recoil polarization in k+ Λ photoproduction at 5 gev

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Volume 40B, number 4 PHYSICS LETTERS 24 July 1972 RECOIL POLARIZATION IN K+A PHOTOPRODUCTION AT 5 GeV G. VOGEL, H. BURFEINDT, G. BUSCHHORN, P. HEIDE, U. KOTZ, K.-H. MESS, P. SCHMOSER, B. SONNE and B.H. WI1K Deutsches Elektronen-Synchrotron DESY and II. Institut fiir Experimentalphysik der Universitdt Hamburg, Germany Received 1 June 1972 The A polarization in the reaction 7P ~ K÷A has been measured using the decay A ~ prr- as an analyzer. It is found to be large and negative in the momentum transfer range 0.2 ~<Itl ~< 1.0 (GeV/c) 2. While pion photoproduction has been extensively studied using both unpolarized and polarized beams as well as polarized targets, the experimental informa- tion on kaon photoproduction for energies above the resonance region is rather scarce. In addition to dif- ferential cross section measurements [1], only a few data points on the polarized target asymmetry for the sum of K+A and K +y~° photoproduction [2] are available. In this paper we report the results of a measurement of the A polarization in the reaction 7P -* K+A, using the parity-violating decay A -~ prr- as an analyzer. For an unpolarized beam and unpolarized target, parity conservation requires the A polarization vector to be perpendicular to the reaction plane, defined by the normalp7 X pK/lp3, X pK I (Basel convention [3] ). In the A rest frame, the angular distribution of the decay protons is given by 1/2(1 +aP cos6), where 6 is the angle between the proton momentum vector and the normal to the reaction plane, P the degree of polarization of the A hyperons, and c~ = 0.645 -+ 0.017 [4] the asymmetry parameter. From this ex- pression, the polarization P is given in terms of the up-down asymmetry as P : (2/oO(U-D)/(U+D) . (1) Here U (D) is the number of protons with positive (negative) momentum component with respect to the normal to the reaction plane. The experiment was performed at the DESY elec- tron accelerator and data were taken at an incident photon energy of 5 GeV and for values of the four- momentum transfer t between -0.2 and - 1.0 (GeV/c) 2. The experimental layout is shown in fig. 1. A well col- limated bremsstrahlung beam with a typical intensity of 1.5 × 1010 equivalent quanta per second was passed through an 11.2 cm long liquid hydrogen target and monitored with a quantameter. The produced K ÷ mesons were detected with a focussing magnetic spectrometer. The particles were identified using three threshold Cerenkov counters set to respond to positrons, pions and kaons. Three hodoscopes, located at appropriate focal planes of the spectrometer, meas- ured the particle momentum p, horizontal production angle 0 and vertical angle ~ with an accuracy of Ap/p = 0.4%, A0 = 3 mrad, A~b = 1.2 mrad (FWHM). A hodoscope at the vertical target image plane measured the vertical coordinate z of the events at the hydrogen target. Protons from the A ~ pn- decay were detected in a scintillation counter system 60 cm wide and 70 cm high. The distance from the target varied between 1.5 m and 2 m, depending on the t value. In the verti- cal direction, the setup was subdivided into three counters: a central counter of 20 cm height placed symmetrically with respect to the spectrometer mid- plane, and a top and a bottom counter, each 25 cm high. A hodoscope consisting of 15 horizontal counters was placed behind the central counter. This made it possible to correct for the tilt of the reaction plane around the beam axis. Except at the smallest momentum transfers of -0.2 and -0.3 (GeV/c) 2, the solid angle of the recoil detector was sufficient to cover the whole decay cone for the protons. In addition, between 6% and 10% of the decay n mesons were also accepted. The appara- tus could be rotated around a horizontal axis at beam height. By frequently interchanging "top" and "bottom", nonsymmetric response of the counter system and the following electronics was cancelled. 513

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Page 1: Recoil polarization in K+ Λ photoproduction at 5 GeV

Volume 40B, number 4 PHYSICS LETTERS 24 J u l y 1972

R E C O I L P O L A R I Z A T I O N IN K + A P H O T O P R O D U C T I O N A T 5 G e V

G. VOGEL, H. BURFEINDT, G. BUSCHHORN, P. HEIDE, U. KOTZ, K.-H. MESS, P. SCHMOSER, B. SONNE and B.H. WI1K

Deutsches Elektronen-Synchrotron DESY and II. Institut fiir Experimentalphysik der Universitdt Hamburg, Germany

Received 1 June 1972

The A polarization in the reaction 7P ~ K÷A has been measured using the decay A ~ prr- as an analyzer. It is found to be large and negative in the momentum transfer range 0.2 ~< Itl ~< 1.0 (GeV/c) 2.

