new jlab / hall b proton radius experiment the prad...

New Jlab / Hall B proton radius experiment The PRad experiment at JLab

EINN2013, Paphos, Cyprus, Oct. 28 – Nov. 2, 2013

Michael Kohl * (in behalf of Ashot Gasparian)

Hampton University, Hampton, VA 23668 Jefferson Laboratory, Newport News, VA 23606

* Supported by NSF grants PHY-0855473, 0959521, and 1207672, and by DOE Early Career Award DE-SC0003884

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

Outline 2

  The proton radius puzzle

  The PRad experiment at Jlab   Method   Overview of the experiment   Expected precision   Status

A full session dedicated to the proton radius DNP2013, Newport News, Oct. 23-26, 2013: Thursday Oct. 24, 2013 4:00-4:12 FD.00001: The PRad Experiment at JLab

Dipangkar Dutta 4:12-4:24 FD.00002: Target Simulation for the PRad Experiment

Yang Zhang 4:24-4:36 FD.00003: Radiative corrections beyond the ultra-relativistic

approximation for PRad – Mehdi Meziane 4:36-4:48 FD.00004: The MUSE Measurement of the Proton Radius at PSI

πM1: Overview – Bill Briscoe 4:48-5:00 FD.00005: Simulation study for the PRad experiment

Chao Peng 5:00-5:12 FD.00006: The MUSE Measurement of the Proton Radius at PSI

πM1: Simulations – Katherine Myers 5:12-5:24 FD.00007: The MUSE Measurement of the Proton Radius at PSI

πM1: Scattering test – Ron Gilman 5:24-5:36 FD.00008: The MUSE Measurement of the Proton Radius at PSI

πM1: Radiative Corrections and Two-photon Exchange Andrei Afanasev

5:36-5:48 FD.00009: The MUSE Measurement of the Proton Radius at PSI πM1: Beam Studies – Vincent Sulkosky

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Hall A PR07-004, PR08-007 (PAC31/33)

• Recoil polarization, completed 2008 • Polarized target, completed 2012

BLAST (polarized target) C. Crawford et al., PRL98 (2007) 052301

X. Zhan, E08-007 + LEDEX update Phys. Lett. B 705 (2011) 59

2-sigma difference lower than BLAST

Charge and magnetic rms radii: RE = 0.875 ± 0.010 fm RM = 0.867 ± 0.020 fm

New proton measurements at low Q2 4

Rosenbluth separation at low Q2 Precise charge and magnetic rms radii: RE = 0.879 ± 0.008 fm RM = 0.777± 0.017 fm

MAMI A1

J. Bernauer et al. PRL105 (2010) 242001

New proton measurements at low Q2 5

•  R. Pohl et al., Nature 466, 09259 (2010): 2S➭2P Lamb shift ΔE(meV) = 209.9779(49) - 5.2262 rp2 + 0.0347 rp3 ➮ rp = 0.84184 ± 0.00067 fm

Possible issues: atomic theory & proton structure

PSI muonic hydrogen measurements 6

•  UPDATE: A. Antognini et al., Science 339, 417 (2013): 2S➭2P Lamb + 2S-HFS ΔEL(meV) = 206.0336(15) - 5.2275(10)rp2 + 0.0332(20)TPE ➮rp = 0.84087±0.00039 fm

Spectroscopy Scattering

Electronic Rp = 0.88 fm

Muonic Rp = 0.84 fm

RP = 0.84184(67) fm

RP = 0.875(10) fm

RP = 0.8775(51) fm

RP = 0.84087(39) fm

The proton radius puzzle 7

  >7σ discrepancy between muonic and electronic measurements

  High-profile articles in Nature, NYTimes, etc.

  Puzzle unresolved, possibly New Physics

  The µp result is wrong Discussion about theory and proton structure for extracting the proton radius from muonic Lamb shift measurement

  The ep (spectroscopy) results are wrong Rydberg constant could be off by 5 sigma Accuracy of individual Lamb shift measurements?

  The ep (scattering) results are wrong Fit procedures not good enough Q2 not low enough, structures in the form factors

 Proton structure issues in theory Off-shell proton in two-photon exchange leading to enhanced effects differing between µ and e Hadronic effects different for µp and ep: e.g. proton polarizability (effect ∝ ml

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 Physics beyond Standard Model differentiating µ and e Lepton universality violation, light massive gauge boson Existing constraints on new physics

Possible resolutions to the puzzle 8

New measurements are on their way

Need more precision for extraction from scattering More insights from comparison of ep and µp scattering

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rp (fm) ep µp Spectroscopy 0.8758 ± 0.077 0.84087 ± 0.00039

Scattering 0.8770 ± 0.060 ???

