Volume Two: Symposia and Invited Papers || Do Mutants Have to Be Slain, or Do They Die of Natural Causes?: The Case of Atomic Parity Violation Experiments

Do Mutants Have to Be Slain, or Do They Die of Natural Causes?: The Case of Atomic ParityViolation ExperimentsAuthor(s): Allan FranklinSource: PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association,Vol. 1990, Volume Two: Symposia and Invited Papers (1990), pp. 487-494Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/193092 .

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Do Mutants Have to be Slain, or Do They Die of Natural Causes?: The Case of Atomic Parity Violation Experiments1

Allan Franklin

University of Colorado

In Constructing Quarks (1984) Andrew Pickering discussed the early experiments on atomic parity violation performed at Oxford University and at the University of Washington and published in 1976 and 1977. The results disagreed with the predictions of the Weinberg-Salam (W-S) theory of unified electroweak interactions. Another experiment, performed at the Stanford Linear Accelerator Center in 1978, on the scattering of polarized electrons from deuterons confirmed the theory. Pickering regards the Oxford and Washington experiments as mutants, slain by the SLAC experiment.

By 1979 the W-S theory was regarded by the high-energy physics community as established, despite the fact that as Pickering recounts, "there had been no intrinsic change [emphasis in original] in the status of the Washington-Oxford experiments." (Pickering 1984, p. 301). In Pickering's view "particle physicists chose [emphasis in original] to accept the results of the SLAC experiment, chose to interpret them in terms of the standard model (rather than some alternative which might reconcile them with the atomic physics results) and therefore chose to regard the Washington- Oxford experiments as somehow defective in performance or interpretation." (Pickering 1984, p. 301). The implication seems to be that these choices were made solely so that the experimental evidence would be consistent with the W-S theory, and that there weren't good, independent reasons for them.

In this paper I will reexamine the history of this episode, presenting both Pickering's interpretation and an alternative explanation of my own, arguing that there were good reasons for the decision of the physics community. It will also ex- plore some of the differences between my view of science and that proposed by the "strong programme" or social constructivist view in the sociology of science. A cen- tral feature of this view is that change in the content of scientific knowledge is to be explained or understood in terms of the social and/or cognitive interests of the scientists involved. The evidence model I advocate explains adherence to scientific beliefs in terms of their relationship to valid experimental evidence. This model also argues that there are good reasons for belief in that experimental evidence.

PSA 1990, Volume 2, pp. 487-494 Copyright ? 1991 by the Philosophy of Science Association



488

1. The Experiments

It had been experimentally demonstrated in 1957 that parity, or left-right symme- try, was violated in the weak interactions. This feature of the weak interactions had been incorporated into the Weinberg-Salam theory. The theory predicted that one would see weak neutral-current effects in the interactions of electrons with the strongly interacting particles. The effect would be quite small when compared to the dominant electromagnetic interaction, but could be distinguished from it by the fact that it violated parity conservation. A demonstration of such a parity-violating effect and a measurement of its magnitude would test the W-S theory. One such predicted effect was the rotation of the plane of polarization of polarized light when it passed through bismuth vapor. This was the experiment performed by the Oxford and Washington groups. They jointly reported prelimimary values for R, the parity violating parameter, of R = (-8 + 3) x 10-8 (Washington) and R = (+10 ? 8) x 10-8 (Oxford), in disagreement with the W-S predictions of (-3 to -4) x 10-7 (Baird 1976).

Pickering offers the following interpretation. "Bismuth had been chosen for the experiment because relatively large effects were expected for heavy atoms, but when the effect failed to materialise a drawback of the choice became apparent. To go from the calculation of the primitive neutral-current interaction of electrons with nucleons to predictions of optical rotation in a real atomic system it was necessary to know the electron wave- functions, and in a multi-electron atom like bismuth these could only be calculated approximately....Thus in interpreting their results as a contradiction of the Weinberg-Salam model the experimenters were going out on a limb of atomic theory." (Pickering 1984, pp. 295-6). Pickering attributes all of the uncertainty in the comparison between experiment and theory to the theoretical calculations and none to the experimental results themselves.

The comparison was even more uncertain than Pickering implies and included such uncertainties. The experimenters reported systematic effects which they believed did not exceed +10 x 10-8, and which were not yet fully understood. Thus, there were possible systematic experimental uncertainties of the same order of magnitude as the expected effect. The novelty of the experiments also tended to make the validity of the experimental results uncertain. The theoretical calculations of the expected effect were also uncertain, with the largest and smallest results differing by a factor of approximately two.

