Bastien, P.

Berge, J.P.

Dahl, O.

FerroiLuzzi, M.

Kirz, J.

Miller, D.H.

Murray, J.J.

Rosenfeld, A.H.

Tripp, R.D.

Watson, M.B.

eScholarship - University of California

Lawrence Berkeley National LaboratoryRecent WorkTitleTHE PION LAMBDA RESONANCE (Y *)Permalinkhttps://escholarship.org/uc/item/8ch8j332AuthorsBerge, J.P.Bastien, P.Dahl, O.et al.Publication Date1961-04-06eScholarship.org Powered by the California Digital LibraryUniversity of California... 'I I' UCRL-964o UNIVERSITY OF CALIFORNIA adiarion TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Division, Ext. 5545 BERKELEY, CALIFORNIA DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal 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 its 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, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. .. For Physical Review Letters UNIVERSITY OF CALIFORNIA Lawrence Radiation Laboratory Berkeley, California Contract No. W-7405-eng-48 UCRL-9640 Limited distribution THE PION-LAMBDA RESONANCE (Y 1•) J. P. Berge, P. Bastien, 0. Dahl, M. Ferro-Luzzi, J. Kirz, D. H. MUler, J. J. Murray, A. H. Rosenfeld, R. D. Tripp, and M. :a. Watson April 6, 1961 -2.- UCRL-9640 THE PION-LAMBDA RESONANCE (Y l *> * J. P. Berge, P. Bastien, 0. Dahl, M. Ferro-Luzzi, t J. Kirz, D. H. Miller, J. J. Murray, A. H. Rosenfeld, R. D. Tripp, and M. B. Watson Lawrence Radiation Laboratory and Department of Physics University of California, Berkeley, California April 7, 1961 Alston et al. 1 have discovered an 1 = 1 Tl'- A resonance with a mass of 1380 Mev and a half-width r /2 consistent with 32 Mev. They reported on 141 A 11'+1f- events made by 1150 Mev/c K- mesons incident on the 15-in, Lawrence Radiation Laboratory hydrogen bubble chamber which produced the sequence ( 1) followed by a strong decay Y 1101± - A + tr± + 130 Mev. (2) a -In the course ofjcontinuing study of K p interactions, we have now * explored the same reactions from Y 1 threshold f PK(lab) = 405 Mev/cl I + - • through ?K = 850 Mev c. We report on -500 A tr 1r and find m(Y 1 ) = 1385 Mev and r /2 closer to 20 Mev; however we are still unable to distinguish between spins J = 1 /2 and J = 3 /Z. * It is conventional to discuss Y data in terms of a simplified model in which they are produced and decay "isolated", that is, one neglects the . * + -interferences: (a} between Y 1 's and background A 'If 1r produced in non-resonant partial waves, as well as {b) the influence of Bose statistics and final-state interactions on the tr11' system. * . . Isolated Y 1 'a will be produced in association with a pion of fixed center-of mass (c.m.) kinetic energy (spread, of course, by -3- UCRJ..-9640 + ·-:1: r /2). Thus in Fig. 1, which is Dalitz' representation of the A 1r -r. data I *+ *- . at PK(lab) = 850 Mev c, Y 1 wUI fall in a horizontal band and Y 1 in a vertical band. That this model is poor is sho-vm by the fact that conservation . * of parity requires that our isolated Y 1 show no fore-aft asymmetry on strong decay, i.e. in the strong-decay distribution in the y* c. m. frame, the odd coefficients must be zero. However, our data show a concentration of backwards A. The asymmetry coefficients a1, (assuming a 2 = 0) in Table 1 show that the "isolated" model is far from realistic, both for our new data and the older events at 1150 Mev /c( l). A major purpose of this Letter is to point out this difficulty. In the following Letter, 2 Dalitz and Miller treat the reaction as a symmetrized three-body