Proton radius puzzle Jan C. Bernauer COFI workshop - San Juan - May 2018 100 years of protons! Proton is a composite system. It must have a size! How big is it? What is ”stuff”? The matter around us is described by non-perturbative quantum chromodynamics. npQCD is hard. Simplest QCD system to study: Protons 2 Proton is a composite system. It must have a size! How big is it? What is ”stuff”? The matter around us is described by non-perturbative quantum chromodynamics. npQCD is hard. Simplest QCD system to study: Protons 100 years of protons! 3 What is ”stuff”? The matter around us is described by non-perturbative quantum chromodynamics. npQCD is hard. Simplest QCD system to study: Protons 100 years of protons! Proton is a composite system. It must have a size! How big is it? 4 Two transitions for two unknowns: Rydberg constant R 1S Lamb shift = radius1 ) Direct Lamb shift 2S 2P ! Motivation: ”Normal” Hydrogen Spectroscopy 8S 4S 3S 3D 2S 2P R1 L1S EnS u 2 + 3 − n n 2 1S L1S = 8171:626(4) + 1:5645 rp MHz 5 Direct Lamb shift 2S 2P ! Motivation: ”Normal” Hydrogen Spectroscopy 8S 4S 3S 3D 2S 8S 2S 8D ! ! 2S 2P R1 L1S EnS u 2 + 3 − n n Two transitions for two unknowns: Rydberg constant R 1 1S 2S 1S Lamb shift = radius ! ) 2 1S L1S = 8171:626(4) + 1:5645 rp MHz 6 Motivation: ”Normal” Hydrogen Spectroscopy 8S 4S 3S 3D 2S 2P 2S ! 2P R1 L1S EnS u 2 + 3 − n n Two transitions for two unknowns: Rydberg constant R 1S Lamb shift = radius1 ) Direct Lamb shift 2S 2P ! 2 1S L1S = 8171:626(4) + 1:5645 rp MHz 7 ”Normal” Hydrogen Spectroscopy Results 8 Elastic lepton-proton scattering Method of choice: Lepton-proton scattering Point-like probe No strong force Lepton interaction ”straight-forward” Measure cross sections and reconstruct form factors. 9 Cross section for elastic scattering dσ dΩ 1 = "G2 Q2 + τG2 Q2 dσ " (1 + τ) E M dΩ Mott h i with: 2 1 Q θ − τ = ;" = 1 + 2 (1 + τ) tan2 e 4m2 2 p Rosenbluth formula Electric and magnetic form factor encode the shape of the proton Fourier transform (almost) gives the spatial distribution, in the Breit frame 10 How to measure the proton radius dG d (G /µ ) r2 6 2 E r2 6 ² M p E = ~ 2 M = ~ 2 − dQ 2 − dQ 2 Q =0 Q =0 D E D E 1 0.995 . 0.99 dip . 0.985 std 0.98 /G 0.975 E dGE G 0.97 dQ2 → Q2=0 0.965 0.96 0 0.05 0.1 0.15 0.2 0.25 0.3 Q2 [(GeV/c)2] 11 History of unpolarized electron-proton scattering 100 10 2 ) /c 1 (GeV 0.1 / 2 Q 0.01 0.001 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year Andivahis Borkowski Janssens Rock Walker Bartel Bosted Litt Sill Berger Christy Price Simon Bernauer Goitein Qattan Stein 12 Mainz Microtron cw electron beam 10 µA polarized, 100 µA unpolarized MAMI A+B: 180-855 MeV MAMI C: 1.6 GeV A1 3-spectrometer facility 28 msr acceptance angle resolution: 3 mrad 4 momentum res.: 10− High-precision p(e,e’)p measurement at MAMI 13 A1 3-spectrometer facility 28 msr acceptance angle resolution: 3 mrad 4 momentum res.: 10− High-precision p(e,e’)p measurement at MAMI Mainz Microtron cw electron beam 10 µA polarized, 100 µA unpolarized MAMI A+B: 180-855 MeV MAMI C: 1.6 GeV 14 High-precision p(e,e’)p measurement at MAMI Mainz Microtron cw electron beam 10 µA polarized, 100 µA unpolarized MAMI A+B: 180-855 MeV MAMI C: 1.6 GeV A1 3-spectrometer facility 28 msr acceptance angle resolution: 3 mrad 4 momentum res.: 10− 15 Measured settings A 1 B C 0.8 ] 2 ) /c 0.6 (GeV [ 2 Q 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 1422 settings JCB et al., Phys. Rev. Lett. 105 (2010) 242001, M. O. Distler, JCB, Th. Walcher, Phys. Lett. B 696, 343 (2011) JCB et al., Phys. Rev. C90 (2014) 015206 16 Cross sections 100 100 10 10 1 ] ] 1 µb µb [ [ 0.1 exp exp σ σ 0.1 180 0.01 MeV 315 585 MeV MeV 0.01 720 450 0.001 MeV MeV 855 MeV 0.001 0.0001 0◦ 20◦ 40◦ 60◦ 80◦ 100◦ 120◦ 140◦ 0◦ 20◦ 40◦ 60◦ 80◦ 100◦ 120◦ 140◦ Central angle of spectrometer Central angle of spectrometer Polynomial Inv. poly. Friedrich-Walcher Poly. + dip Spline Double dipole Poly. dip Spline dip Extended G.K. × × 17 Muonic Hydrogen Spectroscopy Replace electron with muon 200 times heavier = 200 times smaller orbit ) Probability to be ”inside” 2003 higher! 