- Email: [email protected]
Measurement of the polarization of the Λ0 in the reaction γ + p → K+ + Λ0 in the resonance region
Nuclear Physics B137 (1978) 261-268 © North-Holland Publishing Company
MEASUREMENT OF THE POLARIZATION OF THE A° IN THE REACTION 7 + P ~ K+ + A° IN THE RESONANCE REGION R. HAAS * T. MICZA1KA, U. OPARA ** K. QUABACH and W.J. SCHWILLE Physikalisches Institut der Universitiit Bonn, W. Germany Received 2 August 1977 (Revised 1 February 1978)
At the Bonn 2.5 GeV synchrotron the polarization of the A0 was measured at 40 ° and 90° for three energies. The kaon was detected with a strong focussing magnetic spectrometer and separated from other particles with the help of a differential liquid (~erenkov counter. The polarization was determined by means of the angular distribution of the decay proton which was measured with a combination of sonic spark chambers and a scintillation counter hodoscope. The typical statistical errors are about 13%. The systematic errors add up to 8%.
1. Introduction The latest comprehensive review of kaon photoproduction in the resonance region was given at the Electron-Photon Symposium in Bonn in 1973 [1,2]. Since then no new data have been published. So the available data set is still lamentably small, consisting of 141 differential cross-section measurements and 20 A ° polarization points. Accordingly, the work on phenomenological theories has not been stim. ulated much in recent years. The cross-section measurements at small and medium angles show a rather smooth dependence on energy and angle, whereas the data of the backward hemisphere have a considerable energy-dependent structure and thus indicate the influence of resonances. The recoil polarization measurements are concentrated around 1.1 GeV at 90cm and show large negative polarizations of up to 40% [3]. Both data sets are not sufficient to provide clarification on the resonance mechanism in K ÷ photoproduction. This, of course, is mainly due to the lack of experimental data. In addition, the large amount of non-resonant background in K°A ° photoproduction increases the uncertainties in the resonance part of the amplitude. * Now at T0V Essen, W. Germany. ** Now at Gesamthochschule Wuppertal, W. Germany. 261
262
tL Haas et al. / Polarization o f the A 0
Data on hadronically produced reactions that suggest two resonant s- and p-waves near threshold (leading to an appreciable amount of polarization) do not alleviate these problems substantially [4]. Stronger restrictions can be made by additional measurements of the recoil polar ization, especially in an extended energy range, and by measurements of the target asymmetry [5]. In this paper we describe our measurements of the recoil polarization.
2. Set-up of the experiment The experimental set-up is shown in fig. 1. A strong focussing magnetic spectrometer accepted the kaons in a solid angle of 0.54 msr with a momentum resolution of-+ 1.25%. The kaons were detected by four scintillation counters S 1, Sz, $3, $4, and a differential liquid (~erenkov counter. For further details see ref. [6]. The reaction 7p -+ K+A° is separated kinematically from other reactions, e.g. ~o production, by proper setting of the maximum energy of the bremsstrahlungsspectrum. The spin distribution of the A ° is analysed with the help of its parity non-con-
BH
BeQmhardener
QuanIa
Fig. 1. Plan of the experimental set up for this experiment. Long branch: K+ detector. Short branch: A0 polarization detection system.
K Haas et al./Polarization of the A0
263
+A
+FI -TA Production target LC Lead collimator H1,2 Hodoscopes SC Spark chamber T Telescope / / / / Lead shielding
Fig. 2. Side view of A0 branch.
serving weak decay A ° ~ pTr-, which results in an asymmetric angular distribution of the decay proton if the A ° is polarized. The angular distribution of the decay protons in the rest frame of the A ° is given by N(O) d ~ = 4-~(1 + aP cos 0) d[2,
where 0 is the angle between k~ X qK and the direction of proton emission, t~ = 0.645 -+ 0.017 [7], P = polarization of the A °, dI2 = solid angle of the decay proton. The decay distribution was measured by a set of eight sonic spark chambers and two hodoscope planes (fig. 2). Together with the K + branch the hodoscopes served to detect the decay protons in coincidence. In order to obtain an acceptable correlation between the scintillation counters and the tracks in the spark chambers the hodoscopes were arranged in a 3 × 4 matrix.