state which permits nonzero a1• However some of . the features of the isolated model are still discernable, for example the extra * population of the Y 1 bands on the Dalitz plots. Tho~·efore, in order to simplify the discussion, in our Letter we still refer to this "isolated" model. The data to which the model may be most meaningfully applied are those. at PK = 850 Mev /c, (see Fig. 1). (They cross * where the Y 1 bands do not overlap badly in the middle of the Dalitz plot at about 700 Mev /c.) * Figure 2 is an experimental histogram of the Y 1 mass. The individual uncertainty in each mass measurements is typically :1:(3 to 5) Mev. If we fit the histogram with an s-wave resonance curve of the form dn/ dm oc [(m- 1385)2 + ( I/2)2]-l as indicated in Fig. 2. we find r/2 = 15 to 20 Mev (depending upon whether or not some background is subtracted). This experimental value is an agreement with the "KN bound state" prediction of I'/2 = 18 Mev, 3 but it is also not too far from global symmetry's r /2 = 25 Mev (see below). -4- UCRL-9640 *+ ·-Figure 3 displays the excitation data for Y 1 and Y 1 and total A 1r+ 11'-. Du.ta at 1150 Mev /c are from Alston et al. 1 and at 300 and 400 Mev /c are combined from the present experiment and !rom Nordin. 4 The Y t' cross sections at 850 Mev /c can be obtained fairly unambiguously in the sense that + -the A 11' 11' events of Fig. 1 can be interpreted as a fiat background plus an • extra population of Y 1 's at the bands. A ba~kground t~ubtraction is also possible at 760 Mev /c. However at 620 Mev /c, the bands cover almost the * whole ellipse, making it impossible to distinguish Y 1 events from background. At SlO Mev /c, the bands no longer cover the middle of the ellipse, and we . * might expect any appreciable Y 1 population to show up in the bands above background. In the Dalitz plot of the data for PK = 510 Mev/c in Fig. 4, we however, · · ' actually find,.\a concentration of events near the center of the diagram, not in • the bands. This result could be consistent with essentially no Y 1 + -production (as suggested in Fig. 3) plus a nonresonant background of A 1r 11' events tending to favor equal pion mo~enta (for reasons unknown), it is also • conceivable and quantitatively sensible that there is indeed appreciable Y 1 * . production with the Y 1 and the production pion predominantly in a relative p state (or higher). This would tend to depopulate the ends of the ellipse where the production pion momenta would be low and produce the observed distribution of events. Therefore one cannot determine unambiguously from these data the * . cross section for Y 1 production at PK = S 10 Mev /c.· Table 11 illustrates the apportioning of events between the various possible reactions . • The Y 1 _production angular distributions are isotropic within statistics £or all our beam momenta. This would suggest that Y 1 production + -proceteds dominantly through a single J = 1/2 wave. However the A. tr TF . cross section alone comes very close to Tr ~2 /2, and so other partial waves are probably present. * Next we discuss the Y 1 spin. Global symmetry predicts a A'IT -5- UCRJ ... -9610 resonance with spin J = 3/2. 