Cross-section asymmetry in the photodisintegration of the deuteron by polarized photons

Nuclear Physics A381(1982) 330-342 © North-Holland Publishing Company

CROSS-SECTION ASYMMETRY IN THE PHOTODISINTEGRATION OF THE DEUTERON BY POLARIZED PHOTONS V.G. GORBENKO, Yu .V. ZHEBROVSKIJ, L.Ya. KOLESNIKOV, A.L. RUBASHKIN and P.V . SOROKIN

Institute of Physics and Technology, the Ukrainian Academy of Sciences, Kharkov 310108, USSR Received 12 October 1981 Absfet: The asymmetry of cross sections for the photodisintegration of the deuteron has been measured in the linearly polarized photon energy range 80-600 MeVat c.m. proton emission angles 75°-150° . The obtained data are not found to be in agreement with theoretical predictions. E

NUCLEAR REACTIONS ZH(polarized y, p), E=80-6OOMeV; measured Q(B) asymmetry. Linearly polarized beam .

1. Introdactiou Two-body disintegration of the deuteron is one of the elementary processes convenient for extracting fundamental information about the effect of nucleon intrinsic degrees of freedom on the NN interaction in nuclei, the role of meson exchange currents in electromagneticinteractions of hadrons, the AN interaction etc. Considerable interest has recently been shown in the reaction y+d->n+p,

(1)

in connection with the search for two-nucleon resonances. The deuteron has been chosen as an object of study because it is the simplest nucleus for the production of dibaryon states. The first evidence of the existence of dibaryon resonances was obtained by the Tokyo group 1), who observed an anomalously high proton polarization in the photodisintegration of the deuteron at 9* = 90° and E,. = 550 MeV. Analysing these results, together with data on differential cross sections, Kamae and Fujita 2) suggested that the AA dibaryon resonance was produced with quantum numbers IV") = ((3+) and a mass of 2380 MeV . The existence of the dibaryon resonance 1(3-) with a mass of 2260 MeV has been indicated by the analysis of the spin dependence of the cross sections in elastic pp scattering studied at Argonne 3-s) . The inclusion of this resonance in the recent multipole analysis 6) has provided a satisfactory description of the differential cross sections and the proton polarization in the reaction (1) in the energy region between 330

V.G. Gorbenko et a1. / Goss-section asymmetry

33 1

350 and 700 MeV. However, the results of this analysis allow one to assign the quantum numbers 1(2-) to this resonance when it is considered in combination with the 0(1+) resonance of mass 2380 MeV. At Saclay e), the cross section for the yd -oppor - process has been measured by the detection of a pion in coincidence with a proton . Those workers related the peak observed at E,. = 410 MeV to the excitation of the resonance with a mass of 2230 MeV. The problem of the existence of dibaryon resonances has also been treated theoretically by using the group approach 9), the bound-state model with a oneboson exchange potential 2), the quark bag model 1°), the orNN or orvNN molecular model'') etc. In a number of papers the mass spectrum, spins and parities of dibaryon resonances were predicted. However, the amount of available experimental data is insufficient to solve the problem unambiguously. In particular, it would be interesting to measure various polarization parameters in the deuteron photodisintegration process, viz., the cross-section asymmetry with linearly polarized photons, X, the recoil nucleon polarization, P, and the asymmetry of the cross sections on a polarized target, T. Here we present our results on the polarized beam asymmetry _ do,, - doll do,, + dan for the reaction (1) in the energy range E,. = 80-600 MeV at c.m. proton emission angles 75°-150°, taken with the linearly polarized photon beam of the Kharkov 2 GeV electron linear accelerator. Some of the results presented here have been published earlier in refs . 12,13) . 2. Experimental equipment The experimental layout is shown schematically in fig. 1 . This arrangement was almost identical to that used previously in experiments on pion photoproduction on nucleons and very light nuclei 14). It included electron and photon beam formation and control systems, goniometers with diamond single crystals of different thicknesses, a liquid hydrogen-deuterium target and detecting apparatus . 2.1 . ELECTRON AND PHOTON BEAMS

The electron beam of the Kharkov linac, after passing through the parallel transport system, was focused on a single crystal radiator to form a spot of 3 mm in diameter on the target with angular spread ^-10 -4 rad. The initial electron beam energy Ea varied in the range 600-1600 MeV depending on the photon energy E at which the E-asymmetry was measured. The electron beam monochromacy was AF-o/Eo =1 % . The average current (-1 WA) was measured by a secondary emission

