Coincidence experiments at CEBAF

441

Nuclear Instruments and Methods in Physics Research B40/41 (1989) 44-446 North-Holland, Amsterdam

COINCIDENCE

EXPERIMENTS

AT CEBAF

Jean MOUGEY Continuow

Electron

Beam

Accelerator

12000 Jeff ermn Avenue,

Facility,

Newport

News,

VA 23606,

USA

With the 4 GeV Continuous Electron Beam Accelerator Facility (CEBAF) presently under construction in Newport News, Virginia, a new range of nuclear and subnuclear phenomena can be investigated, mostly through coincidence experiments. The accelerator characteristics, some examples of its physics programs, and the related experimental equipment are briefly reviewed.

1. Introduction During on

the

the last few decades,

structure

electro-

nuclei

and photoinduced

the intrinsic weak,

qualities

known

tinuous

in beam

a large

consists

extent, of

potential.

photons

have given the clearest

this

mesonic fects,

“classical”

spectra

adding

and the quasifree Clearly,

excitation

fragments

in

production,

the various The

to narrow

be achieved.

will allow

the nuclear fm,

tween

q >, 0.5

nucleons

0

100

200

off 3He [l].

300

500

100

E=667 MeV

quantitatively peak

more exclu-

of the nuclear are detected

specific

knock-out,

etc) in which

dynamics

of present

play different

electron

difficult

accelera-

[2], being limited

regions

for which

coincidence

motivated

in

channels

a

ratio can

the recent developaccelerators.

Com-

beam with a high energy of 4 GeV, quantitative

many-body

over small distances 0.5

out

- 100% duty cycle electron

a continuous

CEBAF

the reaction

(R/A)

This strongly

mesonic

from the two domi-

kinematical

real-to-accidental

of ef-

in fig. 1, showing

scattering

single

of nuclear cycle

role

of the A resonance.

these experiments

in practice sufficient

bining

to

and

i.e. the quasielastic

two-nucleon

aspects low-duty

tors makes

a mean

the

one must perform

from

order

experimental

detection

explicitly

in which one (or more)

emerging

on

implications

-

relativistic

be explained

structures,

to go further,

coincidence

ment of

cannot

their

confirmed in

exhibiting

electron

and

equipment.

for deviations

resonances,

the contributions

single nucleon

roles.

picture,

nucleon

in inclusive

sive experiments

studies

evidences

ments

and to con-

moving

for introducing

by simply

(pion

and

This is illustrated

The observed

from

probe

quality these

of freedom. results

follows -

and the need

recent

nant

This

from

But, at the same time, electrons

currents,

degrees

obtained

flexible

nucleons

effective from

of information

been

reactions.

and kinematically

To

nuclei

a wealth

has

of the electromagnetic

improvements

capability. that

of

After a short description of the CEBAF accelerator, I shall discuss a few examples of coincidence experi-

aspects.

compared

GeV/c)

short

and precise By probing

studies

to the nucleon range

as well as the internal

hadrons

and its role in the nuclear

revealed

[31.

size (I 5

correlations substructure

medium

of

the nucleus

should

0168-583X/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics publishing Division)

beof be

B.V.

0 ioo

2io

&dO

360 Energy

transfer

500

w [Mev]

Fig. 1. Cross sections for inclusive electron scattering off ‘He (from ref. [l]). III. NUCLEAR

PHYSICS/ASTROPHYSICS

442

J. Mougey / Coincidence experiments

at CEBAF

2. CEBAF overview

The need for a high current, high duty factor electron accelerator in the several GeV energy range, and dedicated to nuclear physics, was first formulated in 1976 by a panel of the National Academy of Science (G. Friedlander, chairman). In winter 1982/83 an ad hoc NSAC Panel (D.A. Bromley, chairman) evaluated five competing proposals, and DOE adopted the NSAC-endorsed panel recommendation to accept the proposal by SURA (Southeastern Universities Research Association). Originally a 2 GeV, SLAC-type linac with two-pass recirculation and pulse stretcher ring, CEBAF was converted in the fall of 1985 to a cw, superconducting recirculating linac, and DOE proposed the project to Congress for construction start in FY87. The CEBAF beam performance objectives are OS
Fig. 3. CEBAF accelerator configuration. The 45 MeV injector provides three interspersed 499 MHz trains [(l, 4, 7, . .), (2, 5, 8, . .), (3, 6, 9, . .)] of < 2ps bunches with individually adjustable current levels. Extraction at fractional energies, i.e. on the 1st to 4th pass, is accomplished by deflecting bunches by rf separators operating at 2495 MHZ, achieving the goal of serving three users with beams of individually adjusted current and up to three different, although correlated, energies. Some details about experimental equipment in the three experimental halls - A, B and C - are given in section 4. In addition to the conventional injector system, plans are made for future implementation of a polarized electron source. The total estimated cost of the project (as of July, 1988) is $265 million, the total laboratory cost (including operation till completion) being $450 million. This includes $41.3 million for experimental equipment. Industrially fabricated prototype cavities exceed specifica-

Fig. 2. CEBAF 5-cell superconducting cavity pair, showing the wave guides for higher order mode damping, and the frequency tuning system.

