Leptonic Decays of Vector Mesons: the Branching Ratio of the Electron-Positron Decay Mode of the Rho Meson J

VOLUME 19, NUMBER 15 PHYSICAL REVIE%' LETTERS 9 OCTOBER 1967

LEPTONIC DECAYS OF VECTOR MESONS: THE BRANCHING RATIO OF THE ELECTRON-POSITRON DECAY MODE OF THE RHO MESON J. G. Asbury, * U. Becker, William K. Bertram, *j P. Joos, M. Rohde, and A. J. S. Smith* Deutsches Elektronen-Synchrotron, Hamburg, Germany

C. L. Jordan and Samuel C. C. Ting Department of Physics, Columbia University, New York, New York/. (Received 25 July 1967)

At the Deutsches Elektronen-Synchrotron (a) BH background must be kept small, as (DESY) 6.2-GeV electron synchrotron, exper- its subtraction contributes to (1) a statistical iments are in progress to investigate the lep- uncertainty as well as an uncertainty caused tonic decays of vector mesons. ' We report by lack of precise knowledge of inelastic car- here a measurement of the branching ratio bon form factors. At symmetry, for a given — pair invariant mass M=2/8 is the momen- B= 1 (p' —e+e )/I'(p' —~+v ), (p turn of the electron and positron, 0 the angle from which the constant be coupling g p may of each with respect to the y beam), the BH obtained directly. To obtain (1), the rates of counting rate varies roughly as 9, whereas the two reactions the p'- e+e pair rate varies as 8 '. y+C-C+ p' (2) Therefore, L e++e- ( p - e+e )/BH = 8' and p' and the BH background can be reduced by ob- y+C-C+ -e+e ~ ~'+~ serving p at large angles. (b) The process e -e e contributes to the were measured under the same kinematical measured e e rate at the ~ mass. However, conditions and with the same apparatus, a sym- by using a spectrometer with a mass resolu- metric double-arm magnetic spectrometer tion comparable with the ~ natural width, one which has been described previously. ' The would see any sizable contribution from this measurement of (3) is the subject of the pre- process as an enhancement in the e+e invari- ceding Letter, s which is referred to as I in ant-mass spectrum. Similarly, a large inter- what follows. ference effect between p and ~ could be detect- To first order, both the Bethe-Heitler (BH) ed, as it would cause a distortion in the shape and Compton diagrams contribute to the reac- of the e+e mass distribution. However, there tion are arguments that the rate of p'- e+e- » rate y+C -C+e++e (4) of e —e+e: Bubble-chamber data show for H2 that e photoproduction =10% of po produc- For the case that the lepton pair e+e has tion. If p and ~ are produced by similar mech- an invariant mass M &600 MeV/c', the Comp- anisms, ~ this holds for carbon as well. Then ton term has been estimated and measured' the SU(6) prediction' of the ratio to contribute less than a few percent to (4) for pair production angles 8 & 7'. Therefore, for y 2:y 2:y '= 9.1:2 (6) p (d cp M. & 600 MeV/c' Reaction (4) can be used as a sensitive test of quantum electrodynamics gives the result that e-e+e is only 10% of at small distances. ' As first suggested by p'- e+e Berman and Drell, ' however, the Compton (c) To obtain an accurate value of the branch- contribution will be greatly enhanced if I is ing ratio (1) both the po production cross sec- equal to the mass of a vector meson (p, tion and the p polarization must be known over or p); hence (4) can also be used to study the the kinematical region for which e+e pairs leptonic decays of vector mesons. are accepted. For this experiment, system- For a measurement of the branching ratio atic errors caused by the p' cross-section mea- (1) to be meaningful, three major error sourc- surement were reduced by measuring both (2) es must be reduced or avoided: and (3) with the same apparatus. Also, the VOLUME 19, NUMBER 15 PHYSICAL REVIEW LETTERS 9 OCTOBER 1967 p polarization was determined indirectly as 11. ~(thear. ) implied in I, the conclusion being that p production at forward angles proceeds via diffrac- 10. tion dissociation, the p', therefore, being com- 1) Asbury ~taL, PR. L. 18, 65(1967) 2) 8~=9.54, 9 =2.250 GeV/c pletely transversely polarized. The angular 3) 8.=15', A=1,400 GeV/c 4) 8a=l5, PI= 1.950 GeV/c distribution W(8*) in the p' rest system is then determined: For pion pairs,

