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(To be submitted to Phys. Rev. Letters)

A TEST OF QUANTUM LECTRODYNAMICS Y ELECTRON-ELECTRONSCATTERING*

W. C. Barber+, B. Gittelman' , G. K. O'Neill', and B. RichteStanford UniversityStanford, California

3c Work supported by the U. S. Office of Naval Research under [Nonr 225(67)1 and [Nonr 1.858(39)1 and the U. S, Atomic EneCommiss ion contract [AT(&-3)-5151. The initial part of thwas supported by the joint program of the Office of Naval Rand the U. S. Atomic Energy Commission.

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I

HEPL SLAC-APRIL

A TEST OF QUANTUM LECTRODYNAMICS Y ELECTRON-ELECTRON CATW. C. Barbert, B. Gittelman* , G. K. O'Neill*, and B. Ri

Stanford University, Stanford, California

We have measured the differential cross-section for elecelectron scattering at a total energy of 600 MeV in the censystem, by a colliding-beam technique using storage rings 1,letter reports the final analysis of the data.

The parameters and operation of the storage rings have described3'4,5. The Stanford Mark III linear accelerator selectrons of 300 MeV, which then circulated in the rings atenergy, in a vacuum of 10-a torr, with a mean storage time A typical counting cycle consisted of 20 minutes of data-takiby 5 minutes for camera unloading, electron injection, and loading. At the start of each cycle the electron beams wer60 ma.

The detection apparatus is shown in Fig. 1. The veto sprovided a factor 3,000 of rejection against cosmic rays;arrangement of absorbers and counterswas measured to veto ol$ of 300 MeV electrons incident from below. The four sparwere triggered on the signature (.5+6+7+81-9tlO) - (any two o

Thirty thousand photographs were obtained in 174 hours time. For 38 of these hours the beams were displaced, eithe

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in the upper and lower chambers were measured independently. Thesystematic errors in angle measurement were found to be less than degree both in the upper and the lower chambers. The fiducial lengof the source region varied from 5 cm to 10 cm as a function of scaing angle. The circulating electron beams were nearly Gaussian inlongitudinal distribution, with full-widths at half-maximum of 2 nseconds. A correction was made for the variation in source densitydue to the longitudinal distribution of the circulating beams.

The background for analysis was 19 events out of 400 (see TableThe number of events per hour with source points outside transversefiducials was the same, within statistical errors, for beams inter-acting or non-interacting, and for no beams in the storage rings. conclude therefore that the principal background for analysis wascosmic-ray muons.

The observed angular distribution was compared with the Mbllerscattering formula 6 modified by a Feynman regulator and by a radiacorrection:

where

4 4+y G; 4:) (1+6) ,90

S2 = 4E2 , q2 = -4E2 sin2612 , q2 = -4~*cos~0/2 ,0Gk(a2) = 1 , and 6 is the radiative correction

1-c

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background to be subtracted. Column 4 is the Mbller cross sein units of l/S r', (rn/E)=l S Column 5 is the radiative correctiothe cut off energy and angle were determined by our experimentaconditions (30 MeV minimum final electron energy and 10 degreefor L@ unless a track was within 10' of the edge of the deteregion. LB is the angle between the tracks in the upper and chambers.) Column 6 i s a correction to the Mbller cross sectioincludes the radiative correction as well as geometrical correresulting from fiducial conditions. Column 7 gives the normatheoretically expected result for our detector if K 2 = 0.

The data were grouped in 1 degree angular bins and given likelihood analysis. All of the corrections and subtractions the data analysis were smaller than the statistical errors of events observed.

The maximum likelihood value of K 2 from the data at 6is

K-2 = + 0.001 i 0.033 F2 The limit of K 1-11; isK 1 < 0.18 F = 1.8 X 10 cm, K > 1.1 GeV/c (onedeviation)

K-l < 0.26 F = 2.6 x lo- 14 cm, K > 0.76 GeV/c (95a .K 2A preliminary analysis of the same data gave = 0.030 2

corresponding to a limit on K 1 for real K of 0.28 F , K(one standard deviation), and of .05 F for K 1 imaginary.

The difference in the best values of K 2 from the prelimand final analyses comes mainly from two sources: 1) The pranalysis did not restrict the collinearity of the tracks in tand lower chambers as tightly as the final analysis did. As there were extra events in the preliminary analysis which wertrated at the smaller angles, The extra events could have co

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The experiment is currently being extended to 1100 MeV total ein the center-of-mass system, with larger detector solid angle.One should note that this experiment is sensitive to the photon

propagator. It is, therefore, not in direct contradiction to theexperiment of Blumenthal et al 9 , which is sensitive mainly to theelectron propagator.

Measurement of the anomalous gyromagnetic ratio of the muon haprovided a sensitive test of quantum electrodynamics at small distaA breakdown of the theory could occur in a number of ways, but if muon moment is analyzed using a cut off on the photon propagator oFeynman form, the experiment 10 gives a limit K > 0.8 GeV (95% conwhich is comparable in sensitivity to our result.

It is a pleasure to thank Professor W. K. H. Panofsky and the Departments of Princeton and Stanford Universities for encouragingthis collaborative experiment, and the engineers and technicians group at both universities for their great effort in construction storage rings and of the detection equipment.

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REFERENCES*

t***

1.2.3.4.5.6.7.a.

9.

Work supported by the U. S. Office of Naval Research unde[Nonr 225(67)1 and [Nonr 18.58(39)] and the U. S. Atomic ECommission contract [AT(&-$5151. The initial part of twas supported by the joint program of the Office of Navaland the U. S. Atomic Energy Commission.High Energy Physics Laboratory, Stanford University, Stanornia.Palmer Physical Laboratory, Princeton University, PrincetoJersey (Internal code: 2456011).Stanford Linear Accelerator Center, Stanford University, California.G. K. O'Neill and J. A. Ball, USAECNYO-8015, July 1957.G. K. O'Neill, Bulletin Am. Phys. Sot. II, 3 No. 3, 170 W. C. Barber, B. Gittelman, G. K. O'Neill, W. K. H. PanoB. 'Richter, HEPL Report No. 170, Stanford University, JunG. K. O'Neill, Proceedings International Conference on HAccelerators and Instrumentation, CERN, Geneva, 1959, p. B. Gittelman, Proceedings of the Particle Accelerator CoWashington, D.C., 1965, p. 1033.C. Mbller, Ann. Physik 14, 531 (1932).Y. S. Tsai, Phys. Rev. s, 269 (1960).W. C. Barber, B. Gittelman, G. K; O'Neill and B. Richter, ings Fifth International Conference on High-Energy AcceleFrascati, Italy, 1965.Blumenthal, Ehn, Faissler, Joseph, Lanzerotti, Pipkin, a

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TABLJ3 IComparison of the Observed Angular Distribution with th-2Expected from Theory if K = 0.

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0.61890.73450.82640.9021)o. 96x31.00481.03681.05631.0515o. 9287

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Fig. 1FIGURE CAPTION

Configuration of the detection apparatus showingsparks from a real event.

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