Results from the Graal experiment

Progress in Particle and Nuclear Physics PERGAMON

Progress

in Particle and Nuclear Physics 44 (2000) 423432

p

http:l/www,elsevier.nl/locate/ppartnuclphys

Results from the Graal Experiment* O.BARTALINI,

M. CAPOGNI,

A. D’ANGELO,

D. MORICCIANI,

C. SCHAERF

INFN, Sezione di Roma II and Universitd di Roma “Tar Vergata” l-00133 Roma, Italy

and P. LEVI SANDRI

C. GAULARD, INFN, Laboratori

M. ANGHINOLFI,

Narionali di Frascati,

l-00044 Frascati,

Italy

and M. BATTAGLIERI, M. CASTOLDI, P. CORVISIERO, M. TAIUTI AND A. ZUCCHIATTI INFN, S&one

M. RIPANI,

M. SANZONE

di Cenova and Universitd di Geneva I-16146 Geneva, Italy

and V. BELLINI, C. SUTERA Laboratori

J. P. BOCQUET,

Nazionali de1 Sud Universitri di Catania I-951 123 Catnnia, Italy

A. LLERES,

and L. NICOLETTI,

IN2P3, Institut des Sciences h’uclhires,

J. P. DIDELEZ,

D. REBREYEND F-38026 Grenoble,

AND F. RENARD

France

and A. JEJCIC AND E. HOURANY

M. GUIDAL,

IN2P3, Institut de Physique Nu&aire,

F-91406 Orsay. France

and F. GHIO AND B. GIROLAMI INFN, Sezione di Roma I and lstituto Superiore

di Sanitd, I-00161 Roma, Italy

and I. KILVINGTON European

Synchrotron

A. LAPIK, Institute

Radiation Facility, F-38026 Grenoble,

and V. KOUZNETSOV, for Nuclear

Research,

France

V. NEDOREZOV

RU-117312 Moscow, Russia

and A. TURINGE RRC Kurchntov Institute ofAtomic

Energy, RU-123182 Moscow, Russia

and N. RUDNEV Institute of Theoretical

and Experimental

The features of the Grail experiment are discussed and future developments

Physics, Moscow, Russia

at ESRF are presented,

The obtained

results

outlined

*presented by P. Levi Sandri 0146-6410/00/$ - see front matter 0 2000 Published by Elsevier Science BV All rights reserved. PII: SOl46-6410(00)00091-0

424

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

1

Introduction

In a large number new generation

of experiments

the baryon spectrum

of measurements

make use of high intensity

beams coupled with large acceptance still incomplete

knowledge

well established,

detectors.

of the nucleon

and many

properties

excited

of the observed

are often poorly known.

resonances[l]

entirely

is therefore

necessary

electro-magnetic knowledge

to deepen

of resonance

If we consider

states

(e.g.

analyses

of baryon

probe (small coupling constant,

Many predicted

electron

states

coupling

The information

from partial-wave

the study

and high polarisation

states.

Most of this

being investigated.

The reason for this huge experimental

ratios, helicity amplitudes) comes almost

is presently

are not sufficiently

induced

by exploiting

easy polarisability

of beams)

branching

in the table of baryon

of pion-nucleon

resonances

effort is in the

constants,

contained

and photon

reactions.

the features

It

of the

in order to improve our

properties.

the simple case of pseudo-scalar

meson photoproduction:

y + p + PS + nucleon we can see that we have eight possible described

by eight matrix

elements

combination

(1)

of spin states.

The scattering

only four of which are independent,

amplitude

due to rotational

is thus

invariance

and parity considerations. With these four complex 16 observables: polarisation three

amplitudes,

the differential

observables.

single polarisation

16 bilinear products

cross section,

To completely observables

can be constructed,

three single polarisation

determine

the scattering

and four appropriately

observables

amplitude,

chosen

double

corresponding

to

and twelve double

the cross section, polarisation

the

observables

must be measured[2] These observables following relations

can be adequately

but its details

the interference

in terms of helicity amplitudes.

In that case, the

hold[3, 4, 51:

It is clear that the general structure section

expressed

c

N

Re(HIH,’

- HzH,*)

(3)

T

N

Im(HIH;

- HsH,*)

(4)

P

N

Re(HIHj

- HzH,‘)

(5)

of the scattering

amplitude

are more clearly

among the helicity

evidenced

amplitudes

in the study

is contained

in the differential

of polarisation

can play a fundamental

observables,

role in revealing

cross where

more subtle

effects[‘l]. The necessary

experimental

tools to perform part of the ambitious

program of a full determination

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

of the transition

amplitudes

a large acceptance

2

are a completely

tagged photon beam, coupled with

detector.

