Measurement of the analyzing power components in photodisintegration of the polarized deuteron

Physics Letters B 302 (1993) 23-28 North-Holland

PHYSICS LETTERS B

Measurement of the analyzing power components in photodisintegration of the polarized deuteron S.I. Mishnev, D.M. Nikolenko, S.G. Popov, I.A. Rachel A.B. Temnykh, D.K. Toporkov, E.P. Tsentalovich, B.B. Wojtsekhowski Budker Institute of Nuclear Physics, 630 090 Novosibirsk, Russian Federation

S.L. Belostotsky, V.V. Nelyubin, V.V. Sulimov St. PetersburgInstitute of Nuclear Physics, 188 350 Gatchina, Russian Federation

and V.N. Stibunov PolytechnicalInstitute, Tomsk, Russian Federation

Received 8 June 1992; revised manuscript received 5 January 1993

Components T~o, T22 tensor and TH vector analyzing power of the d (e, pn)e' reaction with an electron scattered at a small angle (deuteron photodisintegration) were measured. The measurements were performed for photon energy in the range 40-550 MeV and with the center-of-massangle of the outgoing proton being 88 °. Only qualitative agreement with theoretical predictions for T22and/'2o has been observed.

In one o f the most important experimental fields o f deuteron photodisintegration - tensor-polarized target experiments - only one measurement has been carried out [ 1 ]. The experiments reported here were conducted at the 2 GeV electron storage ring VEPP3 in Novosibirsk simultaneously with a measurement o f the asymmetry in elastic d(e, e'd) [2]. Electrodisintegration o f tensor-polarized deuteron with the electron scattered at small angles was studied. This approach is equivalent to photodisintegration [3]. Below we present some confirming arguments. The general expression for the cross section for polarized deuteron photodisintegration has the form [ 4 ] d e _ dao { 1 - ½x/~ P~ sin Oa sin ~a Tll dD dO + ½x/~ P= [ ½(3 COS20d- - 1 ) T2o + x / ~ sin 20a cos ~oa TEl + X//~ sinE0a cos 2~0a 7"22 ] }, Elsevier Science Publishers B.V.

( 1)

where dao/dO is the unpolarized cross section, Pz and Pzz are the vector and tensor target polarizations defined as P z = n + - n _ and P z z = n + + n - 2 n o , where n+, n_, no are the fractional populations o f deuterium magnetic substates relative to the direction of the holding magnetic field, the spherical angles Od and ~0d define the deuteron polarization direction in the frame where the z axis is along the photon trajectory and z - x is the reaction plane. TH is the component of the vector analyzing power and T2o, T21, T22 a r e the components of the tensor analyzing power o f the reaction. They are functions o f the proton knockout angle 0p and the photon energy E v. Analyzing powers are of special interest since they are sensitive to the form of the deuteron wave function, meson exchange currents, final state interactions etc. [ 5-7 ]. It is important to understand that information obtained from polarization experiments often cannot be extracted from non-polarized ones. The experiment was run with two different setups. In the first runs with the same target as ref. [ 1 ], a jet 23

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of polarized deuterium atoms was used as an internal target with thickness 1011 atoms/cm 2 [ 8 ]. The polarization of the jet, measured by the procedure described in ref. [ 8 ], was found to be P°z = 0.97 + 0.07. The background of unpolarized deuterium atoms in the interaction region was taken into account by Monte Carlo simulation, resulting in the correction factor k~ = 0.890 _ 0.033. Calculation of the depolarization by the pulsing magnetic field of the electron beam gave k2=0.924+0.053 [9]. We calculated k3 = 0.975 _+0.025 to describe the capture of non-polarized deuterium ions by electron beam. Combining all corrections one gets the averaged target polarization degree: o Pzz = P~zkl k2 k3 = 0.778 + 0.080.

