Measurement of the spin dependent structure functions of proton and neutron

139c

M~SUREMENT

OF THE SPIN DEPENDENT

STRUCTURE

FUNCTIONS

OF PROTON

AND NEUTRON

Klaus RITH Max-Planck-Institut Fed. Rep. Germany

fiir Kernphysik,

P.O. Box 103980,

D-6900 Heidelberg

1,

Recent results from the EMC experiment on the spin dependent structure function glp(x) of the proton are discussed. They suggest that the nucleon spin does not originate from quark spins but rather from angular orbital rnornentLlm and gluon contributions. A proposed experiment at HERA is presented which will allow a very accurate measurement of the spin dependent structure functions and their integrals of both proton and neutron and a precise test of the Bjorken sum rule.

1. INTRODUCTION The investigation neutron

is very

internal

nance

important

structure

between

of the spin dependent

quired

just

scattering

ture of the nucleon. conventional information

polarized

of the proton, and the neutron

glp(x),

EMC experiment

proposed

electrons storage

with polarized

are, however,

accuracy

system.

lepton beams

on polar-

spin struc-

difficult

and wiih

are very small. Until now only some

the recent

consequences

for the HER4 electron technological

structure

structure

electron

storage

storage

results

and present

developments:

cell gas target of high density. short running

quark

functions

function,

g*p(x),

unexplored.

I will discuss

in a high energy

the A-reso-

the internal

rather

no data exist for the second

and their exciting

on two difficult

force and of the

between

in this 3 valence

the effects

and

carry spin and the forces

for one of the two spin dependent

is completely

In this contribution

experiment

targets

of proton

that about 300 MeV are re-

the ideal tool to study

The experiments

is available

of the strong

and gluons

we can estimate

experiments

are in principle

functions

From the mass difference

for instance

to flip the spin of one quark

Deep inelastic

tively

since quarks

them are spin dependent.

ized targets

based

for the understanding

of hadrons

and the nucleon

structure

longitudinally

ring and an internal

0375~9474/89/$03.50 0 ElsevierSciencePubiishers B.V. (North-HollandPhysicsPubl~~g Division)

an

is

polarized polarized

It will allow us to measure

Bjorken

from the

in some detail

ring. This experiment

time &l(x) and gz(x) for both proton

and to test the fundamental

for glp(x)

and neutron

in a relawith high

sum rule on the 7-10% level.

14oc

K. Rith j Spin dependent

structure ftanctions

2. PHENOMENOLOGY The internal structure

spin structure

functions

gI(x,Q*)

Q2 IS . the negative exchanged

of the nucleon

and g2(x,Q2).

square of the four momentum

between

lepton and nucleon

can be described

x = Q2/2Mv carried

in terms of two

is the Bjorken

variable,

by the virtual

photon

in the deep inelastic

scattering

M is taken to be the proton mass and v = E - E' is the energy the virtual

photon

ble Q* dependence

of these quantities

The structure parton

function

(In the following

by

a possi-

will be neglected).

gl(x) has a transparent

interpretation

in the quark

model:

g,(x)

=

3c 2 f

where

from the lepton to the nucleon.

process,

transferred

*

t

f

(q,+(x) + 4,+(x))

the sum runs over the different

quarks

(in units

(antiquarks)

qf'(x)

of /el).

with helicity

,

- (q,-(x) + 7,-(x)))

flavours

(cf'(x))

f and Zf is the charge

are the densities

the same (+) or opposite

of the

of quarks

(-) to that of the parent

nucleon. Due to angular be absorbed helicity.

Therefore

dinally

polarized

target,

where

the incoming qf-(x)

momentum

by a quark

conservation

(helicity

qf+(x)

charged

photon

its helicity

can be obtained

(helicity

opposite

from a measurement

lepton beam on a longitudinally

the direction photon

a virtual

&J) with

of the nucleon

(cross section:

d2e/(dQ2dxft'

with a longitu-

polarized

spin is antiparallel := of').

rtl) can only

to the photon

nucleon

to the one of

Correspondingly,

can be measured in the case where the target polarization is parallel to t7 spin (o ), and gl(x) is proportional to the difference of the two

the photon

cross sections

otL - oft.

