Progress in Particle and Nuclear Physics PERGAMON
Progress
in Particle and Nuclear Physics 44 (2000) 273-291
p
http://www.elsevier.nl/locate/ppartnuclphys
Recent Results from Jefferson Lab V. D. BURKERT Jefferson Lab, Newport News, Va. 23606, Recent results on studies of the structure strong interaction
QCD are discussed.
polarized
and recoil polarimeters,
targets,
eters and detector
instrumentation
and nuclear structure
of nucleons and nuclei in the regime of
Use of high current polarized electron beams, in conjunction
spectrom-
studies of nucleon
than has been possible in the past. The CEBAF accelerator
is important
an improved understanding
structure
and strong interaction
I discuss how the first experiments
1
with modern
allow much more detailed
at Jefferson Lab was build to study the internal where confinement
USA
already
of hadronic
of hadrons
in a regime
QCD is the relevant theory.
make significant
contributions
towards
structure.
Introduction
Electromagnetic momentum in Figure
production
of hadrons
and energy transfer) 1. For simplicity
are the relevant peripheral
degrees
properties
theory describes
distances,
quarks quarks
and nuclei near threshold
and gluons are relevant,
spectra
and wave functions.
remains
poorly established,
the coupling
however,
interactions
involves elementary distributions
confinement between
and where JLab experiments
theories.
chiral symmetry.
quark and gluon fields.
is important,
these constituents
currently
and nucleons
Chiral perturbation
in the nucleon.
At intermediate
and they appear
of QCD
have their biggest impact.
structures
separated
This is
from each other
may eventually
be described
Because the electro-magnetic
and electro-
they are best suited to provide the data for such an endeavor.
discuss recent data on studies of the intrinsic
as
via their excitat,ion
to the fundamentals
These regions are not strictly
based on fundamental
weak probes are well understood,
mesons
of the probe we study
for pion production.
and the hope is that due to this overlap hadron
in a more unified approach
resolution
This is the region where the connection
the region I will be focusing on in this lecture. but overlap,
with the three regions
At large distances spatial
and time scales (or
and has a direct link to QCD via (broken)
&CD, and we map out parton
We study
to distance
This is illustrated
the time scale.
Due to the limited
(and short time scales),
and glue.
according
in the interaction.
I have omitted
of freedom.
of nucleon
by perturbative
constituent
probed
many of these processes
.4t short distances governed
may be characterized
nucleon structure,
I will
and results from light nuclear targets.
2H and 3He. 0146-6410/00/$ - see front matter 0 2000 Published by Elsevier Science BV All rights reserved. PII: SOl46-6410(00)00077-6
274
J! D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
1.1
Structure
of the Nucleon at Intermediate
QCD has not been solved for processes at intermediate internal
structure
of nucleons
is generally
distance
Distances
- Open Problems
scales. A direct consequence
poorly known in this regime.
is that the
On the other hand, theorists
Exclusive Hadron Production near threshold production
deep virtual meson production
nucleons and mesons Chiral perturbation theory
.y. 1 ,
“.
Quark models QCD sum rules
N* excitations
constituent quarks gluon flux tubes
< 0.1
>l
cbstance Fig. 1. Exclusive meson electroproduction.
fermi w
A subdivision in distance scales is used to illustrate three
kinematic regions and their respective (effective) degrees of freedom.
l! D. Burkert /Prog. Part. Nucl. Phys. 44 (2000) 273-291
are often not challenged
due to the lack of high quality data in many areas
where the lack of high quality contributed, l
or are expected
but also for the proton.
l
is most noticeable,
to contribute
The nucleon
inelastic
spin structure
JLab experiments
To understand
The long-known (local duality) Carrying
connection
as predicted
[2] remained
virtually
out an experimental
due to the availability data transfer
between
for more than two decades
techniques,
the routine
matter
at high energies
regime and the transition
the full excitation
by our most accepted spectrum
in
to the deep
copiously
the deep inelastic unexplored that
accelerators,
as
unknown,
although
gluonic
[l].
regime and the regime of confinement
for decades.
will address modern
these questions
detector
has become
instrumentation
of spin polarization
availability
spectrum
well, and many states
models.
is completely
to be produced
program
of CW electron
of ordinary
to excited states have been studied
of the nucleon are expected
blocks of
at all.
The role of the glue in the baryon excitation excitations
of the basic building
the ground state nucleon we need to understand Few transitions
for the neutron,
unknown.
quarks play in the wave function
regime have not been explored
have already
in the future:
in the universe is virtually
has been explored
are missing from the spectrum
l
significantly
such as CERN and SLAC. The confinement
well as the continuum.
l
and where
This means that the charge distribution
form of matter
We do not know what role strange
laboratories
l
data
The following are areas
The electric form factors of the nucleon Gsnr GEp are poorly known, especially
the most common l
215
feasible
with high speed
in beam and targets
and recoil
polarimetry. The main contributor News, Virginia,
to this field is now the CEBAF
USA. A maximum
halls (A, B, C) can receive polarized or with the same beam energies.
accelerator
energy of 6 GeV is currently beam simultaneously,
with different
This allows a diverse physics
at Jefferson
available.
program
Lab in Newport
The three experimental
but correlated
beam energies,
to be carried
out in a very
efficient way.
