UPDATE: PROTON CHARGE RADIUS EXPERIMENT (PRAD)

Zhihong Ye

Duke University

CLAS Collaboration Meeting

03/06/2014

PHYSICS MOTIVATION

It is one of the primary building blocks of all visible matter

It has finite size – a bag of quarks and gluons

It has a fuzzy boundary

About “Proton”:

We want to know the charge radius of the proton:

A precise test of QED Understanding QCD in the non-perturbative region

Three major experimental methods:

Electron-Proton elastic scattering Electric Form Factor Hydrogen spectroscopy (Lamb Shift) CODATA Muonic Hydrogen spectroscopy (Lamb Shift)

Assuming static charge distribution In Breit frame, the Fourier transform of the charge distribution gives

form factor:

(r )

Charge Radius from Form Factor:

.)2/)(1()()( 323 rdrqrqirderqF rqi

,1),()( 3 rdrr

02

2

2|))(

6

Q

pE

pp

E dQ

QdGr

...||120

1||

6

11)( 4422 rqrqqF

With

The proton r.m.s. charge radius can be obtained from:

)()(, 222 QGqFQq pE

Sachs Form Factor

, where

How to measure the Form Factor:

Unpolarized e-p elastic scattering:

Polarized e-p scattering:

)(1

1')()(

1

1' 2202222 2

QGE

E

d

dQGQG

E

E

d

d

d

d pE

mott

QpM

pE

mott

ep

12

2

222

2tan)1(21,

4,

2sin'4

pM

QEEQwhere

2tan

2

' M

EE

P

P

G

GR

l

tpp

M

pE

p

lt PP ,where are the transverse and longitudinal polarization of the proton.

Rosenbluth Separation GMp can be ignored

Recent e-p Scattering Experiments

– Large amount of overlapping data sets

– Statistical error ≤ 0.2%– Luminosity monitoring with

spectrometer Q2 = 0.004 – 1.0 (GeV/c)2 result: rp

=0.879(5)stat(4)sys(2)mod(4)group

J. Bernauer, PRL 105,242001, 2010

Measurements @ Mainz

(slide from Haiyan)

JLab Recoil Proton Polarization Experimental

Ee: 1.192GeVPb: ~83%

BigBite

• Δp/p0: ± 4.5% ,• out-of-plane: ± 60 mrad• in-plane: ± 30 mrad• ΔΩ: 6.7msr• QQDQ• Dipole bending angle 45o

• VDC+FPP • Pp : 0.55 ~ 0.93 GeV/c

LHRS

• Non-focusing Dipole •Big acceptance.

• Δp: 200-900MeV• ΔΩ: 96msr

• PS + Scint. + SH

X. Zhan et al. Phys. Lett. B 705 (2011) 59-64C. Crawford et al. PRL98, 052301 (2007)

(slide from Haiyan)

Focal-plane polarimeter

The absolute frequency of Hydrogen energy levels has been measured with very high

accuracy.

Since Hydrogen is a very simple system, its energy levels can be precisely calculated

in QED with correction for the finite size of the proton.

Charge Radius from Hydrogen Lamb Shift:

If the proton is point-like, . Corrected for the finite size, r

ZrU

)( 23 ))((

6

4)( p

ErrZ

rU

The energy level is related to the proton charge radius:

0234 )()(

3

2)( l

pEl

pE rmZrE

where mlme for electrons, or mlmu for Muons.

Since mu=200me, Muonic Hydrogen Lamb Shift gives much more precise measurements

Comparing the measurements and the QED calculations can determine the proton

r.ms. Charge radius with high resolution.

Muonic Hydrogen Lamb Shift Experiment at PSI

The proton radius puzzle!Charge Radius Results:

The results from Muonic Lamb Shift measurements are 7-σ away from the e-p elastic measurements and electronic Hydrogen Lamb Shift.

PRAD EXPERIMENT High resolution, large acceptance, hybrid HyCal calorimeter (PbWO4 and Pb-

glass)

Q2 range of 2x10-4 – 2.0x10-2 GeV2 (lower than all previous electron scattering experiments.)

Simultaneous detection of elastic and Møller electrons

Windowless H2 gas flow target

XY – veto counters

Vacuum box, one thin window at HyCal only

Spokesperson: A. Gasparian, Co-spokesperson: D. Dutta, H. Gao, M. Khandaker

High Resolution Calorimeter (HyCal):

A PbWO4 and Pb-glass calorimeter

2.05 x 2.05 cm2 x18 cm (20 rad. Length) 1152 modules arranged in 34x34 matrix at the central

region 5 m from the target, and 0.5 sr acceptance

Target windows are the major sources of background for typical magnetic spectrometer

experiments

PRad will avoid this background by developing a new windowless target cell

Working closely with JLab target group to design and build this target.

Target thickness1.0 x 1018

atoms/cm2 at 25K

Target supported by NSF - MRI grant

Windowless Gas Flow Target:

Windowless Gas Flow Target:

Target density was studied by COMSOL Multiphysics Simulations show that the desired densities can be

achieved. Thickness at the center: 3.42 x 1018 H/cm2

Target construction well underway.

(Simulations by Y. Zhang/Duke)

windowless target5608 sccm

2nd stage1500 Lt/s Trubo Pump

2nd stage, 1500 Lt/s Trubo Pump

1st stage, 3000 Lt/sTurbo Pump

Simulation

Position Detector:

PRad aims to measure GEp at very low Q2. The high precision of angular

measurements at very forward direction is crucial.

