Longitudinal Polarization of the Electrons from the Decay of Unpolarized Positive and Negative Muons

Transcription

1 Proceedings of the Physical Society Longitudinal Polarization of the Electrons from the Decay of Unpolarized Positive and Negative Muons To cite this article: G Culligan et al 1959 Proc. Phys. Soc Related content - NEW EXPERIMENTAL DATA ON - AND - MESON DECAYS A O Vasenberg - The charge-exchange scattering of negative pions by protons at 96.6 MeV S G F Frank, J R Holt, T Massam et al. - The Elastic Scattering of 98 MeV Negative Pions by Hydrogen D N Edwards, S G F Frank and J R Holt View the article online for updates and enhancements. Recent citations - Beam photon Sorin Vlaicu - CP symmetry and its violation in fundamental interactions S D Rindani - Pure leptonic weak processes A. Bertin and A. Vitale This content was downloaded from IP address on 23/08/2018 at 05:37

2 169 Longitudinal Polarization of the Electrons from the Decay of Unpolarieed Positive and Negative Muons BY G. CULLIGAN, S. G. F. FRANK AND J. R. HOLT Nuclear Physics Research Laboratory, University of Liverpool,Communicated by U. W. B. Skinner; MS. received 14th August 1958 Abstract. The polarization of electrons from the decay of unpolarized positive and negative muons has been detected by the method of transmission of bremsstrahlung through magnetized iron. It is shown that the positrons have positive helicity and the negatrons negative helicity, thus providing a clear demonstration of violation of invariance under charge conjugation. It follows on the basis of the two component neutrino theory that the neutrino associated with the decay of the pion has negative helicity and the anti-neutrino positive helicity. The agreement of this with recent results for the neutrino in,%decay lends support to this theory and confirms the law of conservation of leptons INTRODUCTION HE two component neutrino theory (Lee and Yang 1957a, Salam 1957, T Landau 1957) has, up to the present, been very successful in accounting for experimental data concerning the weak interactions between fermions. At the basis of this theory are the ideas that the neutrino and anti-neutrino are fully polarized parallel or anti-parallel with respect to their direction of motion and that in all reactions the number of leptons minus the number of anti-leptons remains constant ( law of conservation of leptons ). The actual sign of the helicity of the neutrino or anti-neutrino, which is + 1 for the parallel case and - 1 for the anti-parallel case, has to be decided experimentally and the leptonic assignment of the particles e+, pi, v and by definition and experiment. The lepton conservation law requires that the processes of positive and negative /3-decay should be associated uniquely with neutrinos of opposite kind. The processes are then p+n+e++v or p+e-+n+v and n+p+e-+v, where in the first two cases the particle is defined to be a neutrino and in the last case an anti-neutrino. The negatron is by definition a lepton, so it follows that the neutrino also is a lepton and the positron and anti-neutrino are anti-leptons. The experiments described below are concerned with the fixing of the neutrino and anti-neutrino helicities and with the applicability of the lepton conservation law to the decay processes of the pion and muon. We have shown that the positrons from the decay of unpolarized positive muons have positive helicity and that the negatrons from the decay of negative muons have negative helicity. The result for positive muons ha8 already been published in brief form (Culligan ut al. 1957) and we have recently learned that similar results for both particle8 have been obtained elrewhere (Crowe, private Communication). The followipg argument \is baaed on the two qomponent theory, 5,. PROC. PHYS. 80C. LXXIII, E M

