Muon capture in hydrogen: First MuCap results and future plans

We survey a new generation of high precision experiments on muon capture in hydrogen and on the positive muon lifetime, performed at the Paul Scherrer Institute (PSI). First results and plans for the future are presented.     

Japanese hadron facility plans and future μSR

The proposed next major science project in Japan, the high intensity 1 GeV proton accelerator with unique beam characteristics, is described here. It will supply a proton beam of more than 100 μA in either de mode or sharply pulsed mode (down to 10 ns), using a specially designed time structure conversion ring. The beam will be used for keV μ⁺ generation at the production target, MeV surface μ⁺ production and 10 MeV decay μ⁺ and μ⁻ production, as well as a possible slow μ⁻ production. All of these unique muon beams will be developed for the next generation of μSR experiments. With the development of the keV μ⁺ source particularly in mind, a pilot station is now under construction at UT-MSL/KEK. Possible new μSR experiments are also reviewed.     

Muon decay

Muon decay, after more than fifty years of experimental and theoretical investigations, has been established as a firm basis for the Standard Model of Electroweak Interactions. The decay products of the muon have been identified, and the Lorentz structure of the decay interaction has been determined completely from existing experiments in a model-independent way. Together with the completely anologous leptonic τ decays it continues to be an important and sensitive testing ground for deviations from the Standard Model.     

Muon spin echo

A muon spin echo method is proposed for determining the contributions of the static and dynamic local fields in muon experiments. It is shown that if for each muon which has stopped in the sample a rf pulse of fixed duration is applied to the sample at a time τ after the muon entered the sample, then after a sufficient number of muon-positron decays have been accumulated a muon spin echo can appear at time 2τ. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 500–503 (10 October 1996)     

Muon induced fission

Fission probabilities following muon capture are calculated for nuclei in the actinide region. The capture process is described in a simple model and cross sections of photon as well as neutron reactions are considered. The energy dependence of both is taken into account. A comparison with experimental data is made.     

Muon tracking underground

Summary We present a new design of plastic streamer tubes, optimized to match the experimental requirements of large-area under-ground detectors, where muon identification is needed with good angular resolution.

---

Riassunto Si descrive un nuovo disegno di tubi a streamer in plastica, ottimizzati per le esigenze sperimentali di apparati sotterranei di grande area, dove si richiede l'identificazione di muoni con buona risoluzione angolare.

---

Рзюме Предлагается новый проект пластических стримерных трубок, оптимально согласованный с эксперименталыми требованиями подэемных детекторов на большой площади, когда небходима идентификция мюоноб с высокими угловым разрешением.

     

Muon capture revisited

The problem of inclusive muon capture in nuclei is studied by calculating the capture rate in asymmetric infinite nuclear matter and using the local density approximation to evaluate the capture rates in nuclei. It is shown that the method is rather reliable and allows one to improve on approximations used in the past. The need for a strong nuclear renormalization is shown, reducing the capture rates by about a factor two in medium and heavy nuclei. By using standard effective interactions in the spin-isospin channel one can account for this renormalization and one finds a remarkable overall agreement with the measured capture rates for a large list of nuclei through the periodic table.     

Muon diffusion in metals

The hopping rate W of the positive muon in metals is studied theoretecally. We consider four interactions: muon-electron interaction, linear as well as quadratic muon-lattice interactions and inhomogeneous broadening of the muon levels due to imperfections. The W-vs-T curve consists of four regions. The highest-T region, where the small polaron theory is applicable, is followed by a minimum, below which the Kagan-Klinger damping may be dominant. Then, the damping due to metallic electrons dominates and the hopping rate obeys a power law of a negative power. When kT is smaller than the inhomogeneous broadening, the hopping rate obeys another power law of a positive power, as Sugimoto has shown. We analyze experimental data on muon diffusion in Cu, Al and Fe.     

