Voj.UMz 9, NUMsxa 5 PHYSICAL REVIEW LETTERS SEpTzMSKR 1, 1962

decreases rapidly to 0 at -t = 1 (GeV/c)', then

has a significant change in slope, so that itsvalue at t= 5-(GeV/c)' seems to be - -~. If thisis the behavior of a(t), the consequent beha. viorof F(t) is a rapid exponential decrease from +1

at t =0 to -4x10 a.t -t =1, followed by a. veryslow decrease to -1 x10 ' at t = 5 (GeV/c)'.

However, the magnitude of the errors in theexperimental points must restrain deductionsbased on these results onl. y. Further measure-ments are planned to determine whether or not

o(t) is indeed approaching an asymptotic limit.In the framework of the Regge-pole hypothesis,the question of the asymptotic limit of n(t) has abearing on the "elementarity" of the pion and onthe nature of the short-range nucleon-nucleoninteraction.

Re wish to acknowledge the invaluable coopera-tion of the AGS staff and operating crew, a.nd

thank Dr. R. L. Cool, Dr. R. H. Phillips, and

Dr. T. F. Kycia for their help and interest in

this work.

~Work supported by the U. S. Atomic Energy Com-

mission and by a grant from the National ScienceFoundation.

~C. Lovelace (to be published). V. N. Gribov, J.Exptl. Theoret. Phys. {U.S.S.R.) 41, 667 (1961)[translation: Soviet Phys. —JETP 14, 478 (1962)].G. F. Chew and S. C. Frautschi, Phys. Rev. Letters7, 394 (1961); 8, 41 (1962); Phys. Rev. 123, 1478(1961). S. C. Frautschi, M. Gell-Mann, and F. Zach-ariasen, Phys. Rev. 126, 2204 (1962). R. Blanken-becler and M. L. Goldberger, Phys. Rev. 126, 766(1962).

~G. Cocconi, A. N. Diddens, E. Lillethun, G. Man-

ning, A. E. Taylor, T. G. Walker, and A. M. Wether-ell, Phys. Rev. Letters 7, 450 (1961); A. N. Diddens,E. Lillethun, G. Manning, A. E. Taylor, T. G. Wal-ker, and A. M. Wetherell, Phys. Rev. Letters 9, 111(1962).

J. B. Cumming, G. Friedlander, J. Hudis, andA. M. Poskanzer, Brookhaven National LaboratoryInternal Report BNL 6034, 1962 (unpublished).

4The extrapolation of the line for Po = 11 GeV/c tothe value -t = 11 (GeV/c)2 investigated in our secondmeasurement seems to indicate that the elastic crosssection there may be a factor of -10 smaller than ourupper limit. Obvious improvements in the experimen-tal techniques ( flat top beam and better geometry)will probably allow measurements at such momentumtrans fer s.

PROPERTIES OF THE g MESON

H. Foelsche, E. C. Fowler, H. L. Kraybill, J. R. Sanford, t and D. StonehillfYale University, New Haven, Connecticut

{Received July 30, 1962)

Since the discovery of the eta meson, ' its spinand parity have been in doubt because of back-ground contamination and poor statistics in avail-able samples of eta decay. The purest samplehas been obtained by Bastien et al. ,

' who havereported 23 charged eta decays (m+w w') fromreactions of the type K +P~ A + q, with esti-mated background of three events. They havetentatively assigned spin zero, negative parity,and positive G-parity to the eta meson.

Since the above conclusions clearly requireverification, we report results from 102 chargedeta decays with estimated background of only 6 /&,

produced in four-pronged reactions of a m+ beamin the Brookhaven 20-in. hydrogen bubble chamberat the Cosmotron, at incident kinetic energies of1090 MeV and 1260 MeV. Our results favor 0for the spin, parity, and G-parity of the eta mes-on, and definitely exclude the simple isospin-conserving assignments 0, 1, and 1 . Wealso find that at most a small amount of the de-

cay mode (~ v y) is present in our data.At 1090 MeV, we have identified 1165 cases of

the reaction

and 85 cases of the reaction+ + + -

O7T +P~P+ 7T + 77 +77 +7t' .

The corresponding numbers for 1260 MeV are265 and 28, respectively. In Reaction (2) either

meson may be combined with the ~ and 7)'

mesons to produce a ~+7t ~' triplet. The effectivemasses for these two triplets have been com-puted for each event. A histogram includingboth effective-mass combinations shows a prom-inent peak lying at 548 MeV at both energies. 'Figure 1 shows the effective-mass spectrumobtained by selecting from each event that~+~ m' triplet whose effective mass lies near-est to 548 MeV. It is clear from Fig. 1 thatalmost every example of Reaction (2) is con-

VOLUME 9, NUMBER 5 PHYSICAL REVIEW LETTERS SEPTEMBER 1, 1962

20—l 090 MEV

85 EVENTS

0.7

0.6

03o o

Q04 o o o r

0OOr o

0.50

0 oa

b)

Q2

500 550I

600 Q. l

Q.p I

QQ Qt 0.2 08 Q.4T.- T-

@Q

TLi

Z 4 e 8 to tZ

EYEHTS / IPtlT AREA

1260 MEV

28 EVENTS

I

500(MEV)

I

600 650

FIG. 1. Histogram of effective mass of T( 7' ~ in thereaction Tt++p-p+ 7t++ p++7t + ~ . From each event,that combination of ~+~ ~0 was selected which had masscloset to 548 MeV.

sistent with production of an eta meson. Thesmall background of events having Tr+v mo effec-tive mass within the peak region of 530 MeV to570 MeV was estimated as follows. Assumingthat the secondaries from the background eventsare uniformly distributed in five-particle mo-mentum space, the expected ~+m & mass dis-tribution was computed. Two thirds of this dis-tribution lies outside the region 530 MeV to570 MeV. Accordingly, the four events at 1090MeV and seven events at 1260 MeV which lie out-side the peak region imply two background eventswithin the peak region at 1090 MeV and four at1260 MeV. Thus the background in our sampleof etas is only about 6%. It is interesting tospeculate that the high purity of the sample at1090 MeV may be connected with the fact that thereaction proceeds at the threshold for ~++/-&~ m +n 0

In order to study the true width of the eta mes-on, the resolution function was obtained by sum-ming the computed Gaussian error functions forthe individual events, and it is shown in Fig. 1.The full width at half maximum of the resolution

FIG. 2. (a) The Dalitz plot of 102 eta decays to7t ~ ~, produced by 7)++p-It++q at 10SO MeV and1260 MeV. Q is the total kinetic energy of the decaypions in the center of mass of the eta. 1'+, T, and

Tp are the pion kinetic energies in the same system.I'b) The variation of density of events with remotenessfrom the center on the Dalitz plot. The horizontalscale denotes the average density within strips bounded

by curves of constant matrix element for the case 1These curves look roughly parallel to the boundary ofthe plot. The vertical scale is the value of 70/Q atwhich these curves intersect the 1O axis (see refer-ence 6). The solid curve is the prediction for 1For the 0, the density should be constant. For 0and 1, the average density should be close to zeronear the center, rising toward the boundary of the plot.

function is 13 MeV. The agreement of our reso-lution function and the experimental peak is ex-tremely close at 1090 MeV. At 1260 MeV theagreement is worse, but adequate within thepoorer statistical significance. At 1090 MeV,there are no events in the regions 510 MeV to530 MeV and 570 MeV to 590 MeV, where aBreit-Wigner shape with I'/2= 5 MeV (half-widthat half maximum) would predict six events. Wetherefore conclude that the half-width of the etais less than 5 MeV. Our best value for the etamass is 548~ 1 MeV from the 1090-MeV dataand 551+ 2 MeV from the 1260-MeV data. Thisis in good agreement with the values previouslyreported by several authors. "~'

The Dalitz triangular plot for the 102 eventsin the mass peaks is given in Fig. 2(a). The ex-pected appearance of this plot for various spinand parity assignments has been described else-where' ' and is summarized briefly in Table I.Our data are unfavorable to all assignments in

VOLUME 9, NUMBER 5 PHYSICAL REVIEW LETTERS SEPTEMBER 1, 1962

Table I. Characteristic appearance of the Dalitz

plot for the simplest assignments of spin, parity, and

G-parity for an isospin zero meson.

