01),2$3& 4)#%-1$&5#,6-$3&,%&78-'1$&9,:;& +!4...

!"#"$%&!"'()%'&*$&+,-*%./0*1),2$3&4)"#%-1$&5#,6"-*$3&,%&7"8"-'1$&9,:;&

+!4<&,$=&>?++4</@@@

A"$%&+,'#BC"

?+5&5D-*$3&E""2$3?$,B"*FG&H?E,.&IG&JKII

>?++4<&H1)),:1-,21$+!4<&H1)),:1-,21$

A"$%&+,'#BC" ?+5&?D-*)&E""2$3G&?$,B"*FG&H?&/&E,.&IG&JKII

LB"&M"(%-,)&H(--"$%&,$=&M(#)"1$&0"#%1-&N1-F&N,#%1-'

5%-,$3"&O(,-C'&"P*'%&*$&%B"&$(#)"1$&,%&'B1-%&=*'%,$#"&'#,)"'Q&R1&%B".&D),.&,&-1)"&*$&"),'2#&D-1%1$&'#,6"-*$3S

?

r [fm]0 0.5 1 1.5 2 2.5 3 3.5 4

]-1

[fm

Bre

it

2 r4

0

0.2

0.4

0.6

0.8

1

1.2

r [fm]0 0.5 1 1.5 2 2.5 3 3.5 4

]-1

[fm

Bre

it

2 r4

-0.05

0

0.05

0.1

0.15

0.2

Figure 2.5: On the left is the distribution of the charge within the neutron, the combined result of experiments around the globe that use polarization techniques in electron scattering. On the right is that of the (much larger) proton distribution for reference. The widths of the colored bands represent the uncertainties. A decade ago, as described in the 1999 NRC report (The Core of Matter, the Fuel of Stars, National Academies Press [1999]), our knowledge of neutron structure was quite limited and unable to constrain calculations, but as promised, advances in polarization techniques led to substantial improvement.

But quarks can have a transverse spin preference, denoted as transversity. Because of e!ects of relativity, transversity’s rela-tion to the nucleon’s transverse spin orientation di!ers from the corresponding relationship for spin components along its motion. Quark transversity measures a distinct property of nucleon structure—associated with the breaking of QCD’s fundamental chiral symmetry—from that probed by helicity preferences. "e first measurement of quark transversity has recently been made by the HERMES experiment, exploiting a spin sensitivity in the formation of hadrons from scattered quarks discovered in electron-positron collisions by nuclear scientists in the BELLE Collaboration at KEK in Japan.

Fueled by new experiments and dramatic recent advances in theory, the entire subject of transverse spin sensitivities in QCD interactions has undergone a worldwide renaissance. In contrast to decades-old expectations, sizable sensitiv-ity to the transverse spin orientation of a proton has been observed in both deep-inelastic scattering experiments with hadron coincidences at HERMES and in hadron production in polarized proton-proton collisions at RHIC. "e latter echoed an earlier result from Fermilab at lower energies, where perturbative QCD was not thought to be applicable. At HERMES, but not yet definitively at RHIC, measure-ments have disentangled the contributions due to quark transverse spin preferences and transverse motion preferences within a transversely polarized proton. "e motional prefer-ences are intriguing because they require spin-orbit correla-

tions within the nucleon’s wave function, and may thereby illuminate the original spin puzzle. Attempts are ongoing to achieve a unified understanding of a variety of transverse spin measurements, and further experiments are planned at RHIC and JLAB, with the aim of probing the orbital motion of quarks and gluons separately.

"e GPDs obtained from deep exclusive high-energy reactions provide independent access to the contributions of quark orbital angular momentum to the proton spin. As described further below, these reaction studies are a promi-nent part of the science program of the 12 GeV CEBAF Upgrade, providing the best promise for deducing the orbital contributions of valence quarks.

The Spatial Structure of Protons and NeutronsFollowing the pioneering measurements of the proton

charge distribution by Hofstadter at Stanford in the 1950s, experiments have revealed the proton’s internal makeup with ever-increasing precision, largely through the use of electron scattering. "e spatial structure of the nucleon reflects in QCD the distributions of the elementary quarks and gluons, as well as their motion and spin polarization.

Charge and Magnetization Distributions of Protons and Neutrons. "e fundamental quantities that provide the simplest spatial map of the interior of neutrons and protons are the electromagnetic form factors, which lead to a picture of the average spatial distributions of charge and magnetism.

