Post on 27-Mar-2015
R. Michaels, Jlab UGM, June, 2011
Lead ( Pb) Radius Experiment : PREX208
208Pb
E = 1 GeV, electrons on lead
05
Elastic Scattering
Parity Violating Asymmetry
Spokespersons
Paul Souder, Krishna Kumar
Guido Urciuoli, Robert Michaels
Ran March – June 2010 in Hall A
Graduate Students
Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse)
Jon Wexler (UMass)
Kiadtisak Saenboonruang (UVa)
R. Michaels, Jlab UGM, June, 2011
)(
sin4122 2
22
QF
QG
dd
dd
dd
dd
AP
WF
LR
LR
)( 2QF n
%1%3 n
n
R
dR
A
dA
Idea behind PREX
Z of Weak Interaction :
Clean Probe Couples Mainly to Neutrons( T.W. Donnelly, J. Dubach, I Sick 1989 )
0
In PWIA (to illustrate) :
w/ Coulomb distortions (C. J. Horowitz) :
0
R. Michaels, Jlab UGM, June, 2011
Measured Asymmetry
Weak Density at one Q2
Neutron Density at one Q2
Correct for CoulombDistortions
Small Corrections forG
nE G
sE MEC
Assume Surface Thickness Good to 25% (MFT)
Atomic Parity Violation
Mean Field & Other
Models
Neutron
Stars
R n
PREX Physics Output
Slide adapted from C. Horowitz
R. Michaels, Jlab UGM, June, 2011
Fundamental Nuclear Physics :
What is the size of a nucleus ?
Neutrons are thought to determine the size of heavy nuclei like 208Pb.
Can theory predict it ?
R. Michaels, Jlab UGM, June, 2011
str
mb
d
d
1fmq
Reminder: Electromagnetic Scattering determines
r
rPb
208
(charge distribution)
1 2 3
d
d
R. Michaels, Jlab UGM, June, 2011
Z of weak interaction : sees the neutrons
proton neutron
Electric charge 1 0
Weak charge 0.08 1
0
Analysis is clean, like electromagnetic scattering:
1. Probes the entire nuclear volume
2. Perturbation theory applies
6
R. Michaels, Jlab UGM, June, 2011
)()()(ˆ5 rArVrV
)(
)(sin41
22 2
22
2
QF
QFQG
d
d
d
d
d
d
d
d
AP
NW
F
LR
LR
neutron weak charge >> proton weak charge
is small, best observed by parity violation
||)()(/
//3 rrrZrdrV )()()sin41(22
)( 2 rNrZG
rA NPWF
22 |)(| QFd
d
d
dP
Mott
)()(4
1)( 0
32 rqrjrdQF PP )()(
4
1)( 0
32 rqrjrdQF NN
Electron - Nucleus Potential
electromagnetic axial
Neutron form factor
Parity Violating Asymmetry
)(rA
1sin41 2 W
Proton form factor
0
Pb is spin 0208
R. Michaels, Jlab UGM, June, 2011
( R.J. Furnstahl )
Measurement at one Q is sufficient to measure R
2
N
proposed error
Why only one parameter ?
(next slide…)
PREX:
R. Michaels, Jlab UGM, June, 2011
ZN
Nuclear Structure: Neutron density is a fundamental observable that remains elusive.
Reflects poor understanding of symmetry energy of nuclear matter = the energy cost of
xn
)21()()2/1,(),( 2xnSxnExnE
n.m. density
ratio proton/neutrons
• Slope unconstrained by data
• Adding R from Pb will eliminate the
dispersion in plot.
N208
Slide adapted from J. Piekarewicz
R. Michaels, Jlab UGM, June, 2011
Skx-s15Thanks, Alex Brown
PREX Workshop 2008
E/N
N
R. Michaels, Jlab UGM, June, 2011
Skx-s20Thanks, Alex Brown
PREX Workshop 2008
R. Michaels, Jlab UGM, June, 2011
Skx-s25Thanks, Alex Brown
PREX Workshop 2008
R. Michaels, Jlab UGM, June, 2011
Application: Atomic Parity Violation• Low Q test of Standard Model
• Needs RN (or APV measures RN )
2
0
Isotope Chain Experiments e.g. Berkeley Yb
13
APV
rdrZrNG
H eePWNF
PNC35/2 )()sin41()(
22
R. Michaels, Jlab UGM, June, 2011
Neutron
StarsWhat is the nature of extremely dense matter ?
