C57-26 d. Low-Energy Muon and Muonium Beam Source at Fermilab

A high-efficiency source of muonium that can be transported as a beam in vacuum provides opportunities for fundamental muon and precision physics measurements such as sensitive searches for symmetry violation. Although PSI is currently the world leader, the intense 800-MeV PIP-II linac beam at Fermilab could provide world-class low-energy muon and muonium beams, with unparalleled intensity, driving the next generation of precision muon-based physics experiments at the intensity frontier. However, it is critical to initiate the prerequisite R&D now to prepare for the PIP-II era. A low-energy secondary muon line recently installed in an operating facility (the MeV Test Area, which utilizes the intense 400-MeV Fermilab Linac beam) could support the required R&D, and potentially compete for new physics in the immediate term, if approved. This beamline was developed for µľ and will need to be re-optimized for surface µ+ production and transport, making it also suitable for muon spin rotation physicsľľa unique research and industrial application for which no U.S. facility exists, and whose facilities are oversubscribed worldwide. The method utilizes the efficient conversion of positive muons to muonium atoms in superfluid helium [1] combined with the predicted expulsion of muonium atoms at the superfluid surface due to the large positive chemical potential of muonium in superfluid helium [2]. The µ+ beam enters the cryostat from below and its energy is controlled such that a significant fraction stops in a superfluid helium film at ~100 mK. A surface electron pool maintains an electric field inside the superfluid helium to drift the stopped µ+ to the surface, where they combine with electrons forming muonium and are expelled vertically into the vacuum. The surface µ+ beam is also suitable for use on its own, as are other configurations of the beam line. In Phase I we will perform advanced simulations to develop the technical approach needed to maintain the required electric field in the helium, design detectors to characterize the produced muonium, and perform studies of the entire system to optimize the beam and target performance for surface µ+ production and transport in the recently installed secondary beamline. (This beamline supports both muon charge species from 4 to 50 MeV and various target geometries.) Optimization will involve target material and geometry (slices) studies, optics and upgraded (now available) beamline components. In Phase II we will apply the techniques and apparatus developed in Phase I to realize muonium-beam production using superfluid helium at ~100 mK in a dilution refrigerator, establishing a muonium beam for experiments such as measurement of antilepton gravity and improved searches for muoniumľantimuonium mixing. This flexible low-energy muon beam can also support precision muon experiments and muon spin- rotation applications in material science such as critical surface studies of quantum computing devices.