PoS(CD2018)024 fm, This ) 67 https://pos.sissa.it/ ( 84184 . 0 = p r ] from muonic hydrogen spectroscopy: 1 ∗† [email protected] ], and sparked an enduring and intriguing puzzle. 2 was five standard deviations smaller that the accepted[ value CODATA of that time (0.8768(69) fm) Speaker. On behalf of the MUSE Collaboration The proton radius puzzle began inment 2010 of when the the CREMA proton Collaboration radius released their [ measure- Since that first measurement, manypears theories smaller have when measured been with formed muons as thanmeasurements it to have does also why when been the measured made with proton electrons.rent in radius status Many an ap- new of attempt the to proton resolve radiusMUon the puzzle, proton and puzzle. Scattering review Experiment future We (MUSE). will experimental plans, detail with the a cur- focus on the † ∗ 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 9th International workshop on Chiral17-21 Dynamics September 2018 Durham, NC, USA Evangeline J. Downie The proton radius: are we still puzzled? The George Washington University E-mail: PoS(CD2018)024 q m G fm. / ) (1.1) E 10 G ( 77 . 0 = p ]. There are also r 6 Evangeline J. Downie 0, as there no scattering = ]. Chambers and Hofstadter ). If one measures only over 3 2 1 q , and fitted to extract a radius. ]. 2 6 q ]. 2 values, reaching as low as possible, ] reported on an extraction of 2 fm [ 5 q 0 [ is the Sachs electric form factor, and ) = (see Figure ) q | 2 q 69 ) q ( ( e 2 et al. q 2 G ( e dq ]. 8768 . 1 8 0 dG 6 = − p r ] reported the largest data set in elastic-proton scattering = 4 [ p 2 r et al. ], more than five standard deviations from the accepted CODATA 1 ], but an order of magnitude more precise. This resulted in a flurry 2 , one must measure incredibly precisely to see the effect of the radius, fm [ 2 ) q fm [ ) 67 ( , range, one might miss higher order effects, resulting in truncation error on 69 2 ( q 84184 from JLab. Both Bernauer and Zhan supported the larger electric charge radius . 8768 . 0 2 0 q = = p is the mean charge radius of the proton, p r r p r There are several weaknesses in the extraction of the charge radius from scattering experi- The first measurement to extract the radius was reported in 1956 [ Thus scattering measurements must cover a range of The proton radius puzzle (PRP) began in 2010 when the CREMA collaboration published their the extracted radius, which can be as large as the 4% PRP effect [ ments. The scattering measurementsfactors. produce cross The sections, resulting which form are then factors used are to plotted extract as form a function of where is the momentum transfer. Theported radius, as extracted from spectroscopy of muonic hydrogen, was re- used electron scattering off a gaseous hydrogen target to determine a radius of a very limited, low but extending far enough outissues from in zero the in extraction order of toextract the both minimize radius large truncation and by error small fitting: radii [ reported from depending at the on the same the 2018 data MITP parameterization set, workshop as used, in can one Mainz be can [ seen from the many extractions measured to date at MAMI. Shortly thereafter, Zhan 2. Electron scattering measurements of calculations, publications, andpuzzle. experimental proposals Indeed, which, in many thusthe cases, far, different they have mechanisms have not of made resolved the measuringpotential the puzzle solutions, the and somewhat proton the more charge experiments puzzling. planned radius, to examine We resolve review the it. status of the PRP, Since then, accelerators and detectorsin have theory improved, and and radius furtherproton developments extraction radius. have techniques been In leading 2010, made Bernauer to increasingly precise determinations of the This primary issue is that it is not possible to measure exactly at value of measurement of the charge radius of the proton, which is defined as: reported in the CODATA 2010 average of as the effect of the radius becomes very small at lower The proton radius: are we still puzzled? 1. Introduction over a wide actually occurs, thus radius extractionsone are measures always at interpolating very outside low of the measured range. If PoS(CD2018)024 6 Q and 4 Q ]. Evangeline J. Downie 10 , ]. For a full review of 9 2 ]. 