Transcript Slide 1

Nucleon Electromagnetic Form
Factors
Experimental Perspectives
Will Brooks
EINN 2005
Basics
Status of Data
Insights and Opinions
Conclusions and Outlook
Basics
Scientific motivations:
 To understand the ground state
structure of the proton and neutron:
Basic
s
 An important experimental constraint
for determining Generalized Parton Distributions
 A challenging test for nucleon models and for lattice QCD
 Required for extracting information on strange quark distributions in
the proton
 To understand nuclei
Nucleon form factors a basic and essential ingredient in models of
nuclei
Several significant and surprising new results in the past
decade
Basic
 Form factors describe extended systems
s
 Nucleon form factors: quarks
 Very general hadron current for elastic scattering (single
photon exchange approximation)
k’
k
q
p
p’
Note: there are numerous other
nucleon form factors, e.g., :
Basic
s
Not in this talk, but see Kent Paschke’s talk this afternoon
Also, F(q2>0) not in this talk, but upcoming Frascati workshop
Recent reviews: Hyde-Wright and de Jager, Annu. Rev. Nucl.
Part. Sci. 2004 54:217; Gao, Int’l J. Mod. Phys. E, 2003.
Basic
s
 Functions F1 and F2: Dirac, Pauli form factors
 Dirac: ‘non-spin flip,’ Pauli: ‘spin flip’
 Proton: F1(0)=F2(0)=1, Neutron: F1(0)=0, F2(0)=1
 In Breit frame, obtain simple
expressions for the charge density and
current densities:
Basic
s
These motivate the definition of the Sachs form factors:
(= - q2)
 The cross section in terms of G’s has no
interference terms:
Basic
s
Dividing kinematic factors out yields the reduced cross
section convenient for ‘Rosenbluth separations’ for
X(e,e’):
(= - q2)
 Can also measure form factor ratio with
polarization techniques, e.g. proton recoil
polarization:
Basic
s
 Dipole form, Galster parameterization:
Basic
s
Describes GMp and GMn reasonably well
Describes GEn reasonably well
Status of data
 Best known
 Rosenbluth separation
 Required to obtain GEp from GEp/GMp
 Recent revisions
The Proton
Magnetic
Form Factor
Full Q2 range
Intermediate Q2 region
 Rosenbluth separation
 Polarization transfer
The Proton
Electric
Form Factor
All data
Data extracted using Rosenbluth separation only
Data extracted using polarization transfer only
B
A POSSIBLE EXPLANATION:
 The Rosenbluth/polarization GEp discrepancy may be
resolved by including corrections for two-photon exchange
(TPE)
 A direct measurement of the TPE contribution is given by
the ratio of positron and electron elastic scattering:
Re+e-(e,Q2) ≡ s(e+p)/s(e-p)
 The correction factor to the e-p elastic cross section due to
TPE is just 1-(Re+e--1)/2
Experiments in Jefferson Lab’s Halls A, B (CLAS),
and C to investigate different aspects of the TPE
problem
Radiative corrections
to single photon
exchange
 Any dependence different
from Re+e- = 1 indicates TPE
Expected size of
two-photon-exchange
 Because GE2 is the slope of the Rosenbluth plot, any
slope/curvature in Re+e- will modify the extraction of GE
BMT - Blunden, Melnitchouk,
Tjon – hadronic model where
intermediate state is a proton
with a form factor.
Axial-VMD
BMT
CABCV
CABCV – Chen, Afanasev,
Brodsky, Carlson,
Vanderhaeghen – partonic
calculation with GPD as
intermediate state
Axial-VMD – Bjorken model
updated by Afanasev and fitted
to data to explain the full GEp
discrepancy.
Experimental method:
producing positrons and
electrons
Untagged, collimated photon beam produced by the standard
Hall B tagger system (5% radiator) is incident from the right:
Experimental method – details of
chicane
Y scale is expanded by a factor of 6 compared to X scale
Incident lepton
momentum spectrum
(0 – 5 GeV/c)
Experimental method:
strategies
 Lepton beam energy is a continuum and is not known event-by-event,
therefore, need energy-independent process identification
 Want leptons over a full range in scattering angle (epsilon), therefore,
cannot use standard CLAS electron identification and pion rejection
Use correlated, over-constrained kinematics to identify the elastic
scattering process
 Measure momentum and angle for electron and for proton
 Distinguish lepton from proton via time-of-flight
 Suppress non-elastic background
 Cross-checks –
Systematic uncertainties
expect same answer
 when reverse chicane field:
• electrons ↔ positrons
• checks asymmetries in magnetic fields and trajectories
 when reverse torus field
• checks any residual asymmetries in e+/e- acceptance
 from each of six sector pairs
 when electron or positron beam is blocked
 Directly measuring incident e+/e- flux asymmetry
 Defining the fiducial volume of event via proton, which is
identical for e+/e Final budget: 1% systematic uncertainty
Expecte
d
accuracy
±1% error band shown for reference
The Neutron
 Nuclear target
Electric
 Besides Rosenbluth separation,
Form Factor
techniques to determine GEn include:
(deuterium, from T20 measurement)
Highly sophisticated, high-precision measurements!
Many (7) techniques from many (7) laboratories!
Data from past few decades
Low Q2 region
Low Q2 region
The Neutron
Magnetic
Form Factor
 Nuclear target
 Required to obtain GEn from GEn/GMn
 Techniques to measure GMn include:
Rosenbluth separation (with deuteron wavefunction
modeling and subtraction of proton contribution)
‘Ratio’ method: 2H(e,e’n)/2H(e,e’p) (more on this later)
with Faddeev or PWIA modeling
New preliminary data set from CLAS/JLab!