Muon scattering at Fermilab

A review of the muon scattering program to date at Fermilab is made. Brief sections are included on muon physics at particle accelerators prior to Fermilab and on technical details of the Fermilab muon beam. A section is included on the theoretical work most closely associated with the deeply inelastic muon scattering program.     

Muon decay in orbit

The decay spectrum of a muon bound in the $\text{1s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ state of an atom is calculated using V-A theory. Accurate muon and electron wave functions including finite nuclear size effects and vacuum polarization are used for the evaluation of the electron emission spectra of various elements. The formalism to perform the angular integration of the matrix elements is developed for the V-A weak interaction.     

Muon facilities at TRIUMF

TRIUMF provides its user community with a wide variety of muon beams for use in μSR and fundamental particle studies. The existing muon channels and their characteristics are described along with a proposed superconducting solenoid to be constructed in 1987.     

Muon induced quadrupole photofission

Muonic X-rays were measured in coincidence with prompt fission in ²³⁸U induced by radiationless transitions. The results demonstrate the dominating role of 3d → 1s transitions.     

Muon science in J-PARC

The intensity of proton accelerator has attained to the order to mega-watt, and several MW-class proton accelerators start to operate in the world. J-PARC is a complex of three accelerators, and generates a variety of secondary beams, i.e. muon beam, neutron beam, meson beam and neutrino beam. The muon facility is established in order to provide a pulsed muon beam for various experimental programs. The first muon beam is transported to the experimental area in September 2008. Although the accelerator is still under commissioning, and the beam power doesn’t reach the design value of 1 MW yet, the world strongest pulsed muon beam will be provided shortly. In this paper, we review the muon beam line in J-PARC, and discuss evolved scientific programs.     

Muon decay in sulphur

The electron spectrum from muon decay in orbit in sulphur was measured at SIN. In the energy range from 54 to 92 MeV, the measurement shows a spectrum compatible with the weak V-A theory within ±35% over the whole range.     

Muon Capture on Light Nuclei

This work investigates the muon capture reactions ²H(?? ^?, ?? _?? )nn and ³He(?? ^?, ?? _?? )³H and the contribution to their total capture rates arising from the axial two-body currents obtained imposing the partially-conserved-axial-current (PCAC) hypothesis. The initial and final A = 2 and 3 nuclear wave functions are obtained from the Argonne v ₁₈ two-nucleon potential, in combination with the Urbana IX three-nucleon potential in the case of A?=?3. The weak current consists of vector and axial components derived in chiral effective field theory. The low-energy constant entering the vector (axial) component is determined by reproducting the isovector combination of the trinucleon magnetic moment (Gamow-Teller matrix element of tritium beta-decay). The total capture rates are 393.1(8) s^?1 for A = 2 and 1488(9) s^?1 for A?=?3, where the uncertainties arise from the adopted fitting procedure.     

Muon Tomography of Large-Scale Objects

Physics of Atomic Nuclei - Cosmic ray muons arriving from the upper hemisphere to the Earth’s surface, are currently used to develop methods of muonography (analogous to radiography) of the...     

Muon investigations of fullerenyl radicals

Four of the five possible isomeric C₇₀Mu radicals have been detected by transversefieldSR in a C₇₀ powder sample at 298 K. Their assignment is based on the results of semi-empirical MNDO calculations. There are significant changes in intensity and lineshape of the signals at low temperature. The first SR spectrum of a fullerenyl radical in solution has been obtained.     

Muon capture on neutron-rich nuclei

Future facilities will allow the exploration of extremely neutron-rich nuclei far from the valley of stability. It is discussed that the strong-neutron excess results in changes in the collective excitations of such nuclei compared to conventional stable nuclei. We propose muon capture as an experimental tool to explore such changes. We will quantify our discussion by the calculation of the total and differential muon capture rates on selected calcium isotopes between ⁴⁰Ca and ⁶⁰Ca. Our calculations are based on the random phase approximation and agree nicely with the measured rates for ⁴⁰Ca and ⁴⁴Ca. Received: 6 April 2001 / Accepted: 14 May 2001