2 2.1
Structure Charge
of the Ground
and current
State Nucleon
distribution
The nucleon ground state has been studied for decades in elastic electron-nucleon the charge and current
distribution
form factors.
The superscript
respectively.
Early experiments
in the nucleon
in terms of the electric
y or 2 are used to describe from Bonn,
DESY,
the electromagnetic
and CEA showed
scattering.
It probes
(G7,)and magnetic
(GL)
and weak form factor,
a violation
of the so-called
276
I! D. Burkert /Prog. Part. Nucl. Phys. 44 (2000) 273-291
“scaling law”, which may be interpreted
that the spatial distribution
not the same, and the corresponding a downward
trend
SLAC data sets confuses other data sets.
form factors have different
RL,+, = GL/GL as
for the ratio
the picture
greatly
Q* dependencies.
a function
of Q2. Adding
situation
and to constrain
Reliable
theoretical
matics
and the first experiments
where the proton
the virtual photon,
polarization
is measured
smaller systematic
quantities.
uncertainties
beautifully
will be continued
than previous
A precision the neutron ep -+ em+
2.2
on the neutron measurement
to proton
for an in-situ calibration
From the analysis of deep inelastic
plane, but transverse
accessed
directly
at high Q2 (Figure
this experiment
to
in electromagnetic
target
magnetic
the Q2 range.
interactions.
[37].
counter
The
The experiment 61 will measure
using a similar techniques.
form factor will be carried
simultaneously
has
2). They confirm the
and extend
in the year 2000. Other experiments[5,
of the neutron
corresponding
out with CLAS using
This experiment detection
will use the reaction
efficiency.
The flavor-neutral
However, the tiny contribution
the strangeness
contribution.
structure
function
experiments
we know that the strange
at the 5 - 10% level to the nucleon
quark contributions
form factors?
or d-quarks.
polarized
and contributes
ask what are the strange
to the nucleon photon
ground
state
Then one may
wave function
coupling does not distinguish
of the 2” is parity violating,
The effect is measurable
spin.
and their
s-quarks from u-
and allows measurement
due to the interference
of
with the single photon
The asymmetry
A
in polarized
electron
term containing corrections. factors
For a specific kine-
Strangeness Structure of the Nucleon
quark sea is polarized,
graph.
transfer
ratio measured
R&,,is
experiments
from a deuterium
of neutron
results.
scattering
at high Q2 significantly,
the power of polarization
to higher momentum
the same quantity
using double polarization
is given by:
Since the ratio
trend of the early data, improve the accuracy data illustrate
needed
k~&., W&w)” + k3 ’
A+ = where the ki are kinematic
in the electron
asymmetry
with the
developments.
of this type have now produced
the doubIe polarization
The data showed
data were urgently
Reliable data for the electric form factors at high Q2 can be obtained measurements,
are
the older and newer
(Figure 2). Part of the data are incompatible
They also do not show the same general trend.
to clarify the experimental
of charge and magnetization
= G~QZ CGLG~ + TGLGL - f(l - 4sin26’w)KG’,Gi
eg &rff
scattering
c(G;)~ + TV
contains
combinations
the axial form factor Gi is suppressed
The weak form factors
(G’). For example,
can be expressed
of electromagnetic
due to the factor (1 - sin20w),
(i - sin20w)G7,
- f(GLn + GS,)
The
and gives small
in terms of the G7 and the strangeness
the weak electric form factor can be written:
Gg =
and weak form factors.
form
RD. Burkert / Prog. Part. Nucl. Phys. 44 [ZOOO)273-291
The same relation G7 are known.
holds for the magnetic
form factors.
The G” form factors can be measured
The elastic $I results of the JLAB HAPPEX
show that strangeness
contributions
G; + 0.4G”, = 0.023 f 0.034(&t)
the analysis.
3
The Nucleon
since the
at Q2 = 0.47GekT2, of Gk and G”,, 141:
i O.O22(syst) f O.O26(G;) when the 1999 data are included
by the uncertainties
GEn! New measurements
measured
in a combination
error will be obtained
The error is then dominated
factor, especially
experiment
are small when measured
,4t least a factor of two smaller statistical
277
in the neutron
of GLn and CL,, should remedy
Spin Structure
electromagnetic
this situation
in
form
[5. 6, 371.
- from Small to Large
Dis-
tances The internal found
that
spin structure
at small distances
small to large distances of the nucleon kinematic
of the nucleon has been of central interest the quarks
carry
the quarks get dressed
spin.
How is this process
Gerasimov
between
Drell-Hearn
theory
s
g,(L7T)dx =
The integral
are performed,
hi12
for the difference
over the entire inelastic
energy
=
-gT2a
in helicity
scale?
difference
Going from
At the two extreme
6
at Q* > 2 GeV2.
is expected 01/z(V) -
s
as
Jc!
