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

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

G.M. Urciuoli

strmb

dd

1fmq

Reminder: Electromagnetic Scattering determines

r

r

Pb208

(charge distribution)

1 2 3

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

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.

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

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

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 ?

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

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…

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

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

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.

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

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

G.M. Urciuoli

Hall A at Jefferson Lab

Polarized e-

SourceHall A

G.M. Urciuoli

PREX in Hall A at JLab

CEBAFHall A

Pol. Source

Lead Foil Target

Spectometers

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

Experimental Method

Flux Integration Technique:HAPPEX: 2 MHzPREX: 850 MHz

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

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 :

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

G.M. Urciuoli

Beam AsymmetriesAraw = Adet - AQ + E+ ixi

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

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“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.

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

G.M. Urciuoli

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

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

G.M. Urciuoli

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.

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

G.M. Urciuoli

G.M. Urciuoli

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

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.

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)

• 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

G.M. Urciuoli

Spares

G.M. Urciuoli

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

G.M. Urciuoli

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

1 GeV optimum

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)

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.