Intermediate Q2 region
Low Q2 region
Low Q2 region
Low Q2 region
Low Q2 region
Low Q2 region
Low Q2 region
Low Q2 region
Measure ratio of quasi-elastic e-n
scattering to quasielastic e-p
scattering off deuterium:
‘Ratio’
Method
~1.0 for Q2>1 GeV2, calculated from deuteron models
→ Measure R, use known GEp, GMp, GEn to extract GMn
(t=Q2/4M2)
 Method insensitive to:
 Luminosity uncertainties
 Electron radiative processes,
reconstruction efficiency, trigger efficiency
 Deuteron wavefunction
‘Ratio’
Method
 Method requires:
 Accurate neutron detection efficiency
 Equivalent solid angles for proton and neutron
 Final state is positively identified by experimental
information
 Can be extended to higher Q2
NEUTRON DETECTION EFFICIENCY:
 Data taken with hydrogen target and
deuterium target simultaneously in beam:
D2
‘Ratio’
Method
in CLAS
H2
 Tag neutrons with H2 target via ep→e’np+
 In-situ efficiency, timing, angular resolution determination
 Completely insensitive to PMT gain
variations/changing backgrounds
 Detect neutrons with the
calorimeters and TOF
Dual-Cell
Liquid
Cryotarget
Neutron Detection
Benefits of
CLAS
 Numerous cross-checks
 Three independent, overlapping measurements of e-n
 Three magnetic field settings allow independent, partially overlapping
measurements of e-p
→Multiple, overlapping measurements of GMn
 Accomodates dual-cell target
 In-situ neutron tagging
 In-situ proton elastic scattering (alignment, proton efficiencies)
 Can veto events with extra charged particles
 Can study accuracy of deuteron model
Overlapping
Measurements of GMn
(Semi-schematic)
Neutrons in
large-angle
Calorimeter
Neutrons in
forward-angle
calorimeters
Neutrons in
TOF counters
Protons in
middle DC
Protons in
forward DC
0
1
2
3
4
5
Q2 (GeV2)
Consistency of Overlapping Data
W vs. qpq
QE Protons
QE Neutrons
Effect of qpq cut on W2 spectrum in
exclusive channel
Inclusive
data
Inclusive
data
Q2>
Quasielastic
signal
CLAS/E5 Data
Inelastic
background
Corrections to the
Data
 Neutron detection efficiency vs. momentum and hit position
 Proton detection efficiency vs. momentum and detector
element
 measured with elastic scattering
 Electron momentum corrections applied (small)
 Nuclear corrections (small, have evaluated in two models)
 Radiative corrections (~0 for electron, small but poorly
known for electron-proton interference and TPE)
Sources of systematic
uncertainties
 Detection efficiencies
 Nucleon solid angle
 Uncertainty in other nucleon form factors
 Inelastic background
 Deuteron wave function
 Photon rejection
 Radiative corrections, including two-photon-exchange
Insights and Opinions
 A critical, thoughtful, synthetic
theoretical analysis of the nucleon
form factors is now needed
Insights and opinions
 Models should compare to all four form factors (plus timelike)
 It is not a failure for a model/lattice to fail to describe data
 Functional form of GMn appears to be quite similar to
proton, slightly smaller (~5%) at larger Q2 > 1 GeV2
 dipole a very good approximation (~5-6 %) to at least 5 GeV2
 With GPDs, can study connections between DIS and elastic FF,
now have much more information with neutron data at larger Q2
– can, e.g., compare u and d quark roles in DIS and FF
 Measuring the same observable with fundamentally
different techniques is very important.
Conclusions and outlook
 Enormous technical progress in
experimentally determining nucleon form
factors over the past decade
Conclusion
s and
outlook
 Can now begin to quantitatively compare proton to
neutron over a wide range in Q2
 Challenge for theoretical models/lattice is much greater –
should compare predictions to all four form factors. A
much more rigorous requirement.
 Near future, can get GMn to Q2 ~ 7.5 GeV2 and GEn up to 4
GeV2; with 12 GeV upgrade, to 14 GeV2 for GMn and GEp
“Beyond the Born Approximation: A Precise Comparison of
Positron-Proton and Electron-Proton
Elastic Scattering in CLAS”
A precise comparison of e+p and e-p elastic scattering over a wide
kinematic range is feasible in CLAS. The ratio s(e+p)/s(e-p) directly
yields the two-photon-exchange contribution to elastic scattering.
JLab experiment: E04-116
J. Arrington*, K. Hafidi, R. J. Holt, P. E. Reimer,
E. C. Schulte, X. Zheng, Argonne National Lab
F. Klein, D. Sober, The Catholic University of America
K. Joo*, M. Ungaro, University of Connecticut
B. A. Raue*, W. Boeglin, M. Moteabbed, Florida International University
A. Afanasev*, W. K. Brooks*, V. D. Burkert, A. Deur,
L. Elouadrhiri, D. W. Higinbotham, B. A. Mecking, W. Melnitchouk, Jefferson Lab
G. E. Dodge, C. E. Hyde-Wright, L. B. Weinstein*, Old Dominion University
and the CLAS Collaboration
*spokespersons
CLAS – the CEBAF Large Acceptance Spectrometer
Charged particle angles 8° - 144°
Neutral particle angles 8° - 70°
Momentum resolution ~0.5% (charged)
Angular resolution ~0.5 mr (charged)
Identification of p, p+/p-, K+/K-, e-/e+
Expected Results – Comparison to Theory
1%
1%
Expected Results –
Comparison to Existing Data with Q2>0.8 GeV2
1%
Expected Results –
Breadth of
Coverage
1%