QCD corrections
and experiment
sum rule (GDH-SR) I =DH
with the distance
for the proton-neutron
rp”1 =
is good agreement
spin.
sum rules: the Bjorken sum rule (Bj-SR) which holds in
limit, and is usually written
At the finite Q2 where experiments
of the nucleon
with gluons and QB pairs and acquire more and more
evolving
regions we have two fundamental
the asymptotic
only a fraction
ever since the EMC experiment
have been calculated.
and there
At the other end, at Q2 = 0, the
to hold:
~3/2b4&/
=
_A,2
4
v
l/2 and helicity 3/2 total absorption K is the anomalous
cross sections
regime.
The quantity
magnetic
between
these regions is given by the constraint
moment
is taken of the
target. One important
connection
- it defines the slope of the Bjorken integral IrD,(Q2 Phenomenological
question
and neutron,
are taken into account
we can go beyond models and describe
SR to the GDH-SR for the proton-neutron &CD. For the proton
+ 0)
regime [8, 9, 71. The data at low Q2 [lo] are in good
if nucleon resonances
is whether
Q2)dz) at Q2 = 0:
to extend the GDH integral for the proton and neutron
it to the deep inelastic
with the predictions
An interesting
= S gr(z,
--t 0) = 2+Q2
models have been proposed
to finite Q2 and connect agreement
(ry(Q2)
due to the GDH-SR
difference
the GDH-SR
within the framework
is nearly saturated
explicitly
[9] (Figure 4).
the transition of fundamental
by low-lying
from the Bjtheory. i.r.
resonances
[ll, 121
I! D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
278 ASNZ(l870)
I
[1En”mn,L ---...>.-.
Jlab
I
-r
dCmbrldge(1871) ODESYl1873)
2.
5
4
1
0
Fig.
=0.5500V'
Results for the ratio RLM of electric
and magnetic form factors of the proton.
0.6
1.2
Fig.
techniques [3]
3.
Raw asymmetry
ZNgs + eX scattering
with the largest contributions absent
coming from the excitation
in the p-n difference.
may take on a smooth
Other resonance
transition
Recent estimates
[13] suggest
be applicable theoretical
using the modern
these techniques
end, at Q* = 0, where hadrons
2
2.2
measured
The latter contribution
in this connection
is: how
of higher order QCD expansion?
are the relevant
degrees of freedom,
chiral perturbation
of the GDH-SR
gap, perhaps
theory
to finite Q*.
utilizing lattice &CD. These efforts
since it would mark the first time that hadronic
structure
is described
theory in the entire kinematic
Experiments
have been carried out at JLAB on NH3 [15], NDa[16], and 3He [17] targets to extract
Q2 evolution
of the GDH integral
and neutrons
and from the elastic to the deep inelastic regime.
will be needed contributions
to determine
an experiment
are expected
on polarized
region, and the changeover energy continuum
NH3.
in the low Q2 range Q2 = 0.1 - 2.0 GeV’ only two data points with large errors exist
especially
at the larger Q2 values. for Q* above 1.3 GeV2
The deep inelastic
[18]. First results from
in the year 2000. Figure 3 shows an uncorrected The positive
the
in machine energy to 6 GeV, some extrapolation
have been measured
back to a positive
are evident.
Currently,
limitations
the full integral,
to the GDH integral
the JLAB experiments
by
regime, from small to large distances!
for protons
Because of the current
may
Significant
fundamental
for Q2 < 2 GeV’.
is
as well and the Q2 evolution
A crucial question techniques
in inclusive
may be valid as low as Q2 = 0.5 GeV2. At the other
efforts are needed to bridge the remaining importance
1.6
at JLAB.
are reduced
at very small Q2, and may allow evolution
are of utmost
1.6
of the A(1232).
contributions
to the Bj-SR regime.
low in Q* the Bj-SR can be evolved
1.4
The full
squares are the results from JLAB obtained with the double polarization
1
elastic asymmetry, asymmetry
the negative
for higher
asymmetry
asymmetry
mass resonances
from
in the A
and the high
I! D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 2 73-291
279
Neutron 0.01
EGl expected SLAC Data
I 0 > 2 9 -0.01
Burkertlloffe Saffer I g2 DIS
-0.02 -0.03 -0.04 -0.05 -0.06 -0.07 -0.08
0
1
2
3
4
5
8
7
8 9 Q*(G~V/C)~
Proton
DHG EGl expected SLAC Data A0 Burkert/ioffe Soffer - g2 DIS
1
2
3
4
5
6
7
8
Y
10
Q’(GeV/c)’
Fig.
4. The first moment
predictions only[ll]. straint.
rl(QZ)
of the polarized
from [7, 91. The curve labelled The straight The points
the measured
portion
line near
along
structure
A0 contains
Q 2 = 0 is the slope
the horizontal
of the integral
given
axis indicate
on the proton
function
s-channel
NH3
gl(r,
resonance
by the GDH
the expected and neutron
Q2).
sum rule con-
statistical NDJ.
