lead ( p b) r adius ex periment : prex
DESCRIPTION
Lead ( P b) R adius Ex periment : PREX. 208. Elastic Scattering Parity Violating Asymmetry . E = 1 GeV, electrons on lead. Spokespersons Paul Souder Krishna Kumar Robert Michaels Guido Urciuoli. 208 Pb. - PowerPoint PPT PresentationTRANSCRIPT
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Lead ( Pb) Radius Experiment : PREX208
208Pb
E = 1 GeV, electrons on lead
05
Elastic Scattering Parity Violating Asymmetry
Spokespersons• Paul Souder• Krishna Kumar• Robert Michaels• Guido Urciuoli
G.M. Urciuoli
Hall A Collaboration Experiment
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neutron weak charge >> proton weak charge
is small, best observed by parity violation
)()()(ˆ5 rArVrV
||)()(///3 rrrZrdrV )()()sin41(
22)( 2 rNrZ
GrA NPW
F
22 |)(| QFdd
dd
PMott
)()(41)( 0
32 rqrjrdQF PP )()(
41)( 0
32 rqrjrdQF NN
)()(
sin4122 2
22
2
QFQFQG
dd
dd
dd
dd
AP
NW
F
LR
LR
Electron - Nucleus Potential
electromagnetic axial
Neutron form factor
Parity Violating Asymmetry
)(rA
1sin41 2 W
Proton form factor
0
%1%3 n
n
RdR
AdA
APV~ 500 ±15ppb, Q2~ 0.01 GeV2
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G.M. Urciuoli
strmb
dd
1fmq
Reminder: Electromagnetic Scattering determines
r
r
Pb208
(charge distribution)
1 2 3
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G.M. Urciuoli
Z of weak interaction : sees the neutrons
0
proton neutron
Electric charge 1 0
Weak charge 0.08 1
Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies
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G.M. Urciuoli
Neutron Densities
• Proton-Nucleus Elastic• Pion, alpha, d Scattering• Pion Photoproduction• Magnetic scattering• Theory Predictions Fit mostly by data other
than neutron densities
Involve strong probes
Most spins couple to zero.
Therefore, PREX is a powerful check of nuclear theory.
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G.M. Urciuoli
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
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G.M. Urciuoli
( 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
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G.M. Urciuoli
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 ?
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G.M. Urciuoli
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.
Horowitz
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G.M. Urciuoli
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.
• 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…
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G.M. Urciuoli
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+
Horowitz
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G.M. Urciuoli
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
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G.M. Urciuoli
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.
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G.M. Urciuoli
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
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G.M. Urciuoli
Corrections to the Asymmetry are Mostly Negligible
Horowitz, et.al. PRC 63 025501
• 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
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G.M. Urciuoli
Hall A at Jefferson Lab
Polarized e-
SourceHall A
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G.M. Urciuoli
PREX in Hall A at JLab
CEBAFHall A
Pol. Source
Lead Foil Target
Spectometers
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G.M. Urciuoli
High Resolution Spectrometers
Elastic
Inelastic
detector
Q Q
Dipole
Quad
Spectrometer Concept:
Resolve Elastic
target
Left-Right symmetry to control transverse polarization systematic
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Experimental Method
Flux Integration Technique:HAPPEX: 2 MHzPREX: 850 MHz
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G.M. Urciuoli
controls
effective
analyzing
powerTune
residual linear pol.Slow helicity
reversalIntensity Attenuat
or(charge
Feedback)
Polarized Source
High PeHigh Q.E.Low Apower
• Optical pumping of solid-state photocathode
• High Polarization
• Pockels cell allows rapid helicity flip
• Careful configuration to reduce beam asymmetries.
• Slow helicity reversal to further cancel beam asymmetries
GaASPhotocato
de
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P I T A Effect
)sin( IA Laser at Pol. Source
Polarization Induced Transport Asymmetry
yx
yx
TTTT
where
Transport Asymmetry
Intensity Asymmetry
drifts, but slope is ~ stable. Feedback on
Important Systematic :
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Intensity FeedbackAdjustmentsfor small phase shiftsto make close tocircular polarization
Low jitter and high accuracy allows sub-ppmCumulative charge asymmetry in ~ 1 hour
In practice, aim for 0.1 ppm overduration of data-taking.
