Results from the G0 Measurement Colleen Ellis, University of Maryland

2

e e pp

Parity-violating electron scattering from the nucleon Hydrogen and deuterium targets Strange quark contributions to electromagnetic form factors

1988 Kaplan and Manohar—sea quark contributions to ground state nucleon properties,

i.e. spin, charge, and magnetic movements from neutral weak probes ( νN scattering)

1989 McKeown then Beck outlined a PVES program that could measure strange quark contributions to the proton’s charge and magnetism.

D.Kaplan and A.V. Manohar, Nucl. Phys. B310, 527 (1988)R.D. McKeown, Phys Lett. B219, 140 (1989) D.H. Beck, Phys. Rev. D39, 3248 (1989)

Q (EM) QW

u 2/3d -1/3s -1/3

charge symmetry

Measured in G0

Access via form factors: contribution to nucleon charge and magnetism

Parity Violating elastic e-N scattering

Q2 behavior well-understood

3

€

APVproton

(θ ≈ 5°,10°)

APVproton

(θ ≈ 110°)

APVdeuteron

(θ ≈ 110°)

⎧

⎨ ⎪

⎩ ⎪

→

GEs (Q2)

GMs (Q2)

GAe (Q2)

⎧

⎨ ⎪

⎩ ⎪

€

A∝ σ + − σ −

σ + + σ −∝M γM Z

M γ 2 ∝{AE + AM + AA

AD}

€

AE = εGEγGE

Z

AM = τGMγ GM

Z

AA = −(1− 4sin2θW)ε 'GAeGM

γ

AD = ε(GEγ )2 + τ (GM

γ )2

€ can be varied between zero and unity for a fixed Q2 by varying the beam energy and electron scattering angle.

Two kinematics, two targets gives 3 linear combinations of EM and weak form factors

Parity Violating elastic e-N Form Factors PV asymmetries from EM and weak interference terms

€

τ = Q2

4M 2

ε = 1+ 2(1+ τ )tan2 θ2 ⎛ ⎝ ⎜

⎞ ⎠ ⎟

⎡ ⎣ ⎢

⎤ ⎦ ⎥

−1

ε '= 1−ε 2( )τ 1+ τ( )

2

e e pp

C. Ellis/UMD

Axial form factor seen by PV electron scattering

)())(()()( 2222 QGRQFQGQG sA

eA

ZA

eA =

e

e'Z 0 PV

e+ ...

e

Z+++ PV

e+ ...

e

Z+

but Q2 behavior is not well constrained

(FA + Re) at Q2=0 computed

by Zhu etal, PRD 62 (2000) 033008

SAMPLE deuterium asymmetry data agree well

4slide thanks to E. Beise

Asymmetries to Form Factors

5

0s s e

phys E E M M A AA a a G a G a G=

Electromagnetic form factors: Kelly (PRC 70 (2004))does not include new low Q2 data from BLAST or JLabeventually use new fits (Arrington & Melnitchouk for p, Arrington & Sick for n)differences in fits become 0.5 – 1 % in the asymmetryalso used in Schiavilla calculation for D(see Diaconescu, Schiavilla & van Kolck, PRC 63 (2001) 044007)

Two-boson exchange corrections to Asymmetry: 0.5 -1.2%(see Tjon, Blunden & Melnitchouk, arXiv:0903.2759v1)

11

1Z Z

meas Born BornA A A

= =

G0 –Path to extracting vector form factors

C. Ellis/UMD

€

AF

AB

Ad

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟=

a1F a2F a3F

a1B a2B a3B

a1d a2d a3d

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

GEs

GMs

GAe

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟+

a0F

a0B

a0d

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

-53.299.5012.4912.12Ad

-38.2812.1062.8421.57AB

-23.682.4642.8779.39AFat Q2 = 0.62 GeV2

At Q2 = 0.62 and 0.23 (GeV/c)2, there are three measurements:AF : forward angle H (recoil protons) (D.S. Armstrong, et al., PRL 95, 092001 (2005))

AB : backward angle H (D. Androic, et al., PRL 104, 012001 (2010))

Ad : backward angle D

a1 (ppm)

a0 (ppm)a2 (ppm) a3 (ppm)

€

A∝ σ + − σ −

σ + + σ −∝M γM Z

M γ 2 ∝{ AE + AM + AA

ε(GEγ )2 + τ (GM

γ )2 }∝ a0 + a1GEs + a2GM

s + a3GAe(T=1)

G0 Forward Angle Experiment

• Forward angle measurement completed May 04

• LH2 target, detect recoil protons • Q2 = 0.12-1.0 (GeV/c)2,

E=3.03GeV• Spectrometer sorts protons by Q2

in focal plane detectors (16 rings in total)

• Detector 16: “super-elastic”, crucial for measuring the background

• Beam bunches separated by 32 ns• Time-of-flight separates protons

from pions• Results published in : • D.S. Armstrong, et al.,

PRL 95, 092001 (2005)

77

G0 Forward angle Results

8

D.S. Armstrong et al., PRL 95 (2005) 092001

NVSphysV

pE

pM

pE

F

sM

sE AA

RGGG

QGGG

= )0(

22

2 124

τEM form factors: J.J.Kelly, PRC 70, 068202 (2004)

HAPPEX-3

G0 Backward

9

G0 Backward Angle

C. Ellis/UMD

• Hydrogen and deuterium targets• Electron beam energy :

• 362 MeV : Q2=0.23 GeV2 • 687 MeV : Q2=0.62 GeV2

• Completed running in March 2007• Particle detection and

identification :• 16 Focal Plan Detectors • 9 Cryostat Exit Detectors

elastic and inelastic electron separation

• Čerenkov detectors electron and pion separation

e- beam

target

CED + Cherenkov

FPD

e- beamline

Beam Parameter Achieved (OUT-IN)/2

“Specs”

charge asymmetry 0.09 +/- 0.08 2 ppm

x position difference -19 +/- 3 40 nm

y position difference -17 +/- 2 40 nm

x angle difference -0.8 +/- 0.2 4 nrad

y angle difference 0.0 +/- 0.1 4 nrad

energy difference 2.5 +/- 0.5 34 eV

Beam halo (out 6 mm) < 0.3 x 10-6 10-6

Sep.-Dec. 2006

April 2006 March 2007

687 MeV Møller Results

Polarized Beam Properties Measurements of Møller asymmetry

at 687 MeV Mott scattering for 362 MeV

measurements Mott asymmetry measured at 5 MeV

in injector New work on effect of background

improves agreement with Møller

Pe(687 MeV) = 85.78 ± 0.07 ± 1.38%Pe(362 MeV) = ± 2.05%

10

362 MeV

Hydrogen raw electron dataHz/uA

d

11

octant # (azimuthal distribution)

blinded raw asymmetries (ppm)

687 MeV Hz/uA

362 MeV

687 MeV

Deuterium raw electron datablinded raw asymmetries (ppm) Hz/uA

d

d

Hz/uA

12

octant # (azimuthal distribution)

Blinding Factors

× 0.75-1.25H, D Raw Asymmetries,

Ameas

CorrectionsScaler counting correctionRate corrections from

electronicsHelicity-correlated corrections

Background correctionsBeam polarization

Q2 Determination

(from simulation)

Analysis Strategy

H, D Physics Asymmetries, Aphys

UnblindElectromagnetic

radiative corrections

(from simulation)

P= 0.8578 ± 0.0007 (stat) ± .014 (sys)

< 0.1 ppm

13

eA

sM

sE GGG ,,

4% on asymmetry

~10-50 ppm

± 0.003 GeV2

< 1% of Aphys

Rate CorrectionsCorrect the yields for random

coincidences and electronic dead time prior to asymmetry calculation

Randoms small except for D-687 (due to higher pion rate)

Direct (out-of-time) randoms measured

Validated with simulation of the complete electronics chain

14

Data set

Correction to Yield (%)

AsymmetryCorrection (ppm)

systematic error (ppm)

H 362 8 -0.31 0.08

H 687 4 -1.28 0.18

D 362 7 -0.58 0.21

D 687 -10 -7.0 1.8

CED

FPD

Cerenkov TriggerCFD

CFD

CFD

CFD

MT

MT

Coincidence1 CED1 FPD1 Trigger

pion

electron

Trigger CEDxFPD

Backgrounds: Magnetic Field Scans

Use simulation shapes to help determine dilution factors

Main contributions are Aluminum windows (~10%), pions (for D-687 data only).