33 2

V.G. Gonbenko et al. / Cross-section asymmetry

as .

wwsi~c~~~o 0s on

MIJL

l -

q*

á

Fig. 1 . T'he experimental layout. (1) electron beam line ; (2,5) bending magnets of the parallel beam transport system ; (4) collimator-monochromator ; (3, 6) quadrupole lenses ; (7) secondary emission monitors ; (8) goniometer with diamond single crystals ; (9, 14) sweeping magnets ; (10) magnet chamber; (11,13) photon collimators; (12) electron beam disposal ; (15,16) chamber with liquid hydrogen deuterium target ; (17) Faraday cup ; (18, 21) scintillation counter telescopes ; (19, 22) counter shields ; (20, 23) magnetic spectrometers ; (24) quantameter.

monitor calibrated preliminarily with the Faraday cup. The beam stability on the monocrystal was controlled during the run by a second monitor with an aperture . The deviations of the beam were found to be less than 1 mm per hour . The quasimonochromatic linearly polarized photon beam was produced by the coherent bremsstrahlung of electrons in a diamond single crystal t°-16). The crystal thicknesses were chosen according to the cross-section values of the reaction under study and were 0.3 and 2 mm. The diamond crystal was oriented relative to the electron beam by means of a goniometer with an accuracy of 5 x 10 -' rad. The crystallographic axes bl = [110], b2 = [110] and b 3 = [001] were directed along the electron beam and the vertical and horizontal axes of the goniometer, respectively . The maximum photon beam polarization was achieved for such a diamond monocrystal orientation when the main contribution to the cross section of the coherent bremsstrahlung came from the reciprocal lattice point (2, 2, 0). The coherent beam parameters, e.g., the principal maximum position, the beam monochromacy and the coherent effect value, were determined by measuring the photon spectra. For this purpose, we used a total absorption y-spectrometer with a CsI(TI) crystal, 275 mm long and 150 mm in diameter 17) . The electron beam intensity was reduced so that the frequency of photon detection was not above 20 s-'. Fig. 2 shows the intensity spectrum of the radiation of 500 MeV electrons in a diamond single crystal, 2 mm thick, for the collimation angle 9r = 5 x 10~ rad. The solid curves in the figure represent the calculations of the intensity and polarization spectra taking into account the finite y-spectrometer energy resolution, the angular spread of the electron beam, the multiple scattering in the monocrystal and the collimation angle.

V.G. Gorbenko et al. / Cross-section asymmetry 3

I OW.ands)

33 3

E~"600nev

2

0 06 04 02 0

01

02 03 04 05 E, (Gev)

06 07

Fig. 2. Intensity and polarization spectra of bremsstrahlung from a 2 mm diamond single crystal at orientation angles 0 = 95 .3 mrad and a = 84 .7° (0 is the polar angle between the electron momentum po and the crystal axis bt =[I10], a is the azimuthal angle between the planes (Po, bi) and (bl , b2)) . The curve represents the calculation with the following experimental parameters : the energy resolution of the spectrometer AEIE, - 896, the angles of photon eollimation 0, = 5 x 10-4 rad, electron beam divergence wo=10-4 rad and multiple scattering in the target m, = 3 x 10-3 rad.

The total intensity of the photon beam was measured by a Wilson guantameter filled with hydrogen to a pressure of 800 mmHg. For the 2 mm and 0.3 mm diamond monocrystals and the collimation angle 9c = 5 x 10-` rad, the mean beam intensities were 2 x 1010 and 4 x 109 equiv. quanta/s, respectively . 2.2 . TARGET

The low-temperature target used in the experiment 18) had three identical appendices for hydrogen, deuterium and background measurements. The walls of the appendices were made from aluminum, 50 wm thick. The target dimensions varied according to the values of differential cross sections of the process (1) and the energy of the detected recoil protons. At photon energies between 400 and 600 MeV the measurements were performed with an extended target, 20 cm long, while at the energies 80-100 MeV and 120-400 MeV, we used the appendices, 20 and 50 mm in diameter, respectively.

