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Lead ( 208 P b) R adius Ex periment : PREX PowerPoint PPT Presentation


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Lead ( 208 P b) R adius Ex periment : PREX. Elastic Scattering Parity Violating Asymmetry . E = 1 GeV , electrons on lead. PAVI – 11 Conference. Spokespersons Kent Paschke, Paul Souder, Krishna Kumar,

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Lead ( 208 P b) R adius Ex periment : PREX

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Lead 208 p b r adius ex periment prex

Lead(208Pb) Radius Experiment : PREX

Elastic Scattering Parity Violating Asymmetry

E = 1 GeV, electrons on lead

PAVI – 11

Conference

Spokespersons

Kent Paschke, Paul Souder, Krishna Kumar,

Guido Urciuoli, Robert Michaels (presenting)

  • PREX-I Results 2010 Run

  • Approved PREX-II Proposal

  • Future : PREX-N ? (N = III, IV, V …)

Thomas Jefferson National Accelerator Facility


Z 0 of weak interaction sees the neutrons

Z0 of weak interaction : sees the neutrons

T.W. Donnelly, J. Dubach, I. Sick

Nucl. Phys. A 503, 589, 1989

C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels

Phys. Rev. C 63, 025501, 2001

Neutron form factor

C.J. Horowitz

Parity Violating Asymmetry


Hall a at jefferson lab

Hall A at Jefferson Lab

Hall A


Lead 208 p b r adius ex periment prex

Measured Asymmetry

PREX

Physics Output

Correct for Coulomb

Distortions

2

Weak Density at one Q

Mean Field

Small Corrections for

s

n

& Other

G

MEC

G

Atomic Parity Violation

E

E

Models

2

Neutron Density at one Q

Assume Surface Thickness Good to 25% (MFT)

Neutron

Stars

Slide adapted from C. Horowitz

R

n


Lead 208 p b r adius ex periment prex

Slide adapted from J. Piekarewicz

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

Reflects poor understanding of symmetry energy of nuclear matter = the energy cost of

ratio proton/neutrons

n.m. density

  • Slope unconstrained by data

  • Adding R from Pb will eliminate the dispersion in plot.

208

N

5


Lead 208 p b r adius ex periment prex

Thanks, Alex Brown

PREX Workshop 2008

Skx-s15

E/N


Lead 208 p b r adius ex periment prex

Thanks, Alex Brown

PREX Workshop 2008

Skx-s20

E/N


Lead 208 p b r adius ex periment prex

Thanks, Alex Brown

PREX Workshop 2008

Skx-s25

E/N

8


Lead 208 p b r adius ex periment prex

  • Application: Atomic Parity Violation

  • Low Q test of Standard Model

  • Needs RN(or APV measures RN )

2

Isotope Chain Experiments e.g. Berkeley Yb

APV

9


Prex neutron stars

PREX & Neutron Stars

C.J. Horowitz, J. Piekarewicz

RN calibrates equation of state (pressure vs density) of Neutron Rich Matter

Combine PREX RN with Observed Neutron Star Radii

Fig from: Dany Page.J.M. Lattimer & M. Prakash, Science 304 (2004) 536.

Phase Transition to “Exotic” Core ?

Strange star ?Quark Star ?

Some Neutron Stars seem too cold

Explained by Cooling by neutrino emission (URCA process) ?

0.2 fm URCA probable, else not

Crab Pulsar


Lead 208 p b r adius ex periment prex

Pol. Source

Hall A

JLAB

CEBAF

PREX Overview

Parity: “The entire lab is the experiment”

Spectometers

Lead Foil Target


Lead 208 p b r adius ex periment prex

Parity Quality Beam !

( why we love Jlab ! )

Helicity – Correlated Position Differences

< ~ 3 nm

Points: Not sign corrected

Average with signs = what exp’t feels

Units: microns

Slug # ( ~ 1 day)


Lead 208 p b r adius ex periment prex

Double Wien Filter

(NEW for PREX)

Crossed E & B fields to rotate the spin

  • 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 !

Electron Beam

SPIN

Joe Grames, et. al.

13


Lead 208 p b r adius ex periment prex

Hall A Compton Upgrade

with Green Laser

Sirish Nanda, et. al. (Megan Friend’s talk)

  • 1 % Polarimetry

  • at 1 GeV


Lead 208 p b r adius ex periment prex

Magnet and Target

Hall A Moller Upgrade

Superconducting Magnet from Hall C

Saturated Iron Foil Targets

< 1 % Polarimetry

Sasha Glamazdin, et.al. (talk Thurs)

DAQ Upgrade (FADC)


Hall a high resolution spectrometers

Hall A High Resolution Spectrometers

  • Resolve Elastic Scattering

  • Discriminate Excited States

Elastic

detector

Inelastic

Pure, Thin 208PbTarget

2.6 MeV

target

Dipole

DETECTOR footprint

Quads

Scattered Electron’s Momentum (GeV/c)

16


Lead 208 p b r adius ex periment prex

Lead / Diamond Target

Diamond

LEAD

  • Three bays

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

  • Liquid He cooling (30 Watts)


Lead 208 p b r adius ex periment prex

Performance of Lead / Diamond Targets

melted

melted

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

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

NOT melted

Last 4 days at 70 uA

Solution: Run with 10 targets.