On the other hand,

the unpolarized

structure

function

Fl(x) is obtained

from

the sum at1 + oT+:

Fl(x)

2

=$z f

f

I

q,+(x) + P,+(x)

+ qf-(x) + 4f- (x))

The second

spin dependent

structure

of quark masses

and transverse

momenta,

glw

where

+ g*(x)

T denotes

=

g*(x) is related

Kt, of quarks

to effects

in the nucleon:

& 8 Zf2 [rnf +-%$(,T+w - 9T-w)

a nucleon

Experimentally

function

1

polarization

one measures

transverse

the asymmetry

to the photon

polarization.

K. Rith / Spin dependent

141c

structure functions

tt

tl

*=gtl

- c7

+ott ’

0

is related

which historically

to the virtual

D is the depolarization

A - D (A1 + 11 A2).

photon

asymmetries

A1 and A2 by

factor of the virtual

photon

given

by y(2-Y)

D=

with y = v/E and R = eL/uT

and 7 is a kinematical

factor

7 =

Since both R and v are usually

tion to the measured surements

asymmetry

at different

transversally

beam energies

polarized

Al and A2 are (in the Bjorken nucleon

structure

functions

one obtains

limit)

A2W

and to a good approximation

one can assume: an asymmetry

N N

IA113

either by mea-

measurement

another

and

contribu-

asymmetry

with a AT = d(A2 -

related

to the spin dependent

gl(x) and g2(x) by:

Experimentally

A =

to be

factors. scaling

Al(x)

one measures

limits

small, Al is the dominant

or by an additional

where

target,

by positivity

Al and A2 can be separated

A.

3 with d and E kinematical

E Al)

2(1-Y) J? E y(2-Y)

Al und A2 are bounded

The asymmetries IA*I~&.

,

’

y2+2(1-y)(l+R)

=

&

(%)1’2 (g1+g2)

gl = A1.Fl = fi Fl

of counting

rates

tl _ Ntt t-L

tf '

+N

which is smaller than the asymmetry A due to the fact that beam polarization, p T , are usually smaller than one and that in conp B , and target polarization, ventional With

polarized

f being

the target

targets

one obtains

of events B . pT A = p

and for the statistical

&Al

only a small fraction

the fraction

For typical

is polarizable. free nucleons

in

D .A1

1

JNtr +N

sured asymmetry

. f.

of nucleons

from polarized

error

1

=

originating

tt .

values

PB

. PT .f.D

of these quantities

is always

smaller

(f=0.15, pB=pT=0.8,

than 0.02

(in the biggest

D=0.2),

the mea-

part of the kine-

142~

K. Rith /Spin dependent

range even <0.002)

matical

times bigger

and the statistical

than the one expected

The x dependence

structure functions

of the structure

functions

From light-cone

obtains famous

relations

of g,_(x) for proton coupling

constants

and neutron IgA/gVI=ga,

from different

and perturbative

of the polarized

sum rule2 which relates

After

p decay.

algebra

for the integrals

is the Bjorken

Teller

current

rates alone.

cannot be calculated

turba'cive QCD, but there are several predictions nueleonl.

than 50

error for Al is more

from the counting

from per-

models

of the

QCD, however,

structure

the difference

one

functions.

Very

of the integrals

to the ratio of the axial-vector-to-vector the axial charge, measured

correction

for QCD effects3

in nuclear

weak

Gamow-

this sum rule is given by

1 (glpW

- gln(x))dx

- i g,(l - >)

s 0 = 0.191 c 0.002 for as = 0.27 This fundamental ergy behaviour

sum rule is of great importance

has been performed Separate

since no measurement

the high en-

of nucleons.

on a polarized

Experimen-

neutron

target

yet.

sum rules for proton

using

since it relates

of quarks with the low energy behaviour

tally it is still untested

Jaffe4

t 0.02.

SU(3) arguments

and neutron

have been derived

with the assumption

by Ellis and

of an unpolarized

strange

quark sea. This sum rule is given by 1

11 p(n) =

g1

s 0

p(*)(x)

dx = $ ga I+(-)1 + 5 X)

F, D are the SU(3) coupling

where baryon

constants

which govern

the decays

in the

octet.

Again

after correcting

0.63 rt 0.024 Ilp =

for QCD effects5

6 the integrals

and with

the current

value F/D =

have values

0.189 -t-0.005,

11n = -0.002 5 0.005.

3. THE EMC EXPERIMENT The experiment7

FOR glp(x)

measured

the spin asymmetry

muon scattering

at CERN, using polarized

larized

of ammonia,

beam

target8

is automatically

and the degree

beams

for the proton of energies

NH3, and the EMC Forward

polarized

from the decay

of the polarization

in deep inelastic

100-200

Spectrometer.

