The Proton Radius Puzzle and the PRad experiment at JLab

1. The Proton Radius Puzzleand the PRad experiment at JLab Outline • The puzzle • Methods of radius measurements • ep-elastic scattering • hydrogen spectroscopy • muonic hydrogen spectroscopy • The PRad experiment at JLab A. Gasparian NC A&T State University, Greensboro, NC USA (for the PRad collaboration)

2. The Proton Charge Radius: the Current Status • Proton radius is one of the most fundamental quantities in physics: • critically important for atomic physics in precision spectroscopy of atom (Rydberg constant) • precision test of nuclear/particle models • connects atomic and subatomic physics • ~ 8 σ discrepancy between the new muonic- hydrogen measurements and all previous results The Proton Radius Puzzle New muonic-hdrogen result R. Pohl et al., Nature 466, 213 (2010). MENU2013

3. The Proton Charge Radius Puzzle Recent muonic deuterium experiment at PSI A. Antognini et al., Science 339, 417 (2013). MENU2013

4. In the limit of first Born approximation the elastic ep scattering (one photon exchange): e- e- GE ,GM p p • Structure less proton: • GE and GM were extracted using the Rosenbluth separations (or at extreme low Q2 the GM can be ignored, like in the PRad experiment) • Definition of the Proton Radius: (m.s. charge radius given by the slope): • The Taylor expansion at low Q2: MENU2013

5. Recent Mainz ep-Experiment (2010) J. Bernauer, PRL 105,242001, 2010 • Q2 = [0.004 – 1.0] (GeV/c)2range • Large amount of overlapping data sets (~1400) • Statistical error ≤ 0.2% • Luminosity monitoring with spectrometer • Additional beam current measurements rp=0.879(5)stat(4)sys(2)mod(4)group • Confirms the previous results from ep→epscattering; • Consistent with CODATA06 value: (rp=0.8768(69) fm) MENU2013

6. Proton Radius Extracted From e-p Scattering Experiments • More different analysis results than actual experiments • Started with: rp ≈ 0.81 fm in 1963 • Reached to: rp ≈ 0.88 fm by 2011 MENU2013

7. Spectroscopic Transition Measurements in Hydrogen Atom • The Lamb shift: effect of quantization of EM field (polarization of physical vacuum) sensitive to proton size! • Hyperfine structure, interaction of e- and p magnetic dipole moments MENU2013

8. Proton Size and Hydrogen Energy Spectrum A simple demonstration in Quantum Mechanics MENU2013

9. Proton Radius Extracted From eH Spectroscopy New muonic-hdrogen result R. Pohl et al., Nature 466, 213 (2010). MENU2013

10. Proton Radius from the Muonic-Hydrogen MENU2013

11. New Results from Muonic Hydrogen Experiments (2010, 2013) • Muonic hydrogen Lamb shift experiment at PSI • rp = 0.84184(67) fm Unprecedented less than 0.1% precision • Different from most of previous experimental results and analysis MENU2013

12. Spectroscopic Transition Measurements(Lamb Shifts in Hydrogen Atom) MENU2013

13. New PSI Results for μD Atom(Recently Published in Science Journal, 2013) A.Antognini et al., Science 339, 417 (2013) MENU2013

14. The Proton Charge Radius Puzzle Again Recent muonic deuterium experiment at PSI A. Antognini et al., Science 339, 417 (2013). MENU2013

15. Open Questions and Potential Solutions • Potential solutions: • Need new high precision and high accuracy experiments: • ep-scattering experiments: • reaching extremely low Q2 range (10-4 Gev/c2) • possibly with new independent methods PRad experiment at JLab • measure absolute cross sections • ordinary hydrogen spectroscopy • new experiments at York University, Canada and Paris, and more new projects • Check lepton universality: • e-p to μp ratio experiment at PSI (MUSE) • Search in K-decays (KEK project) • Possible new Physics beyond the Standard Model !!! MENU2013

16. Can the Data Quality from eH-Spectroscopy be the Solution? muonic-hdrogen (deuterium) results May be, but the ep-scattering avarage is still at 0.879 fm level MENU2013

