The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, $mu^- + p rightarrow n + u_mu$. The experimental approach was based on the
use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10 bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope. The muon and electron detectors together enabled a precise measurement of the $mu p$ atoms lifetime, from which the nuclear muon capture rate was deduced. The TPC was also used to monitor the purity of the hydrogen gas by detecting the nuclear recoils that follow muon capture by elemental impurities. This paper describes the TPC design and performance in detail.
The central detector in the MuSun experiment is a pad-plane time projection ionization chamber that operates without gas amplification in deuterium at 31 K; it is used to measure the rate of the muon capture process $mu^- + d rightarrow n + n + u_mu
$. A new charge-sensitive preamplifier, operated at 140 K, has been developed for this detector. It achieved a resolution of 4.5 keV(D$_2$) or 120 $e^-$ RMS with zero detector capacitance at 1.1 $mu$s integration time in laboratory tests. In the experimental environment, the electronic resolution is 10 keV(D$_2$) or 250 $e^-$ RMS at a 0.5 $mu$s integration time. The excellent energy resolution of this amplifier has enabled discrimination between signals from muon-catalyzed fusion and muon capture on chemical impurities, which will precisely determine systematic corrections due to these processes. It is also expected to improve the muon tracking and determination of the stopping location.
The anomalous magnetic moment (g-2) of the muon was measured with a precision of 0.54 ppm in Experiment 821 at Brookhaven National Laboratory. A difference of 3.2 standard deviations between this experimental value and the prediction of the Standard
Model has persisted since 2004; in spite of considerable experimental and theoretical effort, there is no consistent explanation for this difference. This comparison hints at physics beyond the Standard Model, but it also imposes strong constraints on those possibilities, which include supersymmetry and extra dimensions. The collaboration is preparing to relocate the experiment to Fermilab to continue towards a proposed precision of 0.14 ppm. This will require 20 times more recorded decays than in the previous measurement, with corresponding improvements in the systematic uncertainties. We describe the theoretical developments and the experimental upgrades that provide a compelling motivation for the new measurement.
Measuring the muon flux is important to the Sanford Underground Laboratory at Homestake, for which several low background experiments are being planned. The nearly-vertical cosmic ray muon flux was measured in three locations at this laboratory: on t
he surface (1.149 pm 0.017 x 10^-2 cm^-2 s^-1 sr^-1), at the 800-ft (0.712 km w.e.) level (2.67 pm 0.06 x 10^-6 cm^-2 s^-1 sr^-1), and at the 2000-ft (1.78 km w.e.) level (2.56 pm 0.25 x 10^-7 cm^-2 s^-1 sr^-1). These fluxes agree well with model predictions.
By measuring the lifetime of the negative muon in pure protium (hydrogen-1), the MuCap experiment determines the rate of muon capture on the proton, from which the protons pseudoscalar coupling g_p may be inferred. A precision of 15% for g_p has been
published; this is a step along the way to a goal of 7%. This coupling can be calculated precisely from heavy baryon chiral perturbation theory and therefore permits a test of QCDs chiral symmetry. Meanwhile, the MuSun experiment is in its final design stage; it will measure the rate of muon capture on the deuteron using a similar technique. This process can be related through pionless effective field theory and chiral perturbation theory to other two-nucleon reactions of astrophysical interest, including proton-proton fusion and deuteron breakup.