3 170 G. Culligan, S. G. F. Frank andj. R. Holt The pion decay process r+p + v involves a single neutrino or anti-neutrino, The muon decay is known to be predominantly p+e+v+i; involving one neutrino of each kind, because of the shape of the energy spectrum of the decay electrons (Lee and Yang 1957 a). Experiments have shown that the muon in the first process is polarized (Garwin, Lederman and Weinrich 1957, Friedman and Telegdi 1957, Cassels et al. 1957), the electrons in the subsequent decay of both positive and negative muons being emitted predominantly backwards relative to the direction of emission of the muon from the pion. Using the above information we can, in a simple way, connect the sign of the helicity of the neutrinos or anti-neutrinos emitted in the pion decay with that of the electrons emitted in the subsequent muon decay. Figure 1 illustrates the argument ; the pion may be either positive or negative and the electron helicity has been arbitrarily taken to be positive. Figure 1. To illustrate the connection between the helicities of the electron, muon and first neutrino in the w, p, e decay chain. The electron helicity has been taken to be positive but otherwise the diagram applies equally to positively charged and negatively charged particles. The two particles emitted in the pion decay must spin in opposite directions to conserve angular momentum since the pion has zero spin ; they move in opposite directions and thus have the same helicity. In the subsequent decay of the muon, since the electron energy spectrum is peaked towards the upper limit, most of the electrons are associated with neutrinos moving into the opposite hemisphere, As these neutrinos, of opposite kind, have opposite helicities, their spins will tend to cancel and the electrons and muons will spin in the same sense. The latter two are known to move in opposite directions so they must have opposite helicities. Thus we conclude that the electrons associated with the muon decay and the neutrinos associated with the pion decay have opposite helicities. On the basis of the two component theory the results of our experiment thus fix the helicities of the muons of both signs (negative for positive muons and positive for negative muons) and show that the neutral particle emitted in positive pion decay has negative helicity while that emitted in negative pion decay has positive helicity. Applying lepton conservation to the decay processes of the muon and pion it can be seen that the positive muon is an anti-lepton and the particle emitted with it in the positive pion decay is a lepton, i.e. a neutrino rather than an anti-neutrino. SO we conclude that the neutrino has negative helicity. Recent work on nuclear p-decay (Goldhaber et al. 1958) has led to the same conclusion regarding the neutrino involved in this process. Since both conclurionr are dependent on the law of conrervation of leptonr their agreement provider confirmation of thir law as applied to the procewr of 8-decay, pion decay and muon decay and ala0 rupportr the two component theory.

4 Polartsatzon of Electrons from Muon Decay 171 The sign of the neutrino helicity together with that of the positrons emitted with them in 8-decay, which is known to be positive (Rehovoth Conference Proceedings 1958) enables the type of angular correlation between the directions of emission of the two particles in this process to be determined. Thus in a 0 -P 0 allowed transition the particles must be emitted predominantly in the same direction to conserve angular momentum, whereas in a AJ = 1 allowed transition they must be emitted predominantly in opposite directions. These correlations are characteristic of the vector V and axial vector A types of coupling respectively. This argument provides an important part of the evidence for the V and A types of coupling as opposed to the scalar S and tensor T types which were favoured until recently (Culligan et al. 1957, Sudarshan and Marshak 1958). Calculations of the electron polarization in muon decay have been carried out by Lee and Yang (1957 b), by Uberall(1957) and by Kinoshita and Sirlin (1957). These show on the basis of the two component theory, that electrons of all energies from the decay of unpolarized muons have the same polarization. The value of the polarization is proportional to the parameter e of the theory, which depends on the type of coupling between the particles and can take a value between - 1 and + 1. The extreme values correspond to 100% polarization of the electrons. The magnitude of 6 can be determined also from measurements of the asymmetry of electron emission from polarized muons, a combination of earlier results leading to a value of *12 (Wilkinson 1957) and a recent experiment to the value 0.91 & 0.14 (Swanson 1958). Our measurements of the polarization are considerably less accurate than these, but certainly require a value greater than 0.6. Finally, the result that the electrons from the decay of unpolarized positive and negative muons at rest have opposite. polarization provides a clear demonstration of the violation of invariance under charge conjugation. $2. APPARATUS TO detect longitudinal polarization of the electrons from muon decay we have made use of the fact that such polarization persists in the bremsstrahlung quanta produced by them (McVoy and Dyson 1957). The polarization of the quanta can be detected by comparing their transmission through iron magnetized either parallel or anti-parallel to their direction of motion. There is a difference due to the Compton cross section being dependent on the relative directions of the spina of quantum and scattering electron (Gunst and Page 1953). Above 0.6 MeV the total Compton cross section is greater when the spins are anti-parallel. The difference between the cross sections for the anti-parallel and parallel cases increases from a negative value at low energies through zero at 0.6 MeV to a broad maximum at about 3Mev and then decreases slowly at higher energies, Over the energy region with which we are concerned (10 M ~ to V 50 MeV) the difference in transmission through the 16cm thickness of iron used in the experiment, when the direction of magnetization is reversed, is expected to be about 5 o/" if the y-radiation is fully polarized, The experimental details were different in the work with positive and negative muons. In the first case the pion beam was brought to rest in a carbon target in which the muons were produced by decay. In the second it was necersary to use the muon8 present as a 3 yo contamination of the negative pion beam. The latter experiment was the more difficult of the two becaure the muon intenrity was mmaller by a factor of 4 and alro becauie of a 12% contamination of ~ ~ MI