3. Experimental procedure To accept the full proton decay cone, a large solid angle of the A ° branch is required. Together with the lack of momentum separation this leads to substantial contamination from electromagnetic background. Therefore, special care was taken to keep this contamination as small as possible. Besides normal precautions such as installation of a vacuum tube for the ")'beam, a sweeping magnet, and shieldings, two special methods were employed.
264
R. Haas et al. / Polarization o f the A 0
(i) A beam hardener of LiH and 0.7 radiation length was placed in front of the sweeping magnet to reduce the number of low-energy photons. This resulted in a factor of ~0.5 in the background rate [8]. (ii) The spark chambers were operated in such a way as to reject minimum ionizing particles and to detect only the slow protons from the A ° decay. This was achieved by special adjustment of the high voltage and the clearing field together with a very constant flow of propanol-contaminated Ne-He gas. The fast electronics was conventional. The spark chamber data were extracted via capacitive microphones, amplified, digitized and subsequently recorded on magnetic tape together with pulse height and time-of-flight information. A PDP-9 computer was used on-line to monitor the performance of the apparatus.
4. Data reduction
In the first step of the off-line analysis, the time-of-flight and the pulse height information of the (~erenkov counter in the K+ spectrometer were used to eliminate all concurrent processes to 7 + P ~ K÷ + A° and produce a clean sample of K÷ events. We then tried to reconstruct for each accepted kaon the corresponding track of the decay proton in the A ° branch. This track was determined by using the spark chamber data and had to hit the appropriate counter hodoscope element. To avoid efficiency losses, only rather weak conditions were required for a track at this stage of data reduction. Despite this, the nOn decay channel even with contaminations from electromagnetic background was nearly completely eliminated due to the high redundancy within the spark chamber stack together with the two hodoscope planes. The rejection efficiency was determined to be better than 98%. The mean K÷ coincidence rate for the charged decay channel was 0.63, which is in very good agreement with the published branching ratio. In this prr- channel, simultaneous random triggers in the hodoscope elements caused additional problems in as far as they led to ambiguities in the track reconstruction. Most often, the spark chamber data was sufficient to determine the true track. However, in 20% of the data, when additional electromagnetic background had led to equivocal coordinates in the spark chamber, more than one track had to be considered in the ensuing analysis. In these cases, a probability for each track was calculated, depending on its X2 and the number of responding microphones, and the track with the highest probability was taken. This method was supported by a Monte Carlo calculation. All effects, such as the resolution of the spectrometer, the decay in flight of the A ° and Coulomb scattering were incorporated. This led to track distributions which were compared to the measured ones. As an example in fig. 3 shows, a good agreement was obtained. To avoid a higher than quoted rate of multiple track events, the machine intensity was limited to 2.101° effective quanta/min and thus the systematic error was kept at less than +0.05.
R. Haas et aL / Polarization of the A 0
RADIAL
DISTRIBUTION Solid
curve
= Monte
265
OF T H E DECAY PROTONS Carlo
calculation
,N
2C
•
i
•
10"
!
, ~:::' ,
; ==
,,
=
=
!::'- e 50
10 °
15 °
FiB. 3. Radial distribution of the decay protons, e is the decay angle of the proton direction. This distribution is independent of the polarization.
persus
the A O
X X X
X X
DECAY DISTRIBUTION OF THE kO Solid
curve
= I~ximum
likelyhood
of
~vX~x
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f
fit
Ky = 1.3 GeV
IDire~ti~n
X X X x ×
X X X X X X X
........