Spin Parity G-parity Matrix element vanishes

on the axesat the boundaryin the centernov here (uniform density)'I'

0 axis and the bounclaryTr"

I ()-0

Table I except 0, since the density is not

significantly low at the axes, center, or boundary

of the plot, and the distribution is consistent with

uniform density over the entire area. In Fig. 2(b)we present the density of events as a function of

distance from the center of the Oalitz plot. Thetheoretical prediction for 0 and 1+ is zerodensity at the center, increasing toward the

boundary. For 1 the density should decreasefrom a maximum at the center to zero at the

boundary as shown in the figure. Our data agreewith 0 +, which predicts constant density, and

have negligible probability of representing afluctuation from the theoretical prediction forany of the three simple assignments 0, 1

1, for which the three-pion decay would con-serve G-parity.

The assignment of positive G-parity to theeta meson implies that isospin is not conservedin the decay into three pions, and that electro-magnetic interaction should be involved in thisdecay. Further, if the quantum numbers are0 +, the decay modes (y, y) and (cc, cc, y)should be prominent. Within the limitationsof our present analysis, we can estimate the

upper limit in the rate of the (cc+, cc,y) mode,and we find that the ratio (cc+, &i, y) to (cc, cc, cc')

is small. Our present four-prong analysis testseach measured event on the hypothesis of beingReaction (1), Reaction (2), or a Dalitz pair eventof the type

+ + pcc +P ~P+a +P + (3)~e +e +y.

A fourth category of events consists of thosewhich, though well measured, do not fit any ofthe hypotheses (1), (2), or (3). (%e have found

only four examples of the reaction m++ p n+~++w++cc . ) The kinematic fit to cc++p ~ p+ri ++cc+

+~ +y was not attempted; therefore the et', de-cay into (cc+, cc, y) can be present only among theunidentified events or as a misidentification in

categories (1) and (2).In Fig. 3(b). we present the spectrum of the

(00'-

C1 tvliSS)NCc MASS (+

Q EFFECT(VE cvlASS (~)-) -20

)090 ~y

4J

O

zO

X

O

-l5 m

4J

$io

24J

5

500

( MEV)

c l I c

0I

I

(io4 Mcv~)

I"IG. 3. (a) The spectrum of missing mass (-, , —, , ?), carried away by all secondariesexcept the proton and one — and the spectrum of the effective mass (- 7 ) in the eventsidentified as —P -P~ — + — + —. . If the (- —. y) decay of the eta were hiding in this classof kinematic identifications, the missing mass of (-, — ~) should show a peak above theeffective-mass spectrum of (-, —, ), in the region between 530 MeV and 570 MeV. (b) Spec-trum of the square of the missing mass of the -0 for events identified as ~ +p —p+ ~ + 7c.

+;, + —., o. If in any of these events a gamma ray had been mistakenly identified as a ~co

the missing mass will be close to zero. Some of the missing masses near zero are,however, clue to the tail of the & spectrum. It appears, therefore, that our samplecoulcl contain only about three events in which a gamma is misidentified.

VOLUME 9, NUMBER $ PHYSICAL REVIEW LETTERS SEPTEMBER 1, 1962

square of the missing mass of the &' in thoseevents which mere identified as satisfying thehypothesis (2) at 1090 MeV. Since it shows no

peaking in the vicinity of zero, it appears thatonly about two or three events in this class ofidentifications could be w++ p ~m++ m++ n +y.An appreciable number of (m+, w, y) decayscould presumably hide in the large reservoirof identifications of Reaction (1). To set an