5 172 G. Culiigan, S. G. F. Frank and J. R. Holt electrons in the negative pion beam. Special arrangements had to be made to eliminate the background produced by these electrons. Figure 2 shows the general arrangement of the apparatus with the counters used in the negative beam, while figure 3 shows the details of the polarization detector with the counters used in the positive beam. From counter number 2 unwards the apparatus was essentially the same in both cases. Figure 2. General arrangement of apparatus and counters used in the negative pion beam. CDmpensatng Col 1!.& 3 Lead Screen M Figure 3. Details of polarization analyser and counters used in the posltive pion beam. Muons decay in the carbon target C, 5cm thick, and decay electrons are detected by plastic scintillation counters 3 and 4 each 10 cm square, The electrons produce bremsstrahlung in the lead radiator Pb, 5 mm thick, and this then passes through the 16 cm long cylindrical core of the magnet M to be detected in the 5 inch diameter NaI(T1) crystal. In order to select the required pulses from the crystal a fast coincidence was arranged between the electron counters 3, 4 and pulses greater than about 2 ~ e v from the crystal, This coincidence pulse operated a gate through which the linear pulses from the crystal passed to a pulse height analyser. The main sources of background were random coincidences between single pulses from the cryrtal and double coincidences 3,4 and in the case of the negative muon experiment, real triple coincidences produced by the high energy electron contamination of the beam. The double coincidence rate 3, 4 was reduced by placing counter number 2, 20 cm in diameter, in anti-coincidence, In the positive beam the main function of. a.

6 Polarization of Electrons from Muon Decay 173 this was to reduce the rate in these counters due to pions. In the negative beam it helped to eliminate the effect of the electron contamination. However, it was not very efficient in doing this because of the presence in the beam of bremsstrahlung produced where the electrons impinged on the various collimators and absorbers. A considerable improvement was made by placing 4cm of lead directly before the veto counter to convert the bremsstrahlung to showers, but final elimination was achieved only by taking additional veto pulses from counter number 1, placed before the beam focusing magnet (figure 2). Monitoring of the beam was carried out with the combination of counters 5623 in the positive beam and with 1234 in the negative beam, the bar above a number indicating a veto counter ELECTRONICS A block diagram of the electronics used with the negative muon experiment is shown in figure 4. The difference in the case of positive muons was the absence of counter number 1 and the different monitoring arrangement. The coincidence units were of the Garwin type, the resolving time being 40 mpsec. Scintillators number 2, 3 and 4 were mounted on EM1 6260B photo-electron multipliers followed by simple EFP 60 head amplifiers. Number 1, which counted at a mean rate of about 2 x lo5 per second, was used with an EM1 6097B feeding a 130 ohm cable directly. The sodium iodide crystal was mounted on an EM inch multiplier. c c 3 1 I Pulrt Height c Anrlyaar Sarltr Figure 4. Block diagram of the electronics used in the negative pion beam. Pulses from the crystal to operate the coincidence circuit were taken from dynode number 9 of the multiplier and after passing through a line amplifier clipped to a total width of about 30mpsec. They were then used to operate a trigger circuit which fed pulses of standard size, 40 mpsec wide, to the coincidence circuit. The linear pulses from the crystal were taken from dynode number 7 and after amplification passed through the gate to a 50 channel pulse height analyser.