5 On the other hand, the Da.litz-Tuan s-wave . ~ resonance in the K-p system corresponds to a Y { with J = l/2, and a strong-decay via s112 if the Ki\. parity is odd. 6 lf our model of an isolated y* were correct the strong decay angular distribution of Eq. (3) should be fore-aft symrnetric (a.1 = 0), and the presence of any polar-equatorial anisotropy (a2 #:-C) would be evidence for J> 1/2. We find instead a 1 #:- 0 but within statistics a 2 = 0 at all production angles. After looking for evidence for ~ * . a 21 0 at any (or all) angles of Y production, we have confined our analysis * 7 to polar-produced Y 's where, as Adair has pointed out, an isolated J = 3/2 particle must decay according to 1 + 3 (~· !92 (the unit vector IS is along the K- beam direction) whUe J = 1/2 must decay isotropically. The decay dis-* I * I tribution for polar-produced Y 's ( ! · K ~ 0,80) is given in Fig. S. vVe • • have used A· ! as a measure of the decay angle instead of!}· !5 in order to * diaplay the fact that the Y are not isolated but tend to strong-decay with the A going backwards. (Essentially the same result is achieved for these polar events by choosing+ A·~ for forward-produced Y$, and -A·~ for backwards ~ Y ). These Adair events of Fig. 5 again fail to show any evidence for 1 + 3 (A·.!*) 2 superimposed on the asymmetry, which is linear in (A· x_*). This statement applies individually to both charges of y* at each momentum and to the sum of all events. However. as is shown in the next Letter, one cannot conclude that J = 1/2 since examples are given where Bose-statistics can cause ICt a Y with J = 3/2 to have a relatively isotropic angular distribution in the Adair analysis. • • Next we discuss the branching ratio Y 1 -I: vs A. Global symmetry predicts definite ratios between the decay rate r(N) of the 3/2. 3/2 pion-. * nucleon 1sobar ( N - N + 1r) and the equivalent rates of. for example, the *- 0 . -three decay modes of Y l These are r(N): r( ~ ): r( ~ ) : -6- UCR.L-9640 3 . 3 3 3 r(A) = pN: (1/6) Pz 0: ( 1/6) Pz-: ( 4/6) p A' For PN = 2 30 Mev /c, Pz= 127 I ? . p A= 207. (p A/P·z/ = 1/4 •. the relative rates are predicted to be 1.37: 1/24: 1/24: 2/3. The half-·Nidth r /2 (N) is known to be 45 rvf.ev; so global symmetry predicts a Y; half-width r/2 = l/zfr(1;0) +r(Z-)+ r(/,~.)1 o£ 24.7 Mev, and a branching ratio r(:z0)/r(A) = 1/16. (As already mentioned, our observed f'/2 is 15 to 20 Mev, but the statistics are sufficiently poor and the effects of Bose-Statistics sufficiently great that r /2 = 25 Mev cannot be ruled out). The Dalitz-Tuan resonance is expected to give values of r(:E0)/r(A) ranging from ;::. 12% (if ,the KA and KI; parities are both odd) down to 6o/o (if KA odd but KZ even). 3 The data for the moat easily measured ratio, r(~0)/r(A), are given in Table 111. The "realistic" upper limits on the ratios are obtained by making a background subtraction; the ''maximum possible" upper limits come from counting every -z0 ,/ 1'1'- event with an effective :E0Tr mass falling within 30 Mev of theY; mass of 1385 Mev, The observed limits on the branching ratios are consistent with the predictions of global symmetry and not necessarily inconsistent with thoae of the Dalitz-Tuan resonance. *- I The Y polarization (Table lV) seems to be large at 850 Mev/ c and, by comparison, surprisingly small at 760 Mev /c. This could be a statistical fluctuation. or may be connected with the fact that the 'Z production channels also show a rapid variation with energy in this region. 