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
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