Model
contributions
errors
for
Y D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
280
4
Excitation
of Baryon Resonances
A large effort is being extended factors contain information state.
to the study
We test predictions
of baryon structure
the search for, so far, unobserved QCD inspired nucleon,
of baryons
and mesons
structure
structure
systems.
decay channels
aspect is
but are predicted
by the
Gluonic excitations
of the
and some resonances
may be “molecules”
is important
to clarify
and the role played by the glue and mesons in hadron
Electroproduction
of hadronic
form
of the excited
&CD. Another
Search for at least some of these states of baryons
The transition
and the wave function
models and strong interaction
may be be copious /citeisgur,
and structure.
many of the possible
4.1
of the nucleon.
states which are missing from the spectrum
]Q”QQ >.
quark-gluon
spectroscopy internal
states
of the transition
quark model [22]. Also, are there other than IQ3 > states?
i.e. ]Q3G > states
the intrinsic
of excited
on the spin structure
is an important
tool in these studies
The scope of the N* program[24,
of resonances
in a large kinematic
as it probes the
271 at JLAB is to measure
range.
The yNA transition.
The lowest excitation due dominantly is in measuring
of the nucleon is the A(1232) ground state.
to a quark spin flip corresponding
the small electric and scalar quadrupole
to possible deformation
strength
dipole transition.
transitions
is that in the hard scattering to the magnetic
and gluon exchange
dipole contribution
[25]. An analysis
The interest
at the few percent
contribution
is
today
to be sensitive
at small distances.
limit the electric quadrupole
nonzero values for the ratio EI+/M~+ at Q2 = 3.2GeV2,
excitation
which are predicted
of the nucleon or the A(1232) [23]. Contributions
come from the pion cloud at large distances, prediction
to a magnetic
The electromagnetic
level may
An intriguing
should be equal in
[19] of earlier DESY data found small
showing that the asymptotic
QCD prediction
is far away from the data. An experiment momentum
transfer,
from CLAS indicate
at JLAB Hall C [20] measured and found values for IEI+/Ml+I negative
prrO production
in the A(1232)
region at high
< 5% up to Q2 = 4 GeV*. Analysis
values at small Q* with a trend
towards
positive
of new data
values at higher Q2.
Results should be available in 2000.
4.2
Higher mass resonances
The inclusive spectrum
shows only 3 or 4 enhancements,
the mass region up to 2 GeV. By measuring we can study symmetry nucleon structure. the interaction. amplitudes transition
properties
For example, It predicts
between
excited
amplitudes
situation
and Oia(1520).
Predictions
transition
and obtain
are known in
of many of these states
a more complete
picture
of the
model only one quark participates
in
for a large number of states based on a few measured
is shown in Figure
to the Ls9 = 1 SU(6) 8 O(3) multiplet
for Sii(1535),
states
in the single-quark-transition
transition
[21]. The current
however more than 20 states
the electromagnetic
5, where the SQTM
have been extracted
for other states
belonging
amplitudes
from the measured to the same multiplet
for the
amplitudes are shown
281
FCD. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
in the other panels.
The lack of accurate
even the simple algebraic
region, by measuring
observables.
at JLAB with the CLAS detector
many channels
The yields of several channels
7. Resonance
prevents
a sensitive
test of
SQTM.
The goal of the N* program resonance
data for most other resonances
excitations
in a large kinematic
recorded
sensitivity
to various resonance
excitation
near 1720 MeV while single pion production
MeV [27]. The pw channel
excitations.
shows resonance
resonance
has been observed
measured
throughout
are shown in Figure 6 and Figure
is more sensitive near threshold,
have different
clearly shows resonance
to a resonance
near 1680
similar to the pn channel.
in this channel so far. For the first time n7r+ electroproduction
the resonance
Figure 7 illustrates
excitation
many polarization
how the various channels
the A ++t~- channel
For example,
data in the entire
range, including
simultaneously
seem to be These yields illustrate
is to provide
No
has been
region, and in a large angle and Q* range.
the vast improvement
in data volume for the A++K
channel.
The top panel
shows DESY data taken more than 20 years ago. The other two panels show samples of the data taken so far with CLAS. At higher Q’, resonance
4.3
not seen before in this channel
are revealed.
Missing quark model states
These are states
predicted
in the IQ3 > model to populate
they have not been seen in rrN elastic excitation
scattering,
the mass region around
our main source
2 GeV. However.
of information
on the nucleon
spectrum.
How do we search for these states? states
structures,
Channels
which are predicted
to couple strongly
to these
or AT. Some may also couple to KY or pn’ (281.
are N(p,w)
Figure 8 shows preliminary
data from CLAS in w production rrO exchange
with strong
on protons.
to be dominated
by diffraction-like
and a monotonic
fall-off at large t. The data show clear deviations
range near 1.9 GeV, where some of the “missing”
peaking
resonances
The process is expected
at forward
w angles, or low t.
from the smooth
are predicted,
fall-off for the W
in comparison
with the
high W region. Although
indications
for resonance
wave study are needed before definite
in electron
scattering
in the search for missing significant
coupling
Strangeness
in photoproduction.
for resonance
of n’ has also been
may provide
two resonances
source of information
production
in these
with the CLAS detector,
could open up yet another
was not available in the past.