X X X X,' ~ ~ X X × X
X X Y X X XX,'~X)
,KX .': ~: X :! X :,: X ;': ) :CX'.,; X .~X ~'~ ,XX ) KXXXXY.3CXXX XXXXX.x.7,X)CXX X KX>CXXXX~,YXXXX~XXX\XXXX k X).'X X~ ~,C~'y ~CXXX~
~X xXXX,~,X)! x N~-v.K >':x X X)U':% ' XX ', Xx X XXXXX XXX S):XXXXX',X)): R'KXX X XX X x k X k ~: X X X x ~ 2:', ~ k x x :{ X'~ -~
z I
- -
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• : xxx>
• ..
-..:..: :".:,. !.:.:i' ... • -.. :....:, ,:.:..:.:
~ x x ~ < x x x x ~ x x x ~ ~ x ~ x ~ x x x x x > x x x x x x ~ x x x x x ~ x x x x ~ > xx-
xx
!.. . :.:'.."l,
x x x x ;..,x x x x x ;.
>-...< x x x :< x x >.• ..< >- ~ x x ~.
x x x w x x x x x x x x x x x ~ x ~ ,
.......'.';: -....., ..: :.'..:.:...:-:.
: ~ x x x x x x x . : < x : < ~ x x x x x x x ~ x x w x~
w
:...;. v.:..;..-.-.: :.._ - •
x x x x x x x x x x ~ x x ~ , x ~ x x x ~ x ~ x x
x x x x x x x : ~ x ~ : • " x}-:xxxxxxxx:
• ..... , . . ,
..:..,.
.....L...-.L.....t.......t......r......-*
eprc (up-down)
Fig. 4. D e c a y distribution o f the A 0 with projections.
eproj (right-teft)
R. Haas et al. /Polarization o f the A 0
266
In a last step both the Monte Carlo and the measured track distributions (fig. 4) were used to determine the polarization value in a maximum-likelihood fit.
5. Results The measured data points are listed in table 1 and shown in figs. 5 and 6.
Table 1 3' energy
0 c'm" K+
Polarization
Statistical error
Systematic error
1.3 1.5 1.7 1.7
40 ° 40 ° 40 ° 90 °
-0.36 -0.07 -0.05 +0.05
0.12 0.13 0.13 0.20
0.10 0.08 0.08 0.08
GeV GeV GeV GeV
/./ .r
PA*
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J" J JJ'l I II
J_ -
. Fujii(Tokyo) ~
~g~(F~.oti>
o Thom(Cornell)
• Voget(D~y)
• This experiment
Fig. 5. Existing data of A 0 polarization at 0 = 90 °. The theoretical curves 4, 6b, 6c are the best solutions of the phase-shift analysis done by Schorsch et al. [9].
R. Haas et al. / Polarization of the A0
P~°
,,o
,2
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267
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"
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/ 1/-7
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-0,3-q~-0,5-
Ao POLARIZAT[O N
0=40 °
.-~ Schorsch et al.
Fig. 6. Existing data of A0 polarization at 0 = 40 °.
6. E ~ o ~ Besides the statistical errors, which are typically +0.12, there are Systematic errors to be quoted. Some mesons from the A ° decay are accepted in the detector and thus lead to ambiguities, they should also be rejected because they are minimum ionizing particles. We quote a maximum systematic error of -+0.03P. (P = polarization.) Part of the A ° may be produced off heavy nuclei in the target walls. Due to the smallness of this effect no measurement has been done. We quote +0.03P as the maximum systematic error for this effect. The uncertainty in the weak decay parameter a results in a systematic error of +0.017P. Errors due to a misalignment of the experiment or to asymmetries in the 3, beam are computed to be maximally +0.05. Ambiguities in the data reduction are estimated to be of the order of +0.05. The root mean errors are quoted in table 1. 7. Conclusions The measured data together with the Tokyo points [3] give a 3 standard deviation decrease in the absolute value of the polarization between k~/= 1.2 and k~ = 1.7 GeV. In the phase-shift analysis of Schorch et al. [9] our data favour the solution with a resonant D13(1735). This resonant phase is also found in a newer analysis by Deans et al. [9] of KA ° photoproduction and also in the analysis o f Baker et al. [10] and Baton et al. [11 ] of the rr-p ~ K°A ° reaction. Our measurements as well as new results for the target asymmetry [5] give additional evidence. We wish to thank Prof. K.H. Althoff for constant encouragement and support. Dr. D. Husmann and the synchrotron crew made this experiment possible by their excellent operation of the machine.