upper limit on this number me have computedthe missing mass (w, w, &') carried away byall the secondaries other than the proton andone m . Figure 3(a) shows this missing massspectrum for 1090 MeV, with the spectrum ofeffective mass (v+, w ) inserted as a control.The computation was performed upon the meas-ured track momenta and angles, not upon theadjusted values obtained by the kinematic fittingprocess. If these events are indeed correctlyidentified as Reaction (1), the missing-massand effective-mass histograms should agreeclosely. On the other hand, contamination ofthe sample by m++P « P + m++ g with the etasubsequently decaying to (m+, w, y) would pro-duce a sharp peak at 550 MeV in the spectrumof missing mass (w+, m, 7') but not in the spec-trum of effective mass (m+, m ). Figure 3(a)shows close agreement between the histograms,and it is very unlikely that 20 (w, w, y) decaysin the region of about 540 to 560 MeV mould bemasked by background fluctuations. Finally,the class of unidentified four-prong events con-tains only about four events whose missing mass(m+, v, '?') is consistent with the mass of the etaand they represent part of a continuous back-ground We con.clude that the ratio of the (w+, v, y)

decay to the charged three-pion decay (v+, v, m')

is less than 25%. Our estimate is probably tooconservative, since we expect a larger fractionof events m++P-+P+& +m +m +y to end up inthe category of unidentified events. A closerexamination of the question of kinematical am-biguities in Reactions (1) and w++ p ~p +m++ w+

+ & +y is likely to reduce our estimate of therelative decay rates of the eta. meson.

We acknowledge helpful discussions with Pro-fessor Horace Taft, Professor Jack Sandweiss,Professor Jack Steinberger, and members of thegroup at Berkeley. Dr. Hans Courant participatedin the early stages of the experiment, and Mr. FredJames assisted in the analysis in the later stages.

*This work is supported in part through funds pro-vided by the U. S. Atomic Energy Commission.

TNow at Brookhaven National Laboratory, Upton,New York.

'A. Pevsner, R. Kraemer, M. Nussbaum, C. Rich-a.rdson, P. Schlein, R. Strand, T. Toohig, M. Block,A. Engler, R. Gessaroli, and C. Meltzer, Phys. Rev.Letters 7, 421 (1962}.

P. Bastien, J. P. Berge, O. I. Dahl, M. Ferro-Luzzi, D. H. Miller, J. J. Murray, A. H. Rosenfeld,and M. B. Watson, Phys. Rev. Letters 8, 114 (1962).

T. D. Lee and C. N. Yang, Nuovo cimento 3, 749(1956).

4H. Foelsche, E. C. Fowler, H. L. Kraybill, J. R.Sanford, and D. Stonehill, presented by J. Sandweissto the Annual International Conference on High-EnergyPhysics at CERN, 1962 (to be published).

E. Pickup, D. K. Robinson, and E. O. Salant, Phys.Rev. Letters 8, 329 (1962).

6M. L. Stevenson, L. 7'. Alvarez, B. C. Maglic, andA. H. Rosenfeld, Phys. Rev. 125, 687 (1962).

PHOTOPRODUCTION OF MUON PAIRS IN CARBON

A. Alberigi-Quaranta, M. De Pretis, t G. Marini, f A. Odian, ~] G. Stoppini, f and L. Tauf,Laboratori Nazionali del Sincrotrone, Frascati, Roma, Italy

(Received July 30, 1962)

Evidence for muon-pair photoproduction in shown in Fig. 1. The 1000-MeV bremsstrahlungnuclei was given in a pioneering work by Masek beam of the Frascati Electron Synchrotron wasand Panofsky, who succeeded in separating one collimated into a 2-cm diameter spot to strikemember of the pair from a large background of a carbon target of 5 cm in length (8.8 g/cm').pions and electrons. We report here on an ex- Positive muons emitted at 10 in the momentumperiment in which coincidences between the two interval from roughly 300 to 400 MeV/c weremuons have been detected, at a rate in perfect deflected by a double-focussing magnetic spec-agreement (within our 5% accuracy) with that trometer' to traverse one of the three momentum-deduced from the Bethe-Heitler fomula. ' defining scintillation counters n, P, and y, and

A drawing of the experimental apparatus is two wedge-shaped copper absorbers. After

226