Production
a new tool
in this mass range with
[28].
may yet be another
are being accumulated
production
may be drawn.
The quark model predicts
to the NV’ channel
show some evidence
higher statistics
are strong, analysis of more data and a full partial
for the first time with CLAS. This channel
states.
KA or KC production data
conclusions
5 . lo5 pq’ events
CLAS has collected observed
production
channels
on resonant [30].
states.
New data
Previous with much
both in photo- and electroproduction.
window for light quark baryon spectroscopy,
which
282
K D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291 100
0 0
1
0
3
2
L
2
3
loo-
-20
-40 SO,,( 1535)
-60
-80 m
-loo0 0.5
1
1.5
2
0
05
1
1.5
0
2
0.5
1
1.5
2
1
1.5
2
Sott’““‘“’
,~~:i~
50
0
-50
-loo0
0.6
1
1.6
2
0
0.6
1
1.5
2
0
0.6
0 _--
-20
__-,' /'
-40
,' -_---
,'
D’,,( 1675) h = 3/2
-20 -20 0-
0
0.5
i
1.5
2
0
0.5
1
1.5
2
-100 •_ 0
0.5
1
1.5
2
Q2 (GeV2) Fig. 5. SQTM predictions
for states belonging to the SU(6) @ O(3) multiplet, discussed in the texl
K D. Burkeri / Prog. Part. Nucl. Phys. 44 (2000) 273-291
283
W(GeV) Fig.
6.
Yields
for various
channels
with CLAS at JLAB.
The statistical
smaller
points.
than
the data
measured
error
Fig.
bars are
7. Yields
for the channel
with
CLAS
data
from DESY.
at different
A++K-
Q2 compared
measured to previous
1
cose2.o
Gev
t +
l
.
Fig. ferent
. II -0.5
-
8. Electroproduction W bins.
distribution
from
W bin suggests production.
The
.
. I,
l
1 I 0
I I I I 0.5
of w mesons deviation
a smooth significant
of the
I I I
1
for difcos0
-
fall-off
for the low
s-channel
resonance
Fig. served
9.
Ratio and
of resonance
predicted
cesses using quark-hadron
from
excitations deep
inelastic
duality.[29]
as obpro-
284
K D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
5
Local
Duality
- Connecting
Constituent
Quarks
and Va-
lence Quarks I began
my talk by expressing
of hadronic
structure
section
in the data between
by Bloom and Gilman also describe
the average inclusive
A new inclusive
this aspect of hadron physics. can be predicted of measured
integrals
target
is surprisingly
non-trivial
consequence
cross sections mass effects.
Remarkably,
is possible
then there should
Such strong connections
have indeed been
in the resonance
Until recently,
experiment
elastic form factors or resonance
regions,
good, though of the underlying
this intriguing
scattering
and predictions
not perfect,
indicating
cross
region if a scaling variable
at JLAB [29] helped
from inclusive deep inelastic
over resonance
agreement
If such description
these regimes.
ep scattering
approximately
arrive at a unified description
[2]. They noted that the scaling curves from the deep inelastic
is chosen that takes into account little utilized.
that we may eventually
from small to large distances.
be obvious connections observed
the expectation
data.
observation
rekindle
was
the interest
excitations
in
of the nucleon
Figure 9 shows the ratio
using deep inelastic
data
that the concept
of duality
0.2
0.6
only.
The
likely is a
dynamics. 2
. . f 0
0’ P’ 0’ G’ a’ * 0’
. ..’
= = = = = =
0.45 (W/c) 0.85 (G&‘/c) 1.4 (GeV/c) 2.4 (W/c)’ 3.3 (G&/c)’ 0.2 (Cd/c) JLobfit ““‘.” GRV valemx .... NW10 ‘... NMCL
1.8
1.6
. . v 0
..,.,....... ..,
:-.
‘k,
Q’ = 0.45 (GeV/c)’ C? = 0.85 (G&‘/c) 0’ = 1.4 (G&/c) Q’= 2.4 (G&/c) 0’ = 3.3 (GeV/c)’ + 0’ = 0.2 (GeV/c)’ JLabfit .‘.‘.‘.. GRV valence . . GRV ....
0.6
NW10
0
Fig. 10. Compilation
of resonance data at differ-
0
0.1
0.3
0.4
0.5
0.7
shown together with the xF3 structure
inelastic
of the curve la-
tained from neutrino and anti-neutrino
a new fit to the
latter
belled ‘JLab
fit’ which represents
JLab data.
and constituent
number of resonance
1
represent
function obdata.
the valence quark distribution
The in
the nucleon.