268
R. Haas et al. / Polarization o f the A 0
References [1] H.M. Fischer, Proc. Int. Symp. on electron and photon interactions at high energies, 1973, p. 77. [2] G. von Gehlen, Proc. Int. Symp. on electron and photon interactions at high energies, 1973, p. 117. [3] B. Borgia et al., Nuovo Cim. 38 (1965) 146; H. Thorn, Phys. Rev. 151 (1966) 1322; D.E. Groom et al., Phys. Rev. 159 (1967) 1213; T. Fujji et al., Phys. Rev. D3 (1970) 439; G. Vogel et al., Phys. Lett. 40B (1972) 513. [4] S. Orito et al., Nuovo Cim. Lett. 18 (1969) 936; [51 K.H. Althoff et al., Nucl. Phys. B 137 (1978) 269. [6] A. Bleckmann et al., Z. Phys. 239 (1970) 1. [7] O.E. Overseth, Phys. Rev. Lett. 19 (1967) 391. [8] E.L. Hart et al., Rev. Sci. Instr. 31 (1960) 1. [91 W. Schorsch et al., NucL Phys. B25 (1970) 179; S.R. Deans et al., Phys. Rev. D6 (1972) 1906. [10] R.D. Baker et al., RL-76-060 Rutherford Lab. (1976). [11] J.P. Baton et al., Abstract No. 38, Topical Conf. on Baryon Resonances, Oxford, 1976.
MEASUREMENT OF THE POLARIZATION OF THE A° IN THE REACTION 7 + P ~ K+ + A° IN THE RESONANCE REGION R. HAAS * T. MICZA1KA, U. OPARA ** K. QUABACH and W.J. SCHWILLE Physikalisches Institut der Universitiit Bonn, W. Germany Received 2 August 1977 (Revised 1 February 1978)
At the Bonn 2.5 GeV synchrotron the polarization of the A0 was measured at 40 ° and 90° for three energies. The kaon was detected with a strong focussing magnetic spectrometer and separated from other particles with the help of a differential liquid (~erenkov counter. The polarization was determined by means of the angular distribution of the decay proton which was measured with a combination of sonic spark chambers and a scintillation counter hodoscope. The typical statistical errors are about 13%. The systematic errors add up to 8%.
1. Introduction The latest comprehensive review of kaon photoproduction in the resonance region was given at the Electron-Photon Symposium in Bonn in 1973 [1,2]. Since then no new data have been published. So the available data set is still lamentably small, consisting of 141 differential cross-section measurements and 20 A ° polarization points. Accordingly, the work on phenomenological theories has not been stim. ulated much in recent years. The cross-section measurements at small and medium angles show a rather smooth dependence on energy and angle, whereas the data of the backward hemisphere have a considerable energy-dependent structure and thus indicate the influence of resonances. The recoil polarization measurements are concentrated around 1.1 GeV at 90cm and show large negative polarizations of up to 40% [3]. Both data sets are not sufficient to provide clarification on the resonance mechanism in K ÷ photoproduction. This, of course, is mainly due to the lack of experimental data. In addition, the large amount of non-resonant background in K°A ° photoproduction increases the uncertainties in the resonance part of the amplitude. * Now at T0V Essen, W. Germany. ** Now at Gesamthochschule Wuppertal, W. Germany. 261
262
tL Haas et al. / Polarization o f the A 0
Data on hadronically produced reactions that suggest two resonant s- and p-waves near threshold (leading to an appreciable amount of polarization) do not alleviate these problems substantially [4]. Stronger restrictions can be made by additional measurements of the recoil polar ization, especially in an extended energy range, and by measurements of the target asymmetry [5]. In this paper we describe our measurements of the recoil polarization.