7 207 times as massive as the electron, ∼ 2 6. The cyan dashed and green dotted curves include / 2 r 2 Q − 1 = E G Effect on the cross section from the proton radius being 0.88 fm vs. 0.84 fm. The solid red line While there were two different measurement mechanisms, with completely different system- There is another mechanism by which one can extract the proton radius: hydrogen spec- muonic hydrogen is far more compact than atomic hydrogen. This makes the perturbation to the atics, which produced thestill same the proton dominant radius, uncertainty in theCREMA many collaboration uncertainty QED decided on to processes. measure the In theelectron proton radius an has using charge effort been muonic to radius hydrogen, replaced improve hydrogen was by where this the the situation, muon. the As the muon is 3.2 Muonic Hydrogen Spectroscopy terms taken from the Kellylines parameterization, indicate respectively. the The kinematic horizontal range green, of blue, the and proposed magenta MUSE dashed data. Figure from [ measurements prior to the PRP, and the immediate impact of the PRP, see [ Figure 1: uses a linear approximation, troscopy. There is a smallThe perturbation finite to size the of S-orbitals, the causedelectrons proton by in the has, P finite orbitals however, no radius is zero of impactorbital at the upon transition the proton. the in origin. P-orbitals, hydrogen, This as around meansperturbation, the that 0.014% all there wavefunction the is of other size a components of small theto of perturbation Lamb to the the the Shift. high difference S-P are precision Asideradius from precisely with of this calculable which the radius proton in lasers from QED. can ament, measurement Thus, be of too, due tuned S-P transitions has and within been calibrated, atomic performedthe hydrogen. it inception This at is measure- of multiple possible the institutions, to PRP, with it extract also ever-increasing the precision, agreed with and, the until CODATA 2010 average [ The proton radius: are we still puzzled? 3. Spectroscopy Measurements 3.1 Hydrogen Spectroscopy PoS(CD2018)024 using 2 q ]. 14 disagreement with smaller. Using the Evangeline J. Downie σ σ 5 . from the 2010 CODATA σ 9 . ] gave a preliminary report at 15 fm, but 7 ], which was almost three times ) 12 21 ( . p fm [ r ) 1424 78 . ( ], it was reported, however, that the measure- 2 ]. This is an order of magnitude more precise 8 1 ]. This was then 7 = d 11 3 extracting a radius in agreement with the smaller ]. An effort at MAMI to reach even lower r fm [ 12562 1 radius problem. . ) 16 2 = fm [ 67 ) = Z ], while Fleurbaey extracted a larger radius [ ( . PRad at Jefferson Lab [ et al. ]. d 2 39 13 r ( q 17 84184 . 0 84087 . = 0 p = r p r ]. 8 experiment, known as ProRad [ 2 q published a value of At the MITP workshop in the summer of 2018 [ In addition to the progress in muonic hydrogen, there have been multiple efforts to make more Since the inception of the puzzle in 2010, many different theories have been postulated to The CREMA collaboration continued to work on the proton radius, publishing again in 2013 In order to further investigate the radius problem, CREMA moved to investigating the radius Additionally, several experiments are underway, or under analysis, to measure the radius via To date, results remain divided between the smaller and larger radius, and, in some cases, The CREMA collaboration successfully extracted the proton charge radius from their data, well-known isotope shift, the muonic deuterium radiusthe also smaller gave muonic a hydrogen proton radius. radius in agreement with precise measurements of the protonresulted charge in radius mixed using outcomes, atomic with hydrogen Beyer spectroscopy. This has 4. Recent developments muonic hydrogen proton charge radius [ 5. Potential resolutions to the Proton Radius Puzzle this meeting, and so we will nothave a go low- into more detail here.Initial At PRAE State in Radiation Orsay, there showed are that alsostatistics the plans to to methodology be worked competitive, in thuswhich principle, it is but is currently needs under planned much construction to [ higher repeat this experiment at the MESA accelerator, a report with further analysis ofet the al. data, including an extraction of the magnetic radius. Antognini of deuterium, helium and othersurements, nuclei. with In an 2016, extracted they radius published of the results of the deuterium mea- electron scattering at exceptionally low multiple extractions from theresults same to data settle set on one give radius,the different it existing answers. would data still [ be Even necessary were to explain all the of cause the of the newer discrepancy in the accepted value of CODATA. value. 3.3 Muonic Deuterium and Helium Spectroscopy more precise than the CODATA 2010 value of ments performed on various muonicwere helium forced to agreed acknowledge with that the this is radii an from atomic helium. Thus we reporting in 2010 a radius of The proton radius: are we still puzzled? S-orbitals far greater than that in atomichydrogen hydrogen, is and roughly therefore eight the million radius times extraction more from muonic sensitive to than all proior measurements, but 4% smaller, and, at that time, in roughly 5 PoS(CD2018)024 . 