Collaborative Research: Equipment for the PSI MUSE Experiment

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It is well known that protons and neutrons are made from constituents, called quarks and gluons. The quarks and gluons give rise to a finite size of the proton, which is about 10,000 times smaller than the size of a single atom. The goal of this project is to measure the size of the proton. An on-going puzzle is that the proton size appears to be different when measured with atomic hydrogen as compared with its counterpart, called muonic-hydrogen, where the electron in a hydrogen atom is replaced by its heavier cousin, the muon. This project will specifically build equipment to be used in an experiment to provide an independent measurement of the proton radius. In addition to the direct scientific goals of the project, the experiments provide students and young scientists experience and training in working in the international collaborations of modern scientific experiments, with state of the art technology.The seven standard deviation discrepancy between the proton radius measured with muonic hydrogen and with electron-proton scattering may point to physics beyond the standard model of particle physics, or result from novel aspects of conventional physics or from issues in extracting the radius from experimental data. The Muon Scattering Experiment (MUSE) aims to resolve the proton radius puzzle. MUSE uses a mixed pion, muon and electron beam at the Paul Scherrer Institute (PSI) in Switzerland. MUSE will determine the proton radius through both muon-proton scattering and electron-proton scattering, in addition to performing these reactions with positive and negative leptons. This not only reduces systematic uncertainties but also provides sensitivity to two-photon exchange contributions that may be responsible for some of the discrepancies seen in earlier experiments. This collaborative project supports the construction of MUSE over the period September 2016 - August 2018. There will also be ongoing testing and commissioning activities at PSI, including a dress rehearsal run with a partial spectrometer in late 2017.