 2 p 3 2 state of muonic 27Al from a measurement of the 3 d 5 2 −2 p 3 2 X-ray transition. The spectroscopic quadrupole moment of 27Al is deduced: Q = +0.153 ± 0.008 b.