R. Michaels, Jlab UGM, June, 2011 Lead ( Pb) Radius Experiment : PREX 208 208 Pb E = 1 GeV,...

R. Michaels, Jlab UGM, June, 2011

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

Ran March – June 2010 in Hall A

Graduate Students

Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse)

Jon Wexler (UMass)

Kiadtisak Saenboonruang (UVa)


)(

sin4122 2

22

QF

QG

dd

dd

dd

dd

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 1989 )

0

In PWIA (to illustrate) :

w/ Coulomb distortions (C. J. Horowitz) :

0


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


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 ?


str

mb

d

d

1fmq

Reminder: Electromagnetic Scattering determines

r

rPb

208

(charge distribution)

1 2 3

d

d


Z of weak interaction : sees the neutrons

proton neutron

Electric charge 1 0

Weak charge 0.08 1

0

Analysis is clean, like electromagnetic scattering:

1. Probes the entire nuclear volume

2. Perturbation theory applies

6


)()()(ˆ5 rArVrV

)(

)(sin41

22 2

22

2

QF

QFQG

d

d

d

d

d

d

d

d

AP

NW

F

LR

LR

neutron weak charge >> proton weak charge

is small, best observed by parity violation

||)()(/

//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

Electron - Nucleus Potential

electromagnetic axial

Neutron form factor

Parity Violating Asymmetry

)(rA

1sin41 2 W

Proton form factor

0

Pb is spin 0208


( R.J. Furnstahl )

Measurement at one Q is sufficient to measure R

2

N

proposed error

Why only one parameter ?

(next slide…)

PREX:


ZN

Nuclear Structure: Neutron density is a fundamental observable that remains elusive.

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


Skx-s15Thanks, Alex Brown

PREX Workshop 2008

E/N

N



PREX Workshop 2008


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

• Needs RN (or APV measures RN )

2

0

Isotope Chain Experiments e.g. Berkeley Yb

13

APV

rdrZrNG

H eePWNF

PNC35/2 )()sin41()(

22


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 :


PREX & Neutron Stars

pn RRCrab 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

Phase Transition to “Exotic” Core ?


How to do a Parity Experiment

Flux Integration Technique:HAPPEX: 2 MHzPREX: 500 MHz

(integrating method)

Example : HAPPEX


/)( AA

Pull Plot (example)

PREX Data


PAVI 09

Beam AsymmetriesA

raw = A

det - A

Q +

E+

ix

i

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

Slopes from

18


Slug # ( ~ 1 day)

Units: microns

RLhelicityforXX LR ,

Parity Quality Beam !

Helicity – Correlated Position Differences

< ~ 3 nm

Wien Flips helped !Average with signs = what exp’t feels

Points: Not sign corrected


Hall A Compton Upgrade

with Green Laser

Sirish Nanda, et. al.

1 % Polarimetry

at 1 GeV


Hall A Moller Upgrade

Superconducting Magnet from Hall C

Saturated Iron Foil Targets

1 % Polarimetry

Magnet and Target

DAQ Upgrade (FADC)

Sasha Glamazdin, et.al.


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


Collimators

Septum Magnet

Top view

target

HRS-L

Q1

HRS-R

Q1

PREX Region After Target

Former O-Ring location which failed and caused significant time loss

Planned Tungsten Collimator and Shielding


10 m

-1 m

Geant 4 Radiation Calculations

• benchmarking against LD2 (ran at 100 uA)

• exploring shielding strategies

J. Mammei, L. Zana

Number of Neutrons per incident Electron

Z (distance along beamline)

1 mLD2 tgtPb tgt

1 - 50 MeV

50 - 500 MeV

0 - 1 MeV

Strategy

• Tungsten ( W ) plug

• Shield the W

• x10 reduction in 0.2 to 10 MeV neutrons

(similar to LD2)

00 37.0

W

PREX-II proposal

PREX -I

PREX -II


Pure, Thin 208 Pb Target

Momentum (GeV/c)

Lead 5- State

2.6 MeV

High Resolution Spectrometers

DETECTOR footprint

25


Diamond LEAD

• Three bays

• Lead (0.5 mm) sandwiched by diamond (0.15 mm)

• Liquid He cooling (30 Watts)

Lead / Diamond Target


Performance of Lead / Diamond Targets

Last 4 days at 70 uA

Targets with thin diamond backing (4.5 % background) degraded fastest.

Thick diamond (8%) ran well and did not melt at 70 uA.

meltedmelted

NOT melted

Solution: Run with 10 targets.


y

z

xS

k

+ -

AT > 0 means

)'( eeeT kkSA

Beam-Normal Asymmetry in elastic electron scatteringi.e. spin transverse to scattering plane

Possible systematic if small transverse spin component

New results PREX

ppmAC T 35.036.052.6:12

ppmAPb T 36.019.013.0:208

• Small AT for 208Pb is a big (but pleasant) surprise. • AT for 12C qualitatively consistent with 4He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz


Error Source Absolute (ppm) Relative ( % )

Polarization (1) 0.0071 1.1

Beam Asymmetries (2) 0.0072 1.1

Detector Linearity 0.0071 1.1

BCM Linearity 0.0010 0.2

Rescattering 0.0001 0

Transverse Polarization 0.0012 0.2

Q2 (1) 0.0028 0.4

Target Thickness 0.0005 0.112C Asymmetry (2) 0.0025 0.4

Inelastic States 0 0

TOTAL 0.0130 2.0

Systematic Errors

(1) Normalization Correction applied

(2) Nonzero correction (the rest assumed zero)


PREX Physics Result

LR

LRPVA

)(0130.0)(0604.06571.0 syststat ppm

at Q2 = 0.00906 GeV2

Statistics limited ( 9% )

Systematic error goal achieved ! (2%)

9.2 % 2.0 %


PREX Asymmetry (Pe x A)

ppm

Slug ~ 1 day

(blinded, raw)


PREX Physics Interpretation


Asymmetry leads to RN

Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm

arXiv:1010.3246 [nucl-th] Shufang Ban, C.J. Horowitz, R. Michaels

Establishing a neutron skin at 95% CL


PREX – II Proposal

Future ?


Future ?


Other Nuclei ?

Shape Dependence ?RN

RN

Surface thickness

Surface thickness

arXiv:1010.3246 [nucl-th]

Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels


PREX : Summary • Fundamental Nuclear Physics with

many applications

• Achieved a 9% stat. error in Asymmetry (original goal : 3 %)

• Systematic Error Goals Achieved !!

• Significant time-losses due to O-Ring problem and radiation damage

• Proposal for PREX-II in preparation


Extra Slides


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

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


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.

FP

TM1Solid

Liquid

Neutron Star Crust vs Pb Neutron Skin

208Pb

Neutron Star

C.J. Horowitz, J. Piekarawicz


.../ 3/12

4

AaA

ZNaa

A

Esv

)1(

PREX: pins down the symmetry energy (1 parameter)

( R.J. Furnstahl )

energy cost for unequal # protons & neutrons

PREX

PREX error bar

Pb208 Actually, it’s the

density dependence of a4 that we pin down.

R. Michaels, Jlab UGM, June, 2011 Lead ( Pb) Radius Experiment : PREX 208 208 Pb E = 1 GeV,...

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