8 * Since no strong evidence for Y spin J = 3/2 has yet been found either at Berkeley or elsewhere9 • 10 , it is intriguing to assume J = 1/2 and . to try to distinguish between sl/2 and pl/2 y* strong decay. .. *-We uae about 60 polarized Y at 850 Mev/c (assuming that the polarization is not a statistical fluctuation). In addition to the normal n to the production plane. it is convenient to define another unit vector !:? = 2 (!: · ~) - n. This vector m lies in the plane of n and £:(the direction of -7 •. UCRL-9640 flight of the .i\), and if A makes an angle· 0 with ~· ~ makes an angle 20. 9 \Ve write P for the measured /1. polarization along n and, similarly, ? n ., . - m • • along r.!.!· u the Y polarization is P, then for strong decay via s112• one can show (Sa) For ? 1; 2 decay the roles of !! and~ arc interchanged; i.e. the A has its maximum polarization along ~· and we have ? =-P/3, P = P', P /P = -3. n m. m n (5b) p = (+33±25)% We find P a (-56 d: ZO)%,Aar:id P /P = 0.6. It is not meaningful n '' m n to state statistical errors on a calculated ratio like P /P when the £rae-. m n z. tional standard deviations are large. Instead we apply the x test; the av-2 z erage val~e of X should be 1.0. For the Sl/Z hypothesis, we find Xg = 0.3 (high probabUity), but for P l /Z, X. PZ a 4.5 (probabUity a 3%). Martin . 2 et al. 9 also report P = -0.38:!::0.25 and P .= 0.19:!:0.25, which yield x~ = 0, n m ~ 2 . . • X p = 1. · Thus if it were established (a) that Y is the Da1it~-Tuan resonance of a I<+ pin an s 1/Z bound state and (b) that Eqs. (5) are not badly perturbed by interference phenomena, then our data would strongly suggest odd KA parity. ACKNOWLEDGMENTS It is a pleasure to acknowledge the help and support of Professor L. W. Alvarez and many members of the Alvarez and Gow groups. We thank R. H. Dalitz and A. Pais for their interest and suggestions. -3-FOOTNOTES *work supported by th'e U. S. Atomic Energy Commission. tNational Academy of Sciences Fellow. UCRL-9640 1. M. Alston, L. W. Alvarez, P. Eberhard, M. L. Good, W. Graziano, H. K. Ticho, and S. G. Wojcicki, Phys. Rev. Letters~· 520 ( 1960). 2. R. H. Dalitz and D. H. Miller, following Letter. 3. R. H. Dalitz, Phys. Rev. Letters §_, 239 ( 1961). 4. Paul Nordin, Jr., 5- and P-Wave Interactions of K- Mesons in Hydrogen, Lawrence Radiation Laboratory Report UCRL-9489 Rev., March 13, 1961; submitted to Physi(:al Review. 5. M. Gell-Mann, Phye. Rev. ~· 1296 ( 1957). 6. R. H. Dalitz and S. F. Tuan, Phys. Rev. Letters~. 425 ( 1959); Annals of Physics~. 100 (1959); and Annals of Physics 10, 307 (1960). The resonance is a consequence of the (a_) solution to the a-wave data on the K- + p interaction. It is not yet known whether (a_) is the correct solution.· See R. H. Dalitz, to be published in Revs. Mod. Phys., July 1961. 7. R. K. Adair, Phys. Rev. 100, 1540 ( 1955). 8. Alvarez Group Memorandum No. 252 (Jan. 1961). 9. H. J. Martin, L. B. Leipuner, VI. Chinowsky, F. T. Shively, and R. K. Adair, Phys. Rev. Letters 6, 283, 1961. See also: R. K. Adair, in Proceedings of the 1960 Conference on Strong Interactions, Berkeley, Dec. 1960 (to be published in Revs. Modern Phys.) 10. M. M. Block et al. (submitted to U Nuovo cimento). -9- UCRL-9640 (II .. Table 1. Coefficient a 1 ('~o) in Y 1 etrong-decay angular distribution. 760 850 1150 y*+ 1 -24 :b 20 -92 :!: 26 -70 :t: 26 -16:t20 -24 :d: 24 - l :t: 20 510 6ZO ~ 10- UCRL-9640 Table 11. Numbers and crosa sections for V and two-prong events. Numbers in parentheses are cross sections ln mUlib:r.rns. + - 0 ·1- -}, 'Tf 1T E Tr 'll' Total if·- *+ yl y Three-1 bod If 31( 1.2:1:.3) )I 4(0.2±.1) 0 0 36 I< 54(1.8~.:.) ) 11 0( 0. 3± .1) 0(0) 0(0) 64 760 12l(l.Z:~!.l4)122(l.Z±.l4) 56(0.6±.1) 55(0.6±.1) 20(0.Z±.OS) 2(0.03±.02) 376 850 56(0.9±.25) 62( l.O:t.Z 5) 67( 1.0±.15) 39(0.6:~:.1) 7(0.1±.04) 3(0.03:1:.02 Z 3.:! . : = -= __.