These days I am trying to reconstruct some stories from my old experiment, CDF. The CDF experiment was conceived in 1979 and constructed in the early eighties at the Fermi laboratories in Batavia, near Chicago. CDF took the first proton-antiproton collisions in 1985, and it collected data in1987-88, 1992-96, and 2001-2011, thus becoming the longest-lasting particle physics experiment in the history of science.
In 1994 CDF announced the first evidence of the top quark, measuring its mass at 174 GeV, a value which still holds its ground the increase by one order of magnitude of the precision of today's measurements. But few know that the top quark had been discovered much earlier... By Rubbia in the mid eighties, and then by other CDF researchers who were, for good or for bad, not trusted enough.
I thought the following excerpt, which tells an anecdote of one of these early sightings, would be of interest to some readers here...
In December 1988 a one-day workshop was organized in the Ramsey auditorium, the conference room at the basement floor of the Hirise, the main building of the Fermi National Accelerator Laboratory. The workshop was the first of a series of meetings that would take place in the course of the following few years, and it was specifically devoted to focused discussions on the top quark search, which was being performed independently by several groups of CDF physicists. The Argonne National Laboratory partecipated to the CDF experiment with a strong group of researchers; they were very active in the search for the top quark in "lepton plus jets" events.
I should explain that back then it was not events containing top-antitop quark pairs that were being sought, but rather ones containing a single top quark. In fact, in the first CDF searches the top quark was believed to have a mass not much larger than the fake 40 GeV signal just published, and soon retracted, by Rubbia's UA1 collaboration. A top quark lighter than the W boson could still, of course, be produced in pairs, but it would be much smarter to search for single top quarks produced in the decay of a W boson, since the latter process has a larger production rate.
One was thus looking for a W boson decaying into a top quark and a bottom quark. The bottom quark would produce a low-energy jet of hadrons; the top would itself decay by turning into a lepton, a neutrino, and a second bottom quark which would yield another hadronic jet. These were thus "lepton plus jets" events of much smaller energy than those later called "W+jets" which would be used for the top discovery six years later.
The Argonne group gave a very detailed presentation on lepton plus jet events. The researchers who presented the work were Jimmy Proudfoot and Barry Wicklund, both experienced and careful physicists. They had looked at many things: jet activity, momentum of tracks, lepton isolation, missing energy. The presentation spurred a lively debate on the details of their event selection, and on their definition of the variables used to identify clean electron and muon events. The conclusion of the Argonne researchers was that it looked like there was no clear signal of a top quark in the data.
After the Argonne presentation Pekka Sinervo got on stage. Pekka worked for the CDF group of Pennsylvania State University. He also had looked for the top quark, and he also was reporting a negative result. His talk received wide interest, criticism, suggestions from the audience. It was all in the right spirit of a workshop where the best minds of the experiment would collaborate to get the most out of the data they had worked so hard to produce.
On the other hand, the picture could also have felt a bit depressing: the talks were giving the message that the top quark was not there to be found, and furthermore, one got the feeling that a well-defined strategy for the top search was missing. Indeed, back then it was not even clear to most CDF researchers that the main background to top production was constituted by events featuring a W boson together with hadronic jets produced by QCD radiation.
Finally, the time came for the talk by Kuni Kondo. Prof. Kondo was a Japanese physicist who led a sizable group of researchers from the University of Tsukuba. In his late fifties, he was lean, not tall, with black hair combed straight above an incipient baldness; he usually dressed in black or grey suits. He was a charming and very polite person, who spoke with a soft tone of voice and smiled a lot. It looked like nothing could ever upset him.
Kondo had devised a very complex, deep method to discriminate top quark events from the background, based on an analysