(ordinary muon capture, or OMC), and it's radiative version

(radiative muon capture, or RMC). These processes are uniquely sensitive to the g_p, the induced pseudoscalar form factor, which is the least well-known component of the the axial-vector structure of the nucleon.

The primary goal of these experiments has been to precisely measure the proton's pseudoscalar coupling (at the kinematical point of muon capture). A second goal has been to search for evidence of the long-predicted modifications of the proton's axial structure when it is embedded in the strongly-interacting environment of a complex nucleus.

RMC spectrometer at TRIUMF
           

Experiments are performed with the RMC spectrometer (see above), or with setups involving solid-state (intrinsic Ge), NaI and/or liquid scintillation detectors, and involve detecting high-energy (RMC) and nuclear gamma-rays, muonic x-rays, neutrons, and/or decay electrons.

A key recent experiment is a measurement of the ortho-para transition rate in muonic molecular hydrogen, TRIUMF Expt 766, (Armstrong co-spokesperson, along with T. Gorringe of U. Kentucky). The interpretation of muon capture (both OMC and RMC) measurements in liquid hydrogen depends on knowledge of the initial atomic and molecular states of the muonic system before capture takes place. The two most recent measurements of g_p for the proton (one using OMC and the other using RMC) disagree significantly; a possible explanation is that the mixture of ortho- and para- muonic hydrogen differs from that assumed in the analyses of those experiments. By precisely measuring the time distribution of the 5.2 MeV neutrons from muon capture on hydrogen, one can determine the transition rate between the ortho and para states, and hence deduce the mixture of states relevant for the previous experiments. The data analysis for this experiment is well underway, and Jessica Clark is writing her PhD thesis based upon it.

Other recent and ongoing experiments in this program include:

• Expt. 452:   First measurement of radiative muon capture on hydrogen
  Phys. Rev. Lett. 77(1996)4512 (nucl-ex/9608005)
  Phys. Rev. C 57(1998)373

• Expt 570:   Gamma-neutrino angular correlation in muon capture on ²⁸Si
  (Armstrong spokesperson)
  Phys. Lett. B395(1997)157; TRI-PP-97-3

• Expt 612: Hyperfine dependence of muon capture in nuclei
  Phys. Rev. Lett. 72(1994)3472
  Phys. Rev. C, 54(1996)2714

• Expt 670:   Radiative muon capture on nuclei - survey
  Phys. Rev. C 59(1999)2853; preprint

• Expt 592: Radiative muon capture on ³He
  (data analysis ongoing)

• Expt 674:   Exclusive ordinary muon capture rates in s-d shell
  Phys. Rev. C 60(1999)055501

• Expt 696:   Hyperfine dependence of muon capture in ³⁵Cl
  nucl-ex/0204016, submitted to Phys. Rev. C

• Expt 734:   Radiative muon capture on Ni isotopes
  (Armstrong co-spokesperson, along with T. Gorringe of U. Kentucky)
  Phys. Rev. C 58(1998)1767

Pion experiments

• Expt 744: pi^- + p   ->   e⁺ e^- n (in-flight)

• Expt. 838: pi^- + p   ->   gamma gamma n
  The first measurement of this rare double radiative process; a possible way to probe pion polarizability
  nucl-ex/0204013 (submitted to Phys. Rev. Lett.)

• Expt. 864: pi^- + d   ->   gamma gamma X
  The first measurement of the similar process on the deuterium; sensitive to the possible existence of diBaryon states.

Meson Hall at TRIUMF

UBC Campus, Vancouver in background

Links:

• RMC spectrometer
• RMC page at U. Kentucky
• E766 proposal (ortho-para)