R. Alarcon @ PAV06May 18, 2006

Nucleon Form Factors and the BLAST Experiment at MIT-Bates

Introduction/Motivation

BLAST Experiment

Results and Discussion

Probing Nucleon Structure with Spin-Dependent Electron Scattering at Low Q2

Spin 1/2: Elastic electron scattering from nucleons

[ ])()()1(

1 2222 QGQGE

E

d

dMEMott τε

τεσ

σ+

+

′=

Ω

GMp(0) = 2.79

GMn(0) = –1.91

= Q2/4M2

= [1+2(1+)tan2(/2)]-1

€

JEMμ = −eΨN

' γ μ F1(Q2 ) +

κ N

2MN

iσ μν qν F2(Q2 )

⎡

⎣ ⎢

⎤

⎦ ⎥ΨN

'

0)0(,1)0(

1)0()0(

12

21

==

==nn

pp

FF

FF

Sach’s ff’s:

21

21

FFG

FFG

M

E

κκ

+=−= charge

magnetism

GEp(0) = 1, GE

n(0) = 0

e

e'

)( 222 qQr

−−= ν

Polarized e-N scattering

Polarized beam + polarized target:

Polarized beam + polarization of recoil nucleon:

unpol

EMM GGbGaA

σ

σσσσ )()( *2* +

∝+−

=↑↓↑↑

↑↓↑↑

2

'

tan2

)(e

p

ee

L

T

M

E

M

EE

P

P

G

G +−=

p p

n

)'( needrr

neutron charge ff

)'(3 eeeHrr

neutron magnetic ff

)'( needrr

)'( peeprr

neutron charge ff

proton GE/GM

Donnelly + Raskin, Ann. Phys. 169 (1986)247

Akhiezer+Rekalo, Sov.JPN 3 (1974) 277Arnold,Carlson+Gross, PRC 21 (1980) 1426

€

3r

H e

p

n dr

Why study hadron form factors?

• They give us the ground state properties of (all visible) matter:size and shapecharge and magnetism distributionsspin and angular momentum

• They are required for knowledge of many other things:structure of nuclei at short distancesProton charge radius and Lamb shiftprecision tests of Weak interaction at low Q2

• They should give clues on how to connect QCD to the NN force

They can be measured NOW with high precision!

PT

latticeQCD

cloud

pQCD

quarks/gluons

form factors

Scientific Motivation for BLAST

Comprehensive study of spin observables from few-body nuclei at low Q2. Nucleon form factors provide basic information on nucleon

structure. Low-Q2 region is a testing ground for QCD and pion-cloud inspired

and other effective nucleon models.

GnE is the least known among the nucleon form

factors, with errors of typically 15-20%. Gn

E related to neutron charge distribution.

Precise knowledge of GnE is essential for parity

violation experiments.

MIT-Bates Linear Accelerator Center

Linac: 2500 MeV Beam: 850 MeV / Imax = 225 mA / Pe = 65 % SHR: Siberian Snake + Compton Polarimeter Target: Internal Target = Atomic Beam Source

At

€

ve

Internal Target Physics at MIT-Bates

Ee 1 GeV, Pe= 40-80 % Im= 200 mA, 10 min

€

ve

South Hall Ring

• pure species• thin• high polarization• thin cell• low holding fieldL = 1031-1033 atoms cm-2 s-1

Novosibirsk, AmPS, HERMES, IUCF, COSY

Atomic Beam Source

Isotopically pure H or D Vector Polarized H Vector and Tensor D

Target Thickness/Luminosity Flow 2.2 x 1016 atoms/s Density 6 x 1013 atoms/cm2

Luminosity 6 x 1031 cm-2s-1

Target Polarization typically 70-80%

Atomic Beam Source

Atomic Beam Source

Quasielastic Beam-Target Asymmetry Av

ed(exp) = h·Pz ·Aved(th)

<hPz> = 0.567 ± 0.006

<Pz> = 0.85 ± 0.04

<h> = 0.67 ± 0.04

€

2t

H (r e ,e' p)

Bates Large Acceptance Spectrometer Toroid

€

δp

p≈ 3%

Left-Right symmetry Large Acceptance

0.1 < Q2/(GeV/c)2 < 1.0

Coils: B = 3.8 kG Drift Chambers

PID,tracking δ ≈ 0.5º,

Cerenkov Counters e, separation

Scintillators TOF, PID, trigger

Neutron Counters Neutron ToF

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

The BLAST Collaboration

Experimental Program

Pol. HInclusive Gp

E/GpM N-∆: C2/M1

Vect-Pol. DGn

M D-state GnE Te

11

Tens-Pol. D

T20 D-state

)e',e(prr

€

rp (

r e ,e'p) +o+ n)e',e(p ,p)e',e(p n,)e',e(p

rrrrrr

€

rd (

r e ,e')