7 174 G. Culligan, S. G. F. Frank andj. R. Holt $4. TESTS AND ADJUSTMENTS One of the principle concerns in this investigation was to make sure that there were no direct magnetic effects on any of the counters. All the multipliers were well shielded with soft iron and mu-metal and the stray field of the magnet in the neighbourhood of the 5 inch multiplier was neutralized with compensating coils (figure 3). To check the counters for residual effects pulses were taken linearly from each counter in turn, a discriminator set on a steep part of the pulse spectrum and the counting rate measured with the magnetic field in opposite directions. In this way it was shown that the change in pulse height from counters 2, 3,4 and NaI was less than 0.1%. An overall check was made in the negative pion beam by switching off the veto counters and examining the gated spectrum of the bremsstrahlung from the electron contamination. When the magnetic field was reversed there was no change in the number of pulses greater than 5 MeV within the statistical accuracy of 0.25y0 and no change above 12 MeV within an accuracy of _+ 1 o/o. The above tests were carried out at the beginning of every run with the cyclotron. The arrangement was checked as an analyser by using the bremsstrahlung from the BOYj3-radiation, which is known to be highly polarized (Goldhaber et al. 1957). A 150mc source enclosed in a silver capsule was placed in the position of the carbon target and the electron counters removed. Spectra of pulses from the sodium iodide crystal were recorded for the two directions of magnetization. The difference of these is plotted in figure 5 and shows the expected increase in analysing power with energy. Pulse HrlgM (W) Figure 5. Test of analyser with bremsstrahlung from ""Y 8-radiation. The procedure at the start of each run with the cyclotron was as follows. All the counters were mounted in position, but the magnet was absent. A beam of positrons was provided by placing a lead radiator in the vacuum pipe between the cyclotron and bending magnet to convert some of the electrons in the negative pion beam. The field direction in the magnet was such as to focus the positrons and not the negative particles. The energy of the positrons at the focus could be varied by altering the strength of the magnetic field. The pulses from the ' events ' coincidence unit (figure 4) were taken directly to the pulse height analyser. Then the delays in the cables from counters 3 and 4

8 Polarization of Electrons from Muon Decay 175 and the trigger circuit were adjusted so that the pulses from the coincidence unit formed a well-defined peak with positrons of energies within the range 10 MeV to 50.~ MeV. The linear output from the sodium iodide crystal was then connected to the gate circuit and the gated spectrum viewed on the pulse height analyser. The energy scale was then calibrated by varying the positron energy through the above range. The magnet was next placed in position and, in the full pion beam, the vetoed double coincidence rate 7234 was measured for various thicknesses of absorber to enable this to be set to give the maximum rate. The performance of the veto counters was checked at this stage. Finally, in the case of the negative beam, the gated linear spectrum from the crystal was examined both with optimum absorber thickness and with a sufficiently increased thickness that no muons could reach the carbon target. The number of beam contamination electrons reaching the lead radiator was practically the same in both cases and the spectrum in the second case then revealed any residual effect due to them. This effect was difficult to measure because of the small counting rate, but was less than 10% of the pulses greater than 12MeV due to the muon decay electrons MEASUREMENTS AND RESULTS The spectrum of pulses from the sodium iodide detector was recorded for a certain number of monitor counts with the magnetic field alternately in one direction and the other. There was an interval of about 1 hour between reversals in the experiment with the positive beam and about + hour in that with the negative beam. The total number of reversals during several periods on the cyclotron in each case was 110 for the first and 220 for the second. At regular intervals the spectrum of random coincidences between double counts 3, 4 and single counts in the sodium iodide detector was measured by putting a delay of one r.f. period in the cable from the latter. This spectrum had the same shape as that of single pulses from the sodium iodide and increased very rapidly below 10 MeV. The upper limit of the electron spectrum from muon decay is at about 53 MeV. As the energy spectrum of the bremsstrahlung shows a roughly logarithmic increase with decreasing energy and is very weak above 30MeV the saturation level of the gate was arranged to be at about 40~ev. The amplification was linear up to this level. The lower energy limit was set by the trigger circuit (figure 4) at about 2Mev. The following are the chief counting rates for the two experiments : In the case of positive muons the electron rate 234 was about 300 per second. The rate in the gated spectrum between 12 ~ e and v 30~ev was 5 per minute. The random coincidence rate was about 3 o/o of this, but the proportion inereased to about 20% at 6 MeV. In the case of negative muons the electron rate T134 was about 80 per second. The rate in the gated spectrum between 12 M ~ and V 30 MeV wa8 4 per minute with the random rate accounting for half thi8. The results for both cases are displayed in figure 6, in which the differenoe between the total spectra for the two directions of msgnstization is plottedi as a percentage of the mean. The errors shown are the statistical atandrrd deviations. I,

