r. michaels, jlab seminar, apr 27, 2011 lead ( pb) radius experiment : prex 208 208 pb e = 1 gev,...

R. Michaels, Jlab Seminar, Apr 27, 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)

First Results from


Standard Electroweak Model

Gauge bosons

Left –handed fermion fields (quarks and leptons) = doublets under SU(2)

Right-handed fields = singlets under SU(2)

Also

3 fermion families + single complex Higgs doublet

Spontaneous symmetry breaking of the Lagrangian.

Symmetries Lagrangian

iW Bi = 1,2,3 and

Parity Violation


A piece of the weak interaction violates parity (mirror symmetry)

which allows to isolate it.

Negative longitudinal spin

Positive longitudinal spin

Pb208

P

S (spin)

(momentum)

Incident electron

Target


Parity Violating Asymmetry 624 10~10~

QALR

LRPV

0Z

e e

+

2

Applications of PV at Jefferson Lab

• Nucleon Structure (strangeness) -- HAPPEX / G0

• Standard Model Tests -- Qweak, Moller

• Nuclear Structure (neutron density) : PREX

W2sin

Applications of PV at Jefferson Lab


)(sin41

22 22

2

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

5


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

r

Pb208

(charge distribution)

1 2 3


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

10


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.


)()()(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


rdrZrNG

H eePWNF

PNC35/2 )()sin41()(

22

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

• Needs RN (or APV measures RN )

2

0

APV

Isotope Chain Experiments e.g. Berkeley Yb

17


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 :


)(P

Fig from: Dany Page.

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

Inputs:

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

19


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 ?


PREX: The entire lab is the experiment

CEBAF

Hall A

Pol. Source

Lead Foil Target

Spectometers

21


Hall A at Jefferson Lab

Polarized e-

SourceHall A


How to do a Parity Experiment

Flux Integration Technique:HAPPEX: 2 MHzPREX: 500 MHz

(integrating method)

Example : HAPPEX


/)( AA

Pull Plot (example)

PREX Data


Halfwave plate (retractable, reverses helicity)

LaserPockel Cell flips helicity

Gun

GaAs Crystal

e beam-

• Rapid, random helicity reversal

• Electrical isolation from rest of lab

• Feedback on Intensity Asymmetry

Polarized Electron Source


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 :


Intensity FeedbackAdjustmentsfor small phase shiftsto make close tocircular polarization

Low jitter and high accuracy allows sub-ppmcumulative charge asymmetry in ~ 1 hour

~ 2 hours

27


Methods to Reduce Systematics

Voltage change of 58 Volts, added to both the + and - voltages, would zero the asymmetry.

A rotatable /2 waveplate downstream of the P.C. allows arbitrary orientation of DoLP

A simplified picture: asymmetry=0 corresponds

to minimized DoLP at analyzer

Perfect DoCP

Scanning the Pockels Cell Scanning the Pockels Cell voltage = scanning the voltage = scanning the retardation phase = retardation phase = scanning residual DoLPscanning residual DoLP

Inte

nsit

y A

sym

metr

y

(pp

m)

Pockels cell voltage offset (V)


• 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)

Joe Grames, et. al.


“Spin Flipper”

Joe Grames et.al.

V-Wien = +90°

MFG1I04A = -45° or +45°

MFG1I04B = -45° or +45°


PAVI 09

Beam AsymmetriesA

raw = A

det - A

Q +

E+

ix

i

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

Slopes from

31


Charge Asymmetry 0.087 +/- 0.151 ppm

Slug # ( ~ 1 day)

ppm


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


Detector slopesmeasured during run

Araw

= Adet

- AQ +

E+

ix

i


Hall A Compton Upgrade

with Green Laser

Sirish Nanda, et. al.

1 % Polarimetry

at 1 GeV


PREX Compton Polarimeter Results


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.


Moller Results Example : Comparing old and new DAQ

Thanks, Zafar Ahmed

38


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


Septum Magnet

target

HRS-L

HRS-Rcollimator

50 Septum magnet augments the High Resolution Spectrometers

Increased Figure of Merit

collimator


Collimators inside Q1

Symmetry in all dimensions to 1 mm


Collimators

Septum Magnet

Top view

target

HRS-L

Q1

HRS-R

Q1

PREX Region After Target

O-Rings which failed and caused significant time loss


Water Cell : Measure Nilanga Liyanage, Seamus Riordan, Kiadtisak Saenboonruang

(agrees with survey)


PAVI 09

Q2 Measurements

Use constructed position at target for precise correction of beam position

44


PREX Integrating Detectors

DETECTORS

UMass / Smith

Detector Package in HRS


High Resolution Spectrometers

Momentum (GeV/c)

4.4 MeV

12C


Pure, Thin 208 Pb Target

Momentum (GeV/c)

Lead 5- State

2.6 MeV


Detector integrates the elastic peak.

Backgrounds from inelastics are suppressed.

Momentum (GeV/c)


Backgrounds that might re-

scatter into the detector ?

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.

Detector cutoff


Diamond LEAD

• Three bays

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

• Liquid He cooling (30 Watts)

Lead / Diamond Target

50


Initial Running

After ~ 5 days

Non-stat Noise !

Synching RasterEliminated Noise due to Target Non-uniformity

Flat

NOT Flat

This eliminated the ….


PAVI 09

Target Density

Intact

MeltedFailure happens fairly suddenly after ~1 week

Y

Y

X

X52


PAVI 09

Better

Best

Target thickness vs run #

Thick diamond target degraded the least 0 +/- 1%

4.5 % 5.6 %

8 %

MeltsMelts


integratewait wait

Beam Current Detector (1 of 4)

An instability in Pockel Cell “bleeds” into the itegration gate. Depends on helicity.

Pockel Cell Error : Affected 1st week (discarded data)

Want small time constants, and same for detectors and bcm

time time

Response to pulsed beam


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

Still very

preliminary


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


PREX Errors in the asymmetry

Absolute (ppm) Relative (%)Polarization 0.007 1.1%Detector Linearity 0.007 1.1%BCM Linearity 0.007 1.1%Re-scattering 0 0Transverse Polarization 0.001 0.2%Q2 0.002 0.4%Target Thickness 0.001 0.2%Asymmetry 12C 0.003 0.5%Inelastics 0 0

TOTAL SYSTEMATIC 0.012 2.1%

STATISTICAL ERROR 0.051 9.0%

Asymmetry leads to RN

( theory curve, not integrated over acceptance )

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

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

Preliminary estimate from C.J. Horowitz

60


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


PAVI 09

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 Target!!A 100at ly testedSuccessful

PAVI 09

Liquid Helium Coolant

Pb

C

208

12

Diamond Backing:

• High Thermal Conductivity• Negligible Systematics

Beam, rastered 4 x 4 mm

beam

(2 x I in proposal)


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)


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


Luminosity Monitor

A source of extremely high rate

Established noise floor of 50 ppm


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…

PAVI 09

(slide from C. Horowitz)


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.

PAVI 09

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(

PAVI 09

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.


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.

PAVI 09

Rn-Rp in 208Pb

If Yp > red line NS cools quickly via direct URCA reaction n p+e+

(slide from C. Horowitz)


PAVI 09

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

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

1 GeV optimum

r. michaels, jlab seminar, apr 27, 2011 lead ( pb) radius experiment : prex 208 208 pb e = 1 gev,...

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