Report from: US Muon Workshop 2021: A Road Map for a Future Muon Facility February 1-2, 2021

EXECUTIVE SUMMARY The “US Muon Workshop 2021: A road map for a future Muon Facility” workshop was held virtually on February 1–2, 2021. The workshop aimed to bring together world experts in muon spectroscopy (μSR) and other techniques and interested stakeholders to evaluate the scientific need to construct a new μSR facility in the United States (US). The more than 200 participants highlighted several key scientific areas for μSR research, including quantum materials, hydrogen chemistry, and battery materials, and how each room could benefit from a new, high flux pulsed muon source. Experts also discussed aspects of the μSR technique, such as low-energy μSR, novel software developments, and beam and detector technologies that could enable revolutionary advances in μSR at a next-generation facility. The workshop concluded with a discussion of a concept being developed for a new μSR facility at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory (ORNL). That novel design concept was first envisioned by many of the same μSR experts at a workshop held previously at ORNL in 2016. The participants expressed that the current design had the potential to be a world-leading μSR facility and strongly encouraged the principal investigators to continue their work in order to refine the concept and determine instrument parameters that would enable new scientific opportunities. Muon Spin Rotation/Relaxation/ Resonance (μSR) is a technique that involves the use of spin-polarized muons that are implanted in a material to provide extremely sensitive measurements of the local magnetic field distribution within samples of scientific interest. The μSR technique has led to important results in condensed matter physics, chemistry, and semiconductor physics, among other fields. This technique is highly complementary to neutron scattering, and since the two techniques share a common user base, three of the four existing μSR facilities in the world are co-located with neutron sources. The exception is in North America, where the sole muon source is located at a meson accelerator laboratory in Vancouver, Canada. The United States has not had a μSR facility since the closure of LAMPF at Los Alamos National Laboratory, and never one that was globally competitive. Accordingly, there have been several efforts in recent years to address this shortcoming, most recently at ORNL beginning in 2016 and culminating with this workshop. Several recurring themes were identified during the workshop: the advantage of higher muon fluxes to enable new science, increasing demand for low-energy muon beams, the need for more software tools for muon site determination and analysis, and the role of multi-probe studies combining μSR with neutrons and other spectroscopic techniques. The primary method for enabling new science with μSR is higher flux muon beams. It is important for the detection of weak magnetic field phenomena delivers greater sensitivity to molecular levels and even facilitates broader applications such as using muon beams for fundamental physics experiments. But by far, the largest benefit of a high muon flux would be the expansion of low-energy μSR capabilities. Low-energy μSR beams enable more depth-resolved experiments, creating opportunities for measuring topological materials, novel states in interfaces, layered heterostructures, and other new types of experiments. In particular, there is an opportunity to focus the low-energy muons into a sub-millimeter beam to create a muon microscope for adding spatial resolution. Based on recent history and the state of the community, the consensus was that any opportunity to expand the capabilities of low-energy μSR would be hugely beneficial to the scientific community. The workshop often noted the complementary nature of μSR to other techniques, especially neutron scattering. Researchers always benefit from having access to other types of measurements of the materials. The co-location of μSR and neutron scattering facilities has proven this fact, and a next-generation muon source in the US would miss scientific opportunities by not being closely associated with existing national expertise in the areas of neutron scattering, computing, advanced materials characterization, and other spectroscopic tech-