Biswas, N.N.

Cason, N.M.

Ezell, C.R.

Gaidos, J.A.

Go, P.T.

Kenney, V.P.

Lamsa, J.W.

Shephard, W.D.

Smith, P.H.

Willmann, R.B.

Lamsa, J. W.

Willmann, R. B.

Ezell, C. R.

Gaidos, J. A.

Go, P. T.

Biswas, N. N.

Cason, N. M.

Kenney, V. P.

Shephard, W. D.

Smith, P. H.

UNT Digital Library

T&CQYED; £Y DJIE JAN 111972 Test of Helicity Conservation and Spin-Parity Analysis of the N (1700) J. W. Lamsa, R. B. Willmann, C. R. Ezell, J. A. Gaidos Purdue University, Lafayette, Indiana 47907, USA «sfc P. T. Go, N. N. Biswas, N. M. Cason, V. P. Kenney, W. D. Shephard, P. H. Smith University of Notre Dame, Notre Dame, Indiana 46556, USA ABSTRACT The diffractively produced low-mass TT+TT~P system is studied in the reaction TT p -» TT~TT n~p. We observe strong N (1700) production and determine the branching fraction to rr~A++ to be 65 ± 15%. A spin-p * parity analysis (assuming J < 7/2) yields J = 5/2+ for the N (1700). * The study of the decay angular distribution of the N (1700) indicates that helicity is conserved in the t-channel rather than in the s-channel, Supported in part by the U.S. Atomic Energy Commission and National Science Foundation. NOTICE — This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, com-pleteness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. DISTRIBUTION OF THIS DOCUMENT IS UNUMF DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. The question of helicity conservation in the production of resonances is of current interest. In this paper we study the diffractively produced low­mass TT Tf p system. In particular we examine the N (1700) produced in the reaction: TTp ­ TTN (1700) (1) * 4­ ­where N (1700) ­♦ TT TT p. Helicity conservation in the s­channel has been o 2 demonstrated for p photoproduction and for elastic scattering. It has been argued that s­channel helicity conservation (SCHC) might hold for dif­3 tractive processes in general. Although there is strong evidence for t­channel helicity conservation in the diffractive production of the A1 4 ­ enhancement, this reaction does not provide a good test of SCHC. Most, if not all, of the data in the A enhancement can be explained in terms of a double­Regge model which by virtue of its kinematical nature cannot con­serve s­channel helicity. Reaction (1), on the other hand, is a diffractive process involving a definite resonance that cannot be explained as a kinematic enhancement from a double­peripheral model. To determine the properties of * + ­the N (1700) we have performed a spin­parity decomposition for the TT TT p system as a function of the effc .̂ve mass. The data from this Purdue­Notre Dame collaboration come from the reactions: + + + ­TT p ­» TT TT TT p (2) _ _ 4­and TT p ­♦ TT TT TT p. ^3) The Purdue data consist of 1C." . . .­..i GeV/c in the 82 inch hydrogen bubble chamber at SLAC. The Notre Dame data consist of 4,494 events of reaction (2) at 18.5 GeV/c, 4,800 events of reaction_(3) at 18.5 GeV/c and 2,515 events vof reaction (3) at 8.05 GeV/c in the BNL 80 inch hydrogen bubble chamber. In Fig. 1 (a) is shown the TT rfp effective mass spectrum below 2.4 GeV for the combined data. The N (1700) peak observed in this figura is also very ' ­Jiinent in the individual samples. ­3­A fit using an s­wave Breit­Wigner shape plus a smooth hand­drawn background yields M = 1715 ± 5 MeV. Since the width is sensitive to the shape of the background we only quote an approximate value T ~ 80 MeV. •k In our spin­parity analysis of the N (1700) we assume that the low mass TT TT p system is diffractively produced. Cross sections for the * broad low mass (< 2 GeV) TTTTN region and for the I = 1/2 N peaks, including •k the N (1700) peak, have been found in many experiments to be approximately independent of incident momentum. Our results are consistent with these experiments. We observe a very strong A (1236) signal in the events with TT TT p mass below « 2 GeV. To determine the fraction of N (1700) s which decay ­ ++ + ­to TT A , the following method was used. For a given TT TT p mass region, a phase­space distribution was fitted to the TT p mass distribution in the regions above and below the A mass region. The number of events above the background curve in the region between 1.160 and 1.280 GeV was determined. This A signal, corrected for the Breit­Wigner tails, is plotted in Fig. ­ I I 1(b). We observe strong TT A enhancements in the intervals centered at * ­ ­H­1.480 and 1.720 GeV. The number of N (1700) ­♦ TT A events was determined by counting events above the hand­drawn dotted curve shown in Fig. 1(b) and •k the total number of N (1700) events was obtained from the signal in Fig. 1(a) The branching fraction (N*(1700) ­» TT"A++)/N*(l700) ­> all TT+TT~P) = 65 ±15%."