W (8*)=(3/87r ) sin'8*, (7) 7|'7r and for the electron pairs,

= W (8*) (3/16m ) (1+cos'8*) (6) i2)

0 (8* is the angle in the p' rest system between 0 200 600 1000 M~ (Me Y/C'J the and di- decay products the incident y-ray = rection). FIG. 1. The ratio 1t (Experimental yield)/(Calculat ed BH yield) is shown as a function of the invariant On the basis of considerations (a)-(c) above, mass of the e+e- pairs. the experimental arrangement was chosen to minimize systematic errors as follows: First, on the basis of ten events, for the spectrom- the largest central pair opening angle consis- eter setting 8, =15', P, =1.95 GeV/c (M, =1020 tent with a reasonable counting rate was cho- MeV/c'). Here the observed e e pair rate sen, namely 30'. The uncertainty in inelastic is =7 times that expected from BH; this gives form factors then introduces an error =5'fo in strong evidence that the decay y - e+e has the BH subtraction. Second, hodoscopes (de- been observed. scribed in Ref. 2) giving a mass resolution of The mass spectrum of e+e events is shown

+15 MeV/c' were used to enable &u —e+e con- in Fig. 2(a). The BH background has been sub- tributions to be observed. Finally, as described tracted, and all corrections have been applied. in I, errors in the p' production cross section The spectrum is in agreement with the modi- due to the of nonresonant m+m- back- presence fied Breit-signer mass distribution' of p -m n ground were reduced to +10% by the careful of 1 (1" =130+5 MeV' and M =765 p p MeV/c'). study of Reaction (3) for different target elements, The smoothness and approximate symmetry momentum transfers, and incident peak brems- of the distribution indicates that the cu - e+e strahlung energies k . (The bremsstrahlung contribution is (5%. beam is described in Ref. 2.) The branching ratio (1) is now obtained by = The e+e pair rate at 8, 15' and p, = 1.400 dividing the integrated yield of Fig. 2(a) by the GeV/c (central pair mass Mo =750 MeV/c2) yield of p mesons produced in the same accept- was much larger than that predicted by quan- ed region; this latter yield is shown in Fig. tum electrodynamics (QED). To show that this 2(b). Taking the polarization factors given apparent deviation is caused by an enhancement in Eqs. (7) and (8) into account, we obtain the in the Compton diagram and not by a fundamen- result' tal QED breakdown (modification of Feynman B = (6.5 + 1.4) x 10 propa, gator or vertex), a run was taken at 8, =9.5', p, =2.25 GeV/c (M, also =750 MeV/c'). As a direct application of our knowledge of As shown in Fig. 1, the ratio the behavior of p' photoproduction cross sec- tions on carbon IEq. preceding Letter] Experimental yield (2), R= and B determined in this experiment, the Bethe -Heitler yield p photoproduction amplitude (2) is determined increases rapidly with angle, in a manner con- experimentally to be of the form sistent with Eq. (5), demonstrating that the 2at. ip 2 2 large pair rate for M=750 MeV/c2 can be ex- A =D(k)pe ie /(m -m im 1"), - plained without invoking a fundamental break- p p down of QED. where ie'+ is an arbitrary phase and where Also shown in Fig. 1 is the value of R obtained, D(k) normalizes the amplitude to the observed

870 VOLUME 19, NUMBER 15 PHYSICAL REVIEW LETTERS 9 OCTOBER 1967 zero-degree p' photoproduction cross section. In the region of the p' mass the dominant virtual Comp- — ton scattering amplitude is therefore of the form -A ~ . A —with A —being the AC y~p py-- y~ee+e y ~+e amplitude for timelike y-& pairs. The interference between the BH and Compton processes is then described by the cross section

