Towards next generation fundamental precision measurements with muons

Abstract

Several previous fundamental precision measurements with muons (μ), such as that of the muonium (Mu) 1S-2S transition frequency and ground-state hyperfine splitting, were limited by the statistics and the beam quality, in terms of vacuum yield, low energy and long term stability. A Mu source with a larger flux can be achieved either by improving the μ → Mu conversion rate or by improving the μ beam (smaller phase space, low energy, high intensity). This thesis reports the ongoing efforts at ETH Zurich (ETHZ) and PSI towards a high yield cryogenic Mu source and a new, low energy, high brilliance positive μ beam. The former is based on mesoporous silica materials while the latter is making use of a phase space compression of μ stopping and drifting in a helium gas density gradient within suitable electric and magnetic fields. The first observation of cryogenic Mu emission into vacuum following muon implantation in mesoporous thin silica films is reported. Based on a simple Geant4 Monte Carlo simulation, a yield of Mu into vacuum of F v Mu = 0.38(4) at 250 K and F v Mu = 0.20(4) at 100 K for 5 keV μ implantation energy is extracted from the measurements. Based on the implantation energy dependence of the vacuum yield deduced from a quantum tunneling model, a second experiment was performed for implantation energies below 5 keV where higher Mu vacuum yields are expected. Due to an upgrade in the LE-μSR apparatus at PSI, new full muon transport simulations had to be performed in order to understand the propagation of μ beam at very low energy. Mu physics and energy losses in the carbon foil are implemented and validated using time-of-flight (TOF) measurements. The feasibility of Mu confinement is also demonstrated by using a SiN membrane as the μ entrance window. These two achievements, high vacuum yield and confinement, represent important steps towards next generation Mu spectroscopy. Research and development (R&D) of the new μ beam compression scheme which is ongoing at ETHZ and PSI are very encouraging. The feasibility of the longitudinal compression stage was successfully demonstrated in 2011. This demonstration relies on the agreement between the experimental results and the simulation based on Geant4, after implementing μ physics processes at low energy such as elastic collisions and charge exchange. A neutron radiography experiment has demonstrated the feasibility to sustain the necessary density gradient for the transverse compression stage. A gas density gradient concept was implemented into Geant4 simulations. With this, the simulated time spectra and the experimental data can be compared when data will available. An engineering run towards the test of transverse compression has been done in Dec 2014. The realization of this beam line will enable one to produce a micro-beam of muons and it will have many applications, especially to the μSR and precision physics community.