PREX

1
Lead ( Pb) Radius Experiment PREX
208
E 850 MeV, electrons on lead
Elastic Scattering Parity Violating Asymmetry
0
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)
2
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
3

• Parity Violating Asymmetry

2

Applications of PV

• Nucleon Structure (strangeness) -- HAPPEX /
  G0
• Standard Model Tests ( ) --
  e.g. Qweak
• Nuclear Structure (neutron density) PREX

4
Measured Asymmetry
PREX
Physics Impact
Correct for Coulomb
Distortions
2
Weak Density at one Q
Mean Field
Small Corrections for
s
n
Other
G
G
MEC
Atomic Parity Violation
E
E
Models
2
Neutron Density at one Q
Assume Surface Thickness Good to 25 (MFT)
Neutron Stars
R
n
5
PREX in Hall A at JLab
Spectometers
Lead Foil Target
6
Impact on Nuclear PhysicsWhat is the
size of a nucleus ?
Is the size of a heavy nucleus determined
by neutrons or by protons ?
7
Reminder Electromagnetic Scattering
determines
(charge distribution)
208
Pb
8
Z of weak interaction sees the neutrons
0
Analysis is clean, like electromagnetic
scattering 1. Probes the entire nuclear
volume 2. Perturbation theory applies
proton neutron
Electric charge 1 0
Weak charge 0.08 1
9
Electron - Nucleus Potential
axial
electromagnetic
is small, best observed by
parity violation
neutron weak charge gtgt proton weak charge
Neutron form factor
Proton form factor
Parity Violating Asymmetry
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.
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Example Recent Pion Photoproduction
B. Krusche arXivnucl-ex/0509003 Sept 2005
This paper obtains
!!
Proton Nucleus Elastic
Mean Field Theory
PREX accuracy
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PREX
2
Measurement at one Q is sufficient to
measure R
N
( R.J. Furnstahl )
Why only one parameter ? (next slide)
PREX error bar
13
PREX
pins down the symmetry energy (1 parameter)
energy cost for unequal protons
neutrons
PREX error bar
( R.J. Furnstahl )
208
Pb
PREX
14
Impact on Atomic Parity
Measures atomic overlap with weak
charge. Neutrons carry most weak charge
15

• 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
16
Impact on Neutron Stars
What is the nature of extremely dense
matter ? Do collapsed stars form exotic
phases of matter ?
17
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.
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PREX Neutron Stars
( 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)
0.2 fm URCA probable, else not
Crab Pulsar
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Liquid/Solid Transition Density
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.

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

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-Rplt0.2 fm all EOS models do not have
  direct URCA in 1.4 M stars.
• If Rn-Rpgt0.25 fm all models do have URCA in
  1.4 M stars.

Rn-Rp in 208Pb
If Yp gt red line NS cools quickly via direct URCA
reaction n pe?
22
PREX Experiment Design
Spokespersons P.A. Souder, G.M. Urciuoli,
R. Michaels
Hall A Collaboration Experiment
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Hall A at Jefferson Lab
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Hall A
Spectro SQQDQ
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High Resolution Spectrometers
Spectrometer Concept Resolve Elastic
Elastic
detector
Inelastic
Quad
target
Dipole
Q Q
26
Optimum Kinematics for Lead Parity E
850 MeV,
ltAgt 0.5 ppm. Accuracy in Asy 3
Fig. of merit
Min. error in R maximize
n
1 month run 1 in R
n
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Corrections to the Asymmetry are Mostly
Negligible
Horowitz, et.al. PRC 63 025501

• 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

28
Septum Magnets (INFN)

• Superconducting magnets
• Commissioned 2003-4

Electrons scattered at 6 deg sent to the HRS at
12.5 deg.
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Integrating Detection

• Integrate in 30 msec helicity period.
• Deadtime free.
• 18 bit ADC with lt 10 nonlinearity.
• But must separate backgrounds inelastics
  ( HRS).

- 4
Integrator
Calorimeter (for lead, fits in palm of hand)
ADC
PMT
electrons
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Lead Target
208
Pb
Liquid Helium Coolant
12
beam
C
Diamond Backing

• High Thermal Conductivity

• Negligible Systematics

Beam, rastered 4 x 4 mm
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Polarized Electron Source
Laser
GaAs Crystal
Halfwave plate (retractable, reverses
helicity)
Pockel Cell flips helicity
Gun
-
e beam

• Rapid, random helicity reversal
• Electrical isolation from rest of lab
• Feedback on Intensity Asymmetry

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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
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Beam Asymmetries
Araw Adet - AQ ??E ??i?xi

• natural beam jitter (regression)
• beam modulation (dithering)

Slopes from
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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
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Polarimetry
PREX 1 desirable 2 required
Møller dPe/Pe 3 (limit foil
polarization) Compton 2 syst.
at present
2 analyses based on either electron or photon
detection
Superlattice Pe86 !
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)

37
Moller Polarimetry with Atomic
Hydrogen Target
( E. Chudakov, V. Luppov, D. Crabb)
H atoms
Ultra Cold Traps

• Polarization 100
• Density
• Lifetime gt 10 min

Solenoid 8T
Trap
beam
Polarimetry

• 1 stat. err. in 30 min at 30 A
• Low background
• High beam currents allowed (100 A)
• Goal 0.5 systematic error

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PREX Summary

• Fundamental Nuclear Physics
• HAPPEX to demonstrate most
• technical aspects
• Polarimetry Upgrade needed
• PREX test run Nov 2005 (this weekend !)
• Experiment Runs in 2007 ?