334

V.G. Gorbenko et aL / Cross-section asymmetry

2.3 . MAGNETIC SPECTROMETERS AND DETECTING APPARATUS

The cross-section asymmetry in the photodisintegration of the deuterons was determined from the yield of protons, the angles and momenta of which were set by the magnetic spectrometer . In the experiment, we used two magnetic spectrometers l'); this allowed us to measure two points simultaneously in the angular distribution of the X-asymmetry. Angular and momentum acceptances of the magnetic spectrometers determined the energy resolution AE/E, of the experiment, which varied between 8% to 12% in the energy range investigated . The protons were detected by the telescopes, each consisting of three scintillation counters placed in the shields at the outputs of the magnetic spectrometers. The signals from the counters were analysed by the electronic apparatus 2°) operating on-line with the M-6000 computer. The protons were separated from v+ mesons by using the dE/dx and time-of-flight technique 21) 3. Experimental procedure and background calculations The polarized beam asymmetry X(E, 0*) in the photodisintegration of the deuteron was measured by a method similar to that used previously for investigating 70 meson photoproduction on protons and deuterons 22'23) . The method is based on measurements of orientation dependences of the proton counts C1(D(6, a) with fixed kinematic parameters (momentum and laboratory angle) corresponding to the photon energy E°,.. The direction of the photon polarization vector was fixed normal (parallel) to the reaction plane. The diamond crystal orientation angles 6 and a unambiguously determine the energy Ey of the first interference peak in the coherent bremsstrahlung spectrum . The data for the two directions of the photon polarization vector was collected at the maxima (C~, CIMc) and minima (Có, Có) of the orientation dependences. Có and Cö were equal to and corresponded to the proton yield from the incoherent part of the photon spectrum . After subtracting the background from the walls of the target, the asymmetry parameter was calculated as Emma

P1 RR-1 +1 '

where R = C~/C~ . The effective photon polarization P,, isrelated to the coherent effect value 8 = (CmX +C~;n,=)/2Co [ref. Za)] as _ 2(1-x) ß-1 PY-kl +( /4 .

V. G. Gorbenko et al. / Cross-section asymmetry

335

The factor k, that takes into account the contributions to the coherent bremsstrahlung cross section from other reciprocal lattice points of the diamond, varied from 0.98 to 0.90 in the relative photon energy range (x = EY/Eo) 0.1-0.4. The effective photon polarization P,, is generally below the polarization of the coherent bremsstrahlung beam. This is due, on the one hand, to the averaging of the photon polarization over the finite energy acceptance ofthe detecting apparatus, and, on the other, to the background from pion photoproduction reactions. We shall now discuss the procedure for taking account of the background corrections for the reactions 0 yP -'P ir ~ yn -opi that appear to be the main background processes in our case . As has been shown in ref. 22), the background from the high-energy part of the photon spectrum can be divided into coherent and incoherent parts, the latter being taken into account in Xmea, by using the effective polarization Pr Taking into account the background correction from the coherent part of the spectrum leads to the following expression for the asymmetry parameter: 1 = Imem (1 + Kcoh) ,

where Eo

Kcoh

(d2 tT/d,(1 dP)hh(EY ) dEYlEY (do-/d , ~fï*)(J1P)I(E) coh Y

Here, Icoh(E,,) is the coherent part of the photon spectrum intensity withthe radiation maximum at Ey ; do,/dfl* is the differential cross section for yd-o np; J = d,O* dE,,P/df dPE, is the corresponding jacobian; P is the momentum of the detected proton . The differential cross section d2o/dD dP was calculated in the impulse approximation taking into account the momentum and angular distributions of intranuclear nucleons 25) : 2

a, d, d dP

dir d cos 0* d~P* 2 ,fo~ p (Pe)Pe2 dPe J " ( 1- P° ~ Oc) [( o Ec in* dP d cos 6 dir d cos B* d~p* +( ) , 8 (Ee Pe, df* dP d cos B yn-pw -

Be, q>c , P,

o YP-pa

B) sin Be dOe ,

(7)

where p (Pr) is the momentum distribution of nucleons calculated with the GlendingCramer wave function 26) with a 7% D-state contribution ; .Be, Ee and Pe are, respectively, the polar (relative to the photon momentum direction) and azimuthal angles and the energy and momentum of the nucleon in the nucleus; d cos B*/dP