How can this success be explained partons,
0.9
Fig. 11. The JLab fit to the Fz data from Figure 10
ent Q’. The curves are from the evolution of deep data, with the exception
0.8
in terms of the underlying
quarks, respectively.
degrees
of freedom
- elementary
Part of the answer is shown in Figure 10, where a large
data sets with different Q2 are shown together
with the evolution
curves from deep
c( D. Burkert / Prog. Part Nucl. Phys. 44 (2000) 273-291
inelastic scattering. evolution
curves fail to reproduce
the resonance
using valence quarks only has the same small E behavior.
fit) reproduces neutrino
The deep inelastic
the the Z&(Z) structure
scattering
(Figure
that the constituent the distribution intriguing
function
11) This quantity
quark distribution
of elementary
observation
determined
A new fit to the data (labelled Jlab
valence quarks.
of neutrino
and anti-
The agreement
suggests
region has an < dependence
valence quarks in the deep inelastic
can be translated
data at small <. while an
from the difference
only contains
in the resonance
285
into the development
region.
very similar to
It remains
to be seen if this
of new model approaches
to resonance
physics. In the following I will discuss recent results from experiments
6
on ‘H and 3He.
Elastic Formfactors of the Deuteron
In the same way that elastic electron and current
distributions,
1, the elastic response
functions in studying
This involves measurement significant
interest
contain
3 electromagnetic
these form factors
currents,
within
6.1
the quark
Since the deuteron
form factors
observable
picture,
On the
set of measurements.
(Tzn). On the other hand, there is behavior
to describe
models that include nucleons,
exchange
charge has spin
Gc, GQ, and GM.
a complete
There we probe the short distance
scale, from hadronic
descriptions
reveals their intrinsic
scattering.
A large variety of models have been developed
wide range in distance
perturbative
and neutrons
is to obtain
of at least one polarization
in the high Q2 behavior.
nucleon interaction.
on protons
so does elastic electron-deuteron
one hand,
the interest
scattering
pion, isobars,
to descriptions
of the nucleon-
the form factors for a
within
and exchange
the framework
of
&CD.
Unpolarized
The unpolarized
elastic response
elastic eD scattering
functions
in eD + eD
cross section contains
the two response
functions
A, B:
$ =OM[A(Q*) +B(Q2)tan2(;)] , where A(Q’)
G;(Q')+ +2~;(Q2)+ $G;(Q~)
=
B(Q') = ;r(l Unpolarized scattered separate
electron
electron
measured
at backward
the response
and different
scattering
this process
were detected
scattering
functions
+ r)Gk(Q2);
Q2 r=m
allows determination
of the magnetic
angles.
of Gc and Go is not possible.
A separation
A(Q2) and B(Q2) by measuring
angles (Rosenbluth in a coincidence
in two high resolution
setup,
separation).
by measuring
the
One can only
the elastic cross section
An experiment
where both the scattered
spectrometers.
form factor
at fixed Q”
in JLab Hall A (E-91-026) electron
and recoil deuteron
The results for A(Q2) are shown in Figure 13.
286
Y D. Burkert / Pmg. Part. Nucl. Phys. 44 (2000) 273-291
10-4
JLab
Hall
0 SLAC
El01
l
~(62’)
1
A
10-S
10-6
RIA+MEC Hummel
10-T
_ _
RIA+MEC Van Orden
et
Hummel
10-a
& Tjon
& Tjon
10-g
Fig.
measured in eD +
12. The electric response function A(&*)
eD scattering.
The
JLab data extend the Q2 range of previous SLAC experiments. The data are approximately may therefore
be understood
described
within these models.
of freedom have to be invoked to describe
6.2
by modern hadronic
models.
It is therefore
Even the approach
to scaling
not obvious that quark-gluon
the data even at the highest
momentum
degrees
transfers.
Tensor Polarization in eD + eD
A separation
of the charge and quadrupole
iment, in addition particularly
to the unpolarized
suited to accomplish
of the electromagnetic
form factors of the deuteron
measurement.
this. This tensor polarization
tering experiment
require a measurement
component
using a suitable
analyzing
accelerators,
of the deuteron reaction.
out at lower energy
carried
out in Hall C, using a high power deuterium to analyze the kinematics
for the second scattering
of the tensor polarization can be expressed
t20 is
in terms
7GQ(~GQ -+ 3Gc) G; + %zG2 9 Q
carried
spectrometer
exper-
form factors:
tze = -2 These experiments
A measurement
requires a polarization
experiment,
covering
and deuteron
Previous
recoil polarization experiments
the lower Q2 range. cryogenic polarization.
target,
in a second scat-
of this type have been
The JLab experiment
and a new deuteron
A liquid hydrogen
which was needed to analyse the deuteron
results for tze are shown in Figure 14 [32, 331. Using the known response
function
was
magnetic
target was used
polarization[31].
