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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

• Guido Urciuoli

• Robert Michaels

Run in March 2010 !

PAVI 09

R. MichaelsPREX at

)(

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

Idea behind 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) :

0

PAVI 09

R. MichaelsPREX at

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

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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 ?

PAVI 09

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str

mb

d

d

1fmq

Reminder: Electromagnetic Scattering determines

r

r

Pb208

(charge distribution)

1 2 3

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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

PAVI 09

R. MichaelsPREX at

neutron weak charge >> proton weak charge

is small, best observed by parity violation

)()()(ˆ5 rArVrV

||)()(/

//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

)(

)(sin41

22 2

22

2

QF

QFQG

d

d

d

d

d

d

d

d

AP

NW

F

LR

LR

Electron - Nucleus Potential

electromagnetic axial

Neutron form factor

Parity Violating Asymmetry

)(rA

1sin41 2 W

Proton form factor

0

Pb is spin 0208

PAVI 09

R. MichaelsPREX at

( R.J. Furnstahl )

Measurement at one Q is sufficient to measure R

2

N

proposed error *

Why only one parameter ?

(next slide…)

PREX:

* 2/3 this error if 100 uA, dPe/Pe = 1%

PAVI 09

R. MichaelsPREX at

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

Slide adapted from J. Piekarewicz

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Skx-s15Thanks, Alex Brown

PREX Workshop 2008

E/N

N

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Skx-s20Thanks, Alex Brown

PREX Workshop 2008

PAVI 09

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Skx-s25Thanks, Alex Brown

PREX Workshop 2008

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Interpretation

of PREX

Acceptance

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Application: Atomic Parity Violation• Low Q test of Standard Model

• Needs RN (or APV measures RN )

2

rdrZrNG

H eePWNF

PNC35/2 )()sin41()(

22

0

APV

Isotope Chain Experiments e.g. Berkeley Yb

PAVI 09

R. MichaelsPREX at

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 :

PAVI 09

R. MichaelsPREX at

Fig from: Dany Page.

J.M. Lattimer & M. Prakash, Science 304 (2004) 536.

)(PInputs:

Eq. of state (EOS)

Hydrostatics (Gen. Rel.)

Astrophysics Observations

Luminosity LTemp. TMass M from pulsar timing

PREX helps here

424 TRL B(with corrections … )

Mass - Radius relationship

pres

sure

dens

ity

PAVI 09

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PREX & Neutron Stars

Crab 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 pn RR

Phase Transition to “Exotic” Core ?

PAVI 09

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PREX Design

CEBAF

Hall A

Pol. Source

Lead Foil Target

Spectometers

PAVI 09

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Hall A at Jefferson Lab

Polarized e-

SourceHall A

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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

PAVI 09

R. MichaelsPREX at

Septum Magnet

target

HRS-L

HRS-Rcollimator

50 Septum magnet augments the High Resolution Spectrometers

Increased Figure of Merit

collimator

L / R HRS control vertical A_T.

What about horizontal ?

PAVI 09

R. MichaelsPREX at

Collimator at entrance to spectrometer

Be degrader

Insertable blockerto block Be

Aligned in spectrometers to define identical Left / Right scattering angles as well as good up / down symmetry

A_T hole

Suppressing A_T systematics

Paul Souder

PAVI 09

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Events from A_T hole (Be plug)

Main detector

A_T detector

Measures horizontal A_T systematic.

(Vertical is measured from Left/Right spectrometers)

Thanks:

Dustin McNulty

Krishna Kumar

Paul Souder

PAVI 09

R. MichaelsPREX at

Measure θ from Nuclear Recoil

AM

E

E

E

2

2

Recoil is large for H, small for nuclei

δE=Energy lossE=Beam energyMA=Nuclear massθ=Scattering angle

(3X better accuracy than survey)

Scattered Electron Energy (GeV)

PAVI 09

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P I T A Effect

)sin( IA Laser at Pol. Source

Polarization Induced Transport Asymmetry

yx

yx

TT

TT

where

Transport Asymmetry

Intensity Asymmetry

drifts, but slope is ~ stable. Feedback on

Important Systematic :

See talk by Gordon Cates

PAVI 09

R. MichaelsPREX at

• Two Wien Spin Manipulators in series• Solenoid rotates spin +/-90 degrees (spin rotation as B but focus as B2).

Flips spin without moving the beam !

Double Wien FilterCrossed E & B fields to rotate the spin

Electron Beam SPIN

(NEW for PREX)

See talk by Riad Suleiman

PAVI 09

R. MichaelsPREX at

Beam AsymmetriesA

raw = A

det - A

Q +

E+

ix

i

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

Slopes from

PAVI 09

R. MichaelsPREX at

* The mistake: Helicity signal deflecting the beam through electronics “pickup”

Helicity signal to driver reversed Helicity signal to

driver removed

X Angle BPM

Raw ALL Asymetry

mic

ron

pp

m

The Corrections Work !Shown : period of data during HAPPEX (4He) when beam had a helicity-correlated position due to a mistake * in electronics.