Do collapsed stars form “exotic” phases of matter ? (strange stars, quark stars)
Crab Nebula (X-ray, visible, radio, infrared)
Application :
R. Michaels, Jlab UGM, June, 2011
PREX & Neutron Stars
pn RRCrab Pulsar
( C.J. Horowitz, J. Piekarewicz )
R calibrates EOS of Neutron Rich Matter
Combine PREX R with Obs. Neutron Star Radii
Some Neutron Stars seem too Cold
N
N
Crust ThicknessExplain Glitches in Pulsar Frequency ?
Strange star ? Quark Star ?
Cooling by neutrino emission (URCA)
0.2 fm URCA probable, else not
Phase Transition to “Exotic” Core ?
R. Michaels, Jlab UGM, June, 2011
How to do a Parity Experiment
Flux Integration Technique:HAPPEX: 2 MHzPREX: 500 MHz
(integrating method)
Example : HAPPEX
R. Michaels, Jlab UGM, June, 2011
/)( AA
Pull Plot (example)
PREX Data
R. Michaels, Jlab UGM, June, 2011
PAVI 09
Beam AsymmetriesA
raw = A
det - A
Q +
E+
ix
i
•natural beam jitter (regression) •beam modulation (dithering)
Slopes from
18
R. Michaels, Jlab UGM, June, 2011
Slug # ( ~ 1 day)
Units: microns
RLhelicityforXX LR ,
Parity Quality Beam !
Helicity – Correlated Position Differences
< ~ 3 nm
Wien Flips helped !Average with signs = what exp’t feels
Points: Not sign corrected
R. Michaels, Jlab UGM, June, 2011
Hall A Compton Upgrade
with Green Laser
Sirish Nanda, et. al.
1 % Polarimetry
at 1 GeV
R. Michaels, Jlab UGM, June, 2011
Hall A Moller Upgrade
Superconducting Magnet from Hall C
Saturated Iron Foil Targets
1 % Polarimetry
Magnet and Target
DAQ Upgrade (FADC)
Sasha Glamazdin, et.al.
R. Michaels, Jlab UGM, June, 2011
High Resolution Spectrometers
Elastic
Inelastic
detector
Q Q
Dipole
Quad
Spectrometer Concept: Resolve Elastic
target
Left-Right symmetry to control transverse polarization systematic
1st excited state Pb 2.6 MeV
R. Michaels, Jlab UGM, June, 2011
Collimators
Septum Magnet
Top view
target
HRS-L
Q1
HRS-R
Q1
PREX Region After Target
Former O-Ring location which failed and caused significant time loss
Planned Tungsten Collimator and Shielding
R. Michaels, Jlab UGM, June, 2011
10 m
-1 m
Geant 4 Radiation Calculations
• benchmarking against LD2 (ran at 100 uA)
• exploring shielding strategies
J. Mammei, L. Zana
Number of Neutrons per incident Electron
Z (distance along beamline)
1 mLD2 tgtPb tgt
1 - 50 MeV
50 - 500 MeV
0 - 1 MeV
Strategy
• Tungsten ( W ) plug
• Shield the W
• x10 reduction in 0.2 to 10 MeV neutrons
(similar to LD2)
00 37.0
W
PREX-II proposal
PREX -I
PREX -II
R. Michaels, Jlab UGM, June, 2011
Pure, Thin 208 Pb Target
Momentum (GeV/c)
Lead 5- State
2.6 MeV
High Resolution Spectrometers
DETECTOR footprint
25
R. Michaels, Jlab UGM, June, 2011
Diamond LEAD
• Three bays
• Lead (0.5 mm) sandwiched by diamond (0.15 mm)
• Liquid He cooling (30 Watts)
Lead / Diamond Target
R. Michaels, Jlab UGM, June, 2011
Performance of Lead / Diamond Targets
Last 4 days at 70 uA
Targets with thin diamond backing (4.5 % background) degraded fastest.
Thick diamond (8%) ran well and did not melt at 70 uA.
meltedmelted
NOT melted
Solution: Run with 10 targets.
