PoS(NuFact2019)076 https://pos.sissa.it/ PACS: MUSE; elastic scattering; muon beam ∗† [email protected] Speaker. representing the E1027 collaboration The MUSE experiment at theaim Paul to Scherrer resolve Institut the (PSI) is protonto part radius determine of puzzle. the a proton suite MUSE radiusand of is through electron-proton experiments particularly simultaneous scattering, that interesting measurements in because of additionnegative it leptons. muon-proton to attempts measuring scattering This these not reactions onlytwo-photon with reduces exchange contributions systematic both which uncertainties positive may but and be also responsiblein for provides earlier some experiments. sensitivity of MUSE to the has discrepancies almost seen completedstart its data commissioning collection phase this and fall. is We scheduled report to on the current status and plans of the MUSE experiment. † ∗ Copyright owned by the author(s) under the terms of the Creative Commons c Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). The 21st international workshop on neutrinosAugust from 26 accelerators - (NuFact2019) August 31, 2019 Daegu, Korea Wolfgang Lorenzon Randall Laboratory of Physics, University ofE-mail: Michigan, Ann Arbor, Michigan 48109-1040, USA The MUSE experiment at PSI: Status and Plans PoS(NuFact2019)076 ] p ], 2 4 µ measurements to Wolfgang Lorenzon ep 00067 fm. Confirmation of this ]. . 5 0 ± 84184 . 0 ) form factors, a technique called Rosenbluth = M p 1 r measurement? Both scenarios seemed unlikely. The ]. This more than five-standard-deviation difference [ 3 approaches zero, which is proportional to the charge ra- ep 2 ) and magnetic (G ) as Q E E ]. However in 2010, the CREMA collaboration at the Paul Scherrer 1 0069 fm [ . measurement, or in the , has become known as the proton radius puzzle [ 0 1 p ± µ The proton radius measured with hydrogen spectroscopy and electron scattering is compared to 8768 . 0 The roughly 4.5% smaller proton radius implied that the proton was about 13% denser than Measurements of the proton charge radius started in the 1950’s using electrons scattering off = p measurements had exquisite statistical andbe systematic wrong, precision, both and the for scatteringerror the and in the the theoretical hydrogen calculations? spectroscopy Although datacalculations, extracting had the a to radius calculations from be had hydrogen wrong. been requirespossible complex checked that Or indeed and was no re-checked it mistakes had without an beenpossibly finding made, even and errors. that due the So to disagreement was was physicsthis due it beyond out. to new the physics, Standard Model? Clearly, new data was needed to sort surprising result came three years later [ previously believed. This was notof just the an Strong interesting Interaction result, (QCD),Questions but and arose is therefore why directly tests related exactly ourspeculated to the theoretical where the understanding the muon strength error, of and if the there theerror proton. was in electron in the give fact an different error, proton could come radii, from. and Was it it an was experimental Figure 1: that measured with muonic hydrogen. RadiusThat from means muonic it hydrogen is is 13% 4.5% smaller below (in the volume) previous best and value. therefore the proton is 13% denser than previously believed. dius. More recently, atomicmeasure physicists atomic transitions have in been the using hydrogenlevels atom. depend spectroscopy on Since methods the the to size exact of very valueslations the of that precisely proton, include the corrections they hydrogen’s for compared energy the their finitevalue measurements size for with of the Lamb the proton shift proton to calcu- charge extracttron radius. an proton indirect Until scattering but very about measurements precise and a from decader electronic ago, hydrogen the spectroscopy proton had charge converged to radius from elec- that determined the proton charge radius to be shown in Fig. Institut (PSI) reported results from a novel experiment using muonic-hydrogen spectroscopy [ separation was used to separateof the the contribution electric of form the factor two (G form factors and to extract the slope hydrogen targets at the Stanfording accelerator. depends of Since the the electric cross (G section for electron proton scatter- MUSE Status and Plans 1. The Proton Radius Puzzle PoS(NuFact2019)076 of approxi- 2 Q Wolfgang Lorenzon of about 115, 153, 1 , corresponding to ◦ Straw-Tube Tracker (STT)Tracker 100 Beam Monitor − Target Chamber ◦ ~ 100 cm Veto ], located in the PiM1 area of the Paul Scher- Scintillator . 6 2 2 3 GEM Detectors , the range of the form factor with greatest sensitivity to the Beam 2 Hodoscope πM1 scattering for comparison with the muon scattering; and for a Beam-Line Scintillator (SPS) Scintillator Scattered Particle Scattered ep 08(GeV/c) . 0 − 2 and scattering angles in the range of 20 c A schematic overview of the MUSE experimental setup is shown. The beam hodoscope deter- ]. 6 These particular beam momenta were chosen to allow clean separation of the pions, muons and electrons in the MUSE is an unusual scattering experiment. While high precision electron scattering experi- The low beam intensity eliminates target density fluctuations from beam heating and allows The MUon Scattering Experiment (MUSE) [ 1 PiM1 beamline using RF timing information. secondary beam it is easy toa obtain precise essentially determination of identical two-photon beams exchange of (TPE) both contributions. charge signs, which allows Figure 2: mines the particle type through time-of-flightthe measurement. beam The particles, GEM and detectors the measuretiming veto information the scintillator of trajectories the tags of beam beam particlesparticle halo downstream scintillator of particles. the planes target. provide The track The beam straw-tube and monitor trackers timing