443

J. Mougey / Coincidence experiments at CEBAF

tions and have been assembled in prototype cryostats. Prototype of rf systems have been operated and first klystrons have been ordered. The 100 keV gun and chopping section is in operation. The central helium liquifier is in fabrication and tunnel construction is under way. Commissioning of the first full linac will start mid-1992 and the first beam will be available in experimental areas early 1994 [4].

“an

-

-

Orden

(R)

T/Otcruor

-.-.-.-.-

T/O

-

only

tensor +

SW

10-3

^a

3. Selected coincidence experiments

“B

10-4 1

Among the various coincidence studies which constitute the essential part of the physics program at CEBAF, I have selected three illustrative examples: the (e, e’N) reactions, the study of two-nucleon correlations through (e, e’2N) reactions and the formation of hypernuclei through kaon electroproduction. Other programs, like the photo- and electroproduction of nucleon resonances, are part of V. Burkert’s talk at this Conference

[Sl.

Coincidence (e, e’p) experiments are currently performed at lower energies in several laboratories [6]. The simplest interpretation of the reaction in the “quasielastic” regime is that the electron scatters on a single proton which is ejected. Assuming further no final state interactions (PWIA) the coincidence cross section takes the simple form = Ku&(

t-

10-e

\

\

HO --+

-19

t 10-7

\

\

t-

‘I I

I

I

i.,

I 600

400

I

I

I

I

I1

\ \ I

!.I_

800

1000

P (MeV/c) Fig. 4. Momentum distributions for i60. Various correlated distributions are shown along with single-particle harmonic

3.1. (e, e’N) reactions

da/de’dp’

10-S

p,,

E,),

(1)

where Q, is essentially the free elastic e-p cross section E,), the spectral function, is the joint proband S(P,, ability for removing a proton of momentum pm, leaving the residual system with an excitation energy E,,,. In the shell model frame S(P,,E,)=CI~~,(P,)I*S(E,+E,), a

(2)

where &(p,) is the momentum space wave function, and the sum extent over all single particle states (Y= (nlj). Although final state interactions and meson exchange currents introduce significant corrections to this simple picture, important information has been obtained on single nucleon energy and momentum densities, up to - 600 MeV/c for d, 3He and 4He, and - 300 MeV/c for A < 4 nuclei. CEBAF is uniquely suited for extending these studies to 500 MeV/c and beyond, where two-nucleon correlations are expected to dominate (fig. 4). The high incident energy allows large momentum transfers at forward electron angles so cross sections are larger and the continuous beam provides a factor of - 100 increase in R/A, making formerly impractical measurements possible. High angular (< 1

oscillator indicates

and Woods-Saxon potential ones. The vertical line the highest momentum to which $(p) is presently determined via (e. e’p) reaction (from ref. [lo]).

mrad) and energy (< 0.5 MeV) resolutions would be required. Direct evidence for two-nucleon contributions to (e, e’p) reactions has been obtained at Saclay [7] on 3He (fig. 5). The broad structure which moves from 35 to 60 MeV with increasing pm value was found to result from an interaction with a correlated (np) pair. Similar observations have been made at Bates on 12C at higher energy transfers up to the quasifree A-production region

PI.

Deeper insight into the various mechanisms leading to nucleon emission would be obtained from separate determination of the four nuclear structure (response) functions R,, R,, R,,, R,, associated with different polarization states of the virtual photon, and which appear in the most general form of the (e, e’p) unpolarized cross section da/de’

dp’=a,(uLRL+vTRT+vL~RLT +v,R,cos

cos +

2+),

(3)

where or,,, is the Mott cross section and the vi are well defined functions of the electron kinematics only. A complete separation necessitates noncoplanar kinematics, $I being the angle between the electron scattering plane and the nucleon emission plane. More structure functions appear for specific beam, target or ejectile polarization. Theory predicts different sensitivities to meson currents, virtual excitation of resonances, relativIII. NUCLEAR

PHYSICS/ASTROPHYSICS

J. Mougey / Coincidence experiments at CEBAF transverse cross section. Counting rate estimates under CEBAF conditions (2 to 100 events h-’ for the 3He(e, e’2p)n reaction at 2 GeV) have shown the feasibility of the experiment although a longitudinal/transverse separation would be very difficult to achieve. Moreover, the use of three spectrometers would put constraints on kinematics. A broad survey of twonucleon emission processes using the LAS (see section 4) may be an appropriate way of starting this program. 3.3. (e, e’K ‘) reactions and the formation

