1. Measurement of the Proton Radius
Prospects for a New
Measurement of the Proton Radius
PRAE
Eric Voutier
Institut de Physique Nucléaire d’Orsay
Orsay, France
Proton radius puzzle
Experimental perspectives
PRAE
Summary
Colloque France-Ukraine 2015
Orsay, October 15th-16th, 2015

2. and electron scattering experiments.
Eric Voutier
Proton radius puzzle
Eric Voutier
The Dilemma
Courtesy of M. Distler 7s
The proton radius extracted from muonic hydrogen spectroscopy differs from the one extracted from electronic hydrogen spectroscopy
and electron scattering experiments.
Orsay, October 15th-16th, 2015
2/15
XXVth International Workshop on Nuclear Theory

3. Lamb Shift Measurement
Eric Voutier
Proton radius puzzle
Eric Voutier
Lamb Shift Measurement
R. Pohl et al., Nature 466 (2010) A. Antognini et al., Science 339 (2013) 417
Measurement of the 2S  2P Lamb shift transition in muonic hydrogen.
Confirmed by new data on both the 2S  2P Lamb shift and the hyperfine splitting
rp = (67) fm
rp = (39) fm
Orsay, October 15th-16th, 2015
3/15
XXVth International Workshop on Nuclear Theory

4. Elastic Electron Scattering
Eric Voutier
Proton radius puzzle
Eric Voutier
Elastic Electron Scattering
J.C. Bernauer et al., PRL 105 (2010) ; PRC 90 (2014)
Measurement of ep elastic at different beam energies covering the
(GeV/c2)2 momentum range with
1400 cross-section data points with uncertainties about 0.4%.
Global fitting analysis of all the data points studying different fit function effects
rp = 0.879(8) fm
rm = 0.777(17) fm
Orsay, October 15th-16th, 2015
4/15
XXVth International Workshop on Nuclear Theory

5. Need for more data !! Explanations ? Proton radius puzzle
Eric Voutier
Proton radius puzzle
Eric Voutier
Explanations ?
Experimental issues
Underestimated uncertainties, bad radius extraction…
New physics beyond the Standard Model
Lepton non-universality, new force/particle (dark photon?)…
Novel hadronic physics
Proton polarizability correction (4th power of the lepton mass) leading to enhance
2g exchange contributions for µp …
There is no concensus in the Community for explaining that disagreement.
Need for more data !!
Orsay, October 15th-16th, 2015
5/15
XXVth International Workshop on Nuclear Theory

6. MUSE @ PSI Projected Data Experimental perspectives
Eric Voutier
Experimental perspectives
Eric Voutier
PSI
MUon Scattering Experiment
(MUSE Collaboration) R. Gilman, E.J. Downie, G. Ron et al.
The MUSE experiment intends to measure mp and ep elastic scattering in the Q2 range (GeV/c2)2 for both + and – lepton beams at 115, 158, and 210 MeV/c.
TOF for PID and momentum measurements
Tracking before and after the target
Beam stability monitoring
Test of electron-muon universality
and of two-photon exchange effects
Projected
Data
Expected to run in 2017 and 2018
Orsay, October 15th-16th, 2015
6/15
XXVth International Workshop on Nuclear Theory

7. Initial State Radiation
Eric Voutier
Experimental perspectives
Eric Voutier
MAMI
Initial State Radiation
ISR
FSR
Reaching low Q2 in the Bethe-Heitler reaction by tagging beam electrons radiating energetic g.
Orsay, October 15th-16th, 2015
7/15
XXVth International Workshop on Nuclear Theory

8. Experimental perspectives
Eric Voutier
Experimental perspectives
Eric Voutier
T. Mihovilovič et al.
E=495 MeV
E=330 MeV
FSR
E=195 MeV
ISR
Measurement of the momentum spectrum of scattered electrons
Requires very accurate understanding of QED radiative corrections and FSR (beyond the 2g exchange)
Access 2×10-4 (GeV/c2)2
Orsay, October 15th-16th, 2015
8/15
XXVth International Workshop on Nuclear Theory