In 1977, both groups published more detailed accounts of their experiments with somewhat revised results (Lewis 1977 and Baird 1977). Both groups reported results in substantial disagreement with the predictions of the W-S theory. The Washington group reported a value of R = (-0.7 ? 3.2) x 10-8, which was in disagreement with the new prediction of approximately -2.5 x 10-7. This value was also inconsistent with their earlier result. This inconsistency, although not discussed by the experimenters, was discussed within the atomic physics community and lessened the credibility of the result. The Oxford result was R = (+2.7 + 4.7) x 10-8, again in disagreement with the W-S prediction. They noted, however, that there was a systematic effect in their apparatus of order 2 x 10-7 radians, which certainly cast doubt on the result. Pickering reported that the papers also "described two 'hybrid' unified electroweak models, which used neutral heavy leptons to accommodate the divergence with the findings of high energy neutrino scattering." (1984, p. 297). Although such models were discussed in the literature at the time, there is no mention of such speculation in these two experimental papers.



489

How were these results viewed by the physics community? Frank Close (1976) summarized the situation by noting that as the atomic physics results stood, they ap- peared to be inconsistent with the predictions of the Weinberg-Salam model supple- mented by atomic physics calculations. Pickering states that, "if one accepted the Washington-Oxford result, the obvious conclusion was that neutral current effects violated parity conservation in neutrino interactions and conserved parity in electron interactions." (1984, p. 296). Close discussed this possibility along with another alternative that allowed the high energy neutrino experiments to show parity nonconservation while the low energy atomic physics experiments would not. "Whether such a possibility could be incorporated into the unification ideas is not clear. It also isn't clear, yet, if we have to worry. However, the clear blue sky of summer now has a cloud in it. We wait to see if it heralds a storm." (Close 1976, p. 506).

In Pickering's view, the 1977 publication of the Oxford and the Washington results indicated that, "the storm that Frank Close had glimpsed had materialised..." (Pickering 1984, p. 298), and this is supported in a summary paper by David Miller (1977). Nevertheless, I believe that the uncertainty in these experimental results made the disagreement with the W-S theory only a worrisome situation and not a cri- sis. In any event, the monopoly of Washington and Oxford was soon broken.

The experimental situation changed in 1978 when Barkov and Zolotorev (1978a,b, 1979, 1980b), two Soviet scientists from Novosibirsk, reported a series of measurements on the same transition in bismuth as the Oxford group. Their results agreed with the predictions of the W-S model. According to Pickering, "the details of the Soviet experiment were not known to Western physicists, making a considered evaluation of its result problematic." (1984, p. 299). This is simply not correct. During September, 1979 an international workshop devoted to neutral current interactions in atoms was held in Cargese (Williams 1980). This workshop was attended by representatives of virtually all of the groups actively working in the field, includ- ing Oxford, Washington, and Novosibirsk. At that workshop not only did the Novosibirsk group present a very detailed account of their experiment (Barkov and Zolotorev, 1980a), "but also answered many questions concerning possible systematic errors." (Bouchiat 1980, p. 364)

In early 1979, a Berkeley group reported an atomic physics result for thallium that agreed with the predictions of the W-S model (Conti 1979). Although these were not definitive results - they were only two standard deviations from zero - they did agree with the model in both sign and magnitude.

It seems fair to say that in mid-1979 the atomic physics results concerning the Weinberg-Salam theory were inconclusive. The Oxford and Washington groups had originally reported a discrepancy, but their more recent results, although preliminary, showed the presence of the predicted parity nonconserving effects. The Soviet and Berkeley results agreed with the model. Dydak (1979) summarized the situation in a talk at a 1979 conference. "It is difficult to choose between the conflicting results in order to determine the eq [electron-quark] coupling constants. Tentatively, we go along with the positive results from Novosibirsk and Berkeley groups and hope that future development will justify this step (it cannot be justified at present, on clear- cut experimental grounds [Emphasis added].)" (1979, p. 35).

Pickering states that, "Having decided not to take into account the Washington- Oxford results, Dydak concluded that parity violation in atomic physics was as predicted in the standard model." (1984, p.300). I find little justification for Pickering's conclusion. Dydak was attempting to determine the best values for the parameters describ-



490

ing neutral-current electron scattering and had tentatively adopted the results in agreement with the W-S model. He concluded nothing about the validity of the standard model. Bouchiat was more positive. His summary paper concluded "that parity violation has been observed roughly with the magnitude predicted by the Weinberg-Salam theory (emphasis in original)." (Bouchiat 1980, p. 365). In another summary, Commins and Bucksbaum (1980) regarded the situation with regard to bismuth as unresolved.

