hadron form factors rolf ent jefferson lab
DESCRIPTION
Hadron Form Factors Rolf Ent Jefferson Lab. Science & Technology Review July 2002. Introduction Pion Form Factor G E p /G M p ratio G E n G M n Strangeness Form Factors Outlook. Why are nucleons interacting via V NN such a good approximation to nature? - PowerPoint PPT PresentationTRANSCRIPT
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•Introduction•Pion Form Factor•GE
p/GMp ratio
•GEn
•GMn
•Strangeness Form Factors•Outlook
Hadron Form FactorsRolf Ent
Jefferson Lab
Science & Technology ReviewJuly 2002
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How are the Nucleons Made from Quarks and Gluons?
Why are nucleons interacting via VNN such a good approximation to nature? How do we understand QCD in the confinement regime?
A) The distribution of u, d, and s quarks in the hadrons
(spatial structure of charge and magnetization in the nucleons is an essential ingredient for conventional nuclear physics; the flavor decomposition of these form factors will provide new insights and a stringent testing ground for QCD-based theories of the nucleon)
B) The excited state structure of the hadrons C) The spin structure of the hadrons D) Other hadron properties (polarizability, quark correlations, ….)
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Nucleon and Pion Form Factors
• Fundamental ingredients in “Classical” nuclear theory• A testing ground for theories constructing nucleons from quarks and gluons. - spatial distribution of charge, magnetization• Experimental insights into nucleon structure from the flavor decomposition of the nucleon form factors
PRECISION
•Additional insights from the measurement of the form factors of nucleons embedded in the nuclear medium - implications for binding, equation of state, EMC… - precursor to QGP
dMGn
MG
nEG
dEG
}pMG
p,ZEGp,ZMG
uEGuMG
sEGsMG
pEG
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Historical OverviewStern (1932) measured the proton magnetic moment µp = 2.79 µDirac
indicating that the proton was not a point-like particleHofstadter (1950’s) provided the first measurement of the proton’s radius through elastic electron scatteringSubsequent data (≤ 1993) were based on:
•Rosenbluth separation for proton, severely limiting the accuracy for GE
p at Q2 > 1 GeV2
As yet, no “ab initio” calculations available, waiting for Lattice QCDMain interpretation based on Vector-Meson Dominance
•In simplest form resulting in dipole form factor:
GD 2
2 Q2
2
with 0.84GeV
Adylov et al. (1970’s) provided the first measurement of the pion’s radius through pion-atomic electron scattering.
Subsequent measurements at Fermilab and CERN (1980’s)
“Ab initio” calculations of the pion far simpler• In asymptotic region, F 8s ƒ Q-2
Proton
Pion
2
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Charged Pion Electromagnetic Form Factor
Potential to approach region where perturbative QCD applies
Hall C E93-021 resultsProjected E01-004 and 12 GeV results
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World Data in 1993
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Measurement of GEp/GM
p to Q2 = 5.6 GeV2 (E99-007)
Earlier nucleon form factor data used Rosenbluth separationLeading to large systematic errors in GE
p since GEp < GM
p for Q2 > 1 (GeV/c)2
Best fit
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Measurement of GEp/GM
p to Q2 = 5.6 GeV2 (E99-007)
Earlier nucleon form factor data used Rosenbluth separationLeading to large systematic errors in GE
p since GEp < GM
p for Q2 > 1 (GeV/c)2
Polarization observables resolve this shortcomingf.i. by measuring recoil polarization:
GEp
GMp
Pt
Pl
Ee Ee'
2Mtan
e
2
Key is high beam current high polarization focal plane polarimeter
1H(e,e 'p)
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Measurement of GEp/GM
p to Q2 = 5.6 GeV2 (E99-007)
Earlier nucleon form factor data used Rosenbluth separationLeading to large systematic errors in GE
p since GEp < GM
p for Q2 > 1 (GeV/c)2
E93-027 observed linear decrease of GE
p/GMp
E99-007 extended the data set to 5.6 (GeV/c)2
using a Pb-glass calorimeter
Linear trend is observed to continue