€

rd (

r e ,e'p)

€

rd (

r e ,e'n)

€

rd (

r e ,e'd)

€

td (e,e'd)

t d (e,e'p)

€

High quality data for nucleon and deuteron structureby means of spin-dependent electron scattering

Event Selection

Neutron: Invariant Mass and Time of Flight

Very clean quasi-elastic 2H(e,e’n)p spectrum

Highly efficient proton veto (Wire Chambers)

Neutron: Invariant Mass and Time of Flight

Very clean quasi-elastic 2H(e,e’n)p spectrum

Highly efficient proton veto (Wire Chambers)

Target Spin Orientation

Target Spin Orientation

nucleons

electrons

* ≈ 0

“Parallel” Kinematics

Electron - Right Sector

Target Spin Orientation

Neutrons

electrons

* ≈ 90

“Perpendicular” Kinematics

electrons

nucleons

Electron - left sector

Kinematics and Observables

Electrodisintegration of the Deuteron

Quasi-elastic 2H(e,e’n) Beam + Target

Polarized

€

dσ

dΩedEe'dΩCM

= S0(1+ PdV Ad

V + PdT Ad

T + h(Ae + PdV Aed

V + PdT Aed

T ))

€

AedV =

aGMn 2

cosθ * + bGEnGM

n sinθ * cosφ*

cGEn 2

+ GMn 2 ≈ acosθ * + b

GEn

GMn

sinθ * cosφ*

BLAST Data Collection

BLAST Data Collection

BLAST Data Collection

Nucleon Form Factors Parameterizations

Proton Form Factors

Neutron Magnetic Form Factor GM

n

Neutron Magnetic Form Factor GM

n

Neutron Magnetic Form Factor GM

n

1. New measurement technique.

2. Includes full deuteron structure.

3. Consistent with recent polarization and other data.

4. Provides a tighter fit to form factor in the low Q2 region.

Preliminary BLAST GMn Data

N. Meitanis

Extraction of GnE

Quasielastic Full Monte Carlo Simulation of

the BLAST experiment Deuteron Electrodisintegration

cross section calculations by H. Arenhövel

Accounted for FSI, MEC, IC, RC Spin-perpendicular beam-target

asymmetry AedV(90º,0º) shows

high sensitivity to GnE

€

2r H (

r e ,e'n)p

Extraction of GnE

Quasielastic Full Monte Carlo Simulation of

the BLAST experiment Deuteron Electrodisintegration

cross section calculations by H. Arenhövel

Accounted for FSI, MEC, IC, RC Spin-perpendicular beam-target

asymmetry AedV(90º,0º) shows

high sensitivity to GnE

Compare measured AedV with

BLASTMC, varying GnE

€

2r H (

r e ,e'n)p

Systematic Errors

Uncertainty of target spin angle 5% 12% per degree

Beam-target polarization product 2.5% Radiative effects

<1.0% Small helicity dependency

Uncertainty of GnM 1.5%

Model dependency <3% Effect of potential negligible Final state interaction reliable

Total: 6.6%

GnE World Plot

GnE World Plot

• Preliminary result

• Only 50% of data, final data should reach 0.5 (GeV/c)2

• Use Arenhovel’s calculations for Gn

M and

contribution of GnE

• Need to combine with other BLAST measurements for global fit

• Provide low Q2 data Check bumpPion cloud

V. Ziskin and E. Geis

BLAST Fit to World Polarization Data

Remarkable consistency of all modern polarization experiments!

Global fit determines GEn to better than ±7%

Nucleon Models

Dispersion theory gives the best description

No theory describes GnE at low and high Q2 simultaneously

Conclusions & Outlook

BLAST at MIT-Bates has measured at low Q2 the nucleon form factors using polarized electrons from vector-polarized hydrogen/deuterium.

Very small systematic errors Gn

E overall known to ≈ 5% at Q2 < 1 (GeV/c)2

Dispersion theory gives the best description

No theory describes GnE at low and high Q2 simultaneously

Evidence for enhancement at low Q2 - role of pion cloud?

With new precision data of T20 from BLAST and with improved A(Q2) new attempt to Gn

E determine from GQ

ed elastic analysis: Mainz-Saclay discrepancy 8% in A factor of 2 in GnE

New measurements of A(Q2) at JLab (E-05-004) and MAMI

Extension of GnE at high Q2 < 3.5 (GeV/c)2 (E-02-13)

Proposal of BLAST@ELSA/Bonn Measure Gn

E for Q2 = 0.04-1.5 (GeV/c)2 with both vector-polarized 2H and polarized 3He