First measurements of iT11 in the pion-deuteron breakup reaction

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FIRST M E A S U R E M E N T S OF iT n IN THE P I O N - D E U T E R O N BREAKUP R E A C T I O N E.L. M A T H I E a, G . R . S M I T H 2, E.T. B O S C H I T Z , W. G Y L E S , C . R O T T E R M A N N Kernforschungszentrum Karlsruhe, Institut fftr Kernphysik und Institut fftr Experimentelle Kernphysik der Universiti~t Karlsruhe, D-7500 Karlsruhe, Fed. Rep. Germany

S. M A N G O , J.A. K O N T E R , A. M A T S U Y A M A Schweizerisches Institut fftr Nuklearforschun~ CH-5234 Villigen, Switzerland

R.R. J O H N S O N TRIUMF/ University of British Columbia, 4004 Wesbrook Cres., Vancouver, B.C., Canada V6T 2,43

and R. O L S Z E W S K I Physikalisches Institut, Universiti~tErlangen-N~rnberg, D-8520 Erlangen, Fed. Rep. Germany

Received 26 November 1984

The vector analysing power, iTll, and differential cross section for the ~r+cl--,lr+pn reaction at T,~= 228 MeV were measured in a kinematically complete experiment, using a polarised deuteron target. Results are presented for a pion angle of 85° with six proton angles between 27° and 61 °. A comparison is made with theoretical calculations.

It has been known for a long time that the 7rNN system provides a basic testing ground for nuclear theory, although until recently the theoretical treatments were confined to individual reactions o f this system. More advanced approaches attempt to describe several coupled reaction channels simultaneously within a unified theory [ 1 - 5 ] . Neglecting electromagnetic channels, there are five reactions in the nNN system which are coupled: NN ~ NN, NN "~ ¢rd, rrd -+ lrd, NN -+ ¢rNN and nd --* ¢rNN. For NN -+ NN a sufficient variety o f data have been measured to essentially constrain a phase shift analysis from 0 - 1 0 0 0 MeV [6]. a Present address: University of Regina, Regina, Saskatchewan, Canada $4S 0A2. 2 Present address: TRIUMF, Vancouver, B.C., Canada V6T 2A3. 28

Great progress has been made recently on NN ~ 7rd [7,8] where almost sufficient data exist to perform a meaningful amplitude analysis. The 7rd --* rrd reaction is theoretically relatively well understood, provided that the values of t20 are smooth and negative, as obtained by Holt and co-workers [9, 10], and not oscillatory and positive as found by Gr~iebler et al. [ 11 ]. This is an important issue which remains to be resolved. However, in addition to the precise total cross section [ 12], differential cross section [13] and vector analysing power [14], i T l l , measurements, more data are needed on other observables for a reliable phase shift analysis which could show subtle effects not predicted by the theory. Recent results of Ao L [ 15 ], differential cross section [16] and analysing power [16] for the NN --* 7rNN reaction are difficult to reproduce by conventional theories [ 1 7 - 1 9 ] . 0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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The least well investigated of these basic coupled reactions is rrd ~ fiNN, which will be discussed in this paper. Recently a first set of unpolarized differential cross sections for the 7rd ~ npn reaction were reported [20]. In a kinematically complete measurement at T,r = 228 MeV, angular settings of the charged particle detectors were chosen to span the range between two extreme regions of the phase space. At one extreme the neutron is left with low momentum. Here the data may be expected to be described by the impulse approximation, assuming the dominant process is gp scattering with a neutron spectator. At the other extreme of the phase space, neutron and proton momenta are large compared with the Fermi-momentum in the deuteron and other scattering mechanisms become important. The authors calculated the cross sections in a simple model and suggested the possible existence of the J = 2 dibaryon resonance. Stimulated by this work Faddeev type calculations have been performed [21,22]. The evidence regarding dibaryons from the analyses of these spin averaged quantities is not yet conclusive. In order to provide a more sensitive test to the theories, measurements of iTll and simultaneously the differential cross section covering a wide range of phase space in a new kinematically complete experiment have been made. The experimental arrangement used was similar to the one previously described [14]. The final state pions and protons from the lr~ breakup reaction were detected in the horizontal plane to the right and left respectively of the incident pion beam direction. Both charged particles were detected in coincidence in two-element scintillation counter telescopes. The particle types and momenta were deduced from the pulse heights, and the time-of-flight (TOF) difference between the pion and proton telescopes. The use of the TOF difference rather than the TOF of the proton resulted in improved resolution of the proton energies at high proton momenta. The pion momentum here is low and the pion TOF provides the information in the calculation of the proton momentum. The proton momentum resolution was always better than 40 MeV/c. Data were collected, at 228 MeV incident pion energy, with 6 pion telescopes between 50 ° and 107 °, in coincidence with 6 proton telescopes between 28 ° and 57 °. Complementary pion and proton arms were positioned at the angles appropriate for free np scatter-