Lead 208 p b r adius ex periment prex

y

AT > 0 means

-

x

+

z

Beam-Normal Asymmetry in elastic electron scattering

i.e. spin transverse to scattering plane

Possible systematic if small transverse spin component

New results PREX

Preliminary !

Publication in preparation

  • 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

19


Lead 208 p b r adius ex periment prex

PREX-I Result

Systematic Errors

  • Statistics limited ( 9% )

  • Systematic error goal achieved ! (2%)

(1) Normalization Correction applied

(2) Nonzero correction (the rest assumed zero)

20


Lead 208 p b r adius ex periment prex

PREX Asymmetry (Pe x A)

ppm

Slug ~ 1 day


Lead 208 p b r adius ex periment prex

Asymmetry leads to RN

Establishing a neutron skin at ~90 % CL

*

Neutron Skin = RN - RP = 0.31 + 0.15 - 0.17 fm

fig from C.J. Horowitz

PREX data

*

Preliminary: Awaiting the “final” acceptance function:


Lead 208 p b r adius ex periment prex

Neutron Skin = RN - RP = 0.31 + 0.15 - 0.17 fm

PREX-I Result, cont.

DATA

rN - rP (fm)

theory: P. Ring

rN = rP

Atomic Number, A

DATA

Preliminary: Awaiting the “final” acceptance function

A physics letter is in preparation for publication.

23


Lead 208 p b r adius ex periment prex

PREX-II Approved by PAC (Aug 2011)

“A” Rating 35 days run in 2013 / 2014


Lead 208 p b r adius ex periment prex

PREX Region After Target

Improvements for PREX-II

Tungsten Collimator & Shielding

HRS-L

Q1

Septum Magnet

target

HRS-R

Q1

Former O-Ring location which failed & caused

time loss during PREX-I

 PREX-II to use all-metal seals

Collimators


Lead 208 p b r adius ex periment prex

Geant 4 Radiation Calculations

PREX-II shielding strategies

J. Mammei, L. Zana

scattering chamber

shielding

Number of Neutrons per incident Electron

0 - 1 MeV

beamline

Energy (MeV)

--- PREX-I

--- PREX-II, no shield

--- PREX-II, shielded

1 - 10 MeV

  • Strategy

  • Tungsten ( W) plug

  • Shield the W

  • x 10 reduction in

  • 0.2 to 10 MeV neutrons

Energy (MeV)

10 - 1200 MeV

Energy (MeV)

26


Lead 208 p b r adius ex periment prex

Other Nuclei ?

RN

Shape Dependence ?

Surface thickness

Parity Violating Electron Scattering Measurements of Neutron Densities

Shufang Ban, C.J. Horowitz, R. Michaels

RN

Surface thickness

arXiv:1010.3246  [nucl-th]


Lead 208 p b r adius ex periment prex

Possible Future PREX Program ?

Each point 30 days stat. error only

Not yet proposed.

Just a “what if ?”

Shufang Ban, C.J. Horowitz, R. Michaels

arXiv:1010.3246  [nucl-th]


Future in hall a at jlab

Future in Hall A at JLab

Early Experiments

g2p/GEp

12 mo. Shutdown

Commissioning

no promised beam

SuperBigbite

$

Beam 1st to Hall A

Moller

$$$

SOLID

$$$$

PREX – II ?

2014

2012

2017

2018

2011

2013

2015

2016


Prex summary

PREX : Summary

  • Fundamental Nuclear Physics with many applications

  • PREX-I 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

  • PREX-II approved (runs in 2013 or 2014 we hope )


Extra slides

Extra Slides


How to measure neutron distributions symmetry energy

How to MeasureNeutron Distributions, Symmetry Energy

  • Proton-Nucleus Elastic

  • Pion, alpha, d Scattering

  • Pion Photoproduction

  • Heavy ion collisions

  • Rare Isotopes (dripline)

  • Magnetic scattering

  • PREX(weak interaction)

  • Theory

Involve strong probes

Most spins couple to zero.

MFT fit mostly by data other than neutron densities


Lead 208 p b r adius ex periment prex

Electron - Nucleus Potential

axial

electromagnetic

is small, best observed by parity violation

208

Pb is spin 0

neutron weak charge >> proton weak charge

Proton form factor

Neutron form factor

Parity Violating Asymmetry


Lead 208 p b r adius ex periment prex

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


Lead 208 p b r adius ex periment prex

Optimum Kinematics for Lead Parity: E = 1 GeV if

<A> = 0.5 ppm. Accuracy in Asy 3%

Fig. of merit

Min. error in R

maximize:

n

1 month run

1% in R

n

(2 months x 100 uA

 0.5% if no systematics)

5


Lead 208 p b r adius ex periment prex

Pull Plot (example)

PREX Data


Liquid solid transition density

Liquid

FP

Solid

TM1

Liquid/Solid Transition Density

Neutron Star Crust vs Pb Neutron Skin

C.J. Horowitz, J. Piekarawicz

  • 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

208Pb


Lead 208 p b r adius ex periment prex

Backgrounds that might re-scatter into the detector ?

Detector cutoff

Run magnets down: measure inelastic region

Run magnets up : measure probability to rescatter

No inelastics observed on top of radiative tail. Small systematic for tail.


Lead 208 p b r adius ex periment prex

PREX:

pins down the symmetry energy (1 parameter)

energy cost for unequal # protons & neutrons

PREX error bar

( R.J. Furnstahl )

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

208

Pb

PREX


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