2 GeV 07 . ]. Many of 0 9 ]. − 7 002 . Evangeline J. Downie ). We will use 3.3 MHz of 1 range of 0 2 q ]. As the two-photon effect changes 18 (see Figure ]. This would be beyond-standard-model 2 q 21 , 20 , 4 19 . In so doing, we will cover a π 80% − , and 0 e 98% − . Full details of the MUSE experiment can be found in the MUSE TDR [ , 10 2 µ The first explanation is that the presence of the muon so close to the proton deforms the The second explanation is that there are new electrophobic scalar bosons, a new force which MUSE will run in the PiM1 beam line of the Paul Scherrer Institut in Villigen, Switzer- MUSE will run with both polarities at three different beam momenta of circa 115, 153 and There have been spectroscopy measurements with electrons and muons, and scattering ex- 15% − total beam flux, which, depending2 on momentum and polarity setting, will have a composition of proton, and that the radiusThis we would are be measuring observed with in muons an is enhanced thus two-photon the effect [ radius of a deformed proton. interacts only with the muon and not the proton [ land. PiM1 wasPSI constructed impinging as on a a precision carbonmomentum-selected pion target, and and beam transported the line, to resulting the with mixturemagnet experimental system the of area polarity 590-MeV pions, can by be electrons proton a switched, and precision beamDue giving muons to MUSE magnet of the the being system. mixed ability nature to The of measure theusing in beam, both the MUSE charge has relative states. to timing identify between thedoes, beam the however, particles, accelerator mean particle-by-particle, that RF MUSE and can detectors simultaneouslyand in measure muons, elastic the reducing scattering experimental with the area. both systematic electrons beam This uncertainty particles in can the subsequently decay comparison in-flight, ofiment precise both in timing order cross-sections. is to needed As remove at decay the allusing backgrounds. stages a In of secondary the order beam exper- to have whicheach a has precisely individual reasonably defined beam large scattering particle dispersion angle, on and itsnecessary spot-size, way to MUSE to use large-acceptance, the has non-magnetic target. to spectrometers track As to the detect secondary scattered beam particles. has210 lower MeV/c, flux, giving it six is different data sets at overlapping these theories have been disproved, butbeen there conclusively are refuted, two but major which surviving also explanations are which not have universally not accepted. sign with lepton charge, observingstates the allows difference a in direct cross observation section of this between effect. opposite lepton charge 6.1 The piM1 beam periments with muons, but,sufficient to precision date, to there address the hasperiment, proton been was radius first no puzzle. proposed muon-proton in MUSE, 2012 elasticin the to Figure scattering MUon address data this proton deficit. Scattering with The Ex- MUSE detector setup can be seen physics, breaking lepton universality.other There experiments, are and strong thus constraints theMeV. This placed masses would of on potentially the also this explain proposed proposal the particlesby muon from observing g-2 are modification anomaly. limited in It to the could muon-scattering be across cross seen few- section section in to compared around scattering to tens-of- data the the invariant electron-scattering masses of these new particles. 6. The MUon proton Scattering Experiment (MUSE) The proton radius: are we still puzzled? explain the puzzle, assuming that the experimentally observed discrepancy is correct [ PoS(CD2018)024 Evangeline J. Downie - orientations to give rough y - and x 5 The MUSE detector system. The beam enters from the bottom left of the figure, and travels Immediately after the BH, there is a stack of three 10 cm by 10 cm Gas Electron Multiplier The first detector the beam encounters upon entering the PiM1 area is the Beam Hodoscope (GEM) detectors, which track the incoming beam with an efficiency of 98%, and a position reso- pixels. The number ofless precise BH timing planes is needed employed andplanes depends it are is on used. more the At important higher beam to momenta,so minimize momenta: timing four multiple becomes planes scattering, at more are so critical low used. only and two momenta, than The multiple 80 Beam scattering ps. Hodoscope less The is so, Beam 99.8 Hodoscope % was efficient constructed and by has Tel readout Aviv University timing and of Rutgers better University. 