provides and information the flux of scattered and the scattered particles. ments typically use an intense primary beam,small with solid-angle scattered spectrometer, MUSE particles is detected using by a aelectrons low-intensity high-resolution, and secondary beam muons that where contains the pions, scatteredThe particles resulting are experimental detected setup by is a shown large in acceptance Fig. spectrometer. identification of each individual beamsimultaneous particle; measurement the of electron contamination of the beam allows a rer Institute (PSI) in Switzerland,radius is puzzle. part of MUSE a isdius suite particularly through of interesting muon experiments because proton thatously it aim scattering, with to attempts which electron resolve to has proton the determine only scattering, proton negative the been in leptons. proton addition done ra- to with This performing low nottwo-photon these precision, only exchange reactions contributions reduces simultane- with which systematic positive may and uncertaintiesin be but responsible earlier also for experiments. some provides of sensitivitycross The the to section electric discrepancies data. seen form The factor experimental will kinematics be cover extracted three from beam momenta fits to the experimental MUSE Status and Plans 2. The MUSE experiment and 210 MeV/ radius [ mately 0.002 (GeV/c) PoS(NuFact2019)076 2 p µ that 3 to ep Wolfgang Lorenzon ) target that provides stable density 2 01K level. The 20.67 K target temper- . demonstrates the performance of the LH 3 elastic cross sections. The projected relative p µ elastic cross sections is expected to be below 3% 3 to p − ]. Figure ep µ 7 to p + µ elastic cross sections is below 2%, with an overall systematic uncertainty p µ to ep Target 2 Histogram of target temperature values measured during the first cooldown period. The shape of , as well as a direct comparison of 2 The absolute values of the radius extraction from electron and muon scattering will be of order MUSE has completed 18 test running periods, including beam studies, detector development, At that time, MUSE will have performed a high precision test of TPE for electrons and muons At the heart of the experiment is a liquid hydrogen (LH 0.009 fm each. However, the uncertainty in the difference of the radii extracted from with an overall systematic uncertainty ofin 0.2%, the while ratio the of projected the relative statisticalof uncertainties about 0.5%. Note that thein relative the statistical cross uncertainties sections. in the form factors are half as large as 4. Current Status and Projections and commissioning runs since 2012.tector The simulations, data and collected the during apparatusapparatus these has is periods demonstrated almost agree reliable complete. well performance. withexperiment Construction It de- software of is will the anticipated be that completedthat commissioning during first of the production the November/December data 2019 detector canbe running hardware be collected period, collected and in during so 2020 and that 2021. period.physics While results The a are remaining variety expected of production in physics data 2022. results will will be released in 2020,at the final low Q Figure 3: the distribution is purely Gaussian with a mean of 20.67 K and a standard deviation of 0.01 K. statistical uncertainty in the ratio of MUSE Status and Plans 3. The LH and sufficient cooling power totion minimize measurements uncertainty at in the target sub-percent level lengthtarget. [ in The order variation to of allow theduring cross the target sec- first temperature 72-hr at cooldown period theature was target corresponds constant cell to at operating the an 0 operating temperature pressure ofwas of stable 20.67 about K to 1.1 within bar 0.02%. and a target density of 0.070 g/cm PoS(NuFact2019)076 p , µ to A949 ep Wolfgang Lorenzon , (2010) 213. 4 Elastic Scattering, arXiv:1709.09753v1 , (2016) 035009. , (2008) 633. 466 p 88 80 µ , (2013) 417. , (2013) 175. 339 63 , A Liquid Hydrogen Target for the MUSE Experiment at PSI, Nucl. Instr. Meth. et al. ). The radius difference extraction is almost a factor two more sensitive than the absolute A summary of some recent proton charge radius determinations, relative to the muonic hydrogen 4 Annu. Rev. Nucl. Part. Sci. Studying the Proton “Radius” Puzzle with [physics.ins-det]. (2020) 162874. Physical Constants: 2014, Rev. Mod. Phys. Muonic Hydrogen, Science Physical Constants: 2006, Rev. Mod. Phys. In summary, the discrepancy between muonic and electronic measurements remains a serious [5] R. Pohl, R. Gilman, G.A. Miller, and K. Pachucki, Muonic hydrogen[6] and theR. proton Gilman radius at puzzle, al., Technical Design Report for the Paul Scherrer Institute Experiment R-12-01.1: [7] P. Roy [4] P.J. Mohr, D.B. Newell, and B.N. Taylor, CODATA Recommended Values of the Fundamental [1] P.J. Mohr, B.N. Taylor, and D.B. Newell, CODATA Recommended Values of the Fundamental [2] R. Pohl et al., The size[3] of theA. proton, Antognini Nature et al., Proton Structure from the Measurement of 2S-2P Transition Frequencies of scattering, References problem, although very recentwards electron the scattering muonic and hydrogen spectroscopy result.those, results Several MUSE new seem results will to are provide gravitate expected theisting to- in first the radius precise coming measurements. muon years. scattering Among provide result MUSE a and test will will of also lepton test universality test the by directly values the comparing for magnitude the ex- proton of radii extracted the from TPE contributions and Figure 4: result, along with the expected MUSE result, arbitrarily placed at 0. MUSE Status and Plans scattering will be of order 0.005 fm,(see since Fig. common systematic uncertainties cancel in theradius difference extraction, or almost factor ten better than the current discrepancy of about 0.034 fm.