 Additional measurements needed / in preparation   Spectroscopy with µD, µHe, and regular H; Rydberg constant   ep-, ed-scattering (PRad at Jlab, ISR at MAMI)   µ±p- and e±p-scattering in direct comparison at PSI (MUSE)   Searches for lepton universality violating light bosons

(e.g kaon decay such as TREK/E36 at J-PARC)

Derivative in Q2 → 0 limit:

  First Born approximation (one photon exchange):

  Structureless proton:

  GE and GM from Rosenbluth separation Can ignore GM at extremely low Q2, (assumed in PRad)   Definition of the Proton Radius:

rms charge radius from slope of GE

e- e-

p p

GE ,GM

  Taylor expansion at low Q2:

Proton charge radius: ep elastic scattering 10

  Low intensity beam in Hall B @ Jlab into windowless gas target   Scattered ep and Moller electrons into HYCAL at 0o   Lower Q2 than Mainz. Very forward angle, insensitive to 2γ, GM   Conditionally approved by PAC38 (Aug 2011): ``Testing of this result

is among the most timely and important measurements in physics.’’   Approved by PAC39 (June 2012), graded “A”

The PRad proton radius proposal (JLAB) 11

  Limitation on Q2min ~ 10-3 (GeV/c)2

  Limitation to angles > ~5o   Electron energies from ~ 180 MeV

  Absolute cross sections (typically 2%)   Not statistics limited   Control of systematics

  Target (windows, thickness)   Beam flux   Acceptance   Detection efficiencies

  Suggested solutions by PRad experiment at JLab:   Non-magnetic-spectrometer method !   No target windows !   Calibrate with other well-known QED processes (Moller)

Designing a new ep-scattering experiment Difficulties of experiments with standard magnetic spectrometers

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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 calorimeter for small scattering angles Θ = 0.70 – 3.80

(Q2 = 2x10-4 – 2x10-2 ) GeV/c2 reduced model dependence in rp extraction   Simultaneous detection of ee → ee Moller scattering (control of systematics)   Use high density windowless H2 gas flow target (still much thinner than lqH2)

  low beam background with high quality CEBAF beam   minimize experimental background

  Large acceptance to cover large kinematic range in single setting   Two beam energies E0 = 1.1 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

Mainz low Q2 data set

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The PRad proton radius proposal (JLAB)

HyCal

  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

Proposed experimental setup in Hall B 16

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Electromagnetic calorimeter (HyCal)   Previously used for the PrimEx experiment (π0 lifetime)   PbWO4 and additional Pb glass   2.05 x 2.05 cm2 x 18 cm (20 rad. len.)   1152 modules arranged in 34x34 matr.   5m from target   0.5 sr acceptance

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Electromagnetic calorimeter (HyCal)   Previously used for the PrimEx experiment (π0 lifetime)

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Windowless H2 gas flow target

Funded by NSF MRI grant and presently under construction

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Windowless H2 gas flow target

Funded by NSF MRI grant and presently under construction

Precise normalization 21

Full simulation of PRad 22

Full simulation of PRad 23

Radiative corrections 24

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

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


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%

  Error budget (added quadratically)

  Simultaneous detection of two processes:   ep → ep   ee → ee Moller scattering

  Windowless H2 gas target will significantly reduce major systematic errors typical for all previous ep-scattering experiments

  High rates will provide small statistical errors (~0.2% for all Q2 bins)

  Extraction of proton charge radius was always limited by systematics and fitting uncertainties

Error estimates 27

Projected precision of PRad 28

  The “Proton Radius Puzzle” is still with us after more than three years!   Theory has so far failed to explain the current ~ 4.5% (~ 8σ) difference in rp

  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] (GeV/c)2   ep→ep cross sections normalized to Moller scattering   windowless hydrogen gas flow target and extended vacuum chamber

to minimize experimental backgrounds and

  PRad expected timeline:   preparation of experimental setup: 2013-14   experiment ready to run in Hall B at JLab: Fall 2014

  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

This project is supported in part by NSF MRI award PHY-1229153 and NSF research award PHY-1205962

Summary and outlook 29

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