Transcript The PrimEx Project at Jefferson Lab (a short overview)

The Proton Radius Puzzle
and the PRad experiment at JLab
A. Gasparian
NC A&T State University, Greensboro, NC USA
(for the PRad collaboration)
Outline
 The puzzle
 Methods of radius measurements
ep-elastic scattering
 hydrogen spectroscopy
 muonic hydrogen spectroscopy
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 The PRad experiment at JLab
The Proton Charge Radius: the Current Status
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Proton radius is one of the most fundamental quantities
in physics:
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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 muonichydrogen measurements and all previous results
The Proton Radius Puzzle
New muonic-hdrogen result
R. Pohl et al., Nature 466, 213 (2010).
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The Proton Charge Radius Puzzle
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-
GE ,GM
p
 Structure less proton:
p
 GE and GM were extracted using the Rosenbluth
separations (or at extreme low Q2 the GM can be
ignored, like in the PRad experiment)
(m.s. charge radius given by the slope):
 The Taylor expansion at low Q2:
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 Definition of the Proton Radius:
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Recent Mainz ep-Experiment (2010)
J. Bernauer, PRL 105,242001, 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
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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
A simple demonstration in Quantum Mechanics
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Proton Radius Extracted From eH Spectroscopy
New muonic-hdrogen result
R. Pohl et al., Nature 466, 213 (2010).
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Proton Radius from the Muonic-Hydrogen
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New Results from Muonic Hydrogen Experiments (2010, 2013)
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Muonic hydrogen Lamb shift experiment at PSI
rp = 0.84184(67) fm
Unprecedented less than 0.1% precision
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 Different from most of previous experimental results and analysis
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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)
A.Antognini et al., Science 339, 417 (2013)
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The Proton Charge Radius Puzzle Again
Recent muonic deuterium experiment at PSI
A. Antognini et al., Science 339, 417 (2013).
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Open Questions and Potential Solutions
 Potential solutions:
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Need new high precision and high accuracy experiments:
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ep-scattering experiments:
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new experiments at York University, Canada and Paris, and more new projects
Check lepton universality:
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PRad experiment at JLab
ordinary hydrogen spectroscopy
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reaching extremely low Q2 range (10-4 Gev/c2)
possibly with new independent methods
measure absolute cross sections
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-hdrogen (deuterium) results
May be, but the ep-scattering avarage is still at 0.879 fm level
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Designing a New ep-Scattering Experiment
(Difficulties of Previous Experiments with Standard Magnetic Spectrometers)
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Suggested solutions by PRad experiment at JLab:
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J. Bernauer, PRL 105,242001, 2010
Non-magnetic-spectrometer method !
No target windows !
Calibrate with other well-known QED processes
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The Proposed New Experiment at JLab (PRad, E12-11-106)
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Experimental goals:
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reach to very low Q2 range (~ 10 times less
than the Mainz experiment)
reach to sub-percent precision in rp extraction
Suggested solutions:
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Non-magnetic-spectrometer method:
use high resolution high acceptance crystal calorimeter
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Simultaneous detection of ee → ee Moller scattering
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(best known control of systematics)
Mainz low Q2 data set
Use high density windowless H2 gas flow target:
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reach smaller scattering angles: (Θ = 0.70 – 3.80 )
(Q2 = 2x10-4 – 2x10-2 ) GeV/c2
essentially, model independent rp extraction
beam background fully under control with high quality CEBAF beam
minimize experimental background
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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Proposed PRad Experimental Setup in Hall B at JLab
HyCal
 High resolution, large acceptance HyCal calorimeter
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(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
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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
 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%
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Expected Result from PRad Experiment
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Summary and Outlook
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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
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New high accuracy experiments are critically needed to address this puzzle:
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ep-scattering experiments with new independent methods
ordinary hydrogen spectroscopy
experiments to check lepton universality in SM
New magnetic-spectrometer-free ep-scattering experiment at JLab (PRad, E12-11-106)
with tight control of systematic errors:
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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:
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preparation of experimental setup:
experiment ready to run in Hall B at JLab:
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2013-14
Fall, 2014
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Thank You!
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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:
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ep → ep
ee → ee Moller scattering
 and windowless H2 gas target
 will significantly reduce major systematic
errors typical for all previous ep-scattering
experiments
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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%
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Estimated error budget (added quadratically)
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Electromagnetic Calorimeter (HyCal)
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Separation of ep-Elastic from Moller Events
 Overlap of Ee' spectra of radiated events
 Calorimeter detects a good part of the hard radiated photons
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