&at 2 g 7W P YP G (q')E E 'G (k) int dE dE dQ dQ , m2

(E E i / 1 1 && 2m I I+21 + I[-,''~ (E E )+k.p E k.g, E ] j (kp kp j + — + — — +

ie z /Qe, Qp ~ ——(P P +P P )I — -I D(k)ae (9) ~ +. —. +& -& ip k m 2-m2-zm I p p where k = photon four-momentum; G(k) = bremsstrahlung energy spectrum; M = mass of tar- The interference cross section dajnt is asym- get nucleus; Q =k+P p-p (re—coil four-mo- metrical under interchange of four-momenta mentum of nucleus); P = initial four-momen- of electron (p ) and positron (p+), and thus tum of nucleus; and t=(k —P+ —P )'. The met- can be observed by taking the difference between ric chosen is@»=1, g;;= —1 (i =1, 2, 3), and two asymmetrical yields for spectrometer set- the constant deter- tings of opposite polarity. The resultant yield y-p coupling g p is directly mined from the branching ratio B. is then independent of any possible systematic asymmetry in the spectrometer as well as pure 1 . dd BH and Compton contributions. cf' pp. fl dfl drn nb Our measurements at 15', 1400 ~. 156' 9 events MeVic (m0 [(Gev/c) MeV/c& = rn&), when compared with (9), give us P = 15 1,xlO " + 25', consistent with the prediction of a pure- ly absorptive (Q =0') Compton amplitude. We thank Dr. J. Pumplin for pointing out the .5x10" importance of only including transversely polarized p's in the derivation of Eq. (3). To him and to Dr. J. Fleischer we are grateful 0- for performing careful checks of our original (00 500 600 700 800 900 1000 calculations. We thank Dr. M. Binkley for par- 46 M«[MeV/c~] f'».f, died ticipation in the last part of the run. We are nb t(Gev/c) &Me grateful for the enthusiastic support of Pro- fessor W. Jentschke and Professor P. Stahel- vents j in of DESY and Professor L. M. Lederman (a) and Professor S. Devons of Columbia, which m =/65 MeV/c~ (' ( =130 MeV made this collaboration possible. We also thank Professor H. Joos, Professor G. Weber, Pro- fessor M. Ross, Professor S. D. Drell, and 600 700 %0 1000 Dr. S. J. Brodsky for interesting discussions. Me e IMeV/c~) We thank Dr. H. O. Wuster and Dr. D. Lublow FIG. 2. (a) Pair yield from p z z as function of for extending to us the DESY computing facil- invariant mass. BH background has been subtracted, ities, and Mr. H. Kumpfert and the synchro- and the magnitude of the yield includes the effect of tron staff for their skillful operation of the the complete polarization of the p mesons. The peak machine. bremsstrahlung energy kmax =6.0 GeV. {b) Pair yield from p 7(+x as function of invariant mass. Again the effect of polarization of the p mesons is included in the calculation of the yield. The momentum p is *Volkswagen Foundation Fellow at DESY. 2.8 GeV/c, and km~ =6.0 GeV. )Present address: Physics Department, University