In the second runs, a storage cell [ 10 ] was used to increase the target thickness to 3 × 10 II atoms/cm 2 (visible to the detectors) with an average tensor polarization of 0.572 + 0.053 [2 ]. The polarization of the jet is higher than that of the stored gas. Thus, small corrections (~< 4%) have been made to account for the dependence of detector acceptance with kinematics. The data acquisition was performed with an electron beam current of 0.1-0.2 A. The total charges were 160 kC and 400 kC for the first and second experiments respectively. Most of the data were obtained by detection of proton-neutron pairs. A few measurements were performed with tagged virtual photons, i.e. with detection of a proton in coincidence with a forward scattered electron. The particle-detection apparatus consisted of two identical detection systems (fig. 1 ) [ 11 ]. Each of the proton arms included six drift chamber planes to measure the proton trajectory, three thin plastic scintillators (4, 10 and 10 mm thick) and two layers of NaI(T~) blocks (50 and 110 mm thick) to determine the proton energy. The energy range covered by the proton calorimeter was divided into nine intervals (the corresponding photon energy intervals are presented in table 1 ). The neutron detection arm consisted of three plastic scintillator bars 1 m × 20 cm X 20 cm fronted by a charged-particle veto counter. The bars were connected to two phototubes, one at each end. The timing difference between the tubes and the bar hit de24

18 March 1993

~

AcPS

d

i/..~1~,.. ' ,~c ~2

%

DC PS

aI Q)

b)

Nal c)

Fig. 1. Layout of the detector system. (a) Top view at the VEPP3 bending magnet (M) with tagging stations (TS), (b) site view at one of the pn-pair detection systems, (c) layout of the detector system around the beam. T is the polarized deuterium target, DC are the drift chambers, PS the plastic scintillators, AC the anticoincidence counters.

fined the position of the neutron. The neutron energy was measured by time-of-flight over a 1.7 m flight path, with a timing resolution of about 1 ns. The detectors covered a substantial solid angle: A~= 32 ° and AO~ 10 °. The average angle of the proton emission in center-of-mass frame was 0ucM~88 °. The tagging system included two detectors to tag forward scattered electrons with energy losses of about 200 and 400 MeV. The detectors were a four-strip semiconductor detector (SCD) followed by a sandwich of three converters and three layers of SCD. The detectors were placed between the poles of the storage ring bending magnet. Off-line background rejection included a cut on the location of the scattering vertex. For pn pair events, the kinematic correlations between the proton and neutron scattering angles and energies were used. For tagged-photon events, the correlation between the proton and scattered electron energy was used. In fig. 2 three histograms of one of the correlation parameters, R(q~p, ~ n ) = ~ p + ¢ , - z t , are shown. The first histogram ("pn ~'') includes all (pn) events, the second one ("pn 2'' ) includes those events which survive after applying the cuts on correlation parameters R(Op, O,) and R(Ep, En); and the third histogram, with events having a large signal in the veto counter, shows essentially (pp) events. One can see that the pp-pair yield is substantially smaller than that of the pn-pairs. Since pp-pairs originate from pion production on the deuteron, due to isospin invariance approximately the same number ofpn-pairs from

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Table 1 Experimental results, a, b, c define the experimental conditions: a: pn-pair registration, jet target; b: pn-pair registration, storage cell target; c: e'p-registration, jet target; a + b or a + b + c: a value averaged over the corresponding measurements. Only statistical uncertainties are shown except for the mean values where the second errors are related to the uncertainty of the target polarization P=. Er, average (MeV)

E~ (MeV)

