Workshop on Progress on JPARC Hadron Physics in

Workshop on Progress on J-PARC Hadron Physics in 2014 Measurement of the proton Zemach radius from the ground-state hyperfine splitting energy in muonic hydrogen RIKEN Masaharu Sato 1/Dec/2014

Outline p Introduction and physics motivation p Experimental principle p Feasibility in J-PARC MLF p Summary 2

Introduction proton : a building block of the Universe Ø structure of the proton is one of the most fundamental observables in the atomic and nuclear physics electric/magnetic form factor, proton radius etc Muonic hydrogen atom exotic atom composed with m- and p mμ/me ~ 200, R ~ a. B/200 μp probability in the proton : ~ 8 x 106 (rp/a. B)3 = (ammp rp )3 bound m feels the effect of the proton structure Muonic hydrogen atom is good tool to study the internal structure of proton 3

Lamb shift in muonic hydrogen r. m. s. charge radius RE : measured many times by e-p scattering and H spectroscopy CODATA 2006 RE = 0. 8768(69) fm PSI group: Laser spectroscopy of 2 SF=11/2 2 PF=23/2 (Lamb shift) of muonic hydrogen (μ-p) μ-p Measured value : 206. 2949(32) me. V R. Pohl et al. , Nature 466 (2010) DELamb = : 209. 8778(49)-5. 2262 RE 2 + 0. 0347 RE 3 me. V RE = 0. 84184(67) fm X 10 better precision 4

Proton radius puzzle Ø “electronic” (hydrogen spectroscopy / e-p scattering ) Ø “muonic” (muonic hydrogen Lamb shift ) e-p & H μ-p 7 s ~4 % “Proton radius puzzle” Still unsettled question: errors in the measurements? structure-dependent corrections are wrong? What about magnetic QED needs modification (in m- p interaction)? distribution? new physics beyond the Standard Model? 5

Motivation proton Zemach radius Rz (convolution of charge (r. E ) and magnetic moment (r. M ) distributions) proton electronic & magnetic structure determined from hyperfine splitting energy of H-like atom Hyperfine splitting 13 S 1 (F=1) Ø EF : Fermi energy 1 S DEHF S Ø d. QED : higher order QED correction Ø dstr : proton structure correction 11 S 0 (F=0) F : total angular momentum directly connected to Rz 6

Past measurements on Zmeach radius hydrogen spectroscopy Rz =1. 037(16) fm Dupays et al. , PRA(2003) =1. 047(19) fm Volotka et al. , EPJ(2005) e-p scattering Rz =1. 086(12) fm Friar & Sick, PLB(2004) =1. 045(4) fm Distler et al. , PLB(2011) Ø Latest values of e-p and H spectroscopy are consistent within their errors. muonic hydrogen 2 S HFS Rz = 1. 082(37) fm Ø m-p value differs? But accuracy is insufficient to verify. 7

Our strategy : measurement of μp 1 S DEHFS muonic hydrogen 1 S HFS energy not measured before laser spectroscopy : 0. 183 e. V = ~6. 78 mm (= ~44. 2 THz) Goals : mid-infrared laser is needed determine 1 S DEHFS with an accuracy of ~ 100 MHz (~ 2 ppm) due to accuracy of the 1 st precise measurement of g. s. DEHFS offrequency μ-p fundamental quantity of μ-p system (can determine proton structure correction (dstr) with ~ppm accuracy) derive the proton Zemach radius from DEHFS 8

Expected precision 1130(1) ppm 1700(1) ppm 460(80) ppm 20(2) ppm Dupays et al. , PRA 2003 RZ = 1. 0? ? (12) fm Improved factor ~3 from PSI results dpol = 460(80) ppm Cherednikova et al. , NPA 2002 We need help from theorists for further improvement of precision. improvement of proton polarizability correction (dpol) drastically reduces uncertainty of Rz 9

Experimental principle How to determine DEHFS 10

Experimental principle (1) Laser spectroscopy : signals of the resonance frequency decay asymmetry of polarized muons atomic capture 1) Produce m-p atom by pulsed muon source shoot m- into hydrogen g. s. m--p atom e m muonic hydrogen p lifetime ~ 2. 2 us 2) spin polarization by laser 13 S 1 (F=1 ) 1 S DEHFS ~ 0. 183 e. V polarization selective excitation 11 S 0 (F=0 ) polarization in F=1 state circularly-polarized laser 11

Experimental principle (2) 3) detect decay electrons μ- e- + ν e + ν μ muon decay asymmetry with polarization (P) polarized μ- decay e－ more decay electrons in opposite direction of muon spin Spin polarization (= frequency is on resonanace) can be detected decay asymmetry of muons 12

Conceptual design of experimental setup 1) H 2 target Top view of setup 2) tunable mid-infrared laser 3) decay electron counter(forward and backward) detect forward/backward electrons Asymmetry = NF - NB 13

Feasibility 14

Feasibility e- F=0 F=1 transition probability decay E/S : laser power density [J/m 2], T : temperature [K] excitation by laser NIM B 281(2012)72 & D. Bakalov, private communication laser power need high laser power collisional quench F=1 F=0 collisional quench rate competitive process with muon decay in F=1 mp( ) + p then, polarization is lost tquench VS tm(= ~2. 2 us) 15

Laser system tunable mid-infrared laser (developed by RIKEN Wada group) frequency ~6. 8 um = ~44 THz band width ~50 MHz repetition ~ 25 Hz seeded OPO with Zn. Ge. P 2 non-linear crystal double pulse 10 m. J x 2 sets = 40 m. J laser power is achievable multi-pass cavity mirror Hydroge n laser 16

Collisional quench rate p F=1 F=0 quench by collision with surrounding atoms mp( ) + p F=1 (polarization is lost) J. Cohen, PRA 43(1991)9 quench rate (l. Q) F=0 proportional to H 2 density Quench rate (l. Q) at 20 K timing gate t = 50 ps at liq H 2 gas H 2 is needed If 0. 1% LHD (liquid hydrogen density), then tquench = 50 ns P = 3. 7 % in ~700 ns time 17

Experiment in J-PARC MLF a proposal submitted to MLF PAC “S 1 -type project” Stage-1 approved 18

J-PARC muon facility World highest pulsed muon source μ- (decay) intensity (Kawamura san, private communication) 5 x 105 [s-1] (at 300 protons from RCS k. W) present RCS Pμ = 40 Me. V/c D-line exp. area H 2 cryostat detectors Laser cabin 19

Resonance hunting p scanning scheme Parameters for estimation negative muon 5 x 105 [s-1] (at 300 k. W) Pμ = 40 Me. V/c dp/p = ± 10 % laser ( ~44 THz) Power 40 m. J repetition 25 Hz bandwidth 50 Hz mirror R 99. 95 % H 2 target density 0. 1 % of LHD Interval : 100 MHz range : ± 5. 7 GHz (~ d. Zemach + d pol ) resonance search : ~22 days frequency determination : ~5 days expected spectrum 20

Summary p New measurement of proton Zeamch radius and ground-state hyperfine splitting energy in muonic hydrogen by spectroscopy with a mid-IR laser p Accuracy of DEHFS : ~ 2 ppm and RZ : ~ 1% (We need help from theory for further precision) p Experiment is feasible in J-PARC MLF muon facility ~ 1 month beam time (with present RCS power of 300 k. W) 21

Collaboration 22