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PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
)(
sin4122 2
22
QF
QG
d
d
d
d
d
d
d
d
AP
WF
LR
LR
)( 2QF n
%1%3 n
n
R
dR
A
dA
Lead ( Pb) Radius Experiment : PREX
Z of Weak Interaction :
Clean Probe Couples Mainly to Neutrons( T.W. Donnelly, J. Dubach, I Sick )
0
In PWIA (to illustrate) :
w/ Coulomb distortions (C. J. Horowitz) :
208
208Pb
E = 850 MeV, electrons on lead
06 Elastic Scattering Parity Violating Asymmetry
0
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Nuclear Structure: Neutron density is a fundamental observable that remains elusive.
ZN
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
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
( R.J. Furnstahl )
Measurement at one Q is sufficient to measure R
2
N
Pins down the symmetry energy (1 parameter)
PREX accuracy
PREX accuracy
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
PREX & Neutron Stars
Crab Pulsar
( C.J. Horowitz, J. Piekarweicz )
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 pn RR
Phase Transition to “Exotic” Core ?
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
FP
TM1Solid
Liquid
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.
Neutron EOS and Neutron Star Crust
Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536.
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Pb Radius vs Neutron Star Radius• The 208Pb radius constrains the pressure of neutron
matter at subnuclear densities.• The NS radius depends on the pressure at nuclear
density and above.• Most interested in density dependence of equation of
state (EOS) from a possible phase transition.• Important to have both low density and high density
measurements to constrain density dependence of EOS.– If Pb radius is relatively large: EOS at low density is stiff with
high P. If NS radius is small than high density EOS soft.– This softening of EOS with density could strongly suggest a
transition to an exotic high density phase such as quark matter, strange matter, color superconductor, kaon condensate…
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
PREX Constrains Rapid Direct URCA Cooling of Neutron Stars
• Proton fraction Yp for matter in beta equilibrium depends on symmetry energy S(n).
• Rn in Pb determines density dependence of S(n).
• The larger Rn in Pb the lower the threshold mass for direct URCA cooling.
• If Rn-Rp<0.2 fm all EOS models do not have direct URCA in 1.4 M¯ stars.
• If Rn-Rp>0.25 fm all models do have URCA in 1.4 M¯ stars.
Rn-Rp in 208Pb
If Yp > red line NS cools quickly via direct URCA reaction n p+e+
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Atomic Parity Violation• Low Q test of Standard Model
• Needs R to make further progress.
2
N
rdrZrNG
H eePWNF
PNC35/2 )()sin41()(
22
0
APV
Isotope Chain Experiments e.g. Berkeley Yb
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Neutron Skin and Heavy – Ion Collisions
Danielewicz, Lacey, and Lynch, Science 298 (2002) 1592.
• Impact on Heavy - Ion physics: constraints and predictions
• Imprint of the EOS left in the flow and fragmentation distribution.
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
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 Impact
Heavy Ions
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
PREX: Experimental Issues
Spokespersons:
P.A. Souder, G.M. Urciuoli, R. Michaels
Hall A Collaboration Experiment
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
PREX in Hall A at JLab
CEBAF
Hall A
Pol. Source
Lead Foil Target
Spectometers
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Optimum Kinematics for Lead Parity: E = 850 MeV, <A> = 0.5 ppm. Accuracy in Asy 3%
n
Fig. of merit
Min. error in R
maximize:
1 month run
1% in R
n
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Beam Asymmetries
Araw
= Adet
- AQ +
E+
ix
i
•natural beam jitter (regression) •beam modulation (dithering)Slopes from
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
“Energy” BPM
BPM Y2
BPM Y1
BPM X1
BPM X2
Scale +/- 10 nm
Position Diffs average to ~ 1 nm
• Good model for controlling laser systematics at source
• Accelerator setup (betatron matching, phase advance)
Helicity Correlated Differences: Position, Angle, Energy
slug
slug
slug
slug
“slug” = ~1 day running
Spectacular results from HAPPEX-H show we can do PREX.
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
X (
cavi
ty)
n
m
Y (
cavi
ty)
n
m
X (stripline) nm Y (stripline) nm
Redundant Position Measurements at the ~1 nm level(Helicity – correlated differences averaged over
~1 day)
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Lead Target
Liquid Helium Coolant
Pb
C
208
12
Diamond Backing:
• High Thermal Conductivity• Negligible Systematics
Beam, rastered 4 x 4 mm
A 80at ly testedSuccessful
beam
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
X (dispersive coord)
(m)
Y (m)
Momentum (MeV)
Detector
Pb Elastic
208
1st Excited State (2.6 MeV)
Lead Target Tests
• Check rates
• Backgrounds (HRS is clean)
• Sensitivity to beam parameters
• Width of asymmetry
• HRS resolution
• Detector resolution
IA1
Nu
m.
eve
nts
Nu
m.
eve
nts
Data taken Nov 2005
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Noise • Need 100 ppm per window pair
• Position noise already good enough
• New 18-bit ADCs Will improve BCM noise.
• Careful about cable runs, PMTs, grounds. Will improve detector noise.
• Plan: Tests with Luminosity Monitor
to demonstrate capability.
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Transverse Polarization
TTTTT PAAPAA 00 sin
oT PP 3sin
ppmAT 150
HRS-Left HRS-Right
Transverse Asymmetry
Systematic Error for Parity
“Error in”
TP
Left-right apparatus asymmetry
Need 33 1010 <
ppmAT 10 measure in ~ 1 hr (+ 8 hr setup)
Theory est. (Afanasev)
P
Transverse polarization
Part I: Left/Right Asymmetry
correction syst. err.
<
Control w/ slow feedback on polarized source
solenoids.
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Transverse Polarization
cos0cos 0TT PAA
HRS-Left HRS-
Right
Vertical misalignment
Systematic Error for Parity
P
Horizontal polarization e.g. from (g-2)
Part II: Up/Down Asymmetry
( Note, beam width is very tiny
m100~
up/down misalignment
• Measured in situ using 2-piece
detector.
• Study alignment with tracking & M.C.
• Wien angle feedback ( )
33 1010cos TP
Need
)
<<
sinPPT
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Warm Septum
Existing superconducting septum won’t work at high L
Warm low energy (1 GeV) magnet designed.
Grant proposal in preparation (~100 k$) [Syracuse / Smith College]
TOSCA design
P resolution ok
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Polarimetry
Electron onlyPhoton only
Preliminary: 2.5% syst ( only)
Møller : Pe/Pe ~ 3 % (limit: foil polarization)
(a high field target ala Hall C being considered)
Compton : 2% syst. at present
2 analyses based on either electron
or photon detection
Superlattice:
Pe=86% !
PREX:
1 % desirable
2 % requirede
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
Upgrade of Compton Polarimeter (Nanda, Lhuillier)
To reach 1% accuracy:
• Green Laser (increased sensitivity at low E)
laser on-hand, being tested
• Integrating Method (removes some systematics of analyzing power)
developed during HAPPEX & in 2006
• New Photon Detector
electron
s
in ~ 1.5 years
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
PREX : Summary • Fundamental Nuclear Physics with
many applications
• HAPPEX & test runs have demonstrated technical aspects
• Polarimetry Upgrade needed
• Beam Time Request Unchanged (30 days)
PREXPAC 29 Jan 2006
R. Michaels
Jefferson Lab
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
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