The situation was made even more complex when a group at the Stanford Linear Accelerator Center (SLAC) reported a result on the scattering of polarized electrons from deuterium that agreed with the W-S model. (Prescott, 1978, 1979). Pickering concludes that scientists chose to accept the SLAC results, and the W-S theory, and chose to reject the early Oxford-Wahington results. In his view this was to unify practice within high energy physics. While I do not dispute Pickering's contention that choice was involved in the decision to accept the Weinberg-Salam model, I dis- agree with him about the reasons for that choice. In my view, the choice was a reasonable one based on convincing, if not overwhelming, experimental evidence

The issue seems to turn on the relative evidential weight one assigns to the original Oxford and Washington atomic physics results and to the SLAC E122 experiment on the scattering of polarized electrons. Pickering seems to regard them as having equal weight. I do not. I argued earlier that the situation with respect to atomic parity violation was very uncertain, both experimentally and theoretically

The moral of the story is clear. The early atomic physics experiments were extremely difficult, beset with systematic errors of approximately the same size as the predicted effects. There is no reason to give priority to the earliest measurements, as Pickering does. One might suggest, rather, that these earlier results were perhaps less reliable because not all of the systematic errors were known.

I will now examine the arguments presented by the SLAC group in favor of the validity and reliability of their measurement. I agree with Pickering that, "In its own way E122 was just as innovatory as the Washington-Oxford experiments and its findings were, in principle, just as open to challenge." (Pickering 1984, p. 301). For this reason, the SLAC group presented a very detailed discussion of their experimental apparatus, results, and the experimental checks they had done.

The experiment depended on a new high intensity source of longitudinally polarized electrons. The polarization of the electron beam was changed randomly to mini- mize the effects of drifts in the experiment. It had already been demonstrated that polarized electrons could be accelerated with negligible depolarization. In addition, both the sign and magnitude of the beam polarization were measured periodically by observing the known asymmetry in elastic electron-electron scattering from a mag- netized iron foil. The experimenters also checked whether or not the apparatus pro- duced spurious asymmetries. They measured the scattering using the unpolarized beam from the regular SLAC electron gun, for which the asymmetry should be zero, and found no effect at the level of 10-5. Changes in beam polarization and the effect of beam energy were measured, for two different detection systems, and the results agreed both with each other and with theoretical calculations.

A serious source of potential error came from small systematic differences in the beam parameters for the two helicities. These could, conceivably, have caused apparent, but spurious, parity violating asymmetries. These quantities were carefully moni- tored and a feedback system used to stabilize them. The most significant imbalance was less than one part per million in the beam energy, which contributed -0.26 x 10-5



491

to A/Q2. This is to be compared to their final result of A/Q2 = (-9.5 ? 1.6) x 10-5 GeV/c-2. This result was regarded by the physics community as a reliable and convincing result.

Contrary to Pickering's claim, hybrid models were both considered and tested by E122. The results are shown in Figure 1 and the superiority of the W-S model is obvious. For W-S they obtained a fit to the experimental data with a X2 probability of 40%. The hybrid model had a X2 probability of 6 x 10-4, "which appears to rule out this model." My interpretation of this episode differs drastically from Pickering's. The physics community chose to accept an extremely carefully done and carefully checked experimental result that confirmed the Weinberg-Salam theory. The physics community chose to await further developments in the atomic parity violating experiments, which, as I have argued, were uncertain. The subsequent history of these experiments during the 1980s shows that although other reliable atomic physics experiments confirm the W-S theory, the bismuth results, although generally in agreement with the predictions, are still somewhat uncertain. In addition, the most plausible alternative to the W-S model, that could reconcile the original atomic physics results with the electron scattering data, was tested and found wanting. There certainly was a choice made, but, as the 'scientist's account' or evidence model suggests, it was made on the basis of experimental evidence. The mutants died of natural causes.

,, I I I I N HYBRID

-5--

W-S (,D

0

c0 \ MODEL . \ ~\ INDEPENDENT

< . \ ro \

15 - - * E= 19.4GeV - a EO= 16.2GeV o E = 22.2GeV

-20 1I I 0 0.1 0.2 0.3 0.4

y

Figure 1. Asymmetries measured at three different energies plotted as a function of y = (Eo - E')/Eo. The predictions of the hybrid model, the Weinberg-Salam theory, and a model independent calculation are shown. "The Weinberg-Salam model is an acceptable fit to the data; the hybrid model appears to be ruled out" (Prescott 1979).



492

2. Discussion

There are several points worth making about this episode of atomic parity violation experiments. Perhaps most important is that the comparison between experiment and theory can often be extremely difficult. This is particularly true when, as in this episode, one is at the limit of what one can calculate confidently and what one can measure reliably.

Pickering remarks that by 1979, and presumably to this day, there had been no intrinisic change in the early Washington and Oxford results. In the sense that no one knows with certainty why those early results were wrong, he is correct. Nevertheless, since those early experiments, physicists have found new sources of systematic error, not dealt with in the early experiments. The redesign of the apparatus has, in many cases, precluded testing whether or not these effects were significant in the older apparatus. While one cannot claim, with certainty, that these effects account for the earlier, presumably incorrect, results, one does have reasonable grounds for believing that the later results are more accurate. The consistency of the 1980s measurements enhances that belief.