Heli

city

-corr

. P

osi

tion

diff

With corrections

PAVI 09

R. MichaelsPREX at

Final Beam Position Corrections (HAPPEX-H)X Angle BPM

Energy: -0.25 ppb

X Target: 1 nm

X Angle: 2 nm

Y Target : 1 nm

Y Angle: <1 nm

Beam Asymmetry Results

Corrected and Raw, Left spectrometer arm alone, Superimposed!

pp

mm

icro

n

Total correction for beam position asymmetry on Left, Right, or ALL detector: 10 ppbSpectacular results from HAPPEX-H show we can do PREX.

PAVI 09

R. MichaelsPREX at

X (

cavi

ty)

n

m

Y (

cavi

ty)

n

m

X (stripline) nm Y (stripline) nm

(ii) Resonant microwave cavities

(Helicity – correlated differences averaged over ~1 day) HAPPEX 2005

Redundant position measurements at the ~1 nm level

(i) Stripline monitors (2 pairs of wires)

PAVI 09

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PREX Integrating Detectors

DETECTORS

UMass / Smith

PAVI 09

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Lead Target

Liquid Helium Coolant

Pb

C

208

12

Diamond Backing:

• High Thermal Conductivity• Negligible Systematics

Beam, rastered 4 x 4 mm

!!A 100at ly testedSuccessful

beam

(2 x I in proposal)

PAVI 09

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Target Assembly

PAVI 09

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IA1

06

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

Nu

m.

eve

nts

Nu

m.

eve

nts

Low Rate

at E = 1.1 GeV

IA1

PAVI 09

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Noise (integrating at 30 Hz) • PREX: ~120 ppm

Want only counting statistics noise, all others << 120 ppm

• New 18-bit ADCs improve beam noise.

• Careful about cable runs, PMTs, grounds loops.

.Test: Use the “Luminosity Monitor” to demonstrate noise (next slide)

(HAPPEX data)

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Luminosity Monitor

A source of extremely high rate

Established noise floor of 50 ppm

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PREX : Summary

• Fundamental Nuclear Physics with many applications

• HAPPEX & test runs have demonstrated technical aspects

• Polarimetry Upgrade critical

• 2 month run starting March 2010

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Extra Slides

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“What if ’’ Scenarios

dPe/Pe

= 1%

dPe/Pe

= 2%

50 uA

30 days

dRN/RN

1 % 1.2 %

100 uA

60 days

dRN/RN

-

0.6 % 0.8 %

Assuming other systematics are negligible

PAVI 09

R. MichaelsPREX at

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

PAVI 09

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Polarimetry Accuracy : 1% from 2 methodsHydrogen: 86.7% ± 2%

Helium: 84.0% ± 2.5%

HAPPEX

Compton Polarimety Measuremens

Møller (New High-field design )

Compton

e

PAVI 09

R. MichaelsPREX at

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…

(slide from C. Horowitz)

PAVI 09

R. MichaelsPREX at

~ 50 ppm noise per pulse milestone for electronics

Asymmetries in Lumi Monitors

after beam noise subtraction

Jan 2008 Data

( need << 120 ppm)

PAVI 09

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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 Star Crust vs Pb Neutron Skin

208Pb

Neutron Star

C.J. Horowitz, J. Piekarawicz

PAVI 09

R. MichaelsPREX at

PREX: pins down the symmetry energy (1 parameter)

( R.J. Furnstahl )

energy cost for unequal # protons & neutrons.../ 3/1

2

4

AaA

ZNaa

A

Esv

PREX

PREX error bar

Pb208

)1(

Actually, it’s the density dependence of a4 that we pin down.

PAVI 09

R. MichaelsPREX at

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+

(slide from C. Horowitz)

PAVI 09

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How to Measure

Neutron Distributions, Symmetry Energy • Proton-Nucleus Elastic• Pion, alpha, d Scattering• Pion Photoproduction• Heavy ion collisions• Rare Isotopes (dripline)

• Magnetic scattering

• PREX (weak interaction)

• Theory MFT fit mostly by data other than neutron densities

Involve strong probes

Most spins couple to zero.

PAVI 09

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Target Spectro: SQQDQ

Hall ACherenkovcones

PMT

Compton Moller Polarimeters

PAVI 09

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Isospin Diffusion (NSCL)Probe the symmetry

energy in 124Sn + 112Sn

Heavy Ions(adapted from Betty Tsang, PREX Workshop)

PAVI 09

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At 50 the new Optimal FOM is at 1.05 GeV (+/- 0.05)(when accounting for collimating small angle, not shown)

1% @ ~1 GeV

PAVI 09

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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

PAVI 09

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Good energy resolution

Successful Results of beam tests Jan 2008

2

0 1

E

E

PAVI 09

R. MichaelsPREX at

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

PAVI 09

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Optimization for Barium -- of possible direct use for Atomic PV

1 GeV optimum

PAVI 09

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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