OcM

Experimental conditions

T2o

T22

40

34-47

88.0

a b a+b

- 0.224 + 0.235 0.441 +0.124 0.296+0.110+0.028

- 0.024 + 0.203 -0.450+0.106 -0.359+0.094+0.034

65

47-83

88.1

a b a+b

0.510 + 0.223 0.228 + 0.1 O1 0.276+_0.092+0.026

- 0.769 + O. 188 - 0.456 _+0.087 -0.511 +_0.079+0.048

107

83-130

88.6

a b a+b

0.356+0.264 0.231 +0.117 0.252_+0.107+0.024

-0.889+0.221 -0.622+0.100 -0.667_+0.091 +_0.063

146

130-163

87.5

a b a+b

0.026_+ 0.322 0.498+0.156 0.408+0.140_+0.038

- 0.518 _+0.274 -0.978-+0.132 -0.891 _+0.119_+0.084

184

163-205

87.4

a b a+b

-0.098_+0.359 0.514_+0.156 0.417-+0.143_+0.039

- 1.490_+0.278 -0.780+0.133 -0.912_+0.120-+0.086

230

205-255

86.7

a b c a+b+c

0.635 -+0.369 ' 0.457-+0.153 -0.111 _+0.280 0.362_ 0.126 +-0.034

- 0.729 + 0.314 -0.571 _+0.132 -0.988_+0.259 -0.666_+ 0.110+0.062

296

255-338

88.2

a b a+b

0.422_+0.387 0.578-+0.161 0.555+_0.149_+0.052

- 1.158_+0.315 -0.717-+0.138 -0.788_+0.126_+0.074

378

338-464

89.6

a b c a+b+c

0.930_+0.659 0.631 _+0.265 0.776 +_0.420 0.699_+0.212+0.066

-0.200_+0.583 -0.861 _+0.225 - 0.730_+ 0.367 -0.764-+0.182-+0.072

505

464-606

88.7

b

- 0.402 _+0.465 _+0.038

- 1.040_+ 0.391 _+0.098

pion production should have been observed. Therefore m o s t o f t h e o b s e r v e d (pn) e v e n t s a r e f r o m t h e two-body deuteron disintegration process. A s o n e c a n see f r o m fig. 2, t h e n u m b e r o f (pn) e v e n t s is r e d u c e d s u b s t a n t i a l l y a f t e r a p p l y i n g all c o n s t r a i n t s . M o s t o f t h e r e j e c t e d e v e n t s c o r r e s p o n d to e l e c t r o n s c a t t e r i n g a t large e n O u g h a n g l e s f o r t h e detector to be able to distinguish them from an electron s c a t t e r e d a t z e r o degrees. T h i s t h r e s h o l d a n g l e 0B is defined by the angular and energy resolution of the d e t e c t o r s a n d is e q u a l t o ~ 1 ° a t l o w E~ a n d r e a c h e s ,~ 3 ° at h i g h E r A s it follows f r o m c a l c u l a t i o n [ 12 ],

t h e r a t i o o f l o n g i t u d i n a l t o t r a n s v e r s e cross s e c t i o n s for deuteron electrodisintegration integrated over e l e c t r o n s c a t t e r i n g a n g l e in t h e r a n g e 0--0B is s m a l l e r t h a n 0.02. T o s i m p l i f y t h i s c a l c u l a t i o n t h e m a t r i x elements of vector components of the nuclear current w e r e a s s u m e d t o b e equal. So t h e c o n t r i b u t i o n o f t h e l o n g i t u d i n a l p h o t o n s to t h e cross s e c t i o n is n e g l i g i b l e a n d t h u s t h e d a t a o b t a i n e d is r e l a t e d t o p h o t o d i s i n t e g r a t i o n processes. T h e n u m b e r o f e v e n t s f r o m t h e b a c k g r o u n d react i o n 2H (e, pn)e'~z ° w h i c h a r e left a f t e r a p p l y i n g kin e m a t i c a l c o n s t r a i n t s , was f o u n d f r o m t h e a n a l y s i s o f 25

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4000

800

> q) v cCL z

> (1)

field pointing in the direction j, and the sign o f P= given by k. Then the experimental asymmetry is defined as

at22= (N~+ --N 2l+ - N 2 +I

+N22+

v

63 Ck

z

-2O

-10 0 R(~p,¢n),

10 deq.

2O

Fig. 2. Histograms of the correlation parameters R ((op, ~o,). The vertical scale for the histogram of the (pp) events is chosen to be 5 times larger than that of the (pn) events to reflect the low detection efficiency of neutrons and to get a clear comparison of yields of two reaction channels.