17. Designing a New ep-Scattering Experiment(Difficulties of Previous Experiments with Standard Magnetic Spectrometers) • Suggested solutions by PRad experiment at JLab: • Non-magnetic-spectrometer method ! • No target windows ! • Calibrate with other well-known QED processes J. Bernauer, PRL 105,242001, 2010 MENU2013

18. The Proposed New Experiment at JLab (PRad, E12-11-106) • Experimental goals: • reach to very low Q2 range (~ 10 times less • than the Mainz experiment) • reach to sub-percent precision in rp extraction • Suggested solutions: • Non-magnetic-spectrometer method: • use high resolution high acceptance crystal calorimeter • reach smaller scattering angles: (Θ = 0.70 – 3.80 ) • (Q2 = 2x10-4 – 2x10-2 ) GeV/c2 • essentially, model independent rp extraction • Simultaneous detection of ee → ee Moller scattering • (best known control of systematics) • Use high density windowless H2 gas flow target: • beam background fully under control with high quality CEBAF beam • minimize experimental background Mainz low Q2 data set • Two beam energies: E0 = 1.1 GeV and 2.2 GeV to increase Q2 range • Will reach sub-percent precision in rpextraction (~ 0.5% total) • Approved by PAC39 (June, 2012) with high “A” scientific rating MENU2013

19. Proposed PRad Experimental Setup in Hall B at JLab HyCal • High resolution, large acceptance HyCal calorimeter (including PbWO4 crystals) • Windowless H2 gas flow target • XY – veto counters • Vacuum box, one thin window at HyCal only MENU2013

20. Windowless H2 Gas Flow Target MENU2013

21. Control of Systematic Errors • Major improvements over previous experiments: • Simultaneous detection of two processes • ep → ep • ee → ee Moller scattering Tight control of systematic errors • Windowless H2 gas target Low beam background • Very low Q2 range: [2x10-4 – 2x10-2] (GeV/c)2Model independent rp extraction • Extracted yield for ep → ep • … and for ee → ee, Moller • Then, ep cross section is related to Moller: • Two major sources of systematic errors, Ne and Ntgt, typical for all previous experiments, cancel out. • Moller scattering will be detected in coincident mode inside the HyCal acceptance. MENU2013

22. Extraction of the Proton Charge Radius • Extraction of rp from MC pseudo-data with and without radiation (single parameter fit) • Estimated systematic uncertainty (with radiative corrections) < 0.3% • Estimated total error in rp extraction ~ 0.6% MENU2013

23. Expected Result from PRad Experiment MENU2013

24. Summary and Outlook • The “Proton Radius Puzzle” is still with us after more than three years! • All theory corrections failed to explain the current ~ 4.5% (~ 8 σ) difference in rp so far • New high accuracy experiments are critically needed to address this puzzle: • ep-scattering experiments with new independent methods • ordinary hydrogen spectroscopy • experiments to check lepton universality in SM • New magnetic-spectrometer-freeep-scattering experiment at JLab (PRad, E12-11-106) with tight control of systematic errors: • reach very low Q2 range for the first time: [2x10-4 – 2x10-2] GeV2 • ep→ep cross sections normalized to Moller scattering • windowless hydrogen gas flow target to control the experimental backgrounds • PRad expected timeline: • preparation of experimental setup: 2013-14 • experiment ready to run in Hall B at JLab: Fall, 2014 MENU2013

25. Thank You! MENU2013

26. Estimated Errors • Extraction of proton charge radius was always limited by systematics and fitting uncertainties • High rates will provide good statistical errors (~0.2% for all Q2 bins) • Simultaneous detection of two processes: • ep → ep • ee → ee Moller scattering • and windowless H2 gas target • will significantly reduce major systematic • errors typical for all previous ep-scattering • experiments • Estimated error budget (added quadratically) MENU2013

27. Electromagnetic Calorimeter (HyCal) MENU2013

28. Separation of ep-Elastic from Moller Events • Overlap of Ee' spectra of radiated events • Calorimeter detects a good part of the hard radiated photons MENU2013