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Particle and nuclear

Particle and nuclear

The top quark: an unbiased tale

06 Oct 2004

The Evidence for the Top Quark: Objectivity and Bias in Collaborative Experimentation
Kent W Staley
2004 Cambridge University Press 360pp £45.00/$70.00hb

Carefully does it

In the world of high-energy physics, experimental teams publishing the results of a search for some new phenomenon — let’s call it a snark — usually telegraph the results via a code word in the title of the paper. If the statistical significance for the snark is weak or non-existent, the title of the paper will contain the words “Search for the snark…” or “Limits on snark properties…”. If, however, the statistical significance is beyond reproach, the paper might be called “Observation of the snark?” or, more dramatically, “Discovery of the snark…”.

But there is a nether world where the statistical significance for the snark might be too strong to ignore but too weak to claim a discovery. It is precisely this situation in which the CDF collaboration at Fermilab, near Chicago, found itself in late 1993. The 500 or so physicists in the team were searching for the top quark — the sixth and presumably last fundamental constituent of the Standard Model of particle physics. The result of their efforts was a mammoth 61-page paper in the journal Physical Review D (50 2966) with the unwieldy title, “Evidence for top quark production in p-bar-p collisions at √s = 1.8 TeV”. The word “evidence” was chosen carefully.

This book by Kent Staley tells the story of how the discovery was made. Staley is a philosopher at St Louis University who became interested in the top quark when he was a graduate student at Johns Hopkins University. While carrying out research for his PhD, Staley petitioned for — and was given — unprecedented access to the inner workings of the CDF collaboration, including unpublished internal documents and extensive interviews with some of the participants.

Out of this voluminous raw material of how “big science” is really done, Staley has produced a remarkable book, or, more correctly, two books pressed into a single volume. The first (and in my opinion more successful) is a rollicking good story that emphasizes the human side of an important scientific enterprise — the exhilarations and frustrations of the principal characters, the heated debates within the collaboration, and finally the tedious consensus-building required to produce the published paper. The second part is addressed to Staley’s fellow philosophers of science. It uses the CDF paper as a vehicle for considering epistemological questions, such as the nature of statistical evidence, the role of biases and subjective opinions versus objective reality.

The book begins by describing how the top quark first appeared in the Standard Model. Unfortunately, Staley then wanders off in a detour on the state of theoretical physics in pre-war Japan, how it was perceived in the West, and when or whether proper credit was given to its authors. While some interesting philosophical and historical questions are raised — including never before published details on the recognition of Kobayashi and Masakawa’s work — this material is at best peripheral to the aim of the book. It would have benefited from either more severe editing or being spun off into a separate monograph.

Staley fares much better in his introductory material on the history of the CDF detector and collaboration. There is a good balance between his descriptions of the detector and his explanations of why it was built. This section includes various social and political aspects of the detector’s construction, the lab’s competition with CERN, and the exigencies of funding and schedules. Staley is correct in emphasizing that the design of the detector was driven by a philosophy that the instrument should extract the most (and most precise) information per event — rather than be narrowly optimized for any particular physics signature, even for the top quark. The result was a general-purpose device that adapted gracefully to changing physics interests and requirements.

Staley is most readable in the middle part of the book, where he follows the struggle within the CDF collaboration while the participants debated the strengths and weaknesses of their statistical (and other) evidence for the top quark. As he writes, “Big science presents a big opportunity for methodologists. With their constant meetings and exchanges of e-mail, collaboration scientists routinely put their reasoning on public display, long before they write up their results for publication in a journal.”

I was particularly interested in Staley’s examination of possible bias in the methodology and how the CDF collaboration dealt with it. When particle physicists try to find a particular set of events among the trillions of collisions that occur in an accelerator, they have to focus their search by ignoring data outside a certain range. In the case of the top quark, the CDF physicists knew that they could select their data in two different ways. Although both approaches were valid, the one they chose turned out to produce a stronger signal. Clearly, there is a danger in admitting a non-physicist to such raw material, since a lack of understanding of the issues and conventions can easily lead to very wrong, even silly, conclusions. I am impressed that Staley “got it right”.

At the end of the book, Staley changes from reporter to philosopher. He realizes that the CDF’s methodology does not fit neatly into any conventional models of the history of science. Instead, he offers his own theory of “error-statistical evidence”. I found this section tough going, replete as it is with ponderous sentences like, “The distinction between the epistemic and causal relevance of epistemic states of experimenters may also help to clarify the debate over the meaning of the likelihood principle.”

On the other hand, I did appreciate some of Staley’s humorous examples to illustrate abstruse constructions. Here I freely admit that Staley is a better physicist than I am a philosopher. As evidence of my incompetence, I had to look up the meaning of ceteris paribus (other things being equal). And just as Voltaire’s Monsieur Jourdain was surprised to learn that he had been speaking prose all his life, so I was intrigued to find that I had been practising “Neyman-Pearson hypothesis testing” during my time on the collaboration.

Ultimately, scientific research and discovery is a very human enterprise from solitary author to large collaborations. Personalities, ambitions and communication skills do affect the exact course that scientific enquiry takes in approaching a better approximation to the laws of nature. As a philosopher, Staley is comfortable in embracing this condition, and he makes a good case for the self-correcting nature of the process and the ultimate objectivity of the result.

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