PoS(NuFACT2018)136 https://pos.sissa.it/ . Each of the four sets of data will allow 2 c / 2 for the MUSE Collaboration ∗† [email protected] the extraction of the proton charge radius;electron in and combination, muon the data interactions test and possible additionally differencespresently two-photon of being exchange the commissioned effects. at The PSI. experiment An is overview of the experiment will be presented. Speaker. Supported in parts by the U.S. National Science Foundation: NSF PHY-1505615 While consistent results forelectron-scattering the data and charge through radius the spectroscopy of ofof the atomic muonic hydrogen, proton hydrogen high-precision found studies have notably beenradius smaller extracted puzzle values raises for from the questions elastic charge rangingbeyond radius. from the experimental standard This model. and so-called The methodological proton- puzzle issuesScattering certainly to Experiment calls (MUSE) for physics at new the measurements. Paul Thedata Scherrer MUon Institute proton off (PSI) the will provide proton elasticmomentum-transfer scattering with range electron from and 0.002 to muon 0.08 beams GeV of positive and negative charge in a four- † ∗ Copyright owned by the author(s) under the terms of the Creative Commons c The 20th International Workshop on Neutrinos12-18 (NuFact2018) August 2018 Blacksburg, Virginia Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). University of South Carolina E-mail: Steffen Strauch The MUon Scattering Experiment (MUSE) atScherrer the Institute Paul PoS(NuFACT2018)136 2 is Q 2 ] ob- Q 2 , 1 m. A veto µ , where Steffen Strauch 0 = 2 Q M1 beam line are

π 2 systems. Finally, the dQ , e.g., the proton radius ep / 2 p E Q beams provides for a direct dG scattering data is expected to . The low beam flux requires µ in an elastic scattering experi- 2 ] discusses many suggested so- 1 µ ¯ h 4 p 6 µ and ]. In the scattering experiment the − but not for and 3 e p = e and µ p 2 elastic-scattering cross sections in a universality and hint at physics beyond r ep p e / ± µ µ ] at the Paul Scherrer Institute (PSI). This unique and 1 9 p ± e in overlapping kinematic settings with beam momenta of 2 ] or the Initial State Radiation (ISR) experiment at MAMI c 7 / 2 60 ps, scattered-particle scintillators serve in the event trigger and < 08 GeV . . Measurements with both charges for the lepton allow for studies of c ] proton charge radius. New electron scattering experiments will improve 6 002 and 0 . ] and larger [ ]. The setup and the detectors are mostly commissioned and await production runs in 5 9 005 fm. A detailed discussion of the experiment is given in the MUSE Technical Design . 0 Since then, several pieces of the puzzle are coming together: New hydrogen spectroscopy The experiment aims for systematic uncertainties of the elastic cross sections below 0.5%. The Recent high-precision measurements of the Lamb shift in the muonic hydrogen atom [ ± ]. What is missing, however, are high-precision scattering data with muons. This void is filled 8 measurements have been obtained,smaller but [ with conflicting results, supporting the case for both, a the standard model. Several ideas have been ruled out, and none have gained universal acceptance. sensitivity to differences in the extracted proton radius from 2019. discrepancy may be an indication of the violation of range between 0 115, 153, and 210 MeV/ test of lepton-type-dependent effects.termination In of the the absence lepton of scattering inelastickinematics angle reactions of at off the the a reaction. proton, given A beamis the magnetic momentum not de- spectrometer completely needed. to determines measure A the thelarge-acceptance schematic scattered detectors. diagram particle of momentum Electrons, the muons, setuptimed and is relative to pions shown the in from radio-frequency Fig. of the thetrons secondary accelerator and and muons identified are in then the tracked beam withof hodoscope. a the set Elec- of target GEM chamber. detectors into Thedetector the GEM in liquid-hydrogen detectors front target inside of have the achieved target atracker chamber position reduces provides trigger resolution rate high-resolution of from 70 and backgroundtarget. tracks. high-efficiency The tracking The straw-tube high-resolution, of theprovide scattered timing for particles the rejection from of the muon-decaytarget, background. helps to The measure beam beam monitor, properties downstream and oftering. to the suppress Together background with from the Møller beam and hodoscope Bhabhadetermination scat- it using the also time-of-flight allows technique. for a precise muon- and pion-momentum be Report [ possible two-photon exchange mechanisms. The use of both tained a significantly smaller value ofelectronic the hydrogen proton spectroscopy charge radius and compared on to electron values scattering based [ on, both, lutions to the puzzle. First, thereof may uncertainties, be difficulties problems in with fitting thein the experiment, including slope case underestimation of of the scattering electricdata. experiments, form Second, or factor novel for issues hadronic the physics in radius may QED extraction be calculations important for in the analysis of spectroscopy [ with the Muon Scattering Experiment (MUSE) [ ment. The MUSE experiment will measure on the precision of the form factor extraction and reach lower values of experiment is the only one, which will directly compare MUSE at PSI radius is found from the slope of the electric form factor, (PRad) experiment at JLab Hall B [ the four-momentum-transfer to the proton. A recent review article [ PoS(NuFACT2018)136 Phys. , , (2010) JPS Conf. M1 beam- , π 466 , Steffen Strauch with initial 2 (2013) 417. Nature The rydberg constant , 339 ]. New Measurement of the Science , p Elastic Scattering Straw-Tube Tracker (STT)Tracker µ Proton Structure from the Beam Monitor 1507.07956 Target ]. Chamber The size of the proton ~ 100 cm ]. Veto Scintillator (2017) 79. 2 Muonic hydrogen and the proton radius puzzle 358 (2016) 035009 [ 1612.06707 ]. 88 CODATA Recommended Values of the Fundamental 1301.0905 Technical Design Report for the Paul Scherrer Institute Science The New Proton Radius Experiment at Jefferson Lab , GEM Detectors (2017) 194 [ Beam Hodoscope 1801.08816 [ (2013) 175 [ B771 Rev. Mod. Phys. πM1 , 63 Beam-Line Scintillator (SPS) Scintillator Scattered Particle Scattered P Transition Frequencies of Muonic Hydrogen First measurement of proton’s charge form factor at very low Q 2 − S 2 et al., Phys. Lett. . c , (2018) 183001 120 (2017) 020052. collaboration, A. H. Gasparian, Geant4-based schematic view of the detector setup for the MUSE experiment at the S Transition Frequency of Hydrogen: Contribution to the Proton Charge Radius Puzzle 13 3 AD − S Rev. Lett. Proc. state radiation 1709.09753 Experiment R–12–01.1: Studying the Proton "Radius" Puzzle with Ann. Rev. Nucl. Part. Sci. 213. Measurement of Physical Constants: 2014 and proton size from atomic hydrogen 1 [8] M. Mihovilovi ˇ [9] MUSE collaboration, R. Gilman et al., [5] A. Beyer, L. Maisenbacher, A. Matveev, R. Pohl, K. Khabarova, A. Grinin et al., [7]PR Figure 1: line at the Paul Scherrer Institute including beam-line instrumentation and scattered particle detectors. References [1] R. Pohl, A. Antognini, F. Nez, F. Amaro, F. Biraben et al., [3] P. J. Mohr, D. B. Newell and B. N. Taylor, [4] R. Pohl, R. Gilman, G. Miller and K. Pachucki, [2] A. Antognini, F. Nez, K. Schuhmann, F. Amaro, F. Biraben et al., MUSE at PSI [6] H. Fleurbaey, S. Galtier, S. Thomas, M. Bonnaud, L. Julien, F. Biraben et al.,