1. Lead ( Pb) Radius Experiment : PREX
208
E = 850 MeV, electrons on lead
Elastic Scattering Parity Violating Asymmetry
Z of Weak Interaction :
Clean Probe Couples Mainly to Neutrons
( T.W. Donnelly, J. Dubach, I Sick )
In PWIA (to illustrate) :
208Pb
w/ Coulomb distortions (C. J. Horowitz) :
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

2. Parity Transformation
A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it.
Incident electron S
(spin)
Target
Positive longitudinal spin P
(momentum)
Parity Transformation
208 Pb
Negative longitudinal spin
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

3. Parity Violating Asymmetry2 +
Applications of PV :
Nucleon Structure (strangeness) -- HAPPEX / G0
Standard Model Tests ( ) -- e.g. Qweak
Nuclear Structure (neutron density) : PREX
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

4. Physics Impact PREX Mean Field & Other Atomic Parity Violation Models
Measured Asymmetry
PREX
Physics Impact
Correct for Coulomb
Distortions
Weak Density at one Q 2
Mean Field
Small Corrections for n s
& Other
Atomic Parity Violation G G
MEC E E
Models 2
Neutron Density at one Q
Assume Surface Thickness Good to 25% (MFT)
Neutron
Stars R n
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

5. PREX in Hall A at JLab Spectometers Lead Foil Target Hall A CEBAF
Pol. Source
Spectometers
Lead Foil Target
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

6. Impact on Nuclear Physics: What is the size of a nucleus ?
Is the size of a heavy nucleus determined by neutrons or by protons ?
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

7. Reminder: Electromagnetic Scattering determines
(charge distribution)
208 Pb
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

8. Z of weak interaction : sees the neutrons
Z of weak interaction : sees the neutrons
Analysis is clean, like electromagnetic scattering:
1. Probes the entire nuclear volume
2. Perturbation theory applies
proton
neutron
Electric charge 1
Weak charge
0.08
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

9. Parity Violating Asymmetry
Electron - Nucleus Potential
electromagnetic
axial
is small, best observed by parity violation
neutron weak charge >> proton weak charge
Proton form factor
Neutron form factor
Parity Violating Asymmetry
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

10. Neutron Densities Proton-Nucleus Elastic Pion, alpha, d Scattering
Pion Photoproduction
Magnetic scattering
Theory Predictions
Involve strong probes
Most spins couple to zero.
Fit mostly by data other than neutron densities
Therefore, PREX is a powerful check of nuclear theory.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

11. !! Example : Recent Pion Photoproduction This paper obtains
B. Krusche arXiv:nucl-ex/ Sept 2005
This paper obtains !!
Proton – Nucleus Elastic:
Mean Field Theory
PREX accuracy
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

12. PREX: Measurement at one Q is sufficient to measure R2
Measurement at one Q is sufficient to measure R N
( R.J. Furnstahl )
Why only one parameter ?
(next slide…)
PREX error bar
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

13. Pb PREX: pins down the symmetry energy (1 parameter) PREX error bar
energy cost for unequal # protons & neutrons
( R.J. Furnstahl )
PREX error bar
208 Pb
PREX
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

14. Impact on Atomic Parity
Measures atomic overlap with weak charge.
Neutrons carry most weak charge
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

15. APV Atomic Parity Violation Low Q test of Standard Model
Needs R to make further progress. 2
Isotope Chain Experiments e.g. Berkeley Yb N
APV
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

16. Impact on Neutron Stars
What is the nature of extremely dense matter ?
Do collapsed stars form “exotic” phases of matter ?
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

17. Inputs: Eq. of state (EOS) PREX constraint Hydrostatics (Gen. Rel.)
pressure
density
Inputs:
Eq. of state (EOS)
PREX constraint
Hydrostatics (Gen. Rel.)
Typ. Astro. Observations
Luminosity L
Temp. T
Mass M from pulsar timing
(with corrections … )
Mass - Radius relationship
Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

18. PREX & Neutron Stars Crab Pulsar
( C.J. Horowitz, J. Piekarweicz )
R calibrates EOS of Neutron Rich Matter N
Crust Thickness
Explain Glitches in Pulsar Frequency ?
Combine PREX R with Obs. Neutron Star Radii N
Phase Transition to “Exotic” Core ?
Strange star ? Quark Star ?
Some Neutron Stars seem too Cold
Cooling by neutrino emission (URCA)
Crab Pulsar
0.2 fm URCA probable, else not
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