336

V.G. Gorbenko et al. / Cross-section asymmetry

and dqv*/d cos 0 are the jacobians of the transition from the c.m. photon and intranuclear nucleon emission angles to the momentum and polar angle of the detected proton in the laboratory system ; do,/d,(1* are the cross sections of the corresponding (yp-pir o or yn->plr-) pion photoproduction reactions on free nucleons . The calculated correction values tcroh at E,. = 500 MeV are listed in table 1. For comparison, the second row of the table presents the background correction due to the incoherent part of the spectrum that was also calculated with eq. (6) with the substitution of the Schiff distribution for Icoh (E,.). TABLE 1 The calculated correction values

a

~

BP =45°

60°

75°

1 .254 5 .61

0.0815 0 .308

0 .0075 0.0276

These calculations allowed us to determine the lower boundary of the angular range under study, viz., 9p* > 60°, where the background correction was not above 0 .3. The upper limit of 6p* =150° was set by the experimental equipment. 4. Results The results obtained for the asymmetry parameter E(E 0* ) are presented in table 2 (the data for E,. = 400 MeV were obtained in two series of runs). The statistical error AX was given by Al = kl(Cl+C 1

+2Co)

-V[(1-ki l)AC

+ [(1+k i l)AC

+(2k l ACo) ,

(8)

where ki = k2(1-x)/(1+(1-x)2) is the factor entering into expression (3) for the effective polarization Pr The measured energy dependences and angular distributions of the cross section asymmetry in the reaction yd -+np are shown in figs . 3, 4 and 5 together with the data reported by the groups at Stanford 27) (for angles 450, 90o and 1350), Frascati 28) (9(°) and Bonn 29) (135°) . The curves in the figures present the calculations by Partovi 30), Laget 31) (with the Holinde wave function) and the analysis by the Japanesegroup 6) where dibaryon resonances with the quantum numbers I =1, J° = 3-(2260) +I = 0, J° = 3+(2380) and I =1, J° =1+(2380) were added to the Born and single-pion reabsorption terms. Partovi's calculations are in fair agreement with the measured angular distributions of the E-asymmetry only for E,.=80 MeV. The recent data obtained at Frascati in the photon energy range between 10 and

-0 .260±0 .016 -0 .165±0 .028 -0 .120±0 .040 -0 .020±0 .040 0 .042 :e0 .037 0 .213±0 .030 0 .145±0 .041 0 .187-:k0 .042 0 .185±0 .043 0 .216±0 .047 0 .265±0 .036 0 .203±0 .034 0 .088±0 .032 0.228±0 .058 0 .277 :k0 .086 0 .184±0 .043 0 .209 :t0 .053 .042 .017f0 -0

80 100 120 140 160 180 200 220 240 260 280 300 320 360 400 400* 450* 500* 550* 600*

' Results published earlier in ref.

-O .(>06±0 .081 -0 .380+0 .107

0, =75*

Er

(MeV)

.

1s)

TABLE 2

-0 .280±0 .026 -0 .105±0.015 -0 .043 :E 0.035 0 .159±0.031 0 .076±0.026 0 .159±0.044 0 .258* 0.035 0 .277±0.037 .233t0 0 .044 0 .280±0.045 0 .248±0.029 0 .267 :0.032 0 .292±0.040 0 .267±0.043 0 .267 :e 0.061 0 .257±0.033 0 .164+0.028 0 .150 :0.057 0 .000±0.094 -0.126±0.142

900 -0.345±0 .025 -0.091±0 .034 0.019 :0 .044 0.237±0 .030 .144f0 0 .032 0.252±0 .036 0.261±0 .031 .390t0 0 .038 0.188±0 .030 0.314±0 .039 0.248±0 .025 0.204±0 .036 0.263±0 .033 0.319±0 .039 0.237 :e 0 .061 0.274±0 .040 0.171 :t0 .042 .070t0 0 .067 .116f0 0 .066 0 .068±0.053

1050

0.104±0 .029 0.147 :0 .043 .231t0 0 .045 0.361±0 .034 0.229±0 .032 0 .237±0 .030 0 .210±0 .024 0 .217±0 .029 0 .212 :0 .033 0.220±0 .030 0 .235 ±0 .058 0 .225* 0 .050 0 .258±0 .039 0 .092±0 .047 -0 .026±0 .083 -0 .017±0 .013

0.078±0 .057

-0.328 :0 .025 .026 .074t0 -0

120'

Cross-section asymmetry velues in the yd -+ np reaction

-0 .169±0 .045 0 .043±0 .042 ~0 .029±0 .029 0 .147±0 .035 0 .119±0 .044 0 .160±0 .040 0 .171+0 .052 0 .217±0 .024 .170t0 0 .024 0 .256±0 .038 0.160 :E:0 .022 0 .171±0 .027 0 .187 :0 .029 0 .195 ±0 .038 0.191±0 .049 .191t0 0 .037 .147t0 0 .057 -O.(>60±0 .072 0.039 :0 .015 0.304±0 .128

