Nucleon Form Factors and the BLAST Experiment at MIT-Bates
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Transcript of Nucleon Form Factors and the BLAST Experiment at MIT-Bates
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