The Proton Radius Puzzleand the PRad experiment at JLab

A. GasparianNC 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

The PRad experiment at JLab

The Proton Charge Radius: the Current Status

2MENU2013

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 resultR. Pohl et al., Nature 466, 213 (2010).

A. Gasparian

The Proton Charge Radius Puzzle

3

Recent muonic deuterium experiment at PSIA. Antognini et al., Science 339, 417 (2013).

MENU2013A. Gasparian

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 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):

4

GE ,GM

MENU2013

The Taylor expansion at low Q2:

A. Gasparian

J. Bernauer, PRL 105,242001, 2010

5

Recent Mainz ep-Experiment (2010)

MENU2013

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)

A. Gasparian

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

6MENU2013A. Gasparian

Spectroscopic Transition Measurements in Hydrogen Atom

7MENU2013

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

A. Gasparian

Proton Size and Hydrogen Energy Spectrum

8MENU2013

A simple demonstration in Quantum Mechanics

A. Gasparian

9MENU2013

Proton Radius Extracted From eH Spectroscopy

New muonic-hdrogen resultR. Pohl et al., Nature 466, 213 (2010).

A. Gasparian

Proton Radius from the Muonic-Hydrogen

10MENU2013A. Gasparian

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

11MENU2013A. Gasparian

Spectroscopic Transition Measurements(Lamb Shifts in Hydrogen Atom)

12MENU2013A. Gasparian

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

13MENU2013

A.Antognini et al., Science 339, 417 (2013)

A. Gasparian

The Proton Charge Radius Puzzle Again

14

Recent muonic deuterium experiment at PSIA. Antognini et al., Science 339, 417 (2013).

MENU2013A. Gasparian

Open Questions and Potential Solutions

15MENU2013

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 !!!A. Gasparian

16MENU2013

Can the Data Quality from eH-Spectroscopy be the Solution?muonic-hdrogen (deuterium) results

A. Gasparian

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

17MENU2013

J. Bernauer, PRL 105,242001, 2010

A. Gasparian

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

18

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

MENU2013

Mainz low Q2 data set

A. Gasparian

Proposed PRad Experimental Setup in Hall B at JLab

HyCal

19MENU2013

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

A. Gasparian

Windowless H2 Gas Flow Target

20MENU2013A. Gasparian

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.

21MENU2013A. Gasparian

Extraction of the Proton Charge Radius

Estimated systematic uncertainty (with radiative corrections) < 0.3% Estimated total error in rp extraction ~ 0.6%

22MENU2013

Extraction of rp from MC pseudo-data with and without radiation (single parameter fit)

A. Gasparian

Expected Result from PRad Experiment

23MENU2013A. Gasparian

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

24MENU2013

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

Thank You!

25MENU2013A. Gasparian

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)

26

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

MENU2013A. Gasparian

27MENU2013

Electromagnetic Calorimeter (HyCal)

A. Gasparian

Overlap of Ee' spectra of radiated events Calorimeter detects a good part of the hard radiated photons

28

Separation of ep-Elastic from Moller Events

MENU2013A. Gasparian