the 2H (e, pp)e'n- reaction events. The latter are the events with a large energy deposition in the veto counter. Applying the same rejection procedure for those events, one can get the number of surviving background events (4%). This number should be doubled since the n o can be produced both on the proton and the neutron. In the experiment described here use was made of two identical detection systems, switching the polarization direction and Pz~ sign as it was done in the earlier experiment [ 1 ] in order to measure the components o f the tensor analyzing power and to suppress the systematic uncertainties (note that Pz= always, while P== + 1 or - 1 [ 8 ] ). The detection systems were placed at 90 ° azimuthal angle around the electron-beam axis (fig. 1 ). Two possible directions of guiding magnetic field, H 1 and 112, correspond to the azimuthal scattering angles ~0d= 90 ° and 0 ° for the first detector system and ~od=0 ° and 270 ° for the second one. The angle Od was equal to 90 ° in all cases. One can see from eq. ( 1 ) that in this case the azimuthal asymmetry appears due to the terms involving Tit and T22,while the term involving T21 has vanished. Note that in an earlier experiment [ 1 ] it was T2~ that determined the reaction asymmetry. The difference in the scattering rates due to T22 for the two detector systems changes sign with reversing the magnetic field direction as well as with the sign o f P=. We therefore had four polarization states and eight numbers N}k representing the number of events registered by the detector system i, with the magnetic 26

18 March 1993

-NI_ +N~_ +N~_-N~_)/ Z N~k.

(2)

With the asymmetry defined in such a way, any residual systematic errors arising, for example, from possible differences in detector systems (in their efficiency, background conditions, solid angles etc. ), are substantially reduced. Using eqs. ( 1 ) and (2) one gets T22 =

k~~

4

at22,

(3)

where k~= 1.055 is the correction factor to take into account the acceptance of the detectors in the azimuthal direction. The results are presented in table 1. To check the systematic uncertainties one can compute an expression which must be equal to zero in absence of any systematic errors: a .... = ( N I + - N 2 + + N ~ + - N ~ +

+NI_-N2~_ +N~_ -X21_)/ ~ Njk.

(4)

We found a .... to be 0.011 _+0.044 in the first experiment and - 0.024_+ 0.021 in the second experiment. As for T2o, one can extract its value from the difference in counting rates for the detector system by switching the sign of Pzz:

T2o= v/~ ENj+ - ZNj_ Pzz 5"N}k

(5)

This asymmetry, unlike Tzz, was obtained from non-simultaneous measurements. Therefore, the difference between the integrals of the luminosity for the two signs of Pzz directly affects T2o. We forced those integrals to be close by reversing the sign of P= every 200 seconds. This is by almost two orders o f magnitude less than a variation time of both the electron beam and the target thickness. The results of the measurements are presented in table 1. To check the consistency of the results obtained for

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T22 in different experimental conditions a, b, c (see table 1 ), taking into account both statistical errors and the errors in Pzz, we d i d as follows. First we fitted the averaged values o f T22 with a curve A (0.0208 Er - 0 . 0 8 3 9 x / ~ r ) .

Tll = ~

~ N ~ k -- ~ N ~ k

~N~k

1.00 ,"

.......~ .......... 5

0,50

Teo

4

-0.00

Here A = 1. Then we found the best value o f the par a m e t e r A for the a and c measurements: A~,¢= 1.15 + 0.12 a n d that for the b m e a s u r e m e n t A b = 0 . 9 6 4 + 0.064. After adding in q u a d r a t u r e the corresponding errors o f P= we finally got A~,¢= 1.15+0.17 and A b = 0 . 9 6 4 + 0.103. One can see that these two values are in agreement with each other. Therefore we concluded that there is a satisfactory agreement between the two measurements. Thus we use the m e a n values o f T2o and T22 as the final results o f the experiment. There is the c o m b i n a t i o n for extraction o f the Tt value: 1

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(6)

The t r e a t m e n t was done assuming P~ = ] IP = l . The results are shown in fig. 3 together with theoretical predictions. The error bars c o r r e s p o n d to the statistical uncertainties only. Schmitt a n d ArenhSvel [6] used the Bonn potential O B E P R and included meson exchange currents ( M E C ) , isobar current ( I C ) and relativistic corrections ( R C ) . Korchin, M e l ' n i k and Shebeko [ 7] used the Paris potential, with one-pion exchange taken into account for MEC. The line shown was o b t a i n e d for the value A = 1.2 G e V for the cut-off p a r a m e t e r o f the nNN form factor. Calculations p e r f o r m e d by Levchuk [ 5 ] used a d i a g r a m m a t i c a p p r o a c h with the Paris potential, M E C and IC taken into account. F i n a l state interactions ( F S I ) are included in all the above calculations. Also results o f two simple calculations [5] based on the plane-wave impulse a p p r o x i m a t i o n ( P W I A ) and that plus nucleon final state interactions ( I A ) are shown. F o r the T1, measurements only the calculations o f ref. [ 6 ] are presented. We conclude that the latter two calculations are inconsistent with the experimental results, while the former ones are in much better agreement. These calculations are o f the same shape and roughly the same magnitude as the data, indicating that they include the correct physics - a n d in particular indicate the

