c region

Volume 94B, number 3

PHYSICS LETTERS

11 August 1980

MEASUREMENT OF THE SPIN PARAMETERS A AND Ann IN pp ELASTIC SCATTERING IN THE 1 - 3 GeV/c REGION ¢: D.A. BELL, J.A. BUCHANAN, M.M. CALKIN, J.M. CLEMENT, W.H. DRAGOSET 1, M. FURIC 2, K.A. JOHNS, J.D. LESIKAR, H.E. MIETTINEN, T.A. MULERA 3, G.S. MUTCHLER, G.C. PHILLIPS, J.B. ROBERTS and S.E. TURPIN T. W. Bonner Nuclear Laboratories and Physics Department, Rice University, Houston, TX 77005, USA Received 9 June 1980

We have measured the asymmetry parameter A and the spin correlation parameter Ann in pp elastic scattering, using the Argonne ZGS polarized proton beam and a polarized proton target. Angular distributions of A and Ann for Itl ~>0.2 (GeV/c) 2 were obtained at eight momenta between 1.10 and 2.75 GeV/c. We find significant structure in both the energy and t-dependence of Ann at these energies. At Plab ~ 1.34 GeV/c Ann reaches a very large value of about 0.8-0.9 near 0cm = 90 °.

Recent experiments on pp scattering in pure initial spin states have revealed remarkable energy dependent structures in many spin parameters in the 1 - 3 GeV/c region. Rapid changes are evident b o t h in the longitudinal and transverse pure spin total cross sections [1,2], as well as in the energy dependence o f elastic scattering at 0cm = 90 ° [3]. The interpretation of these structures remains unclear so far: attempts have been made to explain them as a manifestation o f dibaryon resonances [4], or as the effects of crossing o f various inelastic thresholds [5]. More detailed measurements of spin observables in this energy range seem necessary to resolve the origin o f these effects. In this letter we report measurements o f the spin parameters A and Ann in pp elastic scattering at eight incident momenta between 1.10 and 2.75 GeV/c. A and Ann are related to the pure spin differential cross sections through * Supported by the US Department of Energy. 1 Present address: Western Geophysical Co., Houston, TX, USA. 2 Visiting scientist from Institut Ruder Boskovid, Zagreb, Yugoslavia. 3 Present address: Lawrence Berkeley Laboratory, Berkeley, CA, USA. 310

a ( t ' r ) = a(1 + z 4 + A n . ) , o(~$) = o(1 - 2A +Ann ) ,

(1)

(~(I"$) = o($¢) = a(1 - A n n ) , where o is the unpolarized differential cross section. The initial spins are polarized normal to the scattering plane. A has to vanish at 0 = 0 ° because of angular momentum conservation and at 0cm = 90 ° because of particle identity. The experiment utilized the Argonne ZGS polarized proton beam focused onto the A r g o n n e - R i c e polarized proton target (PPT-VI). The beam polarization was measured with the 50 MeV polarimeter [6] just before injection into the ZGS. No significant depolarization in the ZGS has been observed at these low energies due to the absence of strong depolarizing resonances below 3 GeV/c. The beam polarization was reversed each spill and averaged about 70% over the entire run. The polarized target was a conventional 4 H e - 3 H e cryostat operating at 0.5 K in a magnetic field of 2.5 T. The target material, contained in a cavity 7 cm long by 2 cm in diameter, consisted of ethanediol (C2H602) beads d o p e d with chromium paramagnetic complexes.

Volume 94B, number 3

PHYSICS LETTERS

11 August 1980

$6

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Fig. 1. Layout of the experiment. The polarized beam scatters in the polarized proton target (PPT-VI) and elastic events are detected in a spectrometer consisting of a bending magnet (BM), scintillation counters (S 1-$6), and multiwire proportional chambers (P1 -P6). Scintillator telescopes (ML, MR, NL, NR) monitor relative beam intensities and wire chambers (Wl, W2) monitor beam position and profile.