19. Liquid/Solid Transition DensityFP
TM1
Solid
Liquid
Neutron Star Crust vs Pb Neutron Skin
Neutron Star
208Pb
PREX calibrates the EOS at subnuclear densities.
Thicker neutron skin in Pb means energy rises rapidly with density  Quickly favors uniform phase.
Thick skin in Pb  low transition density in star.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

20. 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…
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

21. 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+
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

22. PREX: Experiment Design
Spokespersons:
P.A. Souder, G.M. Urciuoli, R. Michaels
Hall A Collaboration Experiment
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

23. Hall A at Jefferson Lab Polarized e- Source Hall A R. Michaels
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

24. Hall A Compton Moller Polarimeters Target Spectro: SQQDQ Cherenkov
cones
PMT
Compton
Moller
Polarimeters
Target
Spectro: SQQDQ
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

25. High Resolution Spectrometers
Spectrometer Concept:
Resolve Elastic
Elastic
detector
Inelastic
Quad
target
Dipole
Q Q
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

26. Optimum Kinematics for Lead Parity: E = 850 MeV,
<A> = 0.5 ppm. Accuracy in Asy 3%
Fig. of merit
Min. error in R
maximize: n
1 month run
1% in R n
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

27. Corrections to the Asymmetry are Mostly Negligible
Horowitz, et.al. PRC
Coulomb Distortions ~20% = the biggest correction.
Strangeness
Electric Form Factor of Neutron
Parity Admixtures
Dispersion Corrections
Meson Exchange Currents
Shape Dependence
Isospin Corrections
Radiative Corrections
Excited States
Target Impurities
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

28. Septum Magnets (INFN) Superconducting magnets Commissioned 2003-4
Electrons scattered
at 6 deg sent to the
HRS at 12.5 deg.
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

29. Integrating Detection
Integrate in 30 msec helicity period.
Deadtime free.
18 bit ADC with < nonlinearity.
But must separate backgrounds & inelastics ( HRS).
- 4
Integrator
Calorimeter (for lead, fits in palm of hand)
ADC
PMT
electrons
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

30. Lead Target Pb C 208 12 Diamond Backing: High Thermal Conductivity
Liquid Helium Coolant 12
beam C
Diamond Backing:
High Thermal Conductivity
Negligible Systematics
Beam, rastered 4 x 4 mm
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

31. Polarized Electron Source
Laser
GaAs Crystal
Gun
Pockel Cell flips helicity
Halfwave plate
(retractable, reverses helicity) -
e beam
Rapid, random helicity reversal
Electrical isolation from rest of lab
Feedback on Intensity Asymmetry
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

32. P I T A Effect at Polarized Source
Polarization Induced Transport Asymmetry
(G. D. Cates)
Intensity Asymmetry
Laser at Pol. Source
where
Transport Asymmetry
drifts, but slope is ~stable.
Feedback on
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

33. Araw = Adet - AQ + E+ ixi
Beam Asymmetries
Araw = Adet - AQ + E+ ixi
natural beam jitter (regression)
beam modulation (dithering)
Slopes from
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

34. Helicity Correlated Differences: Position, Angle, Energy
BPM X1
Scale +/- 10 nm
slug
Position Diffs avg
~ 1 nm
Redundant Monitors
Stripline Monitors
Resonant Cavities
Negligible
Systematic Error
BPM X2
slug
BPM Y1
slug
BPM Y2
slug
“Energy” BPM
“slug” = ~1 day running
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

35. Polarimetry Pe=86% ! Møller : dPe/Pe ~ 3 % (limit: foil polarization)
PREX:
1 % desirable
2 % required
Møller : dPe/Pe ~ 3 % (limit: foil polarization)
Compton : 2% syst. at present
Electron only
Photon only
Preliminary: 2.5% syst (g only)
2 analyses based
on either electron
or photon detection
Superlattice:
Pe=86% !
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

36. Upgrade of Compton Polarimeter (Nanda, Lhuillier)
electrons
To reach 1% accuracy:
Green Laser (increased sensitivity at low E)
Integrating Method (removes some systematics of analyzing power)
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

37. Moller Polarimetry with Atomic Hydrogen Target
( E. Chudakov, V. Luppov, D. Crabb)
H atoms
Ultra Cold Traps
Polarization ~ 100%
Density
Lifetime > 10 min
Solenoid 8T
Trap
Polarimetry
beam
1% stat. err. in 30 min at A
Low background
High beam currents allowed (100 A)
Goal: ~ 0.5 % systematic error
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab

38. PREX : Summary Fundamental Nuclear Physics HAPPEX to demonstrate most
technical aspects
Polarimetry Upgrade needed
PREX test run Nov (this weekend !)
Experiment Runs in ?
PREX
UVa Seminar, Nov 2005
R. Michaels
Jefferson Lab