D-687CED 7, FPD 13

15

Data set Asymmetry Correction (ppm)

systematic error (ppm)

H 362 0.50 0.37

H 687 0.13 0.78

D 362 0.06 0.02

D 687 2.03 0.37

Experimental “Physics” Asymmetries

16

Data Set Asymmetry Stat Sys pt Global

H 362 -11.25 0.86 0.27 0.43

D 362 -16.93 0.81 0.41 0.21

H 687 -45.9 2.4 0.8 1.00

D 687 -55.5 33 2.0 0.7

Largest correction is due to the 85% beam polarization--dominates H systematic uncertainties--for D-687, contributes about equally with rate corrections to

the systematic uncertainty

all entries in ppm

Form Factor Results• Using interpolation of G0 forward measurements

G0 forward/backwardPVA4: PRL 102 (2009)Q2 = 0.1 GeV2 combined(G0,HAPPEX, SAMPLE & PVA4)

Global uncertainties

G0 forward/backwardSAMPLEZhu, et al. PRD 62 (2000)

17

 

Some calculations: Leinweber, et al. PRL 97 (2006) 022001 Leinweber, et al. PRL 94 (2005) 152001 Wang, et al arXiv:0807.0944 (Q2 = 0.23 GeV2) Doi, et al, arXiv:0903.3232

Summary

18

• Q2 behavior of strangeness contribution to proton’s charge and magnetism: continue to be small• first results for the Q2 behavior of the anapole contributions to the axial form factor• other results to come soon from G0:

transverse beam spin asymmetries (2- exchange) in H and D PV in the N-D transition: axial transition f.f.

PV asymmetry in inclusive production

€

GE ,M(0) = 1

3(GE ,M

u +GE ,Md +GE ,M

s )

The G0 Collaboration (backward angle run)

19

Caltech, Carnegie Mellon, William and Mary, Hendricks College, Orsay, Grenoble, LA Tech, NMSU, Ohio, JLab, TRIUMF, Illinois, Kentucky, Manitoba, Maryland, Winnipeg,

Zagreb, Virginia Tech, Yerevan Physics InstitutePhD students: C. Capuano, A. Coppens, C. Ellis, J. Mammei, M. Muether, J. Schaub, M. Veerstegen, S. BaileyAnalysis Coordinator: F. Benmokhtar

• Backups

G0: N → DMeasurement: Parity-violating

asymmetry of electrons scattered inelasticallyANΔ gives direct access to GA

NΔDirectly measure the axial (intrinsic spin)

response during N →Δ+ transitionWill find GA

NΔ over a range of Q2

0.05 GeV/c2 < Q2 < 0.5 GeV/c2.First measurement in neutral current process

Data: Inelastic electrons measured by G0Scattered from both LH2 and LD2, each at

two energies (362MeV & 687MeV)

Elastic R

egion: G

0

Inelastic R

egion: N

D

BLINDED

Asymmetry (ppm) vs Octant (LH2 @ 687MeV)

Raw Asymmetry (averaged over inelastic region)

IN

OUT

Octant

Asym

met

ry (p

pm)

2

Im

M

MMAn ∝

Transverse Asymmetry 2 photon exchange

Inclusion of the real part of the 2γ exchange in the cross sectionmay account for the difference between measurements of GE/GM from unpolarized cross section and polarization transfer measurements

It also tests the theoretical framework that calculates the contribution of γZ and W+W- box diagrams that are important corrections to precision electroweak measurements

Asymmetry Uncertainties--Hydrogen, 687 MeV

Value(ppm)

Stat(ppm)

Sys Pt(ppm)

Sys Gl(ppm)

Total(ppm)

Measured Asymmetry -38.14 2.43Background Asymmetry -38.27 0.40Dilution Correction 0.47 0.52Transverse Correction 0.008Rate Correction -38.39 0.17Beam Polarization -44.76 0.52 0.53EM Radiative Correction -46.14 0.16Physics Asymmetry -46.14 2.43 0.84 0.75 2.68

Asymmetry Uncertainties--Deuterium, 687 MeV

Value(ppm)

Stat(ppm)

Sys Pt(ppm)

Sys Gl(ppm)

Total(ppm)

Measured Asymmetry -44.02 3.34

Background Asymmetry -46.05 0.050

Dilution Correction 0.38

Transverse Correction 0.009 0.008

Rate Correction -46.35 1.82

Beam Polarization -54.03 0.62 0.64

EM Radiative Correction -55.87 0.19

Physics Asymmetry -55.87 3.34 1.98 0.64 3.92

Correction due to misidentified pions

red: pionsblue: electrons

3 Ways to calculate • runs with special beam structure providing timing reference allowing particle TOF• PMT Multiplicity (two versus three of the cerenkov PMT’s firing)• Pulse heights (waveform digitized)

cerenkov detector4 pmts

aerogel

# single photo electrons

coun

ts

threshold

contamination to electron yield (A ~ 0)D 362: 0.5%D 687: 4%

687 MeV

25

26

€

A(q,ω,θe ) = a0 + a1GEs + a2GM

s + a3GAe(T=1) + a4GA

s

sAA

AVTT

VLL

elem GRRa

DRvRvAa

= 0

0

40

22

2

QGA F

elem =

),(),( qRvqRvD TTLL =

€

a2 = AelemvTRT

s

D× 1GM

s (Q2)