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ing. The electronic configuration was chosen to enable coincidences between any pion arm with any of the six proton arms. The polarized deuteron target has been recently described in detail [14]. The background arising from reactions on the contaminant nuclei of the polarized target were explicitly measured with a background " C 4 0 " target (a mixture of dry ice and graphite) and subtracted for every case. In addition to these two targets, a CH 2 target was also mounted within the cryostat for use in the absolute calibration of the TOF distributions with free 7rp scattering. The deuteron polarization was determined as before [14]. The positive (negative) target polarization and its uncertainty was 0.18 -+ 0.04 (0.20 ± 0.04), with a negligible tensor polarization (Pzz = 0.03). The vector analyzing power is given by 1 iT11 - _ _

o++o

-

X,/-3-P-o + + P + o - - (P+ + P - ) o 0 '

where a + ( o - ) are the differential cross sections measured with target polarization P+ ( P - ) , consistent with the Madison convention, and o 0 is the cross section measured with the background target. The cross sections are given by d 3 o / d ~ n d ~ p dPp = yield/NbeamNtgt eTrepA~27rA~ p APp ,

where Nbeam is the number of incident pions; Ntg t is the area density of deuterons in the target; ep and e~ are the proton and pion detection efficiencies; A ~ . and A~2p are the pion and proton telescope solid angles (29.9 and 23.5 msr respectively); and APp is the proton momentum bite. The unpolarized differential cross section is the average of the o + and o - cross sections. The target magnetic field acts like a momentum analyzer which distorts the pion and proton trajectories. The proton momentum and proton and pion scattering angles were determined from the TOF difference measurements by an iterative method. An estimated pion TOF and the measured TOF difference (TOFp - TOF~) were used to determine an initial proton momentum. From this and the known magnetic field, the laboratory angles, before magnetic rotation, were determined for both particles. With the initial proton momentum and these laboratory angles, 29

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300

200 10 ° 200 60 40 20 0 -20 -40

300

400

500

.

.

.

400 . .

500

600

600

10i0-:

22 24 26 28 - - 8 4 @ a 8688'9'1 ~

tO

200

~ 3

10-

-40 -60

"~

84

86 88

8~,

i0-:

! oo :~

:

10 -z. -20 -40!

,'~ '10-11 '4'0 4'2 ' 4'4 4'6 @P 8'4 " 8'68'8§'/-, 8~

j

"1:D 10-~-] b

4o! 20: ~ O -20: -40:

-60

~~

5'65'8 6'0 ' 6'2 8'4 '8'6'8'8'"

IO-Z1 _ ~ ~ . . . . ~ l O-t,,

"[0 -p.

@p 8~,

200 300 400 500 600 Proton m o m e n t u m (MeV/c)

2oo ~6o

F

460 '~6o '6oo

Proton m o m e n t u m

(MeV/c)

Fig. 1. Vector analysing power, iTll, and corresponding differential cross section for various proton angles and a nominal pion angle of 85 °. The actual scattering angles vary with proton momentum and are shown with the iT11 data; the cross sections are in the same sequence as the iT 11. The lines are calculations discussed in the text. The arrow indicates the position corresponding to quasi-free 7rp scattering. The horizontal bars show the integration range of each point. 30

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a pion TOF was determined with three body kinematics and compared to the previous estimate. The process was iterated until the scattering angles and the proton momentum converged to their true values. A consequence of these angular distortions is that for each pair of coincident pion and proton detectors the real scattering angles are slightly a function of the proton momentum. For all cases the actual scattering angles are shown in fig. 1. The momenta at which the data points are plotted were corrected for the nonlinear variation of the cross section over the momentum bite. The qualitative features of the data can be seen from looking at the results for a fraction of the 36 angle pairs where measurements were made. The results for pions at a single angle in coincidence with protons at six different angles are shown in fig. 1. One observes a rapid falloff in the differential cross section as the proton angle or the proton momentum is shifted away from that for quasi-free 7rp scattering. The experimental i T 11 also shows a slight decrease from its maximum value near the quasi-free 7rp kinematic conditions. Comparison of the cross sections with the previously published data [20] is not straightforward because of the different scattering angles. The cross section may vary by a factor of two for a change in the proton or pion angle of 3 °. Interpolating our data to angles agreeing with Hoftiezer et al. [20] results in agreement between the two experiments within 20% over most of the range of the data. The discrepancy sometimes becomes larger when the pion momentum is less than 200 MeV/c (high proton momentum at angles away from quasi-free kinematics). The effects of the magnetic field here are large. However, since the angular variation of i T 11 is much smaller than that of the cross sections, the uncertainties in the effects of the magnetic fields are expected to be negligible for this observable. Systematic errors in the cross sections, due to uncertainties in the target thickness, beam flux, detector efficiencies and the effects of the magnetic fields are estimated to add to about 20N. In a theoretical calculation, the cross section and the analyzing power in the lab system can be expressed in terms of the Lorenz-invariant T-matrix, Tmp,mn;md (mp, ran, and m d are the z components of the spin of the proton, neutron and deuteron respectively):