18 PSI setup (CREMA) 19 Result Two semi-independent measurements Consistent results Muonic Hydrogen Spectroscopy Results 20 Muonic Hydrogen Spectroscopy Results Result Two semi-independent measurements Consistent results 21 From the 2017 Review of Particle Physics Until the difference between the e p and µ p values is understood, it does not make sense to average the values together. For the present, we give both values. It is up to the workers in this field to solve this puzzle. The proton radius puzzle 7.9 σ µp 2013 electron avg. scatt. JLab µp 2010 scatt. Mainz H spectroscopy 0.83 0.84 0.85 0.86 0.87 0.88 0.89 0.9 Proton charge radius R [fm] ch 22 The proton radius puzzle From the 2017 Review of Particle Physics Until the difference between the e p and µ p values is understood, it does not make sense to average the values together. For the present, we give both values. It is up to the workers in this field to solve this puzzle. 23 seems solid ep experiments wrong? both scattering and H-spectroscopy wrong? Theory wrong? checked thoroughly ...but maybe framework is wrong? Everybody is right? New physics! ”Naive” dark matter models essentially excluded But can play cancelation games E.g.: Electrophobic force (Liu, Cloët, Miller arXiv:1805.01028) WE NEED MORE DATA Solutions? µp experiment wrong? 24 Theory wrong? checked thoroughly ...but maybe framework is wrong? Everybody is right? New physics! ”Naive” dark matter models essentially excluded But can play cancelation games E.g.: Electrophobic force (Liu, Cloët, Miller arXiv:1805.01028) WE NEED MORE DATA Solutions? µp experiment wrong? seems solid ep experiments wrong? both scattering and H-spectroscopy wrong? 25 Everybody is right? New physics! ”Naive” dark matter models essentially excluded But can play cancelation games E.g.: Electrophobic force (Liu, Cloët, Miller arXiv:1805.01028) WE NEED MORE DATA Solutions? µp experiment wrong? seems solid ep experiments wrong? both scattering and H-spectroscopy wrong? Theory wrong? checked thoroughly ...but maybe framework is wrong? 26 WE NEED MORE DATA Solutions? µp experiment wrong? seems solid ep experiments wrong? both scattering and H-spectroscopy wrong? Theory wrong? checked thoroughly ...but maybe framework is wrong? Everybody is right? New physics! ”Naive” dark matter models essentially excluded But can play cancelation games E.g.: Electrophobic force (Liu, Cloët, Miller arXiv:1805.01028) 27 Solutions? µp experiment wrong? seems solid ep experiments wrong? both scattering and H-spectroscopy wrong? Theory wrong? checked thoroughly ...but maybe framework is wrong? Everybody is right? New physics! ”Naive” dark matter models essentially excluded But can play cancelation games E.g.: Electrophobic force (Liu, Cloët, Miller arXiv:1805.01028) WE NEED MORE DATA 28 Deuteron (arxiv:1607.03165) In CODATA, rd is correlated strongly to rp because it uses ”isotope shift” from 1s-2s in both systems. But: Can built independent value from deuteron 1s-2s and 2s-8s/8d/12d. This gives a difference to muonic deuterium! another puzzle. ! Rydberg from electric Hydrogen and Deuterium in perfect agreement. The muonic deuterium R is in slight disagreement with the muonic hydrogen1 R (<3 sigma) 1 29 New hydrogen results: MPQ (A. Beyer et al., Science 358, 79 (2017)) 30 New results: Paris (Fleurbaey et al., Phys. Rev. Lett. 120, 183001 (2018)) 31 Timeline of proton radius results 0.92 0.9 0.88 0.86 [fm] 0.84 > E 0.82 0.8 < r 0.78 0.76 0.74 Paz ’10 Sick ’03 Pohl ’10 Zhan ’11 Hand ’63 Wong ’94 Udem ’97 Simon ’80 Lorenz ’12 Mergell ’96 Akimov ’72 McCord ’91 Blunden ’05 Eschrich ’01 Melnikov ’00 Bernauer ’10 CODATA ’06 CODATA ’02 Belushkin ’06 Antognini ’13 Borkowski ’75 Rosenfelder ’00 Frerejacque ’66 32 Comments on some newer scattering results 2010: >0.870 Hill, Paz: old data, z expansion with disp. bounds Bounds on infinite exp. bounds for truncated exp.? ! 2012: 0.840(10) Lorenz, Hammer, Meissner: Disp. relation fit. Same value but a lot more data. Probably model dominated. 2014: 0.84 Lorenz, Meissner: z expansion without bounds Fit did not converge. In real minimum, large radius is found. 2014: 0.8989(1) Gracyk/Juszczak: Bayesian estimation Interesting technique, unbelievable? small errors 2016: 0.84? Higinbotham: F-Test to select max. order Misunderstood F-test. Absence of proof = proof of absence. 6 2016: 0.84? Horbatsch/Hessels/Griffioen/Carlson/Maddox... Low-Q Low-Q fits with low order don’t work. 2018: XXX Yan/Higinbotham/... Small radius fraction finally does bias testing 33 Volume of Mainz data set 7 2 6 σ σ ∆ 5 P q 4 3 2 Relative statistical weight 1 0 All others Mainz Mainz data will dominate any fit. Need similar data set to validate! 34 Extrapolation to Q2 = 0 Have to extrapolate form factor to Q2 = 0.