The
A(QZ) the deuteron
287
Y D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
charge form factor Gc(Q2)
can be separated
at Q2 = 0.7 GeV’, and remains Hadronic
models describe
negative
(Figure 15). The charge form factor shows a zero crossing
over the complete
large Q2 range.
the data over the entire range in momentum
transfer.
0 NIKHiF’[G] + Jefferson Lab NRIA [6] - NRIA + MEC I61 CIA[lO] pC!CD [16]
’’
‘\ \t’, i 0.6
0.4
0.2 0
7’
”
\
I b c
\
\
”
$1 ‘.
0
_..
6
-.-_
-0.004
l
_
L .
0.2
LL_Lyb 4
k l
\+
-.-~---------
-0.2
7
4
a (Id)
6
6
Fig. 13. The tensor polarizations
7
a (fm.‘)
tzo, tsr and t22
measured in eD + eD. The deuteron polarization
Deuteron
The deuteron
the known A(&‘)
photo-disintegration
is an ideal laboratory
Fig. 14. The deuteron charge and quadruole form factors as extracted
was measured in Dp + ppn scattering.
6.3
I
1
to study
from the t20 measurement response function.
at high momentum where the traditional
transfer
Yukawan
picture
may break down, and the quark picture may provide a more effective description. simplest
nucleus permits
exact hadronic
transfer
to the constituents,
stituent
cross section
counting
Experimentally,
and thus study the approach
One of the indications differential
calculations.
for the relevance
according
rules predict
to the number
of constituents
that the energy dependence
of the nucleus
The deuteron
as the
one can give a large momentum
to scaling at modest
of quark constituents
and
energies.
in the interaction involved
for the two-body
is scaling of the
in the interaction. reaction
Con-
rd -+ np should
scale like: du N&J z=sn_2 where n is the number of elementary While scaling has been observed and up to the maximum [341.
energies
’
fields in the initial and final states,
at center-of-mass
and n-2 = 11 for the yd + np.
angles near 90” for photon
of 4 GeV (Figure
energies as low as 1 GeV
16), no scaling is observed
for smaller
ocrn angles
288
K D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
New models have been developed the parameter
n to be angle-dependent,
models
the number
where
model (“constituent description
e.g. the Regge gluon-string
of constituent
scaling”)
of the reaction
that give a more realistic description
involved
over a larger kinematical
New data have been taken to extend whether
scaling persists
is smaller
These models
range (Figure
the kinematic
for the 90” kinematics,
model [35], and quark exchange
in the reaction
which involves all constituents.
of the process, and predict
than
in the maximal
indeed provide a better
17).
range up to 5.5 GeV photon
and if scaling is approached
energies to see
at different
angles.
_b Y -
s \b -u
E= (I)
2.0 1.5 1.0 0.5 0.0 6.0 4.0 2.0 0.0
Fig. 15. The cross section for yd --t np multiplied
Fig. 16. Preliminary
by the predicted dimensional
E95-001 to measure the magnetic form factor of the
scaling function s*l.
results for JLAB experiment
Scaling is not observed at small angles, where the
neutron.
quark exchange model gives a better representation
measured in scattering
of the data.
a polarized 3He target.
6.4
The experimental
asymmetry
is shown
of polarized electrons from
Polarization asymmetries on 3He
3He has emerged
as an attractive
comes to a pure polarized naive picture
target
neutron
can be calculated
target
material
for polarized
neutrons.
and is a simple enough nucleus,
with some confidence.
At low momentum
It is the closest any nucleus so that corrections transfer,
corrections
to this appear
to be large for some reactions. An experiment get information
in JLAB Hall A measured
on the magnetic
quasi-elastic
form factor of the neutron,
electron
scattering
off 3He in an effort to
and to study asymmetries
in the breakup
%D. Burkert / Prog. Part. Nucl. Phvs. 44 (2000) 273-291 region at small excitation
energies
Figure 18 shows preliminary magnetic
form factor.
extraction
7
[36]. data for the sensitivity
The model dependency
of the quantity
289
of the measured
of final state corrections
asymmetry
to the neutron
seems small enough to allow
of interest.
Outlook
The ongoing experimental in the first decade intermediate to translate physics.
effort at Jefferson
of the next millennium
distances.
the community
many open problems
effort must be accompanied
The experimental
description
with a wealth of data in hadronic
by a significant
structure
theoretical
at
eflort
of the complex regime of strong interaction
this into real progress in our understanding
One area, where a fundamental
nucleon spin structure
may be within
reach,
is the evolution
of the
from small to large distances.
New instrumentation program
Lab will provide to address
will become
in parity violation
available,
to study strangeness
e.g. the G” experiment
at JLAB, allowing a broad
form factors in electron scattering
in a large kinematic
range. Moreover, in exclusive
there
processes
for longitudinal
on the horizon.
the soft (nonperturbative)
photons
then be measured inelastic
are new opportunities
at sufficiently
high Q2.
which are generalizations
scattering.