While pion photoproduction has been extensively studied using both unpolarized and polarized beams as well as polarized targets, the experimental informa- tion on kaon photoproduction for energies above the resonance region is rather scarce. In addition to dif- ferential cross section measurements [1], only a few data points on the polarized target asymmetry for the sum of K+A and K +y~° photoproduct ion [2] are available.

In this paper we report the results of a measurement of the A polarization in the reaction 7P -* K+A, using the parity-violating decay A -~ prr- as an analyzer. For an unpolarized beam and unpolarized target, parity conservation requires the A polarization vector to be perpendicular to the reaction plane, defined by the normalp7 X pK/lp3, X pK I (Basel convention [3] ). In the A rest frame, the angular distribution of the decay protons is given by 1/2(1 +aP cos6), where 6 is the angle between the proton momentum vector and the normal to the reaction plane, P the degree of polarization of the A hyperons, and c~ = 0.645 -+ 0.017 [4] the asymmetry parameter. From this ex- pression, the polarization P is given in terms of the up-down asymmetry as

P : (2/oO(U-D)/(U+D) . (1)

Here U (D) is the number of protons with positive (negative) momentum component with respect to the normal to the reaction plane.

The experiment was performed at the DESY elec- tron accelerator and data were taken at an incident photon energy of 5 GeV and for values of the four- momentum transfer t between - 0 . 2 and - 1.0 (GeV/c) 2. The experimental layout is shown in fig. 1. A well col- limated bremsstrahlung beam with a typical intensity

of 1.5 × 1010 equivalent quanta per second was passed through an 11.2 cm long liquid hydrogen target and monitored with a quantameter. The produced K ÷ mesons were detected with a focussing magnetic spectrometer. The particles were identified using three threshold Cerenkov counters set to respond to positrons, pions and kaons. Three hodoscopes, located at appropriate focal planes of the spectrometer, meas- ured the particle momentum p, horizontal production angle 0 and vertical angle ~ with an accuracy of Ap/p = 0.4%, A0 = 3 mrad, A~b = 1.2 mrad (FWHM). A hodoscope at the vertical target image plane measured the vertical coordinate z of the events at the hydrogen target.

Protons from the A ~ p n - decay were detected in a scintillation counter system 60 cm wide and 70 cm high. The distance from the target varied between 1.5 m and 2 m, depending on the t value. In the verti- cal direction, the setup was subdivided into three counters: a central counter of 20 cm height placed symmetrically with respect to the spectrometer mid- plane, and a top and a bot tom counter, each 25 cm high. A hodoscope consisting of 15 horizontal counters was placed behind the central counter. This made it possible to correct for the tilt of the reaction plane around the beam axis.

Except at the smallest momentum transfers of - 0 . 2 and - 0 . 3 (GeV/c) 2, the solid angle of the recoil detector was sufficient to cover the whole decay cone for the protons. In addition, between 6% and 10% of the decay n mesons were also accepted. The appara- tus could be rotated around a horizontal axis at beam height. By frequently interchanging " top" and "bo t tom" , nonsymmetric response of the counter system and the following electronics was cancelled.

513

Page 2: Recoil polarization in K+ Λ photoproduction at 5 GeV

Volume 40B, number 4 PHYSICS LETTERS 24 July 1972

• ~ beam"

oSCaL m12 • to quantameter

~ recoil arm

b

Fig. 1. (a) Layout of the experiment. T hydrogen target; M bending magnets; Q quadrupoles; S scintillation counters; Ce, Crr , C K electron, pion, kaon ~erenkov counters; q~, 0, z, p hodoscopes. (b) Recoil counter system as seen from the target (schematically).

An equal number of runs was taken with counter R1 at the top and with counter R3 at the top.

The phototubes on the three large counters were gain- stabilized and adjusted such that the counters were close to 100% efficient for minimum ionizing particles. Lucite plates, between 1 and 3 cm thick, were placed in front of the counters to absorb the low energy back- ground due to electromagnetic processes. This was necessary in order to reduce the instantaneous singles rate in any of the three counters to a tolerable level of 2 - 4 Mc/s. Roughly 90% of this rate was due to beam interactions in the liquid hydrogen.