Dynamic polarization of free hydrogen protons was induced by a 70 GHz microwave system. The target polarization was continuously monitored by a 106.5 MHz NMR system, and averaged about 75% over the run. A schematic layout of the experimental apparatus is shown in fig. 1. Relative beam intensities were measured by scintillator telescopes labeled ML, MR, NL, NR, viewing polyethylene targets upstream of the PPT. The incident beam profile was monitored by two segmented wire ion chambers W 1 and W2. Elastic scattering events were detected by a two-arm spectrometer, each arm being moveable about a pivot point under the PPT. The forward scattered proton was momentum analyzed and its direction measured by multiwire proportional chambers (MWPC) P 3 - P 6 and a bending magnet BM. The scattering angles of the recoil proton were measured by MWPC's PI and P2' The scintillation counters S 1-S6"provided a fast event trigger and helped in rejecting background through pulse height and time of flight information. Most inelastic events were easily removed by restrictions on the missing mass recoiling against the

scattered proton. In the ramaining data sample we select coplanar events (I A4)I = I q~s - (~r -- 180°) I ~< 5 °, where ¢~s and ~r are the azimuthal angles of the scattered and recoil proton), and plot the distribution in A0 = Or - 0r (calc), where Or is the polar angle of the recoil proton and 0r (calc) is the corresponding angle calculated from elastic kinematics. A strong elastic peak is observed at A0 ~ 0, superimposed on top of a much broader distribution due to quasi-elastic scattering from bound protons in carbon and oxygen.The width of the peak varied (depending on momentum transfer t) between ~ 2 - 4 °, consistent with our angular resolution and multiple scattering in the PPT. The quasi-elastic background was subtracted by selecting noncoplanar events (I 2xq~J2 5°), and by matching their A0 distributions to the tails on both sides of the elastic peak. The background subtraction was typically 5 - 1 0 % and was done separately for each t-bin. With the beam and the target only partially polarized, the event rates N(ij) in each of the four initial spin states (i, 1"= beam, target), normalized to the incident beam intensity, are given by 311

Volume 94B, number 3

PHYSICS LETTERS

N(i]) = X(j') X {1 + [PB(i/)

+PT(J)IA +PB(if)PT(/)Ann),

measurements [ 7 - 1 3 ] in the same energy range. The errors shown are purely statistical. Possible systematic errors arise mainly from uncertainties in the beam and target polarizations and (for Ann ) in the background subtraction. We estimate a total relative systematic error of +6% in A and -+8-10% in Ann. Our data on A are in good agreement with previous measurements and confirm the well-known features of the asymmetry at low energies: a rapid rise away from t = 0 to about A ~ 0 . 4 - 0 . 5 , and then a smooth decrease down to A = 0 as 0cm -+ 90 °. Above Plab ~ 2

(2)

where PB and PT are the beam and target polarizations and X is a normalization factor. For a given set of runs ( i / = i t , t # , St, ~'~) Pt and X are independent of the beam polarization due to fast reversal of the beam spin. A and Ann are obtained by inverting eqs. (2). Our results on A and Ann at the eight incident momenta are shown in figs. 2 and 3, along with previous l

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Volume 94B, number 3 1.0

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Fig. 3. Spin correlation parameter Ann in pp elastic scattering at Plab = 1.10-2.75 GeV/c. The dashed line at each momentum indicates 0cm = 90°. Previous data are from refs. [7,11-13]. GeV/c, A decreases slowly with increasing energy within the diffraction peak, and remains small for It[

0.8 (GeV/c)2. The angular dependence of Ann exhibits several interesting features. Around 1.1 GeV/c (see ref. [7] for results at several nearby momenta) Ann has a maximum of about 0 . 6 - 0 . 7 at It[ ~ 0 . 1 5 (GeV/c)2, and decreases to ~0.5 at 0cm = 90 °. In the region 1.2-1.5 GeV/c (see also ref. [14]) the behavior of Ann is qualitatively different: it increases monotonically towards larger angles and reaches a very large value of about