6.2 Beam line detectors and target (BH): a series of thin, segmentedmatsu 2mm-thick, S13360-3075PE BC-404 scintillators, silicon read photo-multipliers. out at The4mm both planes wide ends each by by have Hama- 10cm six central long.bars, bars giving These which 16 are are bars per flanked plane. on The each BH side planes are by alternated five in eight-mm-wide by 10-mm-tall towards the upper right, exiting through the Beam-Line Monitor. Figure 2: The proton radius: are we still puzzled? PoS(CD2018)024 . . ◦ ◦ on on was ◦ ◦ 2 m / g to 100 to 100 ◦ ◦ Evangeline J. Downie ], and are being reused ]. They are made from 22 24 at an azimuthal angle of 60 at an azimuthal angle of 60 ◦ ◦ 45 45 and, when including the effects of + + ]. The target cell is a vertical Kapton to to mr ◦ ◦ 5 23 . 0 45 45 . The inner chambers are formed of five − − ∼ m µ 6 laminated on aramid fabric with an areal density of 368 R

. The GEMs were made for the OLYMPUS experiment [ m µ Following the GEMS, the beam passes into the liquid hydrogen target, designed by the Uni- Behind the STTs are two layers of Scattered Particle Scintillators (SPS). These are used to The STTs follow the design made for the PANDA experiment [ Surrounding the entrance window to the target, is an annular ring of plastic scintillator,Finally, read all unscattered beam particles pass through the Beam Monitor (BM) detector, which Scattered particles pass first through an array of two Straw Tube Tracking chambers (STTs) versity of Michigan group in collaboration with CREARE [ provide a fast scattered particle signal forfast triggering, and timing. also to The suppress bars, backgrounds which byHamamatsu providing are R13435 made PMTs. from Eljen This EJ-204 leads scintillator, to are an read out efficiency of at both 99%, ends and by a timing resolution of better in MUSE. They have an intrinsicmultiple angular scattering, resolution better of than 5mr. Hampton University are responsible for the GEM detectors. cylinder, 6 cm in internaltically diameter, moving 11 cm target ladder high, contains filledground the with measurements; filled 280-ml a cell; of thin an liquid carbonwhere identical, hydrogen. foil no empty for target cell, The tracking cell for ver- tests; is targetentry a in cell and thin back- the exit, CH2 beam. foil; and two anddetectors The a large with scattering blank minimum windows chamber energy position to has loss. allowboth two These scattered sides Kapton windows of particles cover the windows to an beamline. for reach azimuthal They beam cover the range a of scattered polar 20 angle particle of thin-walled straws kept rigid bycapable over-pressure, of and resolving are tracks with readlayers a of out resolution 60-cm-long on of straws a orientated 150 layers single vertically, of end. closest straws to orientated The the horizontally, STTschambers target, moving are also radially followed have outwards. by five five horizontal, The identical by followed outer, a by 90-cm-long, collaboration five straw between vertical, Hebrew layers. University of The Jerusalem, Israel, STTs and were Temple constructed University. The proton radius: are we still puzzled? lution of 70 In order to make strongfabric but which thin is windows made of of Mylar this size (337 mm wide by 356 mm high) a sailcloth out by SiPM detectors, which providesreadout a rate VETO caused signal by for beam the particle trigger. decays The or VETO scattering helps from reduce detectors the is upstream of used the target. to monitormeasurements beam to position, verify the flux beamthin and momentum. scintillating composition; elements The and read beam out toThe monitor by scintillator perform is SiPMs, bars separate can formed flanked be on of time-of-fight moved each to twothe the side time-of-flight planes center beam by of of momentum two the 16 measurements. detector thick for scintillator optimal bars. timing resolution during 6.3 Scattered particle detectors for tracking, then into athe double scattered particles. wall of The scattered thicker particle scintillator detectors bars cover used an azimuthal for angle timing of and 20 triggering of employed. The target is fully constructedsuccessfully and with assembled liquid at PSI, hydrogen has in passedsuggesting late all a 2018. safety density tests, stability It and at ran has the 0.02% a level. temperature stability at the 0.01-K level, both sides of the beamline. They cover a polar angle of PoS(CD2018)024 ], 27 Evangeline J. Downie ]. The BH and BM face 26 7 ], which are each equipped with 5 FPGAs, four peripheral 25 ]. The STTs faced large noise issues when they were first installed, 25 ] are employed to provide pulse-height information. 