The Graal Beam

The Graal facility provides

a polarised

and tagged photon

of laser light on the high energy electrons line (350 nm) of an &-Ion 1470 MeV. Its polarisation measured

asymmetries

circulating

laser we have produced is 0.98 at the maximum

to be 16 MeV (FWHM).

polarisation

in the ESRF a gamma-ray

photon

storage

Compton

scatt,ering

ring [6]. Using the UV

beam with an energy

from 550 t,o has been

Using the green line of the same laser we have measured

the bectm

and cross sections

of 2 +‘, w and 7’ are presently

beam by the backward

energy and the energy resolution

region 550-1470 MeV. Data to obtain

Figure

and versatile polarised

425

in the photoproduction

beam asymmetries

of 77 (81 7r” and TT+in thr, cxnerg\

and cross sections

for the photoproduction

being analysed.

1: The Graal experimental

layout

(not in scale).

The gas Cerenkov

counter

is still under

construction.

The experimental mirror and focused, the mirror.

apparatus

scattered

the ring lattice and their position,

the electron

in Fig. 1. Laser photons

by a system of lenses, at the interaction

Electrons,

The tagging

is indicated

detector

by the laser photons,

is located

beam at a distance

region, approximately

are momentum

relative to the circulating inside a rectangular

are deflected

electron

analysed

by the first dipole of

and a solid state Silicon micro-strip

by the tagger.

box with one side parallel t,o

of 10 mm from it. Inside this box are located

box and inside it 10 plastic scintillators

30 meters far from

beam, is measured

vacuum-tight

by a Beryllium

a Densimet

detector

shielding

with 128 channels

426

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

and a pitch of 0.3 mm. The position energy lost by it and therefore The back-scattered uum system

(gamma-ray)

beam traverses

of the liquid Hydrogen

(or Deuterium)

by two cylindrical

of 32 strips of plastic scintillator

controlled

wire chambers

made of 480 crystals

basis and are 21 radiation for photons[9],

monitoring

a good response

and calibration

Particles

moving

two walls of plastic measurement

scintillator

Lead blocks.

Then it enters

vacthe

with cathode

readout,

a barrel made

the AE/Ax

of charged

Its crystals

have a pyramidal

long (24 cm). This calorimeter

shape

has an excellent

with energy

and is very stable in time due to a continuous

than

25” encounter

two plane wire chambers,

at 3 m from the target

for charged particles

(700 ps FWHM resolution)

of four layers of Lead and plastic scintillators detection

(with 95 percent

(xy and uv)

point,

that provide a

followed by a shower

4 cm thick that provides a full

efficiency)

and a 20 percent

efficiency

detection. two disks of plastic

The beam intensity

3

mirror, exits the accelerator

bars 3 cm thin located

of the time-of-flight

coverage in the backward

scintillators

of 16 MeV FWHM.

slow control system[ll].

coverage of the solid angle for photon

Finally,

gives the

of BGO scintillator.

to protons[lO]

at angles smaller

wall made by a sandwich

for neutron

lengths

electron,

target.

The BGO ball covers an angle from 25” to 155”.

resolution

with a resolution

parallel to the beam axis, used to determine

and the BGO rugby-ball

trapezoidal

by the scattered

the Beryllium

in air by four remotely

The target is surrounded

particles,

traversed

the energy of the gamma-ray

and is collimated

vacuum system

of the micro-strip

scintillator

separated

by a disk of Lead complete

the solid angle

direction. is continuously

and by a lead/scintillating

monitored

by a flux monitor,

fibre detector

that measures

composed

by three thin plastic

energy and flux[l2]

Results

The Graal experiment

started

data-taking

sioning during 1996. Two Hydrogen

in 1997 after six months

targets

were used, respectively

the laser green line of 514 nm was used and a tagged photon with average intensity

of 2 . lo6 s-l

line of 351 nm was used. MeV with typical

intensity

and energy between

The corresponding

tagging counter

and the detector

energy collected

by the calorimeter

trigger.

three and six cm long. During 1997

beam, linearly

polarised,

was provided

The latter was initially

100 and 200 s-l.

