As I am revising the book I am writing on the history of the CDF experiment, I have bits and pieces of text that I decided to remove, but which retain some interest for some reason. Below I offer a clip which discusses the measurement of the natural width of the Z boson produced by CDF with Run 0 data in 1989. The natural width of a particle is a measurement of how undetermined is its rest mass, due to the very fast decay. The Z boson is in fact the shortest lived particle we know, and its width is of 2.5 GeV.
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In late summer of 1989, no longer than a month
after the SLAC workshop where CDF had presented their first measurement of the Z boson mass, the 8th Topical Conference on
Proton-Antiproton Collider Physics was held in Castiglione della Pescaia, a
nice seaside town on the Tyrrhenian sea. The conference was attended by many physicists from the CERN
UA2 experiment, the only one of the two SppS experiments still running. They had been measuring the masses of W and Z
bosons, and they were not welcoming of the CDF competition.
Among the pack of UA2 physicists who
swarmed to Castiglione was a young Joe Incandela, the physicist who would 23
years later lead the CMS experiment to the discovery of the Higgs boson. Joe,
who had been a Ph.D. student of Henry Frisch on a magnetic monopole search just
a few years earlier, was a member of UA2 but was already planning to join the
CDF collaboration (he would officially do so in 1991). In UA2 he was working
at the Z mass measurement. As soon as CDF published its preprint, where the Z mass was determined with fivefold increase in accuracy over the previous CERN measurements, he devoured the article, trying to learn all that there was to learn about
the competitors’ advanced techniques. And he soon discovered an inconsistency
in the CDF measurement.
The CDF paper claimed that a combined
fit of mass and width of the Z boson from 65 Z-->ee candidates in the 80-100
GeV mass range produced a width measurement of 3.6 GeV. This result, combined
with the corresponding one from the Z-->μμ sample, yielded the final width measurement of 3.8 GeV. However, any
physicist with some experience in statistical data analysis would have been
surprised by looking at the Z-->ee mass distribution shown in figure 3 of the
article. There, the peak appeared visibly narrower than what a 3.6 GeV natural
width of the resonance would produce, once smeared out by the experimental
resolution on electron energies that CDF was quoting.
Above: the mass distribution published by CDF with its Z-->ee candidates
Joe spent the month of August on his
computer, generating pseudo-experiments to figure out what was wrong in the CDF
determination of the Z width from the electron-positron decays. He tried
different models for the energy spectrum of the detected electrons. The
relative energy resolution of electrons would improve at higher energy, and the
observed Z peak should consequently get narrower. This was practically the only
way to justify the CDF result. However, it turned out to be impossible to
reconcile the quoted width measurement with the distribution shown in the
article. The measured value was six or seven standard deviations larger than
what a correct fit of the histogram could conceivably give.
Incandela was quite unsure on how to
proceed. Publishing a short paper with a rebuttal of the CDF measurement did
not really look like the right thing to do: it was a clear belligerent act, by
which he would certainly create the grounds for enmity with the physicists who
would soon become his colleagues. Worse still, among the physicists involved in
the CDF measurement was his ex-advisor Henry Frisch!
On the other hand, the
ethical scientist must correct an erroneous published measurement. In
consultation with his colleagues, Incandela decided to discuss directly with
the CDF colleagues the calculations he had produced. During a pause in the
conference works, Incandela met privately with the CDF members. To their
credit, the CDF contingent acknowledged the flaws in their Z width measurement.
CDF then assured their UA2 colleagues that they would publish an erratum
themselves. However, after the conference they decided that the best way to
proceed was to supersede the summer 1989 result by a new measurement. But that
in fact never happened.