18 April 1985

d3o

1

dPp dQp d~2~r 32(2705 ~Ef /-1 1

2 2 PpPrr PoMdEpEnErr

23 ITmp,mn;md I ' mp,mn;md

iTll =X/-6Im( m~p,mn Tmp,mn;OTmp,mn;1) X(

~ ITmp,rnn;mdl2)-1 , mp,mn;md

where P0 is the initial momentum of the pion, and Pp, P , , Ep, En, ETr are the final momenta and energies of each particle. M d is the deuteron mass and Ef = Ep + E n + E~r. The transition matrix Tm~ mn.md can be obtained by solving the 7rNN coupleaechannel equation based on the three body formalism, which has already been applied to the 7rd elastic scattering and the rrd -+ 2p reaction [21]. In practice the impulse (single scattering) and the double scattering terms are calculated and the results are shown in fig. 1. While all of the S- and P-wave rrN interaction except the P11 channel are taken into account in the impulse term, only the P33 channel for the 7rN interaction and 3S1-3D1, and 1S0 channels for the NN interaction are considered for the double scattering terms. The 7rd initial state interaction is estimated by multiplying the T-matrix with a distortion factor which is obtained by solving the 7rd elastic scattering. The peaks of the cross sections correspond to the kinematics of the smallest pn relative momentum in the deuteron, where the impulse process dominates. The rather fiat structure of the cross sections in the low momentum regions in fig. 1 reflects the onset of the pn final state interaction which enhances the cross section. The calculated results can reproduce the cross sections and analyzing powers fairly well in the region where the impulse process dominates. In the region off the quasi-free kinematics where the multiple scattering is more important, the agreement is not always as good, due to the relatively crude approximations for the multiple scatterings. At present no necessity for the addition of exotic resonances can be seen, but more precise data for iT 11 away from the quasi-free kinematics would provide a more stringent test for the theory. In particular it will 31

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be interesting to see if the structure predicted for i T l l far away f r o m the quasi-free region can be confirmed. The authors w o u l d like to thank T.-S.H. Lee for valuable discussions during the writing o f this work. This w o r k was supported in part by the Bundesministerium fiir F o r s c h u n g und Technologie o f the Federal Republic o f G e r m a n y .

References [1] A.S. Rinat and Y. Starkand, Nucl. Phys. A397 (1983) 381. [2] T. Mizutani, C. Fayard, G.H. Lamot and R.S. Nahabetian, Phys. Lett. 107B (1981) 177. [3] B. Blankleider and I.R. Afnan, Phys. Rev. C24 (1981) 1572. [4] H. Garcilazo, Nucl. Phys. A360 (1981) 411.

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[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

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M. Betz and T.-S.H. Lee, Phys. Rev. C23 (1981) 375. D.V. Bugg, Nucl. Phys. A416 (1984) 227c. G. Jones, Nucl. Phys. A416 (1984) 157c. G.R. Smith et al., Phys. Rev. C30 (1984) 980. R.J. Holt et al., Phys. Rev. Lett. 47 (1981) 472. E. Ungricht et al., Phys. Rev. Lett. 52 (1984) 333. W. Griiebler et al., Phys. Rev. Lett. 49 (1982) 444. E. Pedroni et al., Nucl. PhyS. A300 (1978) 321. K. Gabathuler et al., Nucl. Phys. A350 (1980) 253. G.R. Smith et al., Phys. Rev. C29 (1984) 2206. I.P. Auer et al., Phys. Rev. Lett. 51 (1983) 1411. A.D. Hancock et al., Phys. Rev. C27 (1983) 2742. J. Dubach et al., Phys. Lett. 106B (1981) 29. E.A. Umland et al., AlP Conf. Proc. No. 69 (Santa F~, 1980) p. 172. W. Jauch, A. Konig and P. Kroll, Wuppertal preprint WU B 84-7. J.H. Hoftiezer et al., Phys. Rev. C23 (1981) 407. A. Matsuyama,Nucl. Phys. A379 (1982)415. H. Garcilazo, Phys. Rev. Lett. 48 (1982) 577.