The Proton Radius Puzzle and the PRad experiment at JLab
A. Gasparian NC A&T State University, Greensboro, NC USA
Outline The puzzle
Methods of radius measurements
ep-elastic scattering
hydrogen spectroscopy
muonic hydrogen spectroscopy
The PRad experiment at JLab
for the PRad collaboration (JLab experiment E12-11-106)
Collaborating Institutions: Jefferson Lab, NC A&T State University, Duke University, Idaho State University,
Mississippi State University, Norfolk State University, University of North Carolina at Wilmington,
Old Dominion University, University of Kentucky, College of William & Mary, Hampton University USA, ITEP, Moscow, Russia
The Proton Charge Radius: the Current Status
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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-hydrogen result
R. Pohl et al., Nature 466, 213 (2010).
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The Proton Charge Radius Puzzle
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Recent muonic deuterium experiment at PSI
A. Antognini et al., Science 339, 417 (2013).
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In the limit of first Born approximation the elastic ep scattering
(one photon exchange): e- e-
p p Structure less proton:
GE and GM were extracted using Rosenbluth
separation (or at extremely low Q2 the GM can be
ignored, like in the PRad experiment) Definition of the Proton Radius:
(r.m.s. charge radius given by the slope):
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GE ,GM
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The Taylor expansion at low Q2:
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J. Bernauer, PRL 105,242001, 2010
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Recent Mainz ep-Experiment (2010)
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Q2 = [0.004 – 1.0] (GeV/c)2 range
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→ep scattering;
Consistent with CODATA06 value: (rp=0.8768(69) fm)
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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
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Spectroscopic Transition Measurements in Hydrogen Atom
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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
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Proton Size and Hydrogen Energy Spectrum
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A simple demonstration in Quantum Mechanics
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Proton Radius Extracted From eH Spectroscopy
New muonic-hydrogen result
R. Pohl et al., Nature 466, 213 (2010).
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Proton Radius from Muonic-Hydrogen
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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 analyses
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Spectroscopic Transition Measurements
(Lamb Shifts in Hydrogen Atom)
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New PSI Results for μD Atom (Recently Published in Science Journal, 2013)
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A.Antognini et al., Science 339, 417 (2013)
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The Proton Charge Radius Puzzle Again
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Recent muonic deuterium experiment at PSI
A. Antognini et al., Science 339, 417 (2013).
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Open Questions and Potential Solutions
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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 !!!
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Can the Data Quality from eH-Spectroscopy be the Solution?
muonic-hydrogen (deuterium) results
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May be, but the ep-scattering avarage is still at 0.879 fm level
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
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J. Bernauer, PRL 105,242001, 2010
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The Proposed New Experiment at JLab (PRad, E12-11-106)
Two beam energies: E0 = 1.1 GeV and 2.2 GeV to increase Q2 range
Will reach sub-percent precision in rp extraction (~ 0.5% total)
Approved by PAC39 (June, 2012) with high “A” scientific rating
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Experimental goals: reach very low Q2 range (~ 10 times less
than the Mainz experiment)
reach 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
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Mainz low Q2 data set
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Proposed PRad Experimental Setup in Hall B at JLab
HyCal
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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
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Windowless H2 Gas Flow Target
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Control of Systematic Errors
Major improvements over previous experiments:
1) Simultaneous detection of two processes
ep → ep
ee → ee Moller scattering Tight control of systematic errors
2) Windowless H2 gas target Low beam background
3) Very low Q2 range: [2x10-4 – 2x10-2] (GeV/c)2 Model 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.
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Extraction of the Proton Charge Radius
Estimated systematic uncertainty (with radiative corrections) < 0.3%
Estimated total error in rp extraction ~ 0.6%
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Extraction of rp from MC pseudo-data with and without radiation (single parameter fit)
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Expected Result from PRad Experiment
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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 magnetic-spectrometer-free ep-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
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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
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This project is supported in part by the NSF MRI award: PHY-1229153
and NSF research award: PHY-1205962
Thank You!
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Estimated Errors
Contributions Estimated Error (%)
Statistical error 0.2
Acceptance (including Q2
determination)
0.4
Detection efficiency 0.1
Radiative corrections 0.3
Background and PID 0.1
Fitting error 0.2
Total Error 0.6%
Estimated error budget (added quadratically)
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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
High rates will provide good statistical
errors (~0.2% for all Q2 bins)
Extraction of proton charge radius was
always limited by systematics and fitting
uncertainties
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Electromagnetic Calorimeter (HyCal)
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