2. Set-up of the experiment The experimental set-up is shown in fig. 1. A strong focussing magnetic spectrometer accepted the kaons in a solid angle of 0.54 msr with a momentum resolution of-+ 1.25%. The kaons were detected by four scintillation counters S 1, Sz, $3, $4, and a differential liquid (~erenkov counter. For further details see ref. [6]. The reaction 7p -+ K+A° is separated kinematically from other reactions, e.g. ~o production, by proper setting of the maximum energy of the bremsstrahlungsspectrum. The spin distribution of the A ° is analysed with the help of its parity non-con-
BH
BeQmhardener
QuanIa
Fig. 1. Plan of the experimental set up for this experiment. Long branch: K+ detector. Short branch: A0 polarization detection system.
K Haas et al./Polarization of the A0
263
+A
+FI -TA Production target LC Lead collimator H1,2 Hodoscopes SC Spark chamber T Telescope / / / / Lead shielding
Fig. 2. Side view of A0 branch.
serving weak decay A ° ~ pTr-, which results in an asymmetric angular distribution of the decay proton if the A ° is polarized. The angular distribution of the decay protons in the rest frame of the A ° is given by N(O) d ~ = 4-~(1 + aP cos 0) d[2,
where 0 is the angle between k~ X qK and the direction of proton emission, t~ = 0.645 -+ 0.017 [7], P = polarization of the A °, dI2 = solid angle of the decay proton. The decay distribution was measured by a set of eight sonic spark chambers and two hodoscope planes (fig. 2). Together with the K + branch the hodoscopes served to detect the decay protons in coincidence. In order to obtain an acceptable correlation between the scintillation counters and the tracks in the spark chambers the hodoscopes were arranged in a 3 × 4 matrix.
3. Experimental procedure To accept the full proton decay cone, a large solid angle of the A ° branch is required. Together with the lack of momentum separation this leads to substantial contamination from electromagnetic background. Therefore, special care was taken to keep this contamination as small as possible. Besides normal precautions such as installation of a vacuum tube for the ")'beam, a sweeping magnet, and shieldings, two special methods were employed.
264
R. Haas et al. / Polarization o f the A 0
(i) A beam hardener of LiH and 0.7 radiation length was placed in front of the sweeping magnet to reduce the number of low-energy photons. This resulted in a factor of ~0.5 in the background rate [8]. (ii) The spark chambers were operated in such a way as to reject minimum ionizing particles and to detect only the slow protons from the A ° decay. This was achieved by special adjustment of the high voltage and the clearing field together with a very constant flow of propanol-contaminated Ne-He gas. The fast electronics was conventional. The spark chamber data were extracted via capacitive microphones, amplified, digitized and subsequently recorded on magnetic tape together with pulse height and time-of-flight information. A PDP-9 computer was used on-line to monitor the performance of the apparatus.
4. Data reduction
In the first step of the off-line analysis, the time-of-flight and the pulse height information of the (~erenkov counter in the K+ spectrometer were used to eliminate all concurrent processes to 7 + P ~ K÷ + A° and produce a clean sample of K÷ events. We then tried to reconstruct for each accepted kaon the corresponding track of the decay proton in the A ° branch. This track was determined by using the spark chamber data and had to hit the appropriate counter hodoscope element. To avoid efficiency losses, only rather weak conditions were required for a track at this stage of data reduction. Despite this, the nOn decay channel even with contaminations from electromagnetic background was nearly completely eliminated due to the high redundancy within the spark chamber stack together with the two hodoscope planes. The rejection efficiency was determined to be better than 98%. The mean K÷ coincidence rate for the charged decay channel was 0.63, which is in very good agreement with the published branching ratio. In this prr- channel, simultaneous random triggers in the hodoscope elements caused additional problems in as far as they led to ambiguities in the track reconstruction. Most often, the spark chamber data was sufficient to determine the true track. However, in 20% of the data, when additional electromagnetic background had led to equivocal coordinates in the spark chamber, more than one track had to be considered in the ensuing analysis. In these cases, a probability for each track was calculated, depending on its X2 and the number of responding microphones, and the track with the highest probability was taken. This method was supported by a Monte Carlo calculation. All effects, such as the resolution of the spectrometer, the decay in flight of the A ° and Coulomb scattering were incorporated. This led to track distributions which were compared to the measured ones. As an example in fig. 3 shows, a good agreement was obtained. To avoid a higher than quoted rate of multiple track events, the machine intensity was limited to 2.101° effective quanta/min and thus the systematic error was kept at less than +0.05.