550 and 1470

by the coincidence

between

formed only by requesting

to be larger than 160 MeV. Later a charged

between

was obtained

beam had energy between

particles

trigger was added allowing forward events with small energy release in the calorimeter The rate of the DAQ was typically

commis-

550 and 1100 MeV. In 1998 the laser UV

backscattered

of 1 . lo6 s- ‘. The trigger

of beam and apparatus

During data taking,

the

the total

multiplicity

to be recorded.

the polarisation

of

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

the gamma-ray polarisation.

beam was rotated,

approximately

Data were also collected

in the residual

without

vacuum of the storage

two orders of magnitude

ring.

lower, with respect

0.2 0.15

-

The intensity

- -

Figure 2: Procedure tion for horizontal

the beam asymmetry.

states

(right) and the final distribution

(unpolarised

beam was typically

beam.

0

100 200 300 cp7)

Top: azimuthal

(left) and vertical (right) polarisations

sum for the two polarisation

of the bremsstrahlung

; -

100 200 300 PI,

to extract

the laser beam

0.2 --_ _-_ 0.15 _ 0.1 - 0.05 0 Lx

O-O;lik_LLJ 0

by rotating

of the bremsstrahlung

to the Compton

-

0.1 - 0.05 Lx 0

minutes,

laser to obtain the contribution

_-_ -

every twenty

427

distributions

of the gamma-ray

for 17photoproducBottom:

normalised

beam) showing the small detector-efficiency

variations

from which the beam asymmetry

beam.

is extracted

after a fitting procedure

(left)

For photons can be written

linearly polarised

with a degree o polarisation

P the count rate for a given reaction

as:

4 is the angle between

the reaction

plane and the horizontal

the angle 4, kll and /cl are the number

of impinging

photons

plane.

~(4) is the detector

relative

efficiency at

to the two beam polarisations,

and C is the beam asymmetry. By summing This quantity small variations either

[6] and (71 properly

should

be a constant.

with cos(24)

by [g] we obtain

behaviour.

we obtain

The small deviations

of the efficiency aa a function

[6] or ]7] and dividing

by fitting

normalised

of the azimuthal

the count rate for an unpolarised from the constant

behaviour

reflect the

angle (see figure 2). Finally, by taking

the plot from which the beam asymmetry

This procedure

beam.

allows to measure

the beam

is extracted

asymmetry

with a

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

428

very reduced

systematic

efficiency variation

error: only relative photon

and the (small) effect of the

is ruled out. 2

+ 2

Nlrlkll Nldkll + Ndki

3.1

fluxes are considered

= 26($) g constant

(8)

= 0.5(1 + PCcos(2f$))

(9)

r) photoproduction

Since the isospin of 77 is I=O, the (7,~)

process

resonances

in a clean way, being insensitive

resonances,

strongly

to the propagation,

beam asymmetry

beam asymmetry

40

80

I20

(de@

Figure 3: C beam asymmetry The publication

160

of I=3/2

For this reason, the analysed

at Graal. In

This result is confirmed

by the data collected

Li and Saghai[l4]

theoretical

investigate

contribution data.

0

40

80

120

160

Wde~~l for 71meson photopro uction on the proton

of the C beam asymmetry

in the existing

the beam asymmetry

state,

N’

Th e main relevance of these data is in the unexpected

at forward angles,

0

find that significant

to study

energies up to 1470 MeV[13].

0

refinements

channel.

and cross section were the first experiments

figure 3 we show the results already published[8].

in 1998 with photon

opportunity

in the intermediate

coupled, exempli gratia, to the pion photoproduction

17photoproduction

large and positive

offers the very attractive

for r] photoproduction

has stimulated

a number

of

approaches:

the process of 77photoproduction

within a quark model approach and

from Dis( 1520), Fis( 1680) and P13( 1720) are required

to reproduce

429

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

Tiator section

and collaborators[l6]

and asymmetry

extracted

data.

their nN branching

N. Mukhopadhyay from Bonn

Lagrangian

approach

analysis

of n photoproduction

the role of Oia(1520)

and of Fis(1680)

cross and have

ratios.

(target

have combined

asymmetry[l9]

have extracted

test for many QCD inspired

a combined

They have confirmed

and N. Mathur[l7]

observables

3.2

have performed

and from Graal,

the electro-strong

hadron

cross section[l8]

and single polarisation

and by making

parameters

use of an effective

for the N*( 1520) providing

a critical

models.