28 In order to trigger system readout, several TRB3s are programmed to provide an FPGA-based MUSE has full approval from the Paul Scherrer Institut BVR committee, which distributes The input to the TRB3s comes from three different types of discriminator. The SPS is read out Timing of signals is the most crucial aspect of the MUSE readout, providing beam particle high rates and potential pile-upwhich issues were and, specially as adapted such, to areTRB3s. output read To in provide out walk-corrections LVDS by format for MESYTEC the inMesytec MCFD-16s SPS, order MQDC-32s [ and [ to to better monitor communicate performance of with the the BH and BM, triggering system. The trigger requires thatfied no by timing pions, in and the at Beam least Hodoscope.particle one The muon which, trigger or further based electron requires on that was the a identi- SPS SPS look-up paddles, identify a indicates table scattered a identifying scattered acceptable particleveto, combinations originating and in of the a front- hydrogen NOT and target of back- region.data the A are NOT read full-system of out, busy the and signalauthored the are by overall also Stefan DAQ Ritt system required of controlled for PSI. by, a Slow MIDAS, control a trigger and data-acquisition to monitoring6.5 software is be performed MUSE issued. in status EPICS. The beam time at the lab.able The platform. detector system is All fullyin detectors constructed earnest meet and with mounted their a on design summer abeam: 2019 resolutions movable, taking beam crane- and measurements time, efficiencies. to during makeboth which Data systematic TURTLE MUSE and comparison taking will G4beamline; between verify begins conclusively the understanding demonstratingprecisely data of that determined and the pion in simulations and time-of-flight in muon calibration momentarelative measurements; momentum can and distributions be that of MUSE electron, muons, canare and map fully pions out verified, in the production the data beam. taking Once should beam begin properties in Fall 2019. and specially constructed PASTTREC cards were ordered,allowing which the have suppressed STTs the to noise issues, reach their designed resolution and efficiency [ by PADIWA level discriminators, which arewith effective the and TRB3 inexpensive, system designed by [ GSI to work FPGAs which read outdata-flow, individual and channels, synchronization. andTRB3 Each one infrastructure. TDC central channel FPGA TRB3s has whichout are an manages system. designed accompanying triggering, Within scalar, to MUSE, built provide theyprovide an provide into a FPGA-based timing the experiment flexible, and trigger. scalable, scalar Theand information, Data programmable programmed Acquisition and by read- System George also (DAQ) Washington was programmed University constructed and to the trigger by Rutgers University. identification, and background suppression. Tochannels, that and end, 500 we are ADC readingwalk channels, out corrections which well to over are 3000 improve usedprovided TDC timing to by in TRB3 monitor read-out the gains boards SPS, and [ provide BH event-by-event and BM. All TDC signals and scalars are The proton radius: are we still puzzled? than 55 ps. The design was basedand on constructed the by FToF12 for the the University CLAS12 of JLab South upgrade, Carolina. and was developed 6.4 Detector electronics and data acquisition system PoS(CD2018)024 to − e / + e and ) . If there is no − 4 µ Evangeline J. Downie / + µ 8 ). By examining the ratio of electron and muon cross 3 effect with even higher significance. σ The predicted uncertainty in the absolute proton charge radius extraction from MUSE. The point Almost a decade after the inception of the Proton Radius Puzzle, the puzzle endures. Muonic The statistical errors in the cross sections are planned to be better than 1% for backwards The charge radius uncertainty is limited by systematics and fit uncertainties. MUSE detectors The MUSE data analysis will be able to directly compare muon and electron cross sections, hydrogen, deuterium, and heliumrather spectroscopy than have simply further a revealed protonsurements this have radius to given problem. conflicting be radius Recent asets values. high-precision Z=1 give different Different hydrogen problem, radius fits spectroscopy values. to mea- In themake the the same midst first electron-scattering of measurement all data of of elastic this, muondress MUSE scattering the presents on proton a the radius. unique proton opportunity In with to addition sufficient precision to to the ad- precise muon scattering