Recent results from Jefferson Lab

Recent results from Jefferson Lab

Progress in Particle and Nuclear Physics PERGAMON Progress in Particle and Nuclear Physics 44 (2000) 273-291 p http://www.elsevier.nl/locate/ppart...

1MB Sizes 0 Downloads 93 Views

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