For example,
while pion production parton distributions
probes
part
and the hard
low-t p production
the polarized
to measure
it was shown[38,
(perturbative)
A new set of “skewed parton
of the inclusive
structure
the small exclusive
structure
probes
functions
the unpolarized
functions.
need to have sufficient energy transfer
regime, high luminosity
Recently,
and momentum cross sections,
parts
transfer
factorize
distributions” measured
parton
Experiments
391 that
can
in deep
distributions,
to study
these
new
to reach the pQCD
and good resolution
to isolate
exclusive reactions. This new area of research
may become
a new frontier
of electromagnetic
physics
well into the
next century. To accommodate proposed
new physics requirements,
for the CEBAF
of a new experimental meson spectroscopy, be upgraded
machine
an energy upgrade
at JLAB. This upgrade
hall for tagged photon and production
of other
to reach higher momenta
experiments
will be accompanied with a 47~solenoid
heavy mesons.
and improvements
in the lo-12 GeV range has been
Existing
by the construction
detector
to study exotic
spectrometers
in Hall C will
of CLAS will allow it to cope with higher
multiplicities. This will give us access to kinematics momentum
transfer
new generalized
can be reached
parton
distributions.
where copious hybrid meson production
for form factor measurements,
is expected,
higher
and we may begin to map out the
290
V D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 273-291
References [l] N. Isgur, hepph/9904494,
(1999)
(21 E.D. Bloom and F.J. Gilman, [3] M.K.
Jones,
PANIC99,
Phys. Rev. D4, 2901 (1970)
et al., nucl-ex/9910005;
June ‘99, Uppsala,
see also:
C. Perdrisat,
plenary
Sweden
[4] K.A. Aniol, et al., Phys. Rev. Lett. 82, 1096(1999) [5] D. Day, et al., JLAB experiment
E93-026
[6] R. Madey et al, JLAB experiment [7] J. Soffer and 0. Teryaev,
E93-038
Phys.Rev.Lett.
[8] V. Burkert
and B. Ioffe, PhysLetts.
[9] V. Burkert
and B. Ioffe, J.Exp.Theo.Phys.
70, 3373(1993)
B296 (1992)223 78, 619 (1994)
[lo] K. Abe et al., Phys. Rev. D58, 2003 (1998) [ll] V. Burkert
and Zh. Li, Phys. Rev. D47, 46(1993)
[12] W.X. Ma, D.H. Lu, A.W. Thomas,
Z.P. Li, Nucl. Phys. A635 (1998) 497
[13] X. Ji, J. Osborne,
(1999)
hepph/9905010
[14] X. Ji, C.W. Kao, and J. Osborne, [15] V. Burkert,
D. Crabb,
R. Minehart
hep-ph/9910256 et al., JLAB experiment
[16] S. Kuhn, M. Taiuti et al., JLAB experiment [17] Z. Meziani, et al., JLAB experiment [18] K. Ackerstaff [19] V. Burkert,
E93-009
E94-010
et al., Phys. Letts. B444(1998)531 L. Elouadrhiri,
Phys. Rev. Lett. 75, 3614 (1995)
[20] V. Frolov et al., Phys. Rev. Lett. 82, 45 (1999) [21] A.J. Hey , J. Weyers, Phys. Letts. 48B, 69 (1974) [22] N. Isgur, G. Karl, Phys. Rev. D23, 817 (1981) [23] V. Frolov et al., Phys. Rev. Lett. 82, 45 (1999) [24] V.D. Burkert,
Nucl. Phys. A623 (1997) 59c-7Oc
E-91-023
talk,
Proceedings
of
l! D. Burkert / Prog. Part. Nucl. Phys. 44 (2000) 2 73-291 [25] C.E. Carlson,
[26] H. Funsten Uppsala,
Phys. Rev. D34, 2704 (1986)
et al., JLab experiment
ESl-024;
J. Manak et al., proceedings
of PANIC99,
June ‘99.
Sweden
[27] M. Ripani et al., Proceedings, [ZS] S. Capstick
and W. Roberts,
[29] I. Niculescu pel,private
29
PANIC99,
June ‘99, Uppsala,
Sweden
Phys. Rev. D49, 4570 (1994)
et al., “Experimental
Verification
of Quark-Hadron
Duality”,
R. Ent,
C. liep-
communication
(301 M. Q. Tran et al., Phys. Lett. B445, 20 (1998) [31] E. Beise, S. Kox et al., Jlab experiment [32] presented
E94-018
by: J.-S. Real, INFN Workshop
on the Structure
of the Nucleon,
1999.) [33] presented
by K. Hafidi, PANIC 99 Uppsala,
[34] C. Bochna et al., Phys. Rev. Lett.81, [35] L. Kondratyuk,
E. DeSanctis,
[36] H. Gao et al., JLAB experiment
4576 (1998)
et al., Phys. Rev. C (1993) E95-001
[37] W. Brooks, et al., JLAB experiment
E94-017
[38] X. Ji, Phys. Rev. D55, 7114 (1997) [39] A. Radyushkin,
Sweden, June 1999.)
Phys. Rev. D56, 5524 (1997)
Trieste,
Italy, May