GeV, a poThe muon

in flight of the parent

can be chosen by selecting

pions

a specific

energy

K. Rith /Spin dependent

I 001

143c

structure functions

I

I 002

005

01

02

05

07

10’

10’

FIGURE 1 The asymmetry alp from the EMC and SLAC experiments

ratio of the parent Monte

Carlo

sections, larized were

FIGURE 2 EMC result for x~lp(x) and the integral over gl (x) (see text)

pion to decay muon.

simulation

360 mm, separated

in opposite

value.

Because

65% in 8 hours

Typical

time was rather

and than slowly within

of this long build-up

was calculated

The target consisted

using a of two

by a gap of 220 mm, which were po-

directions.

in the order of 75%, the build-up

grew to about

The polarization

to be (82+6)% at 200 GeV.

each of length

simultaneously

1

x

x

target polarizations long. The polarization

24 hours

time the polarization

to the saturation

was only reversed

once per 2 weeks. Values

of Alp were obtained

2.5 GeV2 5 Q2 5 70 GeV2. glected,

the effect

but

is much above

of overlap.

in the systematic

The prediction

The spin dependent

error of Al

ing a QCD calculation scale as a function

The results

of Carlitz

and

of the data at large x

for Al(x)

do not vary with

errors.

structure

function

glpW = Alp(x) FlpW = Alp(x) . of asp

of the model

gives a good representation

the data for x < 0.2.

the statistical

The values

of Al from the data A2 was ne-

A2 was included

The results

is also shown, which

Q2 within

range 0.01 5 x 5 0.7 and

for Al are plotted in figure 1 together with SLAC experiments 9 , which are in good agreement with the EMC

data in the region Kaur"

For the extraction

of neglecting

(taking A2 = L fi). those of previous

over the kinematic

were

glp(x) was obtained

from alp

from

Fqp(x) 2x(1+~)

those measured

in a previous

for R. In Fig. 2, xgl(x)

is plotted

EMC experiment 11

of x (right hand axis and dots). The integral

the low edge bin to 1 is also shown in the figure

US-

on a logarithmic of gl(x) from

(left hand axis and crosses).

144c

K. Rith /Spin dependent

The integral

stnrcture functions

was found to be

1 p=

glp(x) dx = 0.114 + 0.012

I1

(stat.) -t 0.026

(syst.)

.

s 0

The contribution

of the extrapolations

The main sources

for the systematic

long time intervals beam polarization, This value pected

between neglect

target polarization

comes more significant,

reversals,

effects

due to too

uncertainties

in F2,

of A2 and target polarization.

of the integral

from the Ellis-Jaffe

to x = 0 and x = 1 is only about 3%.

errors were acceptance

is much smaller

than the value 0.189 ? 0.005 ex-

sum rule. The disagreement

with the sum rule be-

the EMC and SLAC results 12 .

if one combines

One ob-

tains

IIP - 0.116 f 0.009 4 0.019 which

,

is more than three standard

deviations

away from the expected

theoretical

value. Assuming

the validity

for the proton

II*

a value

of the Bjorken

sum rule one obtains

for the integral

Iln for the neutron:

,

= -0.077 + 0.012 (stat.) f 0.026 (syst.)

which means

that gl*(x) must be large and negative

These results

can be used to calculate

ties to the nucleon

helicity.

from the EMC data

over a large range of x.

the contribution

of the quark helici-

If one defines

--

Aqf

- j

(q,+(x)+ ;,‘(x) - qf-W - qf

and uses the quark parton model definition

2.

(x)1

dx

0

Ilp=$Au+;

2 * 11

of glP(x),

Ad = 0.228 + 0.024 + 0.052

n =$Au+$5d-

-0.154 f 0.024 ? 0.052

From these two equations

one obtains

, .

one can derive Au and Ad, the fraction

from the spin of u and d quarks,

obtains

- + (Au + Ad) = 0.068 + 0.047 i: 0.103.

u+d

If one assumes sum rule prediction

that the discrepancy

between

is due to the polarization

of the nucleon

and after QCD corrections

spin originating

the result

one

and the Ellis-Jaffe

of the (strange)

sea quarks one

K. Rith /Spin dependent

obtains

s

polarized

= -0.113 f 0.019 ? 0.039, which means

opposite

to the nucleon

145c

structure functions

that the (strange)

sea is

spin, and finally:

= 0.006 k 0.058 + 0.117.