measurement, MUSE can make section data, we will be able to extract the radius difference to 0.005 fm (Figure 7. Conclusion muons and much better forrelative forward uncertainties muons to a and few electrons. tenthstrons of We a and will percent, be muons allowing able individual to radius to 0.01 extractions fm keep from point-to-point (see elec- Figure are complete and functioning atof the the required systematics, level such to assimultaneous keep the nature target these density, effects of are under the vastly control,verify reduced, MUSE the and or 5.6 electron many completely and eliminated muon by the measurements. Thus MUSE is suited to Figure 3: is arbitrarily placed at 0.875 fm. difference, electron and muon data can be combined to extract the absolute radius to 0.007 fm. form factors, and radii. The first step will be to compare charge states: The proton radius: are we still puzzled? 6.6 Anticipated precision of radius extraction directly extract two-photon effects. Onceboth these charge effects states are to fully eliminate determined,measured two we within photon will the corrections average same and over experiment. directly compare muons and electrons PoS(CD2018)024 , 466 . Nature , Evangeline J. Downie (2011) 59 (1956) 1454 B705 ]. 103 High Precision Measurement of https://arxiv.org/abs/1709.09753 The size of the proton Polynomial fits and the proton radius Phys.Lett. Phys. Rev. , , 2 1007.5076 [ . at low Q 9 M G p Elastic Scattering, / CODATA recommended values of the fundamental physical E µ (2010) 242001 G p . µ High-precision determination of the electric and magnetic form (2012) 1527 105 Structure of the Proton 84 (2014) 045206 collaboration, Technical Design Report for the Paul Scherrer Institute Experiment R-12-01.1: Phys.Rev.Lett. 90 , Rev. Mod. Phys. , ]. . Phys. Rev. C , OLLABORATION ,. The predicted uncertainty in the proton charge radius difference from MUSE. The point is arbitrar- 1102.0318 constants: 2010 (2010) 213 [ factors of the proton Studying the Proton "Radius" Puzzle with the Proton Elastic Form Factor Ratio 2017. puzzle This material is based upon work supported by the National Science Foundation under Grant [2] P. J. Mohr, B. N. Taylor and D. B. Newell, [3] E. E. Chambers and R. Hofstadter, [6] E. Kraus, K. E. Mesick, A. White, R. Gilman and S. Strauch, [4] A1C [5] X. Zhan, K. Allada, D. Armstrong, J. Arrington, W. Bertozzi et al., [7] MUSE collaboration, [1] R. Pohl, A. Antognini, F. Nez, F. D. Amaro, F. Biraben et al., Numbers PHY-1614850 and PHY-1714833. References a simultaneous electron measurementMUSE and uniquely situated subsequently to access provide fresh both data charge with potential states. to resolve the This proton makes Acknowledgments radius puzzle. Figure 4: ily placed at 0.0 fm. Pleaseresult note and that that all of points Antognini. other MUSE than shows MUSE the show difference the between difference MUSE between proton the and published muon measurements. The proton radius: are we still puzzled? PoS(CD2018)024 , , . ]. 73 Phys. EPJ , , . Phys. Rev. (2013) , ]. with initial , 2019. 8 , 2019. , 2019. 2 TRB3: a 264 (2014) 1 (2011) 101702 Readout electronics The rydberg Evangeline J. Downie 1502.05314 83 [ (2016) 669 A741 New Measurement of the ]. 353 EPJ Web of Conferences (2015) 59 , Science Phys. Rev. D 82 , , ]. (2017) 79 ]. https://www.creare.com/ . Journal of Instrumentation , Nucl. Instrum. Meth. 358 ]. , 2016 IEEE-NPSS Real Time Conference (RT) , in (2013) 175 Muonic hydrogen and the proton radius puzzle (2013) 1 1405.4864 [ Science 63 49 , 10 , 2018. 1612.06707 ]. [ 1209.4667 Probing New Physics with Underground Accelerators and Prog. Part. Nucl. Phys. [ , Electrophobic scalar boson and muonic puzzles (2015) 61 (2017) 194 B740 Muonic hydrogen and MeV forces 1801.08816 [ . (2013) 1078 Workshop on the Precision Measurements and Fundamental Physics: The The OLYMPUS Experiment B771 . Laser spectroscopy of muonic deuterium . (2013) 417. B718 First measurement of proton’s charge form factor at very low Q https://www.mesytec.com/products/datasheets/MQDC-32.pdf Phys. Lett. https://www.mesytec.com/products/datasheets/MCFD-16.pdf DOI , Proton structure from the measurement of 2s-2p transition frequencies of muonic 339 The prad experiment and the proton radius puzzle Phys. Lett. (2017) 01012 ˇ The Proton Radius Puzzle c et al., , A new platform for research and applications with electrons: the PRAE project Technical design report for the PANDA (antiproton annihilations at darmstadt) straw ]. (2018) 183001 . Proton Polarizability Contribution: Muonic Hydrogen Lamb Shift and Elastic The European Physical Journal A , Phys. 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