135*

0 .100 :0.032 0.098±0.044 0.073±0 .024 0.046±0.050 0.088±0 .027 0.074±0.042 0.140 :0 .073

0 .050±0.033

1500

w w v

ca

% n

ç

C

0A

§

02

0.3

Q4

05

1

1Z

0.6

1

120'

75 0

OJ

ti

02

`= 1 ,

.~. ~

0 0!~ Er NO

/ ~ '. gs 1

05

05

900

a,

§

02

0 00 D

§

03

§

M

~ t

t

05

0

07

Fig . 3 . Energy distributions of the cross section asymmetry. Points are : 0, 0 our present data ; I Stanford 27); M Frascati 28); V Bonn 29). The dotted curve represents the calculation by " "Y the solid, dashed and dash-dotted curves show the results of the analysis by the Japanese group 6) .

-ID

-05

0

M

A

to

0.

200 MeV

100 MeV

á

220 MeV

120 MeV

á

240 Mev

ff

140 MeV

260 MeV

¢43'

100 MeV

%"[deg]

0 30 60 90 120 50 0 30 60 90 120 150 0 30 60 90 120 150 0 30 60 90 120 150 0 30 60 90 120 150 180

180 MeV

Ei 80 MeV

Fig . 4 . Angular distributions of the cross-section asymmetry for E,. = 80-260 MeV. A double dash-dotted line representing Partovi's calculation 3°) . is added to the notation of fig. 3

-t0

-05

0

0

05

E

W W

3

h

á ua

W

0

a

n 0

n

C

Z05

du'de'

V.G. Gorbenko et at. / Cross-section asymmetry Er 280 MW

o

.f

300 MeV

320 MeV

LIS

~

a

400 MeV

360 MeV

ORFF

450 MW

13-

ILIM

i

500 Mev

-

600 MeV

0,5

0

..

-to

30

60

%À

.

/

-05

0

.

i //

90 20 150

0

e

v

30 60

90 10 150

0 30

+

60 90 t2o 150 180

[deg] Fig . 5 . Same as in fig . 4 for E,. = 280-600 MeV .

341

V.G. Gorbenko et al. / Cross-section asymmetry

F. ^

-d

Ei300 Mev

80 MeV " E1

i 0 aa

-05 -1.0 t0

a

150 0 30 60 90 120 150 180 0 30 60 90 120 8P" [deg] F=

C16" d6". d6L-d5

E," 300 Mev

E,-80 Mev

0 -O5 -1.0

v ,.

i

.w

b

0 30 60 90 180 150 0 30

E

60

90

120 150 180

" [deg]

Fig . 6. Comparison between the measured angular distributions of the E-asymmetry for E,=80 and 300 MeV and the calculations by Laget 31 ) . (a) The dash-dotted curve indicates the calculation with the S-wave only ; the dashed and solid curves illustrate the case when the D-wave is added to the nucleon pole term and the pion reabsorption amplitude, respectively ; (b) For E, =80 MeV, the solid and double dash-dotted curves show the calculations with and without taking account of the monopole form factor in each baryon vertex with p-meson exchange ; dash-dotted and dashed curves correspond, respectively, to the cases when the contact Born term and pion photoelectric term are added to the nucleon pole term . For E,T300 MeV, on calculating the double dash-dotted curve the monopole form factor wag considered while the p-exchange amplitude was neglected .

60 MeV for 0* = 90° [ref.')] are also in agreement with the calculations by Partovi. However, the calculations fail to describe the experimental data at higher energies . Fig. 6 shows the comparison between the measured angular distributions of the cross section asymmetry parameter at photon energies of 80 and 300 MeV and the

342

V.G. Gorbenko et aL / Cross-section asymmetry

calculations by Laget. The sensitivity of the calculation to the D-state contribution to the deuteron wave function is illustrated in fig. 6a. The best agreement with the experiment is observed for E,, = 80 MeV if the nucleon pole term is included. The calculations were made with the Reid soft-core wave function 32). Fig. 6b shows the effect of the contributions from different amplitudes on the asymmetry of the cross sections . In conclusion we note that the data presented here constitute the basic experimental information for the E-asymmetry in the yd-* np reaction in the photon energy range from 80 to 600 MeV. The comparison with existing theoretical models neither gives preference to any of the calculations nor allows one to draw any certain conclusion on the existence of dibaryon resonances . To solve this problem, further effort is needed . References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) 24) 25) 26) 27) 28) 29) 30) 31) 32)

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