4. INTERPRETATIONS From this EMC result rule is strictly cleon

spin originating

contrast bital

expectations,

momentum

(s,‘) = $ 1

one must naively

the assumption conclude

that the Bjorken

that the fraction

from the spin of the quarks

to intuitive

angular

for asp and under

valid,

is consistent

and that it is rather

sum

of the nu-

with zero, in

due to gluon or or-

contributions:

.

Aqf + Ag + Lz

f Furthermore,

the (strange)

of the parent

yond

result

zero caused

that the contribution

a lot of excitement

the scope of this paper

all the publications will

dealing

only list a few ideas.

- the EMC result - perturbative - the effect Gerasimov

and/or

the different

this 'spin crisis' for possible

the extrapolation

sum rule, which

produce

approaches or even cite problem 13 and therefore I are

for x + 0 is wrong,

linked

to the Drell-Hearn-

a rapid Q* dependence not visible

is dominantly

It is be-

sum rule is violated,

twist effects

is, however,

activities.

explanations

of glp(x)

and the Bjorken

is caused by higher

- the spin of the proton

of asp.

Such a

in the data for asp,

due to orbital

angular

momentum

contri-

of the quarks,

- the spin of the valence tion originating since

to that

from the quark spins is essen-

and theoretical

to discuss with

Examples

QCD is wrong

strong Q* dependence

butions

opposite

nucleon.

The startling tially

sea might have a large polarization

quarks

from gluons.

it arises naturally

however,

several

is compensated Many people

from perturbative

units of angular

momentum

by a large sea quark contribu-

favour QCD.

this last explanation The gluons have

to account

to carry,

for the magnitude

of

the effect. In my opinion understood

The existing cide which

all these discussions

and that much more

show how little

specific

experimental

information

of the proposed

solutions

experiments

the nucleon

are required

is far from being is the correct

one.

accurate

is really

to explore enough

it

to de-

146c

5.

K. Rith / Spin dependent

THE PROPOSED

PO~RIZATION

5.1. Introductory

is based on three assumptions:

for the proton

b) the fundamental

Bjorken

Since the result

value

deduced

is so important

structure

of the nucleon

ists have

the task: (disprove)

valid

and

from a) and b).

for the understanding

it is obvious

of the internal

that in the coming years

the EMC result

for the proton

the statistical

and systematic

errors

tally is completely

unknown.

This can be done using

preci-

lOO-150%

for IMP are unacceptably Iln for the neutron

gin(x) and its integral

experimental-

with much better

the error bars of gl p(x) at low x are about

sion. At present

- to determine

is correct,

sum rule is strictly

c) Iln has the large negative

- to verify

AT HERA

remarks

All the discussion a) the EMC result

~PERIMENT

structure functions

and

large;

which

a polarized

experimendeuteron

target.

This has the advantage that the integral over the polarized deuteron d structure function, I1 , is {apart from a small correction due to the strange

sea) directly

inating

from quark spins:

proportional

to the fraction

Another

method would be the use of a polarized

neutron

target to a good approximation;

- to test precisely

the Bjorken

This can be done by combining

of the nucleon

3He target which

spin orig-

is a polarized

sum rule.

the results

for the proton

and the deuteron;

I

J

(glp(x)

- gln(x)ldx

= 2I,_’

-

Ild

;

0

- to measure

also the second polarized

structure

function

g2(x) for both pro-

ton and neutron. At present

there are two possibilities

The first one is to use the technology solid polarized deuteronized

target of ammonia,

versions

sent a collaboration

NH3, or butanol,

in a low intensity is being

to perform

such an experiment:

of the last decades,

electron

formed which

intends

CqHgOH,

With

the statistics

in this experiment

collected

the statistical

to the EMC result and measure

and their

or muon beam. to perform

ment at CERN using a 100 GeV muon beam and the upgraded trometer.

i.e. a conventional

EMC/NMC

in two years beamtime

Indeed at pre-

such an experiforward

spec-

one could reduce

error of glp(x) by a factor of two compared

Aid(x)

with about the same precision

as alp.