9. MesA Gas Internal target eXperiment
Eric Voutier
Experimental perspectives
Eric Voutier
MESA
MesA Gas Internal target eXperiment
A. Denig et al.
The Mainz Energy recovering Superconducting Accelerator is a new machine under construction (2017) at the University of Mainz for high luminosity polarized experiment.
Beam Mode
I (mA)
E (MeV)
L (cm-2s-1)
External
0.15
200
1039
Internal (ERL)
10.0
150
1036
1 mA
ERL beam
Operation of a high intensity electron beam with an internal gas target.
MESA
projection
dp/p < 10-4
DW ~ ±50 mrad
MESA layout
Orsay, October 15th-16th, 2015
9/15
XXVth International Workshop on Nuclear Theory

10. Proton Radius experiment
Eric Voutier
Experimental perspectives
Eric Voutier
Jlab
Proton Radius experiment
(E Experiment) D. Dutta, H. Gao, A. Gasparian, M. Khandaher et al.
The PRad experiment aims at the simultaneous measurements (~2017) of
the proton elastic and Møller in the angular range ° corresponding to the (GeV/c2)2 four-momentum range.
Two beam energies (1.1 and 2.2 GeV) to cover the planned Q2-range leading to a 0.6% determination
of the proton charge radius.
PRad
projection
Window less gas target
High resolution and large acceptance calorimeter
High control of corrections and systematics effects
Orsay, October 15th-16th, 2015
10/15
XXVth International Workshop on Nuclear Theory

11. S. Barsuk, A. Faus-Golfe, B. Genolini, Y. Prezado, E. Voutier et al.
Eric Voutier
PRAE
Eric Voutier
Physics Goals
S. Barsuk, A. Faus-Golfe, B. Genolini, Y. Prezado, E. Voutier et al.
The ProRad experiment at PRAE is a project for a measurement of the electric form factor of the proton at very low Q2 at the projected pluri-disciplinary PRAE facility in Orsay.
Discrete coverage of the 5-50 angular range
Measurements at three different beam energies
Use a solid hydrogen target
Simultaneous measurements of the elastic and Møller cross sections
2×10-5 (GeV/c2)2
Orsay, October 15th-16th, 2015
11/15
XXVth International Workshop on Nuclear Theory

12. Eric Voutier
PRAE
Eric Voutier
CHyMENE
G. Authelet, D. Beaumel, J.-M. Gheller, A. Gillibert, F.-P. Juster, A. Obertelli, J. Relland, I. Vinyar et al.
The CHyMENE project aims to provide the nuclear physics community a thin and pure windowless hydrogen or deuterium target.
Target µm thick per 10 mm wide
Cryogenic power: 15 W at 11 K
Speed of extruder: 10 mm/s
Vacuum reaction chamber: 5×10-5 mbar
Target under development…
Maximum sustainable beam current ? Target homogeneity ?
...
Orsay, October 15th-16th, 2015
12/15
XXVth International Workshop on Nuclear Theory

13. Detector Constraints & Concept
Eric Voutier
PRAE
Eric Voutier
Detector Constraints & Concept
Measurement of Møller scattering provides absolute normalisation of the cross section and monitoring of the luminosity.
Møller scattering above 10° overwhelms elastic scattering while producing low energy electrons.
Orsay, October 15th-16th, 2015
13/15
XXVth International Workshop on Nuclear Theory

14. Scintillating/Čerenkov
Eric Voutier
PRAE
Eric Voutier
Scintillating/Čerenkov
crystal
Mechanically defined aperture mm
thick scintillator
The accuracy of the measurement relies on a mechanical quality defined aperture selecting the scattering angle.
The full detector is built from several elementary detectors covering one specific scattering angle.
Each elementary detection cell have energy, timing, off-line neutral background rejection, and in-situ efficiency measurement capabilities.
In absence of magnetic field, the energy dynamics at small angles allows to separate Møller and elastic scatterings in the same detector.
Additional absorbers at large angle shields detectors from Møller illumination.
In presence of magnetic field, additional masks allows to stop low energy Møller and separate elastic from high energy Møller measured in distinct detectors.
Work in progress…
Orsay, October 15th-16th, 2015
14/15
XXVth International Workshop on Nuclear Theory

15. Lot of issues to address… Construction and performances of PRAE
Eric Voutier
Summary
Eric Voutier
Status
The idea of the measurement of the proton charge radius in electron scattering at very low Q2 develops in Orsay in the context of the PRAE projects.
Lot of issues to address…
Performances of the target
Performances of the detector
Impact of the measurement on the proton radius puzzle
Control of electron mass effects
Control of radiative corrections …
Construction and performances of PRAE
Stay tuned…
15/15
XXVth International Workshop on Nuclear Theory