0 . 8 - 0 . 9 at 1.34 GeV/c. Around 1.6-2.0 GeV/c Ann is roughly constant for I tl ~ 0.2 (GeV/c) 2 and above 2.0 GeV/c it seems to develop a (weak) minimum at l tL ~ 0.8 (GeV/c) 2 while remaining large near 0cm = 90 °. Our results on the energy dependence of Ann at 0crn = 90 ° are shown in fig. 4, together with previous measurements [7,10-17] in the 1 - 3 GeV/c region. Clearly the most distinct feature of these data is the pronounced peak in Ann centered around Phb ~" 1.34 GeV/c. We note that Ann (90 °) is related in a simple 313

Volume 94B, number 3

PHYSICS LETTERS

1.0

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+Ann)l(1 - Ann) = o t / o s,

0cm : 90 ° .

(3)

At 1.34 GeV/c our value Ann = 0.872 + 0.035 implies o t / o s ~ 15, thus indicating very strong triplet dominance in the large angle elastic scattering at this momentum. It is interesting to n o t e that the observed structure occurs in the vicinity o f the r e p o r t e d 1 D2 (2.17 GeV) and 3F 3 (2.22 GeV) d i p r o t o n resonances [4] (Plab = 1.26 and 1.40 GeV/c, respectively), and it m a y therefore have a bearing on the existence and properties o f these states. An alternative interpretation, without any resonant behavior, might arise as a result o f different m o m e n t u m dependences o f the singlet and triplet contributions, reflecting the p r o d u c t i o n properties of the S- and P-wave NA final states and their coupling to the pp system [5].

314

A more detailed analysis of the energy dependence o f these data, as well as a comparison with phase shift analyses, will be presented elsewhere. We wish to thank the ZGS staff for the successful operation o f the polarized b e a m and J. Bywater for his help in setting up PPT-VI. We also thank J.H. Hoftiezer, D.M. Judd, and G.P. Pepin for their help in running the e x p e r i m e n t .

0.4 o B e s s e t e t al. • Borisov e t al. x G o l o v i n e l at. A D o s t et al. • Cozzika e t al.

11 August 1980

References [1 ] I.P. Auer et al., Phys. Lett. 67B (1977) 113; Phys. Rev. Lett. 41 (1978) 354. [2] Ed. K. Biegert et al., Phys. Lett. 73B (1978) 235; W. de Boer et al., Phys. Rev. Lett 34 (1975) 558. [3] I.P. Auer et al., Phys. Rev. Lett 41 (1978) 1436. [4] H. Hidaka et al., Phys. Lett. 70B (1977) 479; N. Hoshizaki, Prog. Theor. Phys. 60 (1978) 1796; 61 (1979) 129; A. Yokosawa, Proc. 1979 INS Syrup. on Particle physics in GeV region (Tokyo, Japan, Nov. 1979), and references therein. [5] S. Minami, Phys. Rev. D18 (1978) 3273; C.L. HoUas, Phys. Rev. Lett. 44 (1980) 1186. [6] H. Spinka, Argonne preprint ANL-HEP-PR-80-02 (1980). [7] D. Besset el al., SIN preprint PR-80-003 (1980). [8] M.G. Albrow et al., Nucl. Phys. B23 (1970)445. [9] P.R. Bevington et al., Phys. Rev. Lett. 41 (1978) 384. [10] G. Cozzika et al., Phys. Rev. 164 (1967) 1672; the asymmetry data shown in fig. 2 are at Plab = 1.91 GeV/c. [11] D. Miller et al., Phys. Rev. D16 (1977) 2016. [12] H.E. Dost et al., Phys. Rev. 153 (1967) 1394. [13] M.W. McNaughton et al., to be published; see also H.B. Willard et al., AIP Conf. Proc. 51 (19793 420. [14] M.W. McNaughton et al., Preliminary data at Plab = 1.27 GeV/c, private communication. [15] N.S. Borisov et al., AlP Conf. Proc. 35 (1976) 59. [16] B.M. Golovin et al., Results quoted by Yu. M. Kazarinov, Rev. Mod. Phys. 39 (1967) 509. [17] A. Lin et al., Phys. Lett. 74B (1978) 273.