~ 2 hours
HAPPEX
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G.M. Urciuoli
Beam AsymmetriesAraw = Adet - AQ + E+ ixi
• natural beam jitter (regression) • beam modulation (dithering)Slopes from
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G.M. Urciuoli
“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.
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G.M. Urciuoli
Integrating Detection
PMT
Calorimeter
ADC
Integrator
electrons
• Integrate in 30 msec helicity period.• Deadtime free.• 18 bit ADC with < 10-4 nonlinearity.• Backgrounds & inelastics separated (HRS).
Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. The x, y dimensions of the quartz determined from beam test data and MC (HAMC) simulations. (11 x 14 cm)Quartz thickness to be optimized with MC.New HRS optics tune focuses elastic events both in x & y at the PREx detector location.
Actually two thin quartz detectors
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G.M. Urciuoli
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G.M. Urciuoli
Lead Target
Liquid Helium Coolant
Pb
C
208
12
Diamond Backing: • High Thermal
Conductivity• Negligible Systematics
Beam, rastered 4 x 4 mm
beam
5 days at 60 uA 1 shift at 80 uA 3 hrs at 100 uA
Successfuly tested
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Upgrade of Compton Polarimeter
To reach 1% accuracy:• Green Laser Green Fabry-Perot cavity (increased sensitivity at low E)
• Integrating Method (removes some systematics of analyzing power)
• New Photon and Electron Detectors (new GSO photon calorimeter, FADC based photon integration DAQ)
electrons
Upgrade Møller polarimeter: 4 Tesla field saturated iron foil, new FADC DAQ
Polarimetry
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G.M. Urciuoli
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G.M. Urciuoli
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.
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G.M. Urciuoli
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
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Figure of Merit M = 1/E * 1/sqrt(R) * sqrt(1 + B/S)where,E = A_T enhancement for A_T hole events = 50.R = Ratio of A_T hole detector to main Pb detector event ratesB/S = Ratio of bkgd under the A_T hole events to A_T signal
The optimum A_T detector dimension is ~7.6cm in x by 0.8cm in y. This gives Figure of Merit = 0.637 and error inflation ~1.186.
A_T detector design
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G.M. Urciuoli
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.
• Tests with Luminosity Monitor to demonstrate capability.
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PREX Workshop Aug 08
~ 50 ppm noise per pulse milestone for electronics
Asymmetries in Lumi Monitors after beam noise subtraction
Jan 2008 Data
( need < 100 ppm)
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• PREX is an extremely challenging experiment:
– APV ≈ 500 ± 15 ppb.– 1% polarimetry. – Helicity correlated beam asymmetry < 100 ± 10 ppb.– Beam position differences < 1 ± 0.1 nm.– Transverse beam polarization < 1%. – Noise < 100 ppm – (Not melting) Lead Target – Forward angle detection Septum magnet– Precision measurement of Q2: ± 0.7% ± 0.02° accuracy in
spectrometer angles
• However HAPPEX & test runs have demonstrated its feasibility.
• It will run in March-May 2010 and will measure the lead neutron radius with an unprecedented accuracy (1%). This result will have an impact on many other Physics fields (neutron stars, APV, heavy ions …).
PREX: Summary
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G.M. Urciuoli
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Spares
G.M. Urciuoli
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G.M. Urciuoli
Optimum Kinematics for Lead Parity: E = 850 MeV, 6<A> = 0.5 ppm. Accuracy in Asy 3%
n
2AddFOM
2FOM
Fig. of merit
Min. error in R maximize:
1 month run 1% in R
n
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Optimization for Barium -- of possible direct use for Atomic PV
1 GeV optimum
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G.M. Urciuoli
X
(cav
ity)
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)
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Integrating Detection• Integrate in 30 msec helicity period.• Deadtime free.• 18 bit ADC with < 10-4 nonlinearity.• Backgrounds & inelastics must be separeted (HRS).
electrons
ADC
Integrator
PMT
Quartz / Tungsten Calorimeter
(Also a thin quartz detector upstream of this)Attempt to improve resolution by replacing Alzak mirrors in light guide with anodized Al or Silver. The x, y dimensions of the quartz determined from beam test data and MC (HAMC) simulations. (11 x 14 cm)Quartz thickness to be optimized with MC.New HRS optics tune focuses elastic events both in x & y at the PREx detector location.