^ The branching fraction was also determined by counting the N (1700) events with and without a narrow A cut and assuming a linear comDinucion of phase space and TTA decays. (This method is fully described in R. B. Willmann et al., Phys. Rev. Letters 24 (1970) 1260.) The branching fraction thus obtained was also 65%. i -4-To study the angular momentum structure of the TT TT~P system we have 8 - -H-used angular functions for a TT A intermediate state. The functions are expressed in terms of two angles: 9, the angle of the TT~ with respect - I i to the target proton direction in the TT A rest system (Gottfried-Jackson — I I Frame) and 3, the angle of the Tr in the A rest system with respect to the line of flight of the A defined in the rr A rest system. We have con-P t ± - + sidered the following spin-parity (J ) states: 1/2 , 3/2 , 5/2 and 5/2 . In each case the lower i-wave has been assumed. Because of the evidence P + for diffraction production we have assumed Pomeron exchange (i.e. J = 0 exchange) and have therefore set the spin-density matrix element p,, = 1/2. The likelihood function (LF) used to fit the data consisted of a sum of I P l 2 squared amplitudes, |A(J )| . For the results which we present here we have used LF = a ̂ (1/2*) |2 + b [A(3/2") |2 + c |A(5/2+) |2, where a + b + c = 1.* We also p _ considered J = 5/2 but it gave negligible contributions to all mass regions. A maximum-likelihood fit was made for 40 MeV intervals of rr rr p effective 9 mass. In an earlier study it was shown that background from final states such as rr"~p p and rr~f p is eliminated if the analysis is restricted to events with cos 9 > 0.0 and M(rr p) < 1.5 GeV. We have therefore incorporated these cuts in our analysis. Results of the fit are shown in Fig. 2. In Fig. 2(a) is shown the effective mass spectrum for all events used in the fit; in Fig. 2(b)-(d) are t P * These J states are sufficient to describe all of the known I = 1/2 N states below ~ 1.9 GeV. * ^(1/2*) |2 = 1/8[1 + 3 cos20], |A(3/2") | 2 » l /4[ l + 1/4(3 cos29 - 1)(3 cos2g - 1 ) ] , |A(5/2+) | 2 » 3/4o[~2 + 4 cos 2 e + 1/2(1 - 10 cos20 + 15 cos49) (3 cos28 - 1 ) 1 -5-P + + shown the contributions from J = 1/2", 3/2 and 5/2 respectively. We note that * p j_ the N (1700) region is consistent with being almost entirely J = 5/2 . The same conclusion was also reached using an analysis which did not assume a rrA intermediate state. The results of tests for helicity conservation in production of the N (1700) in the s-channel or in the t-channel are shown in Figs. 3 and 4. In Fig. 3 we compare the cos 9 distribution in the t-channel (Gottfried-Jackson Frame)with the distribution of cos 0 in the s-channel (helicity frame) for the N (1700) mass region and for adjacent control regions above and below the N (1700). Here Q^ is the angle of the rr in the rr A rest - 1 1 system with respect to the line of flight of the TT A in the overall center of mass. (For distributions in the s-channel we have used only events for which cos 0 > 0.0.) We note that the angular distribution for the N (1700) region is more peaked at cos 9 = +1.0 in the t-channel (Fig. 3(b)) than at cos 0^ = +1.0 in the s-channel (Fig. 3(e)). This might be expected if the N (1700) were produced aligned along the t-channel quantization axis. The control regions show neither the peaking characteristic of high spin nor any dramatic differences between the reference frames. In Figs. 4(a) and 4(b) are shown the value of the spin-density matrix element p., for the t-channel and s-channel as a function of |t|, the four-momentum transfer from the target proton to the N (1700). Here we have assumed that the N (1700) region is entirely J = 5/2 (see Fig. 2(d)) and t A maximum-likelihood fit was made to the distribution of the angle between the normal to the rr n p decay plane and the target proton direction in the rr rr p rest system. The same angular momentum states ( J = 1/2, 3/2 and 5/2) were used in the fit. The contributions obtained by this method were very similar to those shovm in Fig. 2. The N (1700) region is predominantly J = 5/2. -6-have determined p,, in the t- and s- channels from maximum likelihood fits. We find that p, j, for the t- channel is consistent with a value of 1/2 over a wide range of |tj while for the s-channel the value decreases with increasing jt|. This result again suggests that helicity is conserved in the t-channel rather than in the s-channel. Finally, in Figs. 4 (c)-(f) we show results of fits of the function [l + A + cos (cp) + B cos (2cp)] to distributions of the azimuthal angle of the Tr~ in, the t-channel and s-channel reference frames as a function of M(TT TT p) . The distribution of the azimuthal angle is expected to be isotropic in the frame in which helicity is conserved independent of the spin of the resonance or the existence of intermediate decay states such as TTA. We find the distribution in the t-channel (Figs.4(c) and (e)) to be consistent with isotropy for the N (1700) mass region; the distribution in the s-channel (Figs. 4(d) and (f)) is not. We conclude that there is no evidence for s-channel helicity conserva-tion in reaction (1). Instead our observations are consistent with t-channel helicity conservation in this reaction. We are pleased to acknowledge the cooperation of the 80-inch bubble chamber group, the AGS personnel at BNL, the 82-inch bubble chamber group, and the SLAC personnel. The assistance of W. L. Rickhoff and R. L. Erichsen was invaluable in the experiment. The hard work of the Notre Dame and Purdue scanning and measuring staffs is greatly appreciated. Higher order terms in cos (ncp) were found to be unnecessary. REFERENCES (1) Particle Data Group, Rev. Mod. Phys. 43 (1971) Si; R. J. Piano, Rapporteur's Review Talk XVth International Conference ~ on High Energy Physics Kiev, U.S.S.R., 1970. (2) Aachen-Berlin-Bonn-Hamburg-Heidelberg-Munchen Collaboration, Phys. Rev. 175(1968) 1669; J. Ballam et al.s Phys. Rev. Letters 24(1970) 960; G. HShler and R. Strauss, Karlsruhe preprint (1969). (3) F. J. Gilman, J. Pumplin, A. Schwimmer and L. Stodolsky, Phys. Letters 31B(1970) 387. (4) Aachen-Berlin-Bonn-CERN-Cracow-Heidelberg-London-Vienna Collaboration, Phys. Letters 34B(1971) 160; G. Ascoli et al., Phys. Rev. Letters 26 (1971) 929. (5) E. L. Berger, Phys. Rev. 166(1968) 1525; M. L. Ioffredo et al., Phys. Rev. Letters 21(1968) 1212. (6) H. Satz and K. Schilling, Nuovo Cimento Letters 1(1971) 351; see also M. Ross and Y. Y. Yam, Phys. Rev. Letters 19(1967) 546. (7) E. W. Anderson et al., Phys. Rev. Letters 16(1966) 855; E. W. Anderson et al., Phys. Rev. Letters 25(1970) 699; K. J. Foley et al, Phys. Rev. Letters 19(1967) 397; Aachen-Berlin-Bonn-CERN-Cracow-Heidelberg-London-Vienna Collaboration-Kiev conference, reported by D. R. 0. Morrison, Rapporteur's Review XVth International Conference on High Energy Physics Kiev, U.S.S.R., 1970. (8) J. D. Jackson, High Energy Physics (Gordon and Breach, New York, 1965) p. 327; J. Button-Shafer, Phys. Rev. 139(1965)B607; S. M. Berman and M. Jacob, Phys. Rev. 139(1965) B1023. (9) R. B. Willmann et al., Phys. Rev. Letters 24(1970) 1260. (10) G. Cohen-Tannoudji, J. M. Drouffe, P. Moussa and R. Peschartski, Phys. Letters 33B(1970) 183; H. Satz and K. Schilling, ref. 6; A. Bialas, J. Dabkowski and L. Van Hove, Jcgellonian preprint number TPJU-23/70. Figure Captions Figure 1. a) Distribution of the TT rr~p effective mass for the combined data; b) Distrit. . of the rr~A++(1236) effective mass for the combined data using the fitted number of A++ events as a function of rr+n~p mass. Figure 2. a) Distribution of the TT+TT~P effective mass for events with cos 0 > 0 and rr+p effective mass less than 1.5 GeV; b)-(d) Fitted numbers of events for the various spin-parity states 1/2 , 3/2", 5/2+ as a function of the TT+TT"P effective mass. The fit is described in the text. Figure 3. a)-(c) Distribution of cos 0 in the Gottfried-Jackson frame (or t-channel); d)-(f) Distribution of cos 0^ in the helicity frame (or s-channel) Figs, (b) and (e) correspond to the N (1700) region (1.660 to 1.780 GeV) while the other figures correspond to control regions (Figs, (a) and (d) 1.540 to 1.660 GeV, and Figs, (c) and (f) 1.780 to 1.900 GeV). The curve in Fig. 3(b) represents the angular distribution expected for Jp = 5/2+ with p,, = 1/2. Figure 4. a)-(b) Fitted values for p,, in the t-channel (a) and s-channel (b) for the N*(1700) region (1.620 to 1.820 GeV); c)-(f) Fitted parameters of the function 1 + A cos cp + B cos 2 cp to the azimuthal angle for the N (1700) decay. (c) and (e) show A and B in the t-channel (d) and (f) show A and B in the s-channel. , > P" 1.4 8.6 1.8 2.0 2.2 2. Miif^rrp) CGeV) > o 00 o d z > (b) ± JL 1.4 1.6 1.8 2.0 2.2 F/Cr. / EVENTS / 0.040 GeV <5> O O o t=i CI 01 O 5 8.540—1660 GeV 8.660 — 8.780 GeV 8.780 —8.900 GeV 40 Co) in g 20 > Jl L T W uLn JL 0.5 . 8.0 0 40 < d ) C/> Z > 20 irMnh"'^ (e) ^ ^ ( f ) ^ v ^ r 1 - ^ 0.5 8.0 0 . 0.5 . . ' c o s BH 8.0 © 0.5 8.0 FIGr.z f -CHANNEL s-CHANNEL (a) u 0.5 0.4 0.3 0.2[ X -L »'•• 8 ).!■■;•: 0.2 , 0 .3 .. 0,4. OJ «b) 'H* 2.040 !*'$*<5F!p"*> i FIG*.*{-

Test of Helicity Conservation and Spin-Parity Analysis of the N*(1700).

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