26 QCD and the Structure of Hadrons

4πr2

ρ Bre

it[fm

−1]

?''(F"&T&O(,-C&U,V1-&#1$%-*:(21$'&%1&V"#%1-&W1-F&W,#%1-';&X(G&X=G&X'

HB,-3"&5.FF"%-.

+T&"O(,21$'&

Y*%BT&($C$1Y$'

GZE,M = (1− 4 sin2 θW)Gp

E,M −GnE,M −Gs

E,M


E",'(-*$3&5%-,$3"&0"#%1-&N1-F&N,#%1-'

~ few parts per millionFor a proton:

Forward angle Backward angle

For a spin=0,T=0 4He: GsE only! For deuterium: Enhanced GA

! Z! 2

!

~ 10"4Q2

GeV2

“Anapole” radiative corrections are

problematic


4PD"-*F"$%,)&ZV"-V*"Y

5?E+94

>?++4<

>?++4</T;&X4'&[&KQ\J&XE

'&&,%&]J&^&KQ_J&X"0J

+-"#*'*1$&'D"#%-1F"%"-G&*$%"3-,2$3

?`

1D"$&3"1F"%-.G&*$%"3-,2$3G&:,#C/,$3)"&1$).

ZD"$&3"1F"%-.

N,'%&#1($2$3&#,)1-*F"%"-&W1-&:,#C3-1($=&-"a"#21$

N1-Y,-=&,$=&b,#CY,-=&,$3)"'

XK

&&&&&&&&ZD"$&3"1F"%-.

N,'%&#1($2$3&Y*%B&F,3$"2#&'D"#%-1F"%"-&[&LZN&W1-&:,#C3-1($=&-"a"#21$

N1-Y,-=&,$=&b,#CY,-=&,$3)"'&1V"-&,&-,$3"&1W&]J

N1-Y,-=&,$3)"G&,)'1&`>"&,%&)1Y&]J


c1-)=&=,%,&1$&X'

• “Form Factor” error: precision of EMFF (including 2!) and Anapole correction

• Significant systematic uncertainty in higher Q2 points

η =τ Gp

M

� GpE

∼ Q2

!!"# !! !$"# $ $"# ! !"#

!$"!#

!$"!

!$"$#

$

$"$#

$"!

$"!#

E

sG

M

sG

%&'()*+,-./

+0120321.-45&

6

&7

6$+89:.;1<421.9=>

?97?&((*@!

?&((*@!?

2 ~ 0.1 GeV

2Q

At Q2=~0.1 GeV2, Gs < few percent of Gp

all forward-angle proton data all low Q2 data


X)1:,)&d%&1W&,))&Y1-)=&=,%,

•R,%,&'"%&,DD",-'&%1&'B1Y&#1$'*'%"$%&D-"W"-"$#"&W1-&D1'*2V"&"8"#%•5*3$*d#,$%&#1$%-*:(21$'&,%&B*3B"-&]J&,-"&$1%&-()"=&1(%Q&

!"#$"%&'()*+$,''$-./')$),#,$01$2$3456$7*81$73$7'.9,'$*//./$,''.-*)$#.$:.,#$-"#;$(%"#$&.%+#/,"%#

5*FD)"&d%;

7<+$=$>+?@

7A+$=$B+


9",=&/&9(#*%"&H"-"$C1V&5B1Y"-&H,)1-*F"%"-•DB1%1%(:"&#(--"$%&*$%"3-,%"=&1V"-&dP"=&2F"&D"-*1='

3 Data Quality Checks

3.1 Detector Acceptances

Detector acceptances are checked to ensure that the detector is well aligned and not imposing geometric

cut to skew the Q2. The top two plots in Fig. 3.1 are S0 triggered plots, and the bottom two are detector

triggered plots. The S0 paddles are much bigger than the detector and covers the entire detector plane. The

detector x/y distribution plots with detector triggers look identical to the S0 triggered plots, indicating that

the detector does not impose any geometric cuts on the acceptance.

Detector Plane x (m)-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Dete

cto

r P

lan

e y

(m

)

-150

-100

-50

50

100

-310!

1

10

210

Detector Plane x (m)

LHRS Detector Plane Dist.

S0 Trigger


Dete

cto

r P

lan

e y

(m

)

-150

-100

-50

50

100

-310!

1

10

210


RHRS Detector Plane Dist.

S0 Trigger


Dete

cto

r P

lan

e y

(m

)

-150

-100

-50

50

100

-310!

1

10

210


LHRS Detector Plane Dist.

det Trigger


Dete

cto

r P

lan

e y

(m

)

-150

-100

-50

50

100

-310!

1

10

210


RHRS Detector Plane Dist.

det Trigger

Figure 3.1: Detector acceptance plots with S0 and detector triggers. The bounding box is the outline of the

detector with the PMT located at about 1.2m in x.

The cuts used to generate these plots are

LHRS::"P.hapadcL>550 && L.tr.n==1 && abs(ExTgtCor_L.th)<0.07

&& abs(ExTgtCor_L.ph)<0.07 && abs(ExTgtCor_L.dp)<0.05"

RHRS::"P.hapadcR>700 && R.tr.n==1 && abs(ExTgtCor_R.th)<0.07

&& abs(ExTgtCor_R.ph)<0.07 && abs(ExTgtCor_R.dp)<0.05"

D.evtype==2/(D.evtypebits&0x4)==0x4 are added to the LHRS/RHRS cuts to plot S0 triggered events.

10

@$%"3-,2$3&*$&%B"&>*3B&!"'1)(21$&5D"#%-1F"%"-'

Entries 2.694749e+07

RMS 3733

-20 -15 -10 -5 0 5 10 15 20310!

1

10

210

310

410

510

610Entries 2.694749e+07

RMS 3733

parts per million

Psuedo-random, rapid helicity flip

0"-.&#)",$&'"D,-,21$&1W"),'2#&"V"$%'&:.&>!5&1D2#'

&$1&+@R&$""="=e&="%"#%1-&'""'&1$).&"),'2#&"V"$%'

Elastic

Inelastic

detector

Q Q

Dipole

Quad

target

HAPPEX-III Beam Polarizations

HAPPEX-III Electron Beam Polarizations

Final HAPPEX-III polarization results:

Compton: 89.41± 0.21 (statistical) ±0.94 (systematic)%

Moller: 89.22± 1.7(systematic)%

� �� Period 1

� �� Period 2

� �� Period 3

� �� Period 4

M Friend (Carnegie Mellon University) HAPPEX-III Compton Polarimetry APS April Meeting 10 / 12


>?++4</@@@&4--1-&b(=3"%f?+0&&gDDFh

f?+0&i&?+0&

+1),-*j,21$ KQJKJ KQk\l]J&E",'(-"F"$% KQI_K KQ_mlb,#C3-1($=' KQIn` KQkJl9*$",-*%. KQIJn KQ\`lN*$*%"&?##"D%,$#" KQK`k KQJKlN,)'"&?'.FF"%-*"' KQK`I KQImlL1%,)&5.'%"F,2#& KQT\T IQ`nl5%,2'2#' KQmm_ TQJmlL1%,)&4PD"-*F"$%,)& KQk\T TQ\nl

Compton + Moller polarimeters

more later from Megan Friend, CMU

Spectrometer Calibration

more later from Kiadtisak Saenboonruang, UVa

Fle

xio

Boar

d

AD

C B

oar

d

System

PMT

LEDs

Data Acquisition

DIFF ENABLE

BASELINE ENABLE

Pulser Electronics

Linearity StudiesHRS Backgrounds

more later from Rupesh Silwal, UVa

Systematic uncertainties are well controlled -

experiment is statistics

dominated


>?++4</@@@&E",'(-"F"$%&1W&?+0

ARAW = -21.591 ± 0.688 (stat) ppm

Additional corrections are then applied:•backgrounds (1.1%)•acceptance averaging (0.5%)•beam polarization (11%)

This includes•beam asymmetry correction (-0.01 ppm)•charge normalization (0.20 ppm)

TQJ_l&g'%,%ho&IQ`nl&g'.'%h%1%,)&#1--"#21$&pJQ\l&[&D1),-*j,21$

?$,).'*'&b)*$="=&o&JQ\&DDF

0 5 10 15 20 25-40-30-20-10

0102030

= 1.00, P=0.512!OUT A=21.086 +/- 0.975 ppm, N=381, = 1.09, P=0.092!IN A=22.170 +/- 0.989 ppm, N=409,