__ UCRL-9640 Table Ill. Upper limits on the branching 1·atios ro:;0}/I"(A) !or 'y;. 760 850 rt~0)/r(A) (reasonable upper limit,::fiJ) 3 5 r("£0)/I'(A) (maximum possible ·upper limit,%) zo 10 -12- UCRL-9640 Table lV. y* polarb:ation (in o/tl) for f!*· ~ !.( 0.8~ assuming A decay asymmetry coefficient o. a + 1, Up e Y'XK - -• Polarization PK Py , c.m. ~MevLcl ~MevLc) y*- y*+ 760 240 + ll:t:Z 1 -16:21 850 250 • 56:~:20 +12:1:28 -13- UCRL-9640 FIGURE LEGENDS Fig. 1. Dalit:l' plot of ·K- + p- A+ ,/ + 1T- for 185 events at PK(lab) = 850 I *-Mev c. Note that within the Y 1 band, for example (i.e. for constant *-T +), T - is linear in (A· Y ), where A is the unit vector along the A 1f 1f --* - ' momentum in the Y rest frame (!, = - A). Thus A s which are produced •- ·- -forward by the strong decay of the Y (Y - 1r +A) fall at the lower edge of the ellipse, and vice versa. Point A corresponds to maximum, and point B to zero, A kinetic energy. • • • • Fig. z. Experimental histogram of the Y 1 mass, m (Y 1 ), at PK(lab) = 850 .. Mev/c. To each event plotted in Fig. 1 these corresponds two effective • *+ ·- • Y 1 masses, m(Y 1 ) and m(Y 1 ) ; we choose as m(Y 1 ) that which is closer to 1385 Mev. S-wave resonance curves of the form dn/dm ex:(( rn- 1385)Z+( r/Z)Z]-l have been fitted to the data as indicated, with r /Z = 10, 15, and ZO Mev. . + ~ Fig. 3. Excitation data for total A 1T 1T production and its apportioning to · •+ ·- 3 Y and Y • The dashed curves show Py• (c. m.) and Py• (c. m.) as a function of PK(lab) and are included for reference only. The line labeled 1r kz /Z is the maximum reaction cross section for a single isotopic-spin partial wave with J = 1/Z. The point plotted at PK(lab) = 510 Mev /c represent an estimate of the cross section based on the assumption that the number of events actually within the "bands" (see . * F1g. 4) .is the total number of Y 1 's produced and does not represent a unique interpretation of the data. Fig. 4. Dalitz plot of K-+p- A+ 1r++ 1T- for 31 events at. PK(lab) = 510 Mev/c. . •= I Fig. 5. Ada1r analysis of 6Z Y events at PK= 760 and 850 Mev c; I y~ Kl ~ 0.80. r· > Q) 2 E u -14-P.,.+,c.m. ( Mev/c) T.,.+, c.m. Fig. 1 300 . . . \ .· . . ·: . . '. . I I I I I I I I '----y---' y*-200 MU-22752-A UCRL-9640 PK-=850 ± 10 Mev/c ( 226 events) --- f/2 = IOMev 50 f/2= 15Mev .... Ql .&l 30 E :::::1 z ·--·f/2=20Mev -15- UCRL-9640 MU-22747 Fig. 2 -16-• Total A+.,.++.,.- production e y*+ production 8 Y*- production PK (Mev/c) Fig. 3 t UCRL-9640 (.) ' > Q) 300 :::!; 100 E 0 * ,.. a.. MU-22745-A 150 > Q) ::?! e 100 0 I !:: 1-50 50 -17-T1r+, c.m. (Mev) Fig. 4 ---------} • • -----y*+ MU- 22755 UCRL-9640 ,., 20 '-~ 10 E ::l z 0 1 f-1-1--I -18-l I I -l ---I I I -1.0 -0.6 -0.2 0.2 0.6 + 1.0 A·Y* MU-23254 Fig. 5 • This report was prepared as an account of Government sponsored work. Neither the United States, nor the Com-mission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with ~espect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa-ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any infor-mation, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any employee or contractor of the Com-mission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor . 

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THE PION LAMBDA RESONANCE (Y *)

THE PION LAMBDA RESONANCE (Y *)

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