871 VQLUME 19, NUMBER 15 PHYSICAL REVIEW LETTERS 9 ocTQBER 1967 of Illinois, Urbana, Illinois. on Electron and Photon Interactions at High Energies, fA DESY-Columbia collaboration, this work was sup- Hamburg, 1965 (Springer-Verlag, Berlin, Germany, ported in part by the U. S. Atomic Energy Commission. 1966), Vol I, p, 71. ~For theoretical papers on leptonic decay of vector 5J. G. Asbury, U. Becker, William K. Bertram, mesons, see M. Gell-Mann, D. Sharp, and W. G. Wag- P. Joos, M. Rohde, A. J. S. Smith, C. L. Jordan, and ner, Phys. Rev. Letters 8, 261 (1962); S. L. Glashow, Samuel C. C. Ting, to be published. 6 Phys. Rev. Letters 7, 469 (1961); M. Gell-Mann and Cambridge Bubble Chamber Group, in Proceedings F. Zachariasen, Phys. Rev. 124, 953 (1961); Y.Nambu of the International Symposium on Electron and Photon and J. J. Sakurai, Phys. Rev. Letters 8, 79 (1962); Interactions at High Energies, Hamburg, 1965 (Spring- S. M. Berman and S. D, Drell, Phys. Rev. 133, B791 er-Verlag, Berlin, Germany, 1966), Vol. II, p. 1; (1964); N. M. Kroll, T. D. Lee, and B. Zumino, to be Phys. Rev. 146, 994 (1966); Phys. Rev. Letters 13, published; R. van Royen and V. F. Weisskopf, to be 640 (1964). German Bubble Chamber Collaboration, published; H. Joos, Phys. Letters 24B, 103 (1967); in Proceedings of the International Symposium on Elec- K. Kajantie and J. S. Trefil, Phys. Letters 24B, 106 tron and Photon Interactions at High Energies, Ham- (1967). burg, 1965 (Springer-Verlag, Berlin, Germany, 1966), J. G. Asbury, U, Becker, William K. Bertram, Vol. II, p. 36; Nuovo Cimento 41A, 270 (1966); and to P. Joos, M. Rohde, A. J. S. Smith, S. Friedlander, be published; DESY Report No. 66/32 (unpublished). C, Jordan, and Samuel C, C. Ting, Phys. Rev. Letters C. A. Levinson, H. J. Lipkin, and S. Meshkov, 18, 65 (1967). Phys. Letters 7, 81 (1963). G. Asbury, U. Becker, William K. Bertram, J. D. Jackson, Nuovo Cimento 34, 1644 (1964). J. 9 P. Joos, M. Rohde, A. J. S. Smith, C. L. Jordan, and For up-to-date comparison of this work with other Samuel C. C. Ting, preceding Letter fPhys. Rev. Let- related results see Samuel C. C. Ting in Report to the ters 19, 865 (1967)]. 1967 International Conference on Photons and Elec- A. Krass, Phys. Rev. 138, B1268 (1965); S. D. trons, Stanford University, September, 1967 (unpub- Drell, in Proceedings of the International Symposium lished).

TEST FOR TIME-REVERSAL INVARIANCE OF THE ELECTROMAGNETIC INTERACTION USING THE MOSSBAUER EFFECT IN Ru 9 j' O. C. Kistner Brookhaven National Laboratory, Upton, New York (Received 1 August 1967)

The apparent violation of CP invariance in al percent. The present experiment employs the 2v decay mode' of the E,' has led Bern- the Mossbauer effect on the 90-keV Mj.-E2 stein, Feinberg, and Lee to bring attention mixed transition' in Ru" and is an order of to the fact that there is no substantial exper- magnitude more sensitive, thereby bringing imental evidence that the electromagnetic in- the measurement well within the region of interaction of the strongly interacting particles terest. is invariant under C and T. These authors Jacobsohn and Henley' have tabulated the var- show that strong violation of T invariance of ious possible quantities which are proportion- the electromagnetic interaction may reveal al to sing. Of these, the term of the form itself, through higher order effects, as a T- (k ~ j &e) (k j) (e ~ j) can be obtained experimen- noninvariant impurity of 0.1 to I /o in the am- tally by measuring the recoil-free resonant plitude of the nuclear gamma-decay matrix cross section of an appropriate magnetic hyper- element. The invariance under T requires fine component in a, magnetically oriented ab- that the relative phase angle y between the sorber for linearly polarized garnrna rays. individual multipole amplitudes of a mixed Let the propagation vector k and linear polar- gamma transition be either 0 or w. A finite ization vector & of the incident resonant radi- va, lue of sing would be equa, l to the fraction ation be directed along the z and x axes of a of T-noninvariant amplitude present. The most Cartesian coordinate system, respectively. sensitive experiments previously reported have Let the direction of the magnetic field at the employed P -p, -&2 triple coincidence measure- nuclei of the magnetized absorber be specified ments and y, -y, coincidence measurements by the azimuthal and polar angles & and P re- 4 following polarized-neutron capture, a,nd quote spectively. The cross section for a &ni=+1 an upper limit for sing of the order of sever- hyperfine component in the absorber is then

872