€

a3 = Aelem

(1− 4 sin2θW )vT ' RLTA( )

10+ RLT

A( )11

[ ]D

× 1GA

e(T=1)(Q2)

€

a4 = Aelem

(1− 4sin2θW )vT ' RLTA( )

00+ RLT

A( )01

[ ]D

× 1RA

0 +GAs (Q2)

⎡ ⎣ ⎢

⎤ ⎦ ⎥

absorbs isoscalar axial term

broken down by isospin

€

a1 = Aelem

vLRLs + vTRT

cs[ ]D

× 1GE

s (Q2)

D asymmetries: Calculation provided by Rocco Schiavilla

C. Ellis/UMD

(see Diaconescu, Schiavilla & van Kolck, PRC 63 (2001) 044007)

TTLL

ATTW

VTT

VLLF

e RvRvqRvqRvqRvQGqA

=),(')sin41(),(),(

22),,(

22

Quasielastic PV (ee’) in Deuterium

E. Beise, U Maryland

d

nnppd

AAA

=

Parity conserving nuclear corrections to the asymmetry are generally small, 1-3% at backward angles. Calculation provided to us by R. Schiavilla includes final state interactions and 2-body effects.

Use Quasielastic scattering from deuterium as lever arm for GAe(Q2)

Diaconescu, Schiavilla + van Kolck, PRC 63 (2001) 044007Schiavilla, Carlson + Paris, PRC 67 (2003) 032501

See also Hadjimichael, Poulis + Donnelly, PRC45 (1992) 2666 Schramm + Horowitz, PRC 49 (1994) 2777 Kuster + Arenhovel, NPA 626 (1997) 911 Liu, Prezeau, + Ramsey-Musolf,

PRC 67 (2003) 035501

27

Deuterium model comparison to cross section data

E. Beise, U Maryland 28

data from:S. Dytman, et al., Phys. Rev. C 38, 800 (1988)B. Quinn, et al., Phys. Rev. C 37, 1609 (1988)

calculation from R. Schiavillasee alsoR.S., J. Carlson, and M. Paris, PRC70, 044007 (2004).

• AV18 NN potential• relativistic kinematics• J.J. Kelly fit to nucleonform factors

2-body effects in the D asymmetry

E. Beise, U Maryland 29

leading termof the asymmetry

axial formfactor coefficienthas ~15% correction from2-body effects

calculations from R. Schiavilla, see also R.S., J. Carlson, and M. Paris, PRC70, 044007 (2004).

Scaler Counting Issue with Electronics

Electronics sorts detector coincidences (CEDi and FPDj) into separate scaler channels FPGA-based system in North American electronics (4

octants) Because of error in FPGA programming, two short

(~3 ns) pulses could be sent to scaler in < 7 ns ~ 1% of events have such miscounts

Such pulse pairs can cause scaler to drop or add bits Detailed simulation of ASIC with propagation delays

between (flip flop) elements Effect on asymmetry is <0.01 Aphys

Test by cutting data; compare with French octants

Data

Simulation

30

College of William and Mary, Institut de Physique Nucléaire d'Orsay, Yerevan Physics Institute, Laboratoire de Physique Subatomique et de

Cosmologie Grenoble, University of Illinois, University of Maryland, Thomas Jefferson National Accelerator Facility, University of Manitoba,

Carnegie Mellon University, California Institute of Technology, University of Kentucky, TRIUMF, Louisiana Tech University, Virginia Tech, University of Northern British Columbia, New Mexico State University ,University of Winnipeg, Ohio University, Hampton University, Hendricks College,

University of Zagreb

The Gzero Collaboration

U.S. G0 participation jointly supported by DOE and NSF. Significant additional contributions to G0 from NSERC (Canada) and CNRS (FR)

C. Ellis/UMD 31

Summary of the G0 Backangle Run

Beise Run Coordinator report March 2007

Run Coulombs Hours Gbytes dataH 687 86 400 600H 362 119 550 825D 362 66 520 800D 687 I 40 520 780D 687 II 15 240 340total 330 2300 3450

Run start to run end ~ 8940 hours