Photonpolarisation and asymmetry in the 4He(γ ,np) reaction

Progress in Particle and Nuclear Physics PERGAMON

Progress

in Particle and Nuclear Physics 44 (2000) 461463

-

http:/iwww.elsevier.nMocateippartnuclphys

Photonpolarisation

and Asymmetry in the 4He(y,np) Reaction F. A. NATl-ER

For the PiP/ToF Group of the Maim A2 Collaboration Physikalisches Institut, Universitlit Tiibingen, Tiibingen, Germany

The photon asymmetry

measurement

insights in the nature of correlations NN interaction.

Additionally

A very precise description presented

of 2N knock-out

reactions on 4He provide new

caused by the short distance

the results show first indications of the production

which reduces systematic

of coherent

repulsion

of the

of tensor correlations. bremsstrahlung

will be

errors from the degree of polarisation.

Polarization degrees of freedom and NN correlations The importance treatment

of short

is an indispensable

most promising emission

range correlations ingredient

(SRC) are known for a long time

in any nuclear model [2], i.e. for the binding

way for a direct experimental

[4]. Although

those experiments

confirmation are challenging

but clear SRC effects are still not undoublty role to the absolute the medium ingredient

cross section

[3], because

seen.

and represents

it is far less subject

is the precise determination

[l] and their proper of nuclei.

is the study of photo-induced they have provided

The photon

asymmetry

an additional

method

to uncertainties

of the degree of polarization,

two-nucleon

many empirical

hints,

plays a complementary

for studying

with respect

The

correlation

in

to FSI. One important

to minimize

the systematic

error

of the asymmetry. Production and determination of polarization Linearly

polarized

provides

a plane of reference

photons

pared to an amorphous

are produced

from electron

for the photon

radiator

polarization

this yields additional

process, when the crystal as a whole has absorbed that

the cross section

perimental

data

treatment

and polarisation

equally

well.

(MCB) [5], were implemented.

distributions,

like primary

electron

finite beam spot size (BS) which decrease if the photon incident

beam is collimated

at s(BS) on the radiator

the momentum

transfer

(ED), multiple

analytical

enhances

0146-6410/00/$ - see front matter 0 2000 Published PII: SOl46-6410(00)00099-5

It is assumed,

reproducing

the ex-

one and a Monte

(MS) in the radiator

Their impact

the polarization.

momentum

which

[5]. Com-

Carlo

suited to cope with experimental

scattering

the degree of polarization.

and has a transverse

(Fig. 2 top).

with the same precision

an approximative

which in general

radiator

which stem from a bremsstrahlung

The latter one is perfectly

divergence

off a crystal

vector due to its regular structure

contributions

are calculated

Two methods,

bremsstrahlung

is especially

An electron

p(BS) is deflected -

by Elsevier Science BV. All rights reserved.

and a strong

which is

by m(MS) due

462

R A. Natter / Prog. Part. Nucl. Phys. 44 (2000) 461-463

to small angle scattering

(see Fig. 1, underlined

the lattice basis vector ai). The bremsstrahlung electron

(e) which means a transformation

the momentum tor condition

of the produced is checked:

With an improved contribution

photon

vectors denote transverse

components

process is than calculated

in the reference frame of the

of the crystal

orientation:

(0,cr)

(r9;, @‘) <

(ogk,&.). Finally, the collima-

is retransformed:

l&j < r, where 2, and r, denote

description

of the angle dependence

the collimator

4,

with respect to

(Of,&);

distance

thereafter

and length,

resp.

of the cross section and the e-e bremsstrahlung

[5] and the use of the precise MCB code, a very good agreement

tal spectra could be achieved, thus reducing the systematic

with the experimen-

error of the polarization

prediction

to z 2%.