R. Haas et aL / Polarization of the A 0
RADIAL
DISTRIBUTION Solid
curve
= Monte
265
OF T H E DECAY PROTONS Carlo
calculation
,N
2C
•
i
•
10"
!
, ~:::' ,
; ==
,,
=
=
!::'- e 50
10 °
15 °
FiB. 3. Radial distribution of the decay protons, e is the decay angle of the proton direction. This distribution is independent of the polarization.
persus
the A O
X X X
X X
DECAY DISTRIBUTION OF THE kO Solid
curve
= I~ximum
likelyhood
of
~vX~x
. 10 •
X
X x X X X X Xx x x XX x X xxxx ).X
XX):XX X X X X X X XX>, :% X X X XX X ~::.CX X I X X>:XX:,:X×XXXXX×>X×I r×x X y): %:,::x X X '4 X X X ),: X X X) X X Y X ~:Y X.v x X X X !(X ): X ::): x ) XX X XXT," X)~ ;~ :'(X X X X :.: X X x X XXX> :
f
fit
Ky = 1.3 GeV
IDire~ti~n
X X X x ×
X X X X X X X
........
X X X X,' ~ ~ X X × X
X X Y X X XX,'~X)
,KX .': ~: X :! X :,: X ;': ) :CX'.,; X .~X ~'~ ,XX ) KXXXXY.3CXXX XXXXX.x.7,X)CXX X KX>CXXXX~,YXXXX~XXX\XXXX k X).'X X~ ~,C~'y ~CXXX~
~X xXXX,~,X)! x N~-v.K >':x X X)U':% ' XX ', Xx X XXXXX XXX S):XXXXX',X)): R'KXX X XX X x k X k ~: X X X x ~ 2:', ~ k x x :{ X'~ -~
z I
- -
xx~(
• : xxx>
• ..
-..:..: :".:,. !.:.:i' ... • -.. :....:, ,:.:..:.:
~ x x ~ < x x x x ~ x x x ~ ~ x ~ x ~ x x x x x > x x x x x x ~ x x x x x ~ x x x x ~ > xx-
xx
!.. . :.:'.."l,
x x x x ;..,x x x x x ;.
>-...< x x x :< x x >.• ..< >- ~ x x ~.
x x x w x x x x x x x x x x x ~ x ~ ,
.......'.';: -....., ..: :.'..:.:...:-:.
: ~ x x x x x x x . : < x : < ~ x x x x x x x ~ x x w x~
w
:...;. v.:..;..-.-.: :.._ - •
x x x x x x x x x x ~ x x ~ , x ~ x x x ~ x ~ x x
x x x x x x x : ~ x ~ : • " x}-:xxxxxxxx:
• ..... , . . ,
..:..,.
.....L...-.L.....t.......t......r......-*
eprc (up-down)
Fig. 4. D e c a y distribution o f the A 0 with projections.
eproj (right-teft)
R. Haas et al. /Polarization o f the A 0
266
In a last step both the Monte Carlo and the measured track distributions (fig. 4) were used to determine the polarization value in a maximum-likelihood fit.