= 1.05, P=0.182!AVG A=21.620 +/- 0.694 ppm, N=791, part

s pe

r mill

ion

data “slug”

combined 2-arm data

L-,a"#%1-.&,%&%,-3"%&,V"-,3"=&%1&qT$FGqKQ\$-,=&

OUT / IN from “slow” spin reversals to cancel systematics0 5 10 15 20 25 30

-4-3-2-1012

OUT A=-0.368 +/- 0.224 ppmIN A=-0.032 +/- 0.221 ppm

AVG A=-0.202 +/- 0.157 ppmasym_bcm1

0 5 10 15 20 25 30-0.08-0.06-0.04-0.02

00.020.040.060.08

0.1OUT A=-5.990 +/- 3.246 nm

IN A=0.261 +/- 3.465 nmAVG A=-2.916 +/- 2.372 nm

diff_bpm4ax

0 5 10 15 20 25 30-0.06-0.04-0.02

00.020.040.06

OUT A=-12.992 +/- 4.184 nmIN A=9.842 +/- 4.178 nm

AVG A=-1.763 +/- 2.957 nm

diff_bpm4ay

0 5 10 15 20 25 30-0.08-0.06-0.04-0.02

00.020.040.060.08

0.1OUT A=-2.220 +/- 3.258 nm

IN A=0.872 +/- 3.579 nmAVG A=-0.700 +/- 2.416 nm

diff_bpm4bx

0 5 10 15 20 25 30-0.06-0.04-0.02

00.020.040.06

OUT A=-13.068 +/- 3.957 nmIN A=8.999 +/- 3.901 nm

AVG A=-2.217 +/- 2.779 nm

diff_bpm4by

0 5 10 15 20 25 30-0.1

-0.050

0.050.1

0.15OUT A=-35.402 +/- 5.332 nm

IN A=61.049 +/- 5.186 nmAVG A=12.025 +/- 3.721 nm

diff_bpm12x

slug

mic

ron

Position Differences


>?++4</@@@&!"'()%'APV = -23.742 ± 0.776 (stat) ± 0.353 (syst) ppm

Q2 = 0.6241 ± 0.0028 (GeV/c)2


>?++4</@@@&!"'()%'APV = -23.742 ± 0.776 (stat) ± 0.353 (syst) ppm

Q2 = 0.6241 ± 0.0028 (GeV/c)2

?gX'^Kh&^&/J`QI\k&DDF&C&KQ__T&DDF


>?++4</@@@&!"'()%'APV = -23.742 ± 0.776 (stat) ± 0.353 (syst) ppm

Q2 = 0.6241 ± 0.0028 (GeV/c)2

?gX'^Kh&^&/J`QI\k&DDF&C&KQ__T&DDF

X'4&[&KQ\J&X'

E&^&&&&&&&&&&&&&&&KQKK\&C&KQKIKg'%,%h&C&KQKK`g'.'%h&C&KQKKkgNNh


N*%G&c1-)=&R,%,

X'4&[&KQ\J&X'

E&^&&&&&KQKK\&C&KQKIKg'%,%h&C&KQKK`g'.'%h&C&KQKKkgNNh

Wtih HAPPEX-3 result at Q2 = 0.624 GeV2:


H1$'*="-*$3&1$).&%B"&`&>?++4<&F",'(-"F"$%'

NS / A NS - APV A -0.3 -0.2 -0.1 0 0.1 0.2 0.3

HAPPEX-I (1999) 2 = 0.479 GeV2QMs+0.39GE

sG

HAPPEX-II (2006) Ms+0.09GE

sG 2 = 0.107 GeV2Q

HAPPEX-II He (2006) EsG 2 = 0.078 GeV2Q

HAPPEX-III (2011) Ms+0.52GE

sG 2 = 0.624 GeV2Q

•High precision•Small systematic error•Clean theoretical interpretation


c1-)=&R,%,&1$&5%-,$3"&NN

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4-0.15

-0.1

-0.05

0

0.05

0.1

0.15

EsG

MsG

H)1

G0-backw

ard (

G0-forward

HAPPEX-3

68.3%95%

•&>?++4</@@@&D-1V*="'&,&#)",$G&D-"#*'"&F",'(-"&1W&?+0&,%&]J^KQ_J&X"0JG&,$=&d$='&%B,%&*%&*'&#1$'*'%"$%&Y*%B&$1&'%-,$3"$"''&#1$%-*:(21$Q&&

