Muon Lifetime and Muon Capture

We present an introduction to the MuLAN and MuCAP experiments at PSI, which aim at high precision determinations of two fundamental Weak Interactions parameters: the Fermi constant GF and the induced pseudoscalar form factor gp, respectively. MULAN MUON LIFETIME The Fermi coupling constant GF is one of the fundamental constants of the Standard Model. GF is obtained from the muon lifetime via a calculation in the Fermi Model, in which weak interactions are represented by a contact interaction. FIGURE 1. The experimental principle of the lifetime method is to measure the time difference between a muon stopping in a target and its decay electron or positron. In the case of μ this ideally results in a single exponential with λ0 = 1/τμ+ (MuLAN); for μ in hydrogen the rate is increased due to the capture process to be λ = λ0 +λcapture. The capture rate consequently follows by comparing the lifetimes of both muon charge states in hydrogen (MuCAP). The goal of the MuLAN (Muon Lifetime Analysis) experiment [3] is the determination of the positive muon lifetime, τμ+ , with a precision of 1 ppm; the method is sketched in Fig.1. In order to achieve this higher precision than all combined existing experimental results, the statistics and systematics of the measurement have to be dramatically improved. The ingredients to achieve this challenging goal are: 1) Construction of a multisegment (170 tiles) detector, read out via fast (500 MHz, 8 bit) waveform digitizers; both are crucial in order to separate pile-up events of two simultaneous decay electron hits in one detector module. The layout of the detector and its elements is shown in Fig.2. A depolarizing and dephasing target material e.g. sulfur will be used in a 70 Gauss magnetic field to control the muon spin rotation from residual muon polarization. 2) Construction of a new kicked muon beam line in the high intensity πE3 muon channel at the Paul Scherrer Institute (PSI). This will allow us to collect 1012 events in a few weeks. Muons will be stored in the target in a ∼ 5μs period followed by a 22μs (10× τμ) detection period with the electrostatic kicker off. FIGURE 2. Individual scintillation counters, double layered scintillator tiles; their arrangement to a detector module; schematic view of the full MuLAN soccer-ball detector and electronic racks. In a recent run we successfully installed and tested the kicker in the πE3 beam line, which demonstrated the feasibility of the measurements timing. A offline run with the detector is under way to test the full setup with LEDs simulating hits. Fall’03 will see a first beam test. MUCAP MUON CAPTURE FIGURE 3. View of the MuCAP experiment located in PSI’s μE4 area. From left to right: the final beamline quadrupoles; the cylindrical scintillator hodoscope (eSc) with photomultiplier tubes; the TPC with the surrounding magnet rolled back from its center position inside the hodoscope; the hydrogen purification and filling apparatus. The goal of MuCAP[3] is the high precision measurement of the singlet Muon Capture rate on the proton in low-density, ultra-clean H2 gas. The muon capture rate on the proton λcap can be directly related to the induced pseudoscalar form factor gp of the nucleus, which was calculated very accurately with recent heavy baryon chiral perturbation theory approaches. Existing experimental data are outdated, lack accuracy, and show a unresolved discrepancy between results from ordinary muon capture and radiative muon capture. MuCAP determines the capture rate via the lifetime method as sketched in Fig.1. Goal of MuCAP is a 1% precision on λcap, which in turn yields a 7% error on gp. This is very challenging, due to the large difference in involved rates, (λcap ∼ 700s−1, λ0=455000s, λtrans f er to Z>1 ∼ 109s−1 ), and due to the complex chemistry of negative muons in hydrogen. FIGURE 4. a) Typical MuCAP event: Hits in the entrance scintillator (mu) and wire chambers (muPC1/2) are followed by a muon stop in the TPC. The stopping muon triggers a higher threshold as it deposits more energy at the end of the Bragg range. The dashed lines demonstrate the allowed range of 24 μs drift time in the TPC after the initial entrance scintillator hit. The muon decay electron is observed in one wire chamber (ePC1) and in the four-fold hodoscope coincidence (eSC). b) Detected impurity capture event in the TPC, most likely on a nitrogen nucleus. The muon stops and after a short time a very large signal from a charged particle occurs. The time difference is defined by the muon transfer rate times the impurity concentration. As part of the effort to control the molecular processes, ultra-clean target conditions are selected which enhance only muonic atomic singlet states and suppress muonic molecular formation. A unique high pressure (10 bar) pure hydrogen time projection chamber (TPC) serves as an active target detector. To meet stringent purity conditions it is made out of quartz-glass and bakeable up to 130 degree C. It is surrounded by a μSRcontrolling saddle-coil magnet which provides a 70 Gauss field (relevant only for the μ measurement, as negative muons are effectively depolarized in the atomic cascade); two large cylindrical wire chambers (ePC1/2); and a 16-tile, two-layer scintillator hodoscope (eSc), which sees approximately 2/3 of all decay electrons in coincidence. A view of the setup is presented in Fig.3. The timing start with a hit in the entrance scintillator (mu) and stops with a hit in the scintillator hodoscope counters, both with excellent timing resolution. The eSc is read out with fast waveform digitizers. The TPC is essential for control of the systematics, for several reasons: 1) It allows the unambiguous identification in 3D of the muon stopping positions in hydrogen (and consequently excludes wall stops). 2) It can detect impurity captures (our high Z contamination in the hydrogen after passing the palladium filter is smaller than 0.1 ppm and can actively be monitored via muon capture events on the contaminant nuclei Fig.4b). 3) Muon transfer to deuterium can be observed: A mismatch between muon stopping position and back-tracked decay electron in the two surrounding wire chambers indicate μd diffusion events. Fig.4a shows a typical event where a muon, after being seen in all entrance counters, stops in the TPC. Most of the time muons only trigger a low threshold, but in the final part of the track, where there is a large energy deposition, the high threshold is also fired. The decay electron is observed in surrounding counters. Fig.4b shows a detected impurity event: A stopping muon followed shortly after by a very high threshold trigger. As of summer’03 a MuCAP commissioning run is in progress and first physics data are expected soon.