Angles are determined by the position of the scattered electron.

HyCal provides 2.5 mm position resolution which gives 7% uncertainty of Q2

measurement at 10-4 GeV2.

We are working on designing a new position detector which must has the features:

Thin Not too much space

Minimum radiation materials Control the background events at a small level.

Allow a hole at the center to allow the electron beam goes throughPossible candidates: GEM, Drift Chambers, and Scintillator Fiber Tracker (SFT).

Position Detector: Scintillator Fiber Tracker

X-Plane

Y-Pla

ne

Use 1mm scintillator fibers which gives 0.3 mm

position resolution

Use Silicon Photomultipliers (SiPMs) as read-outs.

Replace Veto-Counter; Provide timing and position at

the same time.

Thin, light weight, relatively easy to build.

A prototype is developing to study the design of the full

SFT.

1300 mm

1300 mm

50 mm

50 mm

100 x Pre-A

mp

lifiers

100 x NIM

D

iscrimin

ators

50 x SiP

Ms

Analog Signals

Analog Signals

PowerSupplies

SFT Prototype (Not to Scale)

50 x SiPMs 100x shortBNC-LEMO

100 x FastB

us

TD

C

100x longRibbon Cables

100x longBNC->LEMO (Ribbon?)

100 x FastB

us

AD

C

Receiving great helps from all four halls. Special thank to Stepan and other Hall-B staffs.

See my talk in this week’s Hall-B meeting minutes.

Extracting the e-p elastic cross sections:

Will detect e-p and Møller electrons simultaneously

Extract e-p->e-p event yields Same for e-e->e-e

Normalizing the ep cross section to the Møller:

Main sources of systematic uncertainties Nbeam and Ntgt

other sources can be canceled out in the normalization.

Main sources of systematic uncertainties Nbeam and Ntgt

other sources can be canceled out in the normalization.

Extracting the e-p elastic cross sections:

Will detect e-p and Møller electrons simultaneously

Extract e-p->e-p event yields Same for e-e->e-e

Normalizing the ep cross section to the Möller:

0.8 degree line

Møller

ep

Simulation

Signal to noise ratio of 107.

Can be improved with fine tuning of the accelerator

Add a collimator in front of the target?

Beam e-

Halo e-Target cell

Beam Halo:

prel

imin

ary

A detailed study of backgrounds and radius extraction.

Subtraction has been performed using this simulation

Study shows that we need 20% beam time for empty

target runs Empty target Full target

GEANT4 Simulation:(by Chao Peng/Duke)

Sim

ulat

ion

Sim

ulat

ion

GEANT4 Simulation:(by Chao Peng/Duke)

Better precision anticipated by extending the Q2 range using Pb-glass part of calorimeter, and adding position detector

Including lead glass part up to 10 deg

rp = 0.8773 (50) fm

Including 3.3 GeV beam up to 4 deg

rp = 0.8753 (52) fm

Sim

ulat

ion

Sim

ulat

ion

Input radius, rp = 0.8768 fm

Radiation Correction:(by M. Meziane/Duke)

ep elastic radiative corrections were simulated using ELRADGEN1

Møller radiative corrections were simulated using MERADGEN2

Both are modified to include the electron mass

1 I. Akushevich, O. Filoti, A. Ilyichev, and N. Shumeiko, arXiv:hep-ph/1104.0039v1, (2011).2 A. Afanasev, E. Chudakov, V. A. Zukunov and A. N. Ilyichev, Comp. Phy. Comm, 176, 218 (2007)

ep

Møller Møller

ep

Sim

ulat

ion

Radiation Correction:(by M. Meziane/Duke)

ep elastic radiative corrections were simulated using ELRADGEN1

Møller radiative corrections were simulated using MERADGEN2

Both are modified to include the electron mass

Correction to the Møller cross section ~2-3%

Corrections to the ep cross section: ~8 -13%

1 I. Akushevich, O. Filoti, A. Ilyichev, and N. Shumeiko, arXiv:hep-ph/1104.0039v1, (2011).2 A. Afanasev, E. Chudakov, V. A. Zukunov and A. N. Ilyichev, Comp. Phy. Comm, 176, 218 (2007)

STATUS & PLAN

Windowless target design is nearly done. Target vacuum pump, motion system, chiller

and cryo-cooler are in hand.

The target & secondary chambers are under production in Germany.

The HyCal will be refurbished soon.

A prototype of a new Scintillator Fiber Tracker (SFT) is being developed. The SFT will

improve the Q2 resolution by a factor of 3.

Geant4 simulation is undergoing smoothly to study the background, systematic

uncertainty and so on.

Experimental techniques to suppress beam halo are under discussion, e.g. adding a

collimator

Radiation Correction can be well handled.

Still many things are needed to finished but we are working very hard to make the

experiment ready!

THANK YOU!

This project is supported by DOE under the contract# DE-FG02-03ER41231 and NSF MRI award PHY-1229153

Expected Uncertainties

Contributions Estimated Uncertainty (%)

Statistical 0.2

Acceptance (including

Q2 determination)0.4

Detection efficiency 0.1

Radiative corrections

0.3

Background and PID

0.1

Fitting 0.2

Total 0.6