•&Recent lattice results indicate values smaller than these FF uncertainties

•&N(-%B"-&*FD-1V"F"$%'&*$&D-"#*'*1$&Y1()=&-"O(*-"&,==*21$,)&%B"1-"2#,)&,$=&"FD*-*#,)&*$D(%&W1-&*$%"-D-"%,21$

Q2 = 0.62 GeV2


&c",C&HB,-3"&R*'%-*:(21$&1W&>",V.&M(#)"*M(#)",-&%B"1-.&D-"=*#%'&,&$"(%-1$&

r'C*$s&1$&B",V.&$(#)"*

208Pb

M"(%-1$&=*'%-*:(21$&*'&$1%&,##"''*:)"&%1&%B"&#B,-3"/'"$'*2V"&DB1%1$Q

D-1%1$ $"(%-1$

4)"#%-*#&#B,-3" I K

c",C&#B,-3" pKQKk I

!

!

MEM =4"#

Q2Fp Q

2( )

!

MPVNC =

GF21" 4sin2#W( )Fp Q2( ) " Fn Q2( )[ ]

!

APV "GFQ

2

4#$ 2Fn Q

2( )Fp Q

2( )

Q2 ~ 0.01 GeV2 APV ~ 0.6 ppmRate ~1.5 GHz5o scattering angle

PREX (Pb-Radius EXperiment)


MFT fit mostly by data other than neutron densities

Involve strong probes

Most spins couple to zero.

• !Proton-Nucleus Elastic

• Pion, alpha, d Scattering

• Pion Photoproduction

• Heavy ion collisions

• Rare Isotopes (dripline)

• Magnetic scattering

• PREX

• Theory

E",'(-"F"$%'&1W&$"(%-1$&'C*$

Electroweak probe

5.6 5.65 5.7 5.75 5.8

rn in

208Pb (fm)

0.25

0.26

0.27

0.28

0.29

0.3

Fn(Q

2)/

N

Skyrme

covariant mesoncovariant point coupling


?&#-(#*,)&#,)*:-,21$&D1*$%&W1-&$(#)",-&%B"1-.

( R.J. Furnstahl )

D,%)$E*,+(/"%F$/G$H"%+$).-%$#;*$+IEE*#/I$*%*/FI

24 26 28 30 32 34 36 38 40 42 44

symmetry energy a4 (MeV)

0.05

0.1

0.15

0.2

0.25

0.3

0.35

r n!

rp (

fm)

24 26 28 30 32 34 36 38 40 42 44


0.05

0.1

0.15

0.2

0.25

0.3

0.35

r n!

rp (

fm)

Skyrme

relativistic mesonrelativistic point coupling

24 26 28 30 32 34 36 38 40 42 44


0.05

0.1

0.15

0.2

0.25

0.3

0.35

r n!

rp (

fm)

The single measurement of Fn translates to a measurement of Rn via mean-field nuclear models


Nuclear Structure: Symmetry energy variation with neutron density is a fundamental observable that remains elusive.

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

n.m. densityratio proton/neutrons

•Slope unconstrained by data

•Adding Rn from 208Pb will eliminate the dispersion in the plot.

Slide adapted from J. Piekarewicz

N-1F&JKk+:&%1&,&M"(%-1$&5%,-Rn calibrates the equation of state of

neutron rich matter

Crust ThicknessExplain Glitches in Pulsar Frequency ?

Combine PREX Rn with observed neutron star radii

Some neutron stars seem too cold

Strange star ? Quark Star ?

Cooling by neutrino emission (URCA)0.2 fm URCA probable, else not

Phase Transition to “Exotic” Core ?

Crab Nebula

pres

sure

dens

ity


Measured Asymmetry

Weak Density at one Q2

Neutron Density at one Q2

Correct for CoulombDistortions

Small Corrections forG n

E GsE 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

20% corrections, calculated to precision by multiple groups

see later talk in last session


HB,))"$3*$3&4PD"-*F"$%

Compton Polarimeter

10X more precise than any previous e--nucleus scattering!

Similar to the HAPPEX measurements• Use Hall A spectrometers• integrating technique

Injector magnetic spin manipulation

Source optimization - reduce position difference and spot-size asymmetry

Precise kinematics calibration

Low energy electron beam polarimetry

Target survivability

Electronics noisenew low-noise ADCs

Integrating photon detection

Beam False Asymmetries

New modulation system for calibrating corrections

Water cell calibrationHigh rate tracking with GEMSLow current beam position monitors

20 ppb absolute measurement3% relative error

Ultimate goal:

upgrade to SC magnetFADC DAQ upgrade

upgrade IR to Green light

Moller Polarimeter

"(APV)/APV ~ 3%"(Rn)/Rn ~ 1%

see later talk by Zafar Ahmed

see later talk by Luis Mercado

Transverse Asymmetry see later talk by Bob Michaels


target

HRS-L

HRS-RcollimatorSeptum Magnet collimator

calibration collimators

\1&5"D%(F&%1&,(3F"$%&%B"&>!5


Detector integrates the elastic peak.Backgrounds from inelastics are suppressed.