K. Rith /Spin dependent

The second

States

izable,

in my opinion

by a collaboration

is much more

of 12 institutions

favourable

from Europe,

1s . to use the technology of the nineties,

and CanadaL4,

gas target

which

approach,

been proposed

147c

structure functions

of hydrogen

where

and deuterium,

and the high current

longitudinally

and has

the United

i.e. a polarized

all the target nucleons polarized

are polar-

beam of an electron

stor-

age ring like HERA or LEP. The essential and deuterium

prerequisites

than it has been the electron of around

achieved

beam

which

age cell target15

electron

is fed by a high intensity

beam polarization

the degree

of polarization

like HERA or LEP. Longitudinal rotators,

a technique

the first

time at HERA.

teresting 16 rings .

which

polarization

are necessary

the difficulties.

new prospects

it is not yet possible

with

results

The advantages - the fraction terium, around

ring for

it is a chal-

could open in-

targets

in storage

has enormous

advantages

and

are the following: f of polarizable

while

nucleons

for conventional

targets

is large:

f=l for hydrogen

like ammonia

or butanol

and deu-

it is only

15%;

solid targets

is large: pH , p D > 0.7, while

an optimistic

- one has a clean pure target.

value

the copper

coils

the two target halfs

beam source,

to hours

try by frequent

nucle-

the target vessel,

of target polarization,

in ms by an rf transition

for a solid target,

rf-insulators

cancel

spin reversals;

out completely

in the atomic

and changes

which were the biggest

and acceptance,

in the EMC experiment,

from unpolarized

and so on;

can be reversed

compared

tor performance

is no background

to cool the solid target,

for the measurement

- the spin direction

for large deuteronized

is pD - 0.4;

There

ons, the 3He/4He bath required

errors

storage

spin

with very high precision.

- the target polarization

between

machines

by special

progress

internal

atoms.

to pre-

at bigger

has to be produced

The technological

stor-

ring has been ob-

for this kind of experiment

for experiments

of

density

of polarized

storage

can be achieved

The effort will pay since this new technology will provide

target

a thin walled

which will be tested at a high energy

Since new techniques lenge to overcome

source

in an electron

larger

polarization

the required

has to be used, namely

hydrogen

of magnitude

until now, and a large longitudinal

at PEP, DORIS and PETRA, but at present

dict firmly

are a polarized

of about two orders

in the order of 50%. To achieve

lOl4 cme2 a new technique

Transverse served

for such an experiment

gas target with a density

source

in the detecfor systematic

in the measured

asymme-

K. Rith /Spin dependent

148c

- the measured

asymmetry

conventional

targets.

Am = f

pT * pB

l

asymmetry

also the statistical

will be -7 times smaller which meana 50 times longer

5.2. Electron

to achieve

beam and storage

The experiment

but conventional

than for

beam polariza-

target

than

technol-

error for A1

that with the old technique

the same statistical

one has to

accuracy.

cell target

could be installed

set of spin rotators

larger

the longitudinal

will be about 7 times larger

with the same luminosity

ogy. Consequently,

measure

D * Al is much

l

Even in the case where

tion is only 50% the measured for an experiment

structure functions

will be mounted

in the East Hall of HERA where for test purposes.

the first

It is proposed

to

straighten

out the beam line over about 2135 m. In this configuration

electron

and proton

ring will be vertically

separated

by 88 cm in the region

of the experiment.

tages for the optimization electron

at the same level and horizontally

polarization.

This arrangement

of spin dynamics

In addition

and facilitates

the amount

of magnets

in the intersection

regions

advan-

high longitudinal

of synchrotron

the target region will be reduced by orders of magnitude dard arrangement

has important

radiation

compared North

hitting

to the stnn-

and South where

ZEUS and HI will be installed. The stored

electron

e/bunch)

3.6.1010

beam

gree of beam polarization ized laser light. With beam polarization

(210 bunches,

corresponds

will be measured

standard

terium beam atoms/s

on Stern-Gerlach

in a single

structed

substate

for the FILTFX

Heidelberg

Test Storage

circulating

the horizontal

section.

of polar-

to measure

the

of a thermal

is fed by an atomic beam atomic hydrogen

and deu-

an intensity

of lO1'

to the cell. Such a target

is presently

being con-

experiment

at LEAR/CERNl'

and a test experiment

ring is guided

can be opened around

recently.

also for the HERA experiment.

and +2 mm in the vertical

shell device which

cell which

is designed

through

This will be chosen

ized atoms are confined forming

storage

Ring TSR, which came into operation

in the storage

cross

backscattering

it should be possible

s-l,

60 mA. The de-

to deliver

used with minor modifications

narrow

4.7010~

of about

of l-28.

separation

(fig. 3). The source

frequency

current

by Compton

techniques

with an accuracy

The target will be a thin walled source based

revolution

to a circulating

during

the cell by beam

injection.

without

being

beam

tubes of ?S mm in

The cell will be a clam Due to the walls

the beam axis. They diffuse

about 1000 wall collisions

The electron

at least to 200 which means direction.

at the It can be

the polar-

out of the cell per-

depolarized,

provided

the cell

K. Rith /Spin dependent

structure functions

149c

magnet High frequency transition HFTl

6-p& magnet

HFT

Detector

r----

2 r-

EbXVOlT

rqjpl

beam

j I

I

/-

Sbxage

_

------_--_ --_I

cell target

Principle

walls

Target Chamber

FIGURE 3 of the atomic beam source

are coated with

special

and storage

cell target

By this technique

materials.

the target

density

can be increased

by about 2 orders of magnitude compared to a free atomic beam of about 10 14 cm-2 . This gives a total luminosity of about 3.5~10~1

to a value

cm -2 s -1 which experiment.

compares

A magnetic

for the measurement

Details

guiding

to the value

of -5.1031

for the measurement

by the magnetic

field of the electron

will be measured

by rf spectroscopy

of the target design

ization by the magnetic

cme2 s-l of the EMC

field of about 0.33 T (in longitudinal

of gl(x), vertical

target depolarization polarization

comfortably

field of the electron

and so on can be found in the contribution

bunches.

to an accuracy

and of problems

like possible

bunches,

direction

of g2(x)) prevents The target

of 2-3%.

target

depolar-

synchrotron

of E. Steffens

radiation 18 to this workshop

5.3. The detector We want a minimal

to perform

model.

tive corrections

An upper

scattering

x range extends

can be nearly

cut at y=O.85

and high hadronic

from elastic

accessible

with a beam energy

Q2 of 1 GeV2 to allow an interpretation

quark parton

events

the experiment

completely

background,

and resonance

We demand

of the data in terms of the. the region

of large radia-

a lower cut at y=O.15 production.

With

suppresses

these cuts the

from 0.02 to 0.8, the Q2 range from 1 to 20 GeV2. It

covered

of 40 5 E 5 200 mrad, where

removes

of E=35 GeV.

by a spectrometer

0 is the electron

with an angular

scattering

angle.

acceptance

15oc

K. Rith /Spin

dependent

Electronspectrometer

structure

functions

for HERA fransilion radiation detector

Triggerwall

Proportional chambers

\

lm L&d

glass

(+ EGO)

side view

lm

FIGURE 4 side view of the proposed

Schematic

To suppress positrons quired.

low energy

charged background,

and to improve pion rejection

Such a field must however

detector

to discriminate

a substantial

be shielded

electrons

magnetic

from

field is re-

from the electron

and proton

beam. Fig. 4 shows a schematic analysis

is accomplished

into two symmetrical ton beam

traverse

can be easily divided

parts by a horizontal

this plate

locally

septum plate.

Momentum

which

is divided

Both electron

and pro-

Due to this arrangement

from the beampipes

behind

the spectrometer the magnet

by a shielding

is

can be

of tungsten

lead of up to 20 cm thickness.

tically

accepts

particles

with angles between

and t200 to -200 mrad horizontally.

by sets of silicon electron

strip detectors

energy will be measured

scintillating

fiber blocks,

terial, which provides (30-loo),

and multiwire

possibly

including

resolution

cles will be suppressed ens will be rejected

in conjunction electrons

by an energy

by the combined

detector.

40 and 140 m-sad ver-

particles

proportional

by a shower wall,

good energy

will help to discriminate

radiation

Charged

as well as a coarse position

of the first level trigger

sition

detector.

a bore with very little rest field which

halfs and the detectors

against background

The spectrometer

jection

through

compensated.

into two identical

protected and/or

side view of the envisioned

by the use of a 1.5 Tm dipole magnet

either

a preshower

(of order 5%/B), measurement

The

part of the same magood pion re-

which will form part

from high energy photons.

hodoscope.

This

Low energy parti-

of the shower wall of 5 GeV. Pi-

information

The expected

chambers.

lead glass or lead

with a scintillator

threshold

will be tracked

of the shower wall and a tran-

pion/electron

suppression

will be bet-

K. Rith / Spin dependent

151c

structure functions

0.6

. HERA

(400

h)

-0.2

glP(x)

from

FIGURE 5 the EMC measurement and the projected of this experiment

ter than 10000. hadron

Possibly

an additional

accuracies

functions

will be installed

for

gree of beam

accuracy

nucleons

or ammonia

but is unity

GeV2,

0.15~~~0.85)

(100% efficient) The projected In addition be taken

(luminosity

3.5.1031

one could achieve beam

of events

by the de-

originating

is small for solid targets

new muon experiment

determination

at low x, could at most be decreased experiment

limited

is shown which has been obtained

In the proposed

are too large for a reliable

posed

structure

like

in our case.

for glp(x)

120 days of data taking.

is mainly

and the fraction

in the target, which

In fig. 5 the EMC result

rors, which

of the spin dependent

of such an experiment

and target polarization

from polarisable butanol

for the measurement

and the sum rules

The statistical

glP(x)

counter

identification.

5.4. Expected

about

Cerenkov

data for 400 hours beamtime

in

these er-

of the x dependence

by a factor

of two.

of

With our pro-

cme2 s-l, f = 1, pH = 0.8, pB = 0.5, Q*>l this accuracy

in about

10 hours

(!!) of

time.

data for 400 hours

to the statistical

of beam

time are also shown

error an overall

7-108 systematic

in the figure. error has to

into account.

It is obvious

that such a new experiment

mination

of the x dependence

tistical

accuracy

of glp(x)

would

allow a rather

and also of its integral

of about 5%. The result

for gld(x)

precise

deter-

IMP with a sta-

of the deuteron

and

K. Rith / Spin dependent stmcture functions

152C

its integral

- which,

the nucleon

apart from a small correction,

spin originating

from the quark spins

determines

the fraction

- will have a similar

of

accu-

racy. The statistical which

errors

experimentally

the necessary "R/Q'

subtraction

which

x. However, largely

enters

asymmetry

unknown

of deuterium

the weights

the accuracy

Aln(x)

quantities,

and hydrogen

and for gin(x),

will be bigger

data and the fact that

of 6Ald and 6~1p, increases

in Aln(x) will be good enough

and x dependence

of gin(x)

0.2

continuously

to see whether

0.8

0.4

0.6

This is demonstrated

in fig. 6 which

from 400 hours

(pD -0.6),

of this quantity, measured

running

together

shows the expected

time with both hydrogen

with a curve which

in 20 days running

atic error. The errors

indicates precision

time with a 3He target

the error bars

include

from in total 800 hours

will be only slightly

the

FIGURE 7 The expected accuracy fo$ gin in 20 days of beamtime with a He target. A 15% systematic error is included.

Fig. 7 shows the resulting

pn = 0.5). In this figure

deuterium

it is

x

FIGURE 6 The expected statistical accuracy for AIn for 400 hours running on both hydrogen and deuterium

terium

with

over a wide range of x.

X

for Aln(x)

due to

at low x and rises towards one for x -) 1 and to determine

negative

magnitude

for the neutron

are completely

statistical

a possible with which

(luminosity

x dependence gin(x) can be

10 32 cmR2 s-l,

a very pessimistic

running

accuracy

(pH = 0.8) and deu-

15% system-

time on hydrogen

and

bigger.

6. CONCLUSION This proposed developments

experiment

on the machine

is a challenge

It will allow a very precise functions ning

determination

gl(x) and g*(x) for both proton

times.

since it involves

new technological

side as well as for target and detectors. of the spin dependent

and neutron

in relatively

structure short run-

K. Rith /Spin dependent

in principle information

the combination

neccessary

of glp(x)

to determine

The data will allow us to separate of the (strange)

and gin(x)

the internal

the spin distributions

sea and to determine

with an accuracy

spin originating

from quark spins.

a precise

test

(7-10% accuracy)

of the fundamental

as a very

tight constraint

Therefore physics

improved

together

on existing

models

contains

spin structure

of the nucleon

to developing

153c

structure functions

all the

of the nucleon.

of u and d quarks and

of about 5% the fraction

Furthermore Bjorken

they will provide

sum rule and serve and also as an aid

of the nucleon

ones.

this fundamental

experiment

is an exciting

example

of hadronic

in the early 1990's with 30-35 GeV electrons.

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