PREXPAVI06 May 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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

Impact on Nuclear Physics:

What is the size of a nucleus ?

Is the size of a heavy nucleus determined by neutrons or by protons ?

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

str

mb

d

d

1fmq

Reminder: Electromagnetic Scattering determines

r

r

Pb208

(charge distribution)

1 2 3

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

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.

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

( R.J. Furnstahl )

Measurement at one Q is sufficient to measure R

2

N

PREX error bar

Why only one parameter ?

(next slide…)

PREX:

)1(

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

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(

PREXPAVI06 May 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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

Impact on Neutron Stars

What is the nature of extremely dense matter ?

Do collapsed stars form “exotic” phases of matter ?

PREXPAVI06 May 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 ?

PREXPAVI06 May 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.

PREXPAVI06 May 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…

( C.J. Horowitz, J. Piekarweicz )

PREXPAVI06 May 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+

( C.J. Horowitz, J. Piekarweicz )

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

Impact on Atomic Parity

Measures atomic overlap with weak charge.

Neutrons carry most weak charge

PREXPAVI06 May 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

PREXPAVI06 May 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

PREXPAVI06 May 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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

PREX: Experimental Issues

Spokespersons:

P.A. Souder, G.M. Urciuoli, R. Michaels

Hall A Collaboration Experiment

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

PREX in Hall A at JLab

CEBAF

Hall A

Pol. Source

Lead Foil Target

Spectometers

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

Hall A at Jefferson Lab

Polarized e-

SourceHall A

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

High Resolution Spectrometers

Elastic

Inelastic

detector

Q Q

Dipole

Quad

Spectrometer Concept:

Resolve Elastic

target

Left-Right symmetry to control transverse polarization systematic

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

Integrating Detection

PMT

Calorimeter (for lead, fits in palm of hand)

ADC

Integrator

electrons

• Integrate in 30 msec helicity period.

• Deadtime free.

• 18 bit ADC with < 10 nonlinearity.

• But must separate backgrounds & inelastics ( HRS).

- 4

PREXPAVI06 May 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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

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

1 GeV optimum

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

Beam Asymmetries

Araw

= Adet

- AQ +

E+

ix

i

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

PREXPAVI06 May 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.

PREXPAVI06 May 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)

PREXPAVI06 May 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

PREXPAVI06 May 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

PREXPAVI06 May 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

PREXPAVI06 May 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

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

PREX : Summary

• Fundamental Nuclear Physics with many applications

• HAPPEX & test runs have demonstrated technical aspects

• Polarimetry Upgrade needed

• Will run 1 month, perhaps in 2008

PREXPAVI06 May 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.

PREXPAVI06 May 2006

R. Michaels

Jefferson Lab

B. Krusche arXiv:nucl-ex/0509003 Sept 2005

Example : Recent Pion Photoproduction

This paper obtains

PN RR

Mean Field Theory fmRR PN 35.005.0

PREX accuracy

fmRN 05.0

!!

fmRR PN 111.0083.0 Proton – Nucleus Elastic:

PREXPAVI06 May 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.

PREXPAVI06 May 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

PREXPAVI06 May 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.

PREXPAVI06 May 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

PREXPAVI06 May 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