After empty-target subtraction, about 60% of the K + triggers were accompanied by a coincident proton as compared to an expected fraction o f 65% from the charged decay mode A ~ pTr-. Losses were due to dead time in the recoil counters (about 4%) and ab- sorption of the decay protons (2 -4%) . These losses are unlikely to distort the analyzing power of the ap- paratus.

The time-of-flight and pulse height of the decay protons were measured. At small momentum transfers, a pulse height cut was quite effective in separating the protons from minimum ionizing background particles as well as from A-decay pions. The time-of-flight spectra of the three recoil counters showed pro- nounced peaks from K+p coincidences on a 2 - 5 % background due to accidentals.

Great care was taken to eliminate any artificial up- down asymmetries in the apparatus. The relative verti- cal alignment of the kaon spectrometer and the recoil arm was bet ter than 0.3 ram. The recoil counter setup was frequently rotated to cancel different response of

the counters or electronics. The vertical alignment of the photon beam was controlled by taking beam pho- tographs, and its vertical intensi ty distr ibution was continuously monitored by means of the hodoscope at the vertical target image plane. The pho ton beam was about 1 cm high at the hydrogen target but was measured to be symmetric with respect to the spec- trometer mid-plane within +-0.25 mm. As an overall check on the symmetry of the apparatus, coincidence measurements of the reaction 3'd ~ r r -p(ps) were per- formed. Due to the Fermi mot ion in deuterium, the recoil protons have sizable momentum components perpendicular to the reaction plane. At t = - 0 . 2 (GeV/c) 2, the vertical angular distr ibution of these protons has about the same shape as that of the protons from A decay, but is of course up-down sym- metric. The measured up-down asymmetry

A.rd~r-p(ps ) = - 0 . 0 1 -+ 0.02 is in fact consistent with no artificial asymmetry in the apparatus.

The data collection and analysis was as follows. For each K + trigger, the contents of all hodoscopes as well as pulse height and time-of-flight information of the three recoil counters were read into a PDP-8 com- puter. From the K + momentum and angle the missing mass was calculated, assuming that the photon energy k was equal to the maximum bremsstrahlung energy kma x. The missing mass spectra obtained at t = - 0 . 4 and - 0 . 8 (GeV/c) 2 are shown in fig. 2. They show a step at the A mass, followed by a continuous spec- trum towards higher masses which is due to K+A photoproduct ion by photons with energies below

514

Page 3: Recoil polarization in K+ Λ photoproduction at 5 GeV

Volume 40B, number 4 PHYSICS LETTERS 24 July 1972

a) t = -04 (GeVlc) 2 I ~ b) t = -Q8 (GeV/c) 2

J , , t2 ~L, ~'.6 1.2 t4 1.6

MZ [(GeV/c2)2]

Fig. 2. Missing-mass-squared spectra for 3' + p ~ K + + hyperon. Also shown are fits to the A and Z ° steps using the spectrom- eter resolution as determined from measurements of the re-

action 3"p ~ 7r+n.

kma x. Superimposed is a second step from the process

7P ~ K + 2 ° . From the measured angles 0 and ¢ of the kaon the

intercept of the reaction plane with the hodoscope in the recoil arm was computed. I f only one of the three recoil counters showed a trigger which fulfilled the pulse height and time-of-flight criteria, the event was classified as "up" or " d o w n " according to whether a counter above or below the reaction plane had fired. At momentum transfers I tl > 0.4 (GeV/c) 2, about 8% of the events were associated with two recoil particles since the protons and pions from A decay could not be completely separated by means of a cut in pulse height. Abou t 50% of these non-unique events could be analyzed using the kinematical constraint that the protons have to have smaller angles with respect to the reaction plane than the pions. (I t was not possible to save all those events because only the central recoil counter was covered by a hodoscope.)

Equation (1), which relates the A polarization to the up-down asymmetry , was modified to account for the 4% fraction of non-unique events at larger I tl values, and for the fact that at small momentum trans- fers the solid angle was not quite sufficient to detect all the A-decay protons. A Monte Carlo simulation of the process showed that the corrections were less than 5%.