4- Momentum (GeV/c)C 1st excited state

Pb excited states3-5- PbC

Ground States

Carbon Ground State

>*3B&!"'1)(21$&5D"#%-1F"%"-

2.6 MeV

Negligible contributions from inelastic events rescattering in spectrometer

p (GeV/c)p (GeV/c)

LeadCarbon


Slug # ( ~ 1 day)

mic

rons

Average with signs = what exp’t feels

Points: Not sign corrected

mic

rons

+,-*%.&](,)*%.&b",FHelicity – Correlated Position Differences < ~ 4 nm

Injector spin manipulation proved important for cancellation


&&4--1-&&&51(-#" ?:'1)(%"&&&gDDFh !"),2V"&&g&&l&h

J.',/"K,L.%$gIh 3433MN N4N

"#$%!!&'(%%#)*+#'&(2) 3433M1 N4N

O*#*&#./$$P"%*,/"#I 3433MN N4N

QRA$$P"%*,/"#I 3433N3 341

S*+&,T*/"%F 34333N 3

U/,%+V*/+*$$J.',/"K,L.%$ 3433N1 341$

01$$$$gIh 34331W 34X$

U,/F*#$$U;"&Y%*++ 343336 34NN1R$$Z+IEE*#/I$$$gJh 343316 34X

[%*',+L&$$\#,#*+ 3 3

LZL?9 KQKITK JQK

Systematic Errors

(1) Normalization Correction applied(2) Nonzero correction (the rest assumed zero)


+!4<&?'.FF"%-.

Ara

w (p

pm)

Slug ~ 1 day

ARAW = 0.593 ± 0.051 (stat) ppmThis includes•beam asymmetry correction (-40 ppb)•charge normalization (96 ppb)

OUT / IN, L/R from “slow” spin reversals to cancel systematics


?$,).'*'&b)*$="=&o&JKK&DD:


+!4<&?'.FF"%-.

Ara

w (p

pm)

Slug ~ 1 day

ARAW = 0.593 ± 0.051 (stat) ppmThis includes•beam asymmetry correction (-40 ppb)•charge normalization (96 ppb)

OUT / IN, L/R from “slow” spin reversals to cancel systematics


?$,).'*'&b)*$="=&o&JKK&DD:

at Q2 = 0.00906 GeV2 ! Statistics limited ( 9% )! Systematic error goal achieved ! (2%)

ppm 9.2 % 2.0 %


( theory curve, not integrated over acceptance )

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

Neutron Skin = RN - RP = 0.34 + 0.15 - 0.17 fm Preliminary estimate from C.J. Horowitz

?'.FF"%-.&)",='&%1&!$


+!4<&@$%"-D"%,21$


+!4<&@$%"-D"%,21$

First electroweak observation of the neutron skin of a heavy nucleus (CL =95%)


N(%(-"&!($&#,$&-",#B&DB.'*#'&31,)

Pb

Major experimental questions have been answered!• Lead sandwich target• Precision polarimetry at 1 GeV• HRS optics optimization• Source performance• Transverse Asymmetry

Must address:• Beam vacuum issues• Neutron radiation in the Hall (shielding, reduced beam collimation)

Proposal for a 2nd run is under development


RN

Surface thickness

RN Surface thickness

ZD21$'&W1-&1%B"-&$(#)"*

E (GeV)Rate

(MHz @ 50 µA)

APV (ppm)

days to 1% on

Rn

208Pb 1.05 1700 0.6 30

120Sn 1.25 810 1.1 20

48Ca 1.7 270 2.5 12

2.2 15 2.8 18

arXiv:1010.3246 [nucl-th]

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

Additional measurements could address • Rn in other nuclei• shape dependence?• isotope dependence?

Plans to pursue 48Ca•far from 208Pb•closer comparison to microscopic calculation•compatible with 12 GeV!

5o scattering at 2 GeV : new septum required


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