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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

2 MENU2013

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

6 MENU2013 A. Gasparian

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

8 MENU2013

A simple demonstration in Quantum Mechanics

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9 MENU2013

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


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


Spectroscopic Transition Measurements

(Lamb Shifts in Hydrogen Atom)


New PSI Results for μD Atom (Recently Published in Science Journal, 2013)

13 MENU2013

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).


Open Questions and Potential Solutions

15 MENU2013

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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16 MENU2013

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

17 MENU2013

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


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.


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


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

24 MENU2013

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

A. Gasparian

This project is supported in part by the NSF MRI award: PHY-1229153

and NSF research award: PHY-1205962

Thank You!


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


27 MENU2013

Electromagnetic Calorimeter (HyCal)

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