From the missing mass spectra in fig. 2 it is obvious that our statistics in the mass region free of any E ° contaminat ion is rather l imited. For the asymmetry calculation however, the mass range can be safely ex- tended somewhat beyond the E ° step, since the A hyperons from the magnetic dipole transition E ° ~ A7

P

0.2

0

-0.2

-0,4

-0.6

-0.8

-1.0 tt t t 02 04 0.6 0.8 10

l t l [(GeV/c) 2]

Fig. 3. The A polarization in 3'P ~ K÷A at 5 GeV. (Basel sign convention [ 3 ] .)

are not strongly polarized. Averaged over all decay angles, the A and E ° polarization vectors are related by PA = -- 1/3P2° [5]. Assuming unpolarized A hyperons from the E ° decay, the E ° contr ibut ion simply corresponds to a dilution of the A asymmetry. This can be corrected for by multiplying the observed asymmetry with the ratio (N A +N z)/NA, where N A a n d N ~ are the numbers of K+A and K+Y~ ° events within the missing mass range considered. In the anal- ysis of the present experiment an upper cut in the squared missing mass of 1.46 (GeV/c2) 2 was chosen,

corresponding to a ratio (NA+Nz)/N A ~ 1.05. Then the error in the A polarization due to the unknown E ° polarization is less than -+0.02.

The t dependence of the A polarization P obtained in this experiment is shown in fig. 3. The error bars represent statistical errors only. In addition, there is an overall systematic uncertainty of-+0.10 which is mainly given by the error on the deuterium data and uncertainties in the background subtraction.

The A polarization P is large and negative in the whole t range covered by the experiment. Since the polarization has to vanish in the forward direction, P has to drop rather steeply from P = 0 at t = tmi n to - 0 . 9 at t = - 0 . 2 (GeV/c) 2. Outside the forward direc- tion, a smooth rise towards larger momentum transfers is observed. As in the differential cross section, no sig- nificant structure is seen around t = 0.4 (GeV/c) 2. Quite a different behaviour has been observed in the corresponding pion-induced reaction n - p ~ KCA [6].

515

Page 4: Recoil polarization in K+ Λ photoproduction at 5 GeV

Volume 40B, number 4 PHYSICS LETTERS 24 July 1972

Here do/dt has a break and the A polarization changes sign at t ~ - 0 . 4 (GeV/c) 2. This difference could be related to the different helicity structure of the am- plitudes in photon and pion induced reactions.

The magnitude and t dependence of our polariza- tion data are in qualitative agreement with the predic- tion of a Regge fit to the 3'p ~ K+A, K +E° cross sec- tions [1] by Capella and Tran Thanh Van [7]. The sign of the polarization is arbitrary in their model. The model is based on exchange degenerate K*(890) and K**(1420) trajectories and a weak K*-Pomeron cut. The authors claim that the K*(890) alone is not sufficient because an unreasonably high cut contribu- tion would be required to fill in a dip at the zero of the K* trajectory at t ~ - 0 . 4 (GeV/c) 2 which is not observed in the differential cross section. The A po- larization data support this conclusion, because from K*(890) exchange plus cut alone one would expect the polarization to change sign at t = - 0 . 4 (GeV/c) 2. Capella and Tran Thanh Van predict a zero crossing

of the polarization at t ~ - 1 . 1 5 (GeV/c) 2 which is not incompatible with the trend of our data towards larger momentum transfers.

We wish to thank Drs. C. Geweniger, R. Kotthaus and K. Wegener for their help in the early stages of the experiment. We also thank our technicians and the DESY Hallendienst for their support.

References [1] A.M. Boyarski et al., Phys. Rev. Letters 22 (1969) 1131. [2] C.C. Morehouse et al., Phys. Rev. Letters 25 (J970) 835. [3] Proc. Intern. Syrup. on Polarization phenomena of

nucleons, Basel 1960, Helv. Phys. Acta, Suppl. VI (1961) 436.

[4] O.E. Overseth and R.F. Roth, Phys. Rev. Letters 19 (1967) 391.

[5] G. Feldmann and T. Fulton, Nucl. Phys. 8 (1958) 106. [6] M. Abramovich et al., Nucl. Phys. B27 (197J) 477. [7] A. Capella, J. Tran Thanh Van, Lett. Nuovo Cimento 4

(1970) 1199. In this paper the polarized target asymmetry is calculated. Within the assumptions of the model, the recoil polarization should be the same.

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