220 MeV - - 280 MeV

” .6 z u 6 ._ s .I

4-

$ :

2-

0, 0

Fig 1: Vectors

(electron:

angles with respect Asymmetry The photon

220,e, photon:

to the crystal

&) and

Fig 2: Nickel and diamond

axes &i

polarization

asymmetry

is defined from cross sections

photon

frame the structure

function

polarisation

cross section depends (IC) and correlated WTT. Therefore,

with the reaction

W m has a trivial dependence

(ClB)

the contribution

currents,

the interference

from the ClB is enhanced,

of the asymmetry

to correlations.

vide additional

to the theoretical

microscopic

constraints

on 4He was carried at about

terms esp.

by the interference

Furthermore,

energy (Fig.

settings

demonstrates,

with the IC,

functions.

corresponding

2 bottom).

versus the missing energy Ezm = Er - Tp - T, - T,,, , missing momentum

through

these measurements

model and its correlation

the

(MEC), isobar

enter the asymmetry

are shown in Fig. 3 and 4, where the cross section

energy E7, resp. Their behaviour

angle Cp= (q$, + &)/2

from meson exchange

out at three diamond-radiator

220, 280 and 350 MeV photon

30% of the data)

(011+ CL). In the c.o.m.

2@), or i.e. ~11,~= a0 (1 f P!C) = 00 + A. Whereas

only on WT which is a sum of contributions one-body

plane parallel and perpendic-

on the azimuthal

leading to a high sensitivity

(only

for 3 crystal settings

as: C = - WTT/ WT = (ali - ol)/

plane: o(O) = a0 (1 + PJcos

of the reaction

polarization

and

of the 4He(y’,np) reaction

ular to the incident

measurement

prediction

cross sections

The

to maximal

Its preliminary

and asymmetry

pro-

results

is plotted

&, = & - & - & and photon

that the direct 2N absorption

reveals a non-vanishing

463

I? A. Natter / Prog. Part. Nucl. Phys. 44 (2000) 461-463

asymmetry

A, as well as 3N absorption

asymmetry

qualitatively.

Inelastic

cesses, reem to ‘forget’ the photon The cont:ibutions

whereas

reactions,

which results

showing

Fig. 3 right, black). Fig. 4 together

a non-vanishing

the A resonance

might be explained

very likely multi-step

in an asymmetry

direct 2N absorption

value only in the momentum

These events enter the energy dependend

with those for carbon

involving

does not change the

compatible

by shaded areas and stem from a calculation

Valenci.; code [2]. A cut on low E zrn highly enhances pzm asymmetry

at all, obviously

like pion production

polarisation

involved are indicated

FSI, if present

(same experiment,) [3] by the interference

with zero.

[6] based on the

which is supported

by the

range of the *He 1s state (see

asymmetry

and deuteron

pro-

C(E,) which is plotted

data.

The discrepancy

of tensor correlations

in

around

which are not present

in deuteron. A-

I

I

I

I

-0.3

-0.4

-I50

l--LA&& 100 200

200

300 400

miss. energy [MeV]

Fig 3: Asymmetry

400

-0.5 + _ 150

600

i

oD l 3He I

I

200

250

miss. momentum [MeV]

vs. Ez,

photon energy

Fig 4: Comparison

and pzm

r 350

I

300

[MeVI

of asymmetries

Conclusion These first results of SRC effects.

show that asymmetry

Furthermore,

to SRC and tensor correlations a comparison

to microscopic

measurements

they allow the conclusion

provide an additonal that the asymmetry

(esp. in the np channel). calculations

More stringent

for 4He which are underway

handle to investigations exhibits

a high sensitivity

results can be expected

from

[3].

References

PI R. PI

Jastrow,

Phys. Rev. 98 (1955) 1479; K. Gottfried,

Nucl. Phys. 5 (1958) 557

C. Giusti and F.D. Pacati, Nucl. Phys. A 571 (1994) 694; R.C. Carrasco, M.J. Vicente and E. Oset, Nucl. Phys. A 570 (1994) 701; 3. Ryckebusch, Phys. Lett. I3 383 (1996) 1

[31 J. Ryckebusch,

Vacas

Nucl. Phys. A 624 (1997) 581; private communication

PI T. Lamparter et al., Z. Phys. A 355 (1996) 1; P. Grabmayr, W.H.A. Hesselink and G. Rosner, contrib. to this School and to Proc. of 4th Workshop on “Electromagnetical Induced Two Hadron Emission”, Granada May 1999, editor P. Grabmayr and A. Lallena 151F.A. Natter,

ibid p. 59

161T. Hehl, ibid p. 70