9 176 G. Culligan, S. G. F. Frank and?. R. Hoh Corrections have been made to the experimental points to allow for the effect of the random coincidence component in diluting the percentage difference. It is clear that there is a magnetic effect which is considerably greater than the errors and in opposite directions for positive and negative muons. The integrated experimental effect above 12MeV is +4.7 f 1.2% for positive muons and f 2.3% for negative muons. In the positive case the transmission of the magnet was greater when the north pole was towards the source and since the Compton cross section is least when the spins of quantum and electron are in the same direction this means that the quanta, and hence the positrons, have positive helicity. Likewise the electrons from negative muon decay have negative helicity. I I Figure 6. Difference in counting rates for the two directions of magnetization with poiitive muons (above) and negative muons (below). The broken curves represent attempts to fit the measurements on the assumption of 100% electron polarization. The broken curves in figure 6 represent an attempt to set limits between which the experimental points should lie assuming the electrons to be fully polarized. The curves showing the greater effect for both positive and negative muons were calculated on the assumption that the bremsstrahlung radiation also was fully polarized. The formula of Gunst and Page (1953) for the spin dependence of the Compton cross section as a function of energy was used, putting in the thickness of the iron and its degree of saturation. The curves showing the smaller effect were calculated by considering 50 MeV fully polarized electrons to be incident on the lead radiator and carrying through a Monte Carlo calculation on the showers produced, using the formulae given by

10 Polarization of Electrons from Muon Decay 177 Dyson (1957) for the polarization of the secondary quanta and electrons. The calculations were not carried below 12 MeV. It can be seen from Dyson s formulae that incident electrons of lower energy give rise to a greater effect for a given quantum energy. The experimental points are in satisfactory agreement with some curve between the estimated limits in both cases. The integrated difference above 12MeV from the theoretical upper limit is f 6%, which is to be compared with a figure of 4-9 f 1.1% obtained by combining the results for positive and negative muons and ignoring the difference in sign. From a consideration of the errors we feel justified in saying that the results point to a degree of polarization between 60 and loo:,. ACKNOWLEDGMENTS We wish to record our thanks to Professor H. W. B. Skinner for his close interest in this work and to Professor J. M. Cassels, Dr. H. Uberall and Dr. H. P. Noyes for valuable discussions. We are indebted to Dr. J. C. Kluyver for his collaboration during the first stage of the experiment. Messrs. T. Massam and S. Rodgers have rendered much useful assistance. The work with the cyclotron was greatly facilitated by the co-operation of the operating crew under Mr. B. Halliday. G. C, wishes to acknowledge the receipt of a grant from the Department of Scientific and Industrial Research. REFERENCES CASSELS, J. M., O KEEFFE, T. W., RIGBY, M., WETHERELL, H. M., and WORMALD, J. R., 1957, Proc. Phys. Soc. A, 70, 543. CULLIGAN, G., FRANK, S. G. F., HOLT, J. R., KLUYVER, J. C., and MASSAM, T., 1957, Nature. Lond., 180, 751. DYSON, F. J., 1957, Proceedings 7th Rochester Conference (New York: Interscience). FRIEDMAN, J. I., and TELEGDI, V. L., 1957, Phys. Rev., 106,1290. GARWIN, R. L., LEDERMAN, L M., and WEINRICH, M., 1957, Phys. Rev., 105,1415. GOLDHABER, M., GRODZINS, L., and SUNYAR, A. W., 1957, Phys. Rev., 106, , Ibid., 109, 1015, GCKST, S. B.. and PAGE, L. A., 1953, Phys. Rev., 92,970. KINOSHITA, T., and SIRLIN, A., 1957, Phys. Rev., 108, 844. LANDAU L., 1957, Nuclear Physics, 3, 127. LEE, T. D., and YANC, C. N., 1957a, Phys. Rev., 105, b, Elementary Particles and Weak Interactions (Brookhaven National Laboratory). MCYOY, K. M., and DYSON, F. J., 1957, Phys. Rev., 106, REHOVOTH CONFERENCE PROCEEDINGS 1958 (Amsterdam: North Holland Publishing CO.). p. a46. SALAM, A., 1957, Nuouo Cim., 5, 299. SUDARSHAN, E. C. G., and MARBHAK, R. E., 1958, Phys. h.. 109,1860. SWANSON, R. A., 1958, Phys. Rev., 112, 580. UBBRALL, H., 1957, Nuouo Cim., 6,376. WILKINSON, D. H., 1957, Nuouo Cim., 6, 516.