E, (MeVl Fig. 5. Missing energy spectra form 3He(e, e’p), showing evidence for an interaction on a two-nucleon (from ref. [7]).

correlated

pair

istic and off-shell effects. Complete studies of selected cases, in particular of disintegration of few nucleon systems, are considered in the CEBAF physics program. Thanks to the high energy and duty cycle of the CEBAF beam, (e, e’n) reactions can be studied with counting rates comparable to those presently achieved for (e, e’p). From polarization measurements in quasifree scattering of longitudinally polarized electrons off neutrons from a deuteron target - reactions ‘H(Z,e’n)p, polarized deuteron target, and 2H(Z, e’ii)p, neutron polarization measurement - one should substantially improve our knowledge of the neutron electric form factor GE. The (e, e’N) reaction on nuclei can also be used to study how the electromagnetic properties of nucleons, sensitive to quark distributions, are modified inside the nuclear medium. 3.2. (e, e’2N) reactions Coincidence studies of two-nucleon emission processes provide the most direct access to two-nucleon densities and correlation functions in nuclear medium. At short distances, they should give insights on twonucleon clustering in nuclei - exchange of heavy mesons ( p, w, . . . ), 6-quark clusters, . . . - and the origin of high momentum components in nuclear wave functions. Especially promising is the (e, e’2p) reaction as the absence of dipole moment in the (pp) system strongly suppress the contribution of two-body currents in the

of hypernuclei

Photo- or electroproduction of kaons can be used to produce hypemuclei [9], i.e. nuclei in which one nucleon is replaced by a hyperon, usually a A, of strangeness S = - 1. In the nuclear medium, the hyperon can be viewed as an impurity living long enough (- 1OW”) to interact with its environment, being not restricted by the Pauli principle (except maybe at the quark level). At present most of our knowledge about the structure of hypemuclei comes from hadronic reactions, mainly the strangeness exchange (K-, n-) one, over which electromagnetic production has the important advantages of cleaner mechanism and minimal distortion. Another specific feature of electromagnetic production is that, due to the vector spin nature of the photon, non-natural parity (spin-flip) states can be excited as well as natural parity (non-spin-flip) ones. Finally, the kinematics is different: recoiless A-production is not possible in (y, K+), in contrast with (K-, T-). Therefore, high momentum transfers and high-spin state excitations are favored. For example, the electromagnetic interaction can directly convert a proton from an outer shell into a hyperon in a deep lying state. Serious experimental problems have to be solved before considering an extensive program on hypernuclei at CEBAF: counting rates are low, of the order of SO-100 events/day per level in the most favorable configuration in which both the kaon and the scattered electron are detected at very forward angles ( < 15 o ). To clearly separate hypernuclear levels, in particular partners within a spin doublet, an energy resolution of - 100 keV is required. Several experimental setups are under study, including one in which both the K+ and the scattered electron are detected near 0 O, and a missing mass resolution of 100-200 keV looks achievable. Kaon electro- and photoproduction on proton and deuteron to study kaon form factor, the KN and AN systems and possibly strangeness + 1 baryons and - 1 dibaryons are part of the LAS physics program.

4. Experimental

equipment

for coincidence

experiments

At present, coincidence experiments are planned in all three CEBAF experimental halls.

J. Mougey / Coincidence experiments Hall

A is designed

part of the program, a pair of vertical design

will

electron

be

angle

acceptance

Ap/p

scattering

of

out the high resolution

early

in Fig.

uniform

accep-

For

is a QQDQ

replica

with similar

each

kinematics vertically techniques

The

be

to better

hadron

or a shorter

could

the (horizontal) than 0.5 mrad.

based

30 cm gap dipole

on a non-

and conven-

arm may either

2.7 GeV/c

QQD

Both are designed

(10 cm) targets

before

event,

configuration

performances.

with extended

a

- 8 msr and a momentum

*5’%.

quadrupoles.

have

a solid

field, 45 o bend,

identical

6, will

than

the

4 GeV/c

10-4,

angle will be defined

design

The

CEBAF Large Acceptance Spectrometer

with

of which

1989.

better

6p/p

of

initially

spectrometers,

in

shown

resolution

tance

tional

magnetic frozen

spectrometer,

momentum

The

to carry

and will be equipped

be its design,

to operate

along beam. Non-coplanar

achieved

by

it hits the target.

bending

the

Dispersion

will be used to achieve

445

at CEBAF

beam

0

Fig. 7. Sketch (LAS) planned

2

3m

of the large acceptance toroidal for Hall B. Overall length and about 8 m.

matching

resolutions

I

spectrometer diameter are

of a few

10-5. Hall B will contain large acceptance be devoted coverage portant

mainly

spectrometer

to experiments

coils producing

six sectors

will be filled

gion.

counters

multiplicities a toroidal

with drift

and shower photon

enough

room in a low field region

operate

polarized

experiments.

envisaged,

scintilla-

electron

around

re-

beams, provides

the target

to

targets.