Pion photoproduction

Pion photoproduction

(x0 and 7r’) is one of the most extensively

source of information

on the structure

differential

cross section

The importance

and nuclei.

*symetry

Versus

and systematic measurements

e,,

for K’ meson photoproduction

In Figure 4 is shown a small sample of the collected

and analysed

on the proton data for C beam asymmetry

Data will soon be available from 500 to 1500 MeV. The curve shown is from

ref[7]. We can see that the fine details of the asymmetry

togheter

statistical

accurate.

data base from 500 to 1500 MeV.

Figure 4: C beam asymmetry

error is a really strong constraint

In Figure

a huge data base on

are not sufficiently

in this field lies in the excellent

Y+P-P+x,

statistical

and the main

and in the large energy and angular range covered by asymmetry

a new, consistent

in K’ photoproduction.

photoreaction

For this reason,

exists but many of the measurements

of the Graal contribution

error that is achieved thus providing

already

of nucleons

studied

5 the backward

with the existing

reproduced:

the small

for all models and analyses.

angles C beam

old data

are only qualitatively

asymmetry

from Daresbury[ZZ]

in R+ photoproduction and SLAC[21].

data

The full Graal

are shown data

set,

0. Bartalini et al. / Pmg. Part. Nucl. Phys. 44 (2000) 423-432

430

including

all angles,

will be published

ysis of the VP1 group[20] collaborators[l5](dashed

3.3

The curves shown are from the partial

(full curve) and from a recently

developed

wave anal-

isobar model by Drechsel

and

curve).

Other results

The Graal experiment

has collected

now being analysed. sections

soon[23].

for w, K+h

data for many other photoproduction

Soon the collaboration and 27r” final states.

up to 1.5 GeV incident

photon

energy.

will make available Also the Compton

The 7’ photoproduction

channels.

These data are

data on beam asymmetry

and cross

process y + p -+ y + p is being analysed threshold

was reached

with the laser

UV line and the signal for v’ decay was clearly seen but, in that case, useful results will be available only by running to measure

at higher photon

energies with the 320 or the 300 nm laser lines).

some rare decays of the 77meson is in progress.

decay channels

In particular

Finally, a program

the branching

17+ 7r”yy and the Dalitz plot for the decay n -+ 3~’ are of great interest

checks of chiral perturbation

Figure

5: C beam

asymmetry

results,

full circles:

Daresbury,

ratio of the as sensitive

theory[24].

for R+ meson photoproduction triangles

and squares:

SLAC

on the proton.

Open circles:

Graal

0. Bartalini et al. / Prog. Part. Nucl. Phys. 44 (2000) 423-432

4

431

Conclusions

The Graal experiment

started

data taking in 1997. It was run for one year with the green laser line

giving rise to a photon

beam of maximum

and the corresponding

gamma-ray

sections

have been produced

the most extended

energy of 1100 MeV and for one year with UV multi-line

beam of 1470 Mev maximum

for n, K’ and rr+ photoproduction

and coherent

on the quasi-free

access double polarisation neutron.

Finally

neutron.

observables

the increase

details n’ photoproduction

channels

Asymmetry providing,

with deuteron

target

The use of a polarised

photon

and to reach the threshold

of the experiment

and the study of the photoreaction

target

for meson photoproduction

in the maximum

data and cross

for these reactions,

data base available until now. Future developments

will include the study of the same channels mechanism

energy.

of new concept[25] on polarised

proton

will allow to and polarised

energy, up to 1.8 GeV will allow to study in for scalar mesons production.

References [1] Particle [2] W.T.

Data Group, Eur. Phys. Journ.

Chiang and F. Tabakin,

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[4] T. Feuster and U. Mosel, Phys. Rev. C59, (1999), 460. [5] B. Saghai and F. Tabakin, Fasano,

F. Tabakin

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Phys. Rev. C55, (1997), 917 and Phys. Rev. C53, (1996), 66 and C.

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(111 F. Ghio et al., Nucl. Inst. and Meth. in Phys. Research [12] V. Bellini et al., Nucl. Inst. and Meth. in Phys. Research 1131 The Graal collaboration,

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[15] D. Drechsel, [16] L. Tiator,

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D. Drechsel,

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G. Knochlein

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Phys. Rev., C53, (1996), 430. lOA, (1972) 10. and G. Knies et al., Phys. Rev. DlO

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