-0.50

-1.00

0

160

260

360 E r , MeV

460

560

660

0.50

1

T22

(b)

0.00

-0.50

-1.00

-1.50

-2,00

16o

z6o

z60 E~,

460

560

660

MeV

o.5

I

I T 11

(c)

0.0

-0.5

-1.0

-1,5

-2.o 0

I

I

I

I

100

200

300

400

, 500

I 600

E 7 MeV Fig. 3. Experimental results and theoretical predictions: (a) for T2o, (b) for T22 and (c) for TH, as a function of the photon energy Ey. The error bars represent statistical uncertainties only. 1 - PWIA; 2 - IA; 3 - IA + MEC (all three are from Levchuk [ 5 ] ); 4 - from Korchin et al. [ 7 ]; 5 - from Schmitt and ArenhSvel [ 6 ]. i m p o r t a n c e o f M E C - although in detail they are certainly wrong. N o t e that the experimental bins in E~ are large; this should be taken into account in comparison with theories.

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We acknowledge the contributions o f R.J. Holt, R. G i l m a n , E.R. Kinney, R.S. Kowalczyk, J. Napolitano, D.H. Potterveld and L. Young, the participants o f our j o i n t experiment on elastic electron scattering from polarized deuterons [2], p e r f o r m e d simultaneously with this work. We are grateful to M.I. Levchuk who was o f great help to us in the interpretation o f the experimental results. We thank H. Arenh6vel, V.F. Dmitriev, E.R. Kinney, Yu.P. Mel'nik, A.V. Shebeko and K.-M. Schmitt for very useful discussions and providing the calculations. We are grateful to Yu.I. Eidelman, V.A. Kiselev, V.M. Petrov, I.Ya. Protopopov, G.M. T u m a j k i n and the staff o f VEPP3 for the maintenance o f the storage ring. We also thank L.G. Isaeva, B.A. Lazarenko, K.T. Ospanov, U.V. Schebiot, Yu.G. Ukraintsev, A.P. Usov and D.K. Vesnovsky for their participation in designing, construction and maintenance o f the experimental set up and M.D. M i n a k o v and S.I. Serednyakov for providing us with NaI counters.

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References [ 1] M.V. Mostovoy et al., Phys. Lett. B 188 ( 1987 ) 181. [ 2 ] R. Gilman et al., Phys. Rev. Lett. 65 ( 1990 ) 1733. [3] H. 1Dberal, Electron scattering from complex nuclei (Academic Press, New York, 1971 ). [4] H. Arenh~vel, Few-Body Syst. 4 (1988) 55. [ 5 ] M.I. Levchuk, Institute of Physics preprint No 609 (Minsk, 1990). [ 6] K.-M. Schmitt and H. Arenh6vel, Few-Body Syst. 7 ( 1989 ) 95. [7] A.Yu. Korchin, Yu.P. Mel'nik and A.V. Shebeko, Sov. J. Nucl. Phys. 48 ( 1988 ) 387; and private communication. [8] A.V. Evstigneev, S.G. Popov and D.K. Toporkov, Nucl. Instrum. Methods A 238 (1985) 12. [ 9 ] E.R. Kinney, Nucl. Instrum. Methods, to be published. [ 10] R.J. Holt, in: Proc. Workshop on Polarized targets in storage rings (Argonne, IL, 1984), ed. R.J. Holt (ANL report No. 84-50) p. 103. [ 11 ] B.B. Woitsekhowskiet al., Novosibirsk preprint INP 88-120 ( 1988 ); Nucl. Instrum. Methods, to be published. [ 12 ] V.F. Dmitriev, private communication.