The data do not approach basic pQCD scaling F2/F1 1/Q2 (Bjørken)
Ralston et al. include quark orbital angular momentum Lq
F2/F1 1/Q
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Measurement of GEp/GM
p to Q2=5.6 GeV2
Hall A E93-027 and E99-007 results
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Radial Charge Distribution
In Breit frame
k first-order correction for Breit-frame transformation
•Fourier-Bessel analysis
(r) an j0(kn r)(R r)n1
nnmax
with kn n / R
Jim Kelly
2 20( ) ( ) ( )p
EG k p r j kr r dr with 2
2
1
Qk
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Extensions
J. Arrington and R. SegelE01-001 (Hall A)
Super Rosenbluth separation
at Q12=1.9, 2.8 and 4.2 GeV2
and Q22=0.5 GeV2
C.F. Perdrisat et al.E01-109 (Hall C)
Use HMS (with new Focal PlanePolarimeter)and larger Pb-glass calorimeter
2 2 1 21 1 1 1
1 12 2 1 21 1 1 1
1
( , )
( ,A A
B B
PEPM
E Q KQR K
E Q KQ
G
G
with
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GnE Experiment with Neutron
Polarimeter2 H( , ' )e e n
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MicrowaveInput
NMRSignal Out
Refrigerator
LiquidHelium
Magnet
Target(inside coil)
1° K
NMR Coil
To Pumps
7656A14-94
LN2LN2
To Pumps
B5T
e–Beam
Frequency
LiquidHelium
GnE Experiment with DNP ND3
Target2 H( , ' )e e n
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Neutron Electric Form Factor GEn
•GEn(Madey,Kowalski) – high current polarized beam, unpolarized LD2 target, neutron
polarimeter & neutron precession magnet.•GE
n(Day) – low intensity polarized beam ND3 polarized target and neutron detector.
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Neutron Electric Form Factor GEn
Hall C Experiment E93-038 (Madey, Kowalski)
•GEn (Hall A) – polarized beam, polarized 3He target, and neutron detector
Pion cloud not sufficient
Relativistic effectsimportant ingredient
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Measurement of GnM at low Q2 from
Q2=0.1,0.2 (GeV/c)2
extracted from full calculation (W.Xu et al. PRL 85, 2900 (2000))
Q2=0.3-0.6 extracted from PWIA, more reliable extraction requires improved theory (in progress)
Hall A E95-001
3 He(e,e')
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Measurement of GnM at low Q2 from
Q2=0.1,0.2 (GeV/c)2
extracted from full calculation (W.Xu et al. PRL 85, 2900 (2000))
Q2=0.3-0.6 extracted from PWIA, more reliable extraction requires improved theory (in progress)
Hall A E95-001
3 He(e,e')
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Measurement of GnM from CLAS
2 2H( , ' ) / H( , ' )e e n e e p GnM
• 6 GeV Projections 12 GeV Projections
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Strange Quark Currents in the Nucleon GEs,
GMs
weak charge of the proton
Qpweak
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What we have on the books now
Strange Form Factors GEs and
GMs
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Strange Form Factors GEs and
GMs
Expected Forward Angle Results by late 2003
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Strange Form Factors GEs and
GMs
Rosenbluth separation of GEs and GM
s
Projected data indicated by open symbols are not approved yet
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High Precision Nucleon Form Factors at JLab
Present Planned (12 GeV)
Comments
GEp 5.6 9.0
(14.0)
Precision Measurements
Does GEp/GM
p keep dropping linearly?
GMp
(20.0)
Q2 > 14 makes assumptions about GEp
GEn 1.5 3.4
(5.5)
Precision Measurements
for Q2 > 1.5
GMn 5.0
(14.0)
Precision Measurements
GEs + GM
s 0.5 1.0 small (non-0), now only at Q2=0.5
GMs 0.8 Presently only approved at Q2=0.1 and 0.8
Q2 range
3He(e,e 'n)
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Summary
• F First measurement away from Q2 0 no Q-2 behavior yet• GE
p Precise data set up to Q2 =5.6 (GeV/c)2
charge differs from current distribution Q2 = 9 (GeV/c)2 planned• GE
n 2 successful experiments, precise data anticipated higher Q2 possible and approved• GM
n Q2 < 1 data from 3He(e,e’) high Q2 data from 2H(e,e’n)/2H(e,e’p) anticipated• GE
s, GMs Happex-2, Happex-He, G0 coming up
+ Sample, Happex, Mainz Stringent constraints on strangeness contributions Enables Q-Weak Standard Model test