5. Results The measured data points are listed in table 1 and shown in figs. 5 and 6.
Table 1 3' energy
0 c'm" K+
Polarization
Statistical error
Systematic error
1.3 1.5 1.7 1.7
40 ° 40 ° 40 ° 90 °
-0.36 -0.07 -0.05 +0.05
0.12 0.13 0.13 0.20
0.10 0.08 0.08 0.08
GeV GeV GeV GeV
/./ .r
PA*
*0.3-
*0,2- IKrXqK'I
6C./S///
*0.1-
/t..//6b ~.o
~
'
[
~,~
/~,,~
~
~,~
T
~o ~ ic,. l
-0.1-
I
-0.2~ -(~3~
I~l.~
I~ lit &~
-0't't -0,5
J" J JJ'l I II
J_ -
. Fujii(Tokyo) ~
~g~(F~.oti>
o Thom(Cornell)
• Voget(D~y)
• This experiment
Fig. 5. Existing data of A 0 polarization at 0 = 90 °. The theoretical curves 4, 6b, 6c are the best solutions of the phase-shift analysis done by Schorsch et al. [9].
R. Haas et al. / Polarization of the A0
P~°
,,o
,2
~$
1
,6
l
267
~8
%0 Ky
-(
\!, 9,
"
r/
/ 1/-7
£-6c
-0,3-q~-0,5-
Ao POLARIZAT[O N
0=40 °
.-~ Schorsch et al.
Fig. 6. Existing data of A0 polarization at 0 = 40 °.
6. E ~ o ~ Besides the statistical errors, which are typically +0.12, there are Systematic errors to be quoted. Some mesons from the A ° decay are accepted in the detector and thus lead to ambiguities, they should also be rejected because they are minimum ionizing particles. We quote a maximum systematic error of -+0.03P. (P = polarization.) Part of the A ° may be produced off heavy nuclei in the target walls. Due to the smallness of this effect no measurement has been done. We quote +0.03P as the maximum systematic error for this effect. The uncertainty in the weak decay parameter a results in a systematic error of +0.017P. Errors due to a misalignment of the experiment or to asymmetries in the 3, beam are computed to be maximally +0.05. Ambiguities in the data reduction are estimated to be of the order of +0.05. The root mean errors are quoted in table 1. 7. Conclusions The measured data together with the Tokyo points [3] give a 3 standard deviation decrease in the absolute value of the polarization between k~/= 1.2 and k~ = 1.7 GeV. In the phase-shift analysis of Schorch et al. [9] our data favour the solution with a resonant D13(1735). This resonant phase is also found in a newer analysis by Deans et al. [9] of KA ° photoproduction and also in the analysis o f Baker et al. [10] and Baton et al. [11 ] of the rr-p ~ K°A ° reaction. Our measurements as well as new results for the target asymmetry [5] give additional evidence. We wish to thank Prof. K.H. Althoff for constant encouragement and support. Dr. D. Husmann and the synchrotron crew made this experiment possible by their excellent operation of the machine.
268
R. Haas et al. / Polarization o f the A 0
References [1] H.M. Fischer, Proc. Int. Symp. on electron and photon interactions at high energies, 1973, p. 77. [2] G. von Gehlen, Proc. Int. Symp. on electron and photon interactions at high energies, 1973, p. 117. [3] B. Borgia et al., Nuovo Cim. 38 (1965) 146; H. Thorn, Phys. Rev. 151 (1966) 1322; D.E. Groom et al., Phys. Rev. 159 (1967) 1213; T. Fujji et al., Phys. Rev. D3 (1970) 439; G. Vogel et al., Phys. Lett. 40B (1972) 513. [4] S. Orito et al., Nuovo Cim. Lett. 18 (1969) 936; [51 K.H. Althoff et al., Nucl. Phys. B 137 (1978) 269. [6] A. Bleckmann et al., Z. Phys. 239 (1970) 1. [7] O.E. Overseth, Phys. Rev. Lett. 19 (1967) 391. [8] E.L. Hart et al., Rev. Sci. Instr. 31 (1960) 1. [91 W. Schorsch et al., NucL Phys. B25 (1970) 179; S.R. Deans et al., Phys. Rev. D6 (1972) 1906. [10] R.D. Baker et al., RL-76-060 Rutherford Lab. (1976). [11] J.P. Baton et al., Abstract No. 38, Topical Conf. on Baryon Resonances, Oxford, 1976.