It seems clear that the evidence model fits this episode better than Pickering's model. Scientists chose, on the basis of reliable experimental evidence provided by the SLAC E122 experiment, to accept the Weinberg-Salam theory. They chose to leave an apparent, but also quite uncertain, anomaly in the atomic parity violation experiments for future investigation.

This episode also demonstrates that scientists make judgements about the reliability of experimental results that coincide with what one would decide on epistemo- logical grounds. The SLAC group argued for the validity of their experimental result using strategies that coincide with an epistemology of experiment. (See Franklin 1990, ch. 6). As we have seen, the scientific community accepted their arguments. If the social constructivist view were correct then one would expect to find at least one episode in which the decision of the scientific community went against the weight of experimental evidence. No such episode has been provided. Nevertheless, I believe that the social constructivists are correct in insisting that in some cases the cognitive interests of the scientists do play a role in establishing experimental results, and thus in theory choice. (See Galison 1987, ch. 2 and Franklin 1986, ch. 5 for examples).

Suppose, however, that someone did present a case in which the decision went against the experimental evidence. Would that destroy the evidence model? I think not. I believe that the evidence model describes not only what scientists should do, but also what they, in fact, do most, if not all, of the time. That scientists, being human, are fal- lible and do not always behave as they ought to, should surprise no one.

Note

1This paper is a very abbreviated version of the arguments presented in Chapter 8 of Franklin (1990).



493

References

Baird, P.E.G. et al. (1976), "Search for Parity Nonconserving Optical Rotation in Atomic Bismuth", Nature 264: 528-9.

-__--__- _.- et al. (1977), "Search for Parity-Nonconserving Optical Rotation in Atomic Bismuth", Physical Review Letters 39: 798-801.

Barkov, L.M. and Zolotorev, M.S. (1978a), "Observations of Parity Nonconservation in Atomic Transitions", JETP Letters 27: 357-61.

_-_-___-_________? -_. (1978b), "Measurement of Optical Activity of Bismuth Vapor", JETP Letters 28: 503-6.

?-?-_-_--_-_-_-_-_-_-_-_-_-_---.( (1979), "Parity Violation in Atomic Bismuth", Physics Letters 85B: 308-13.

?-?-?-?-?-?--_-_- _--?-?-?--__?--. (1980a), "Parity Violation in Bismuth: Experiment", in Williams (1980): 52-76.

__ __?-?-____ _- ______ _. (1980b), "Parity Nonconservation in Bismuth Atoms and Neutral Weak-Interaction Currents", Zhurnal Eksperimental'noi i Teoreticheskoi Fiziki (JETP) 52: 360-9.

Bouchiat, C. (1980), "Neutral Current Interactions in Atoms", in Williams (1980): 357-69.

Close, F.E. (1976), "Parity Violation in Atoms?", Nature 264: 505-6.

Commins, E. and Bucksbaum, P. (1980), "The Parity Non-Conserving Electron- Nucleon Interaction", Annual Reviews of Nuclear and Particle Science 30: 1-52.

Conti, R. et al. (1979), "Preliminary Observation of Parity Nonconservation in Atomic Thallium", Physical Review Letters 42: 343-6.

Dydak, F. (1979), "Neutral Currents," in Proceedings of the International Conference on High Energy Physics, Geneva, 27 June-4 July, 1979. Geneva: CERN, pp. 25-49.

Franklin, A. (1986), The Neglect of Experiment. Cambridge: Cambridge University Press.

_ _ _ _ _ . (1990), Experiment, Right or Wrong. Cambridge: Cambridge University Press.

Galison, P. (1987), How Experiments End. Chicago: University of Chicago Press.

Hollister, J.H. et al. (1981), "Measurement of Parity Nonconservation in Atomic Bismuth", Physical Review Letters 46: 643-6.

Lewis, L.L. et al. (1977), "Upper Limit on Parity-Nonconserving Optical Rotation in Atomic Bismuth", Physical Review Letters 39: 795-8.

Miller, D.J. (1977), "Elementary Particles-A Rich Harvest", Nature 269: 286-8.



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Pickering, A. (1984), Constructing Quarks. Chicago: University of Chicago Press.

Prescott, C.Y. et al. (1978), "Parity Non-Conservation in Inelastic Electron Scattering", Physics Letters 77B: 347-52.

_ _ -____ _ _ _ . (1979), "Further Measurements of Parity Non- Conservation in Inelastic Electron Scattering", Physics Letters 84B: 524-8.

Williams, W.L. (ed.) (1980), Proceedings, International Workshop on Neutral Current Interactions in Atoms, Cargese, 10-14 September, 1979. Washington: National Science Foundation.



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Volume Two: Symposia and Invited Papers || Do Mutants Have to Be Slain, or Do They Die of Natural Causes?: The Case of Atomic Parity Violation Experiments