R. Michaels, Jlab UGM, June, 2011
y
z
xS
k
+ -
AT > 0 means
)'( eeeT kkSA
Beam-Normal Asymmetry in elastic electron scatteringi.e. spin transverse to scattering plane
Possible systematic if small transverse spin component
New results PREX
ppmAC T 35.036.052.6:12
ppmAPb T 36.019.013.0:208
• Small AT for 208Pb is a big (but pleasant) surprise. • AT for 12C qualitatively consistent with 4He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz
R. Michaels, Jlab UGM, June, 2011
Error Source Absolute (ppm) Relative ( % )
Polarization (1) 0.0071 1.1
Beam Asymmetries (2) 0.0072 1.1
Detector Linearity 0.0071 1.1
BCM Linearity 0.0010 0.2
Rescattering 0.0001 0
Transverse Polarization 0.0012 0.2
Q2 (1) 0.0028 0.4
Target Thickness 0.0005 0.112C Asymmetry (2) 0.0025 0.4
Inelastic States 0 0
TOTAL 0.0130 2.0
Systematic Errors
(1) Normalization Correction applied
(2) Nonzero correction (the rest assumed zero)
R. Michaels, Jlab UGM, June, 2011
PREX Physics Result
LR
LRPVA
)(0130.0)(0604.06571.0 syststat ppm
at Q2 = 0.00906 GeV2
Statistics limited ( 9% )
Systematic error goal achieved ! (2%)
9.2 % 2.0 %
R. Michaels, Jlab UGM, June, 2011
PREX Asymmetry (Pe x A)
ppm
Slug ~ 1 day
(blinded, raw)
R. Michaels, Jlab UGM, June, 2011
PREX Physics Interpretation
R. Michaels, Jlab UGM, June, 2011
Asymmetry leads to RN
Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm
arXiv:1010.3246 [nucl-th] Shufang Ban, C.J. Horowitz, R. Michaels
Establishing a neutron skin at 95% CL
R. Michaels, Jlab UGM, June, 2011
PREX – II Proposal
Future ?
R. Michaels, Jlab UGM, June, 2011
Future ?
R. Michaels, Jlab UGM, June, 2011
Other Nuclei ?
Shape Dependence ?RN
RN
Surface thickness
Surface thickness
arXiv:1010.3246 [nucl-th]
Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels
R. Michaels, Jlab UGM, June, 2011
PREX : Summary • Fundamental Nuclear Physics with
many applications
• Achieved a 9% stat. error in Asymmetry (original goal : 3 %)
• Systematic Error Goals Achieved !!
• Significant time-losses due to O-Ring problem and radiation damage
• Proposal for PREX-II in preparation
R. Michaels, Jlab UGM, June, 2011
Extra Slides
R. Michaels, Jlab UGM, June, 2011
Corrections to the Asymmetry are Mostly Negligible
• Coulomb Distortions ~20% = the biggest correction.
• Transverse Asymmetry (to be measured)
• Strangeness
• Electric Form Factor of Neutron
• Parity Admixtures
• Dispersion Corrections
• Meson Exchange Currents
• Shape Dependence
• Isospin Corrections
• Radiative Corrections
• Excited States
• Target Impurities
Horowitz, et.al. PRC 63 025501
R. Michaels, Jlab UGM, June, 2011
PAVI 09
Optimum Kinematics for Lead Parity: E = 1 GeV if <A> = 0.5 ppm. Accuracy in Asy 3%
n
Fig. of merit
Min. error in R
maximize:
1 month run
1% in R
n
(2 months x 100 uA 0.5% if no systematics)
5
R. Michaels, Jlab UGM, June, 2011
Liquid/Solid Transition Density
• Thicker neutron skin in Pb means energy rises rapidly with density Quickly favors uniform phase.
• Thick skin in Pb low transition density in star.
FP
TM1Solid
Liquid
Neutron Star Crust vs Pb Neutron Skin
208Pb
Neutron Star
C.J. Horowitz, J. Piekarawicz
R. Michaels, Jlab UGM, June, 2011
.../ 3/12
4
AaA
ZNaa
A
Esv
)1(
PREX: pins down the symmetry energy (1 parameter)
( R.J. Furnstahl )
energy cost for unequal # protons & neutrons
PREX
PREX error bar
Pb208 Actually, it’s the
density dependence of a4 that we pin down.