Hall C is designed tion

of the

in the forward

Its geometry

for high energy,

A large

including

trometers,

non-focusing

a toroidal

multigap

variety

moderate large spectrometer

moderate

resolu-

of

instruments

are

resolution

focusing

spec-

aperture

magnetic

for parity

and out-of-plane trometer

measurements,

and a dedicated

setup for hypemuclear

spec-

physics.

im-

of six super-

chambers,

detectors beams.

kinematical

field. Each

It will be used with low intensity

or real “tagged”

the

are more

The LAS consists

conducting tion

4n detector,

see fig. 7) and will

for which broad

and high particle than resolution.

a nearly (LAS,

devices,

experiments

5. Conclusion CEBAF study tances,

will open

of nucleon allowing

complete

final states through concerning equipment

the

of intent

same

time.

to welcome

new possibilities

nuclear

structure

determination

coincidence

initial

will be taken

letters

collaborations

exciting

and

early

1989,

for experiments

A CEBAF

User

of

A decision experimental

and a first

set of

will be discussed

at the

Group,

have been formed, all interested

dis-

of the hadronic

experiments.

complement

for the

at short

and

active

user

and would be pleased

physicists.

References

111C. Marchand,

4.0 GeV/c High Resolution Spectrometer

Fig. 6. Sketch of the 4 GeV/c high resolution spectrometer planned for Hall A (distance in meters). Its optical length is 25 m and its total weight is about 400 tons.

P. Barreau, M. Bemheim, P. Bradu, G. Fournier, Z.E. Meziani, J. Miller, J. Morgenstern, J. Picard, B. Saghai, S. Turck-Chieze, P. Vernin and M. Brussel, Phys. Lett. B153 (1985) 29. 121 For review, see S. Frnllani and J. Mougey, Adv. Nucl. Phys. 14 (1984) 1. [31 For an overview of the planned physics program, see the annual reports on the Research Program at CEBAF (RPAC) published by CEBAF (January 1986, 1987,198s). B.K. Hart[41 H.A. Grunder, J.J. Bisognano, WI. Diamond, line, C.W. Leemann, J. Mougey, R.M. Sundelin and R.C. York, Nucl. Phys. A478, (1988) 831~ and refs. therein; see also the proceedings of the 1988 Linear Accelerator Conference, Williamsburg, VA (Oct. 3-7, 1988) to be published. [51 For a review, see V. Burkert, CEBAF Report PR 88-012 (1988) and Proc. Conf. on Excited Baryons ‘88, Troy, NY (Sept. 3-6, 1988) to be published. III. NUCLEAR

PHYSICS/ASTROPHYSICS

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J. Mougey / Coincidence experiments

[6] For recent reviews, see J.M. Finn, W. Bertozzi and R.W. Lou&, Proc. 3rd Workshop on Perspective in Nuclear Physics at Intermediate Energies, Trieste (May 18-22, 1987) eds. S. Boffi, C. Ciofi degli Atti and M.M. Giannini (World Scientific, Singapore, 1988) and Proc. 5th Miniconference, Amsterdam, (Nov. 19-20, 1987) published by NIKHEF-K, Amsterdam. [7] C. Marchand, M. Bemheim, P.C. Dunn, A. Gerard, J.M. Laget, A. Magnon, J. Morgenstem, J. Mougey, J. Picard, D. Reffay-Pikeroen, S. Tuck-Chieze, P. Vemin, M.K. Brussel, G.P. Capitani, E. desanctis, S. Frullani and F. Garibaldi, Phys. Rev. Lett. 60 (1988) 1703.

at CEBAF

[8] R.W. Laurie, H. Baghaei, W. Bertozzi, K.I. Blomqvist, J.M. Finn, C.E. Hyde-Wright, N. Kalantar-Nayestanaki, J. Nelson, S. Kowalski, C.P. Sargent, W.W. Sapp, P. Ulmer, J. Wiggins, B.H. Cottman, P.K. Teng, E.J. Winhold, M. Yamazaki, J.R. Calarco, F.W. Hersman, J.J. Kelly and M.E. Schulze, Phys. Rev. Lett. 56 (1986) 2364; H. Baghaei, MIT Ph.D. Thesis (1987) and Phys. Rev. C, to be published. [9] A.M. Bernstein, T.W. Donnelly and G.N. Epstein, Nucl. Phys. A358 (1981) 195. See also reference [3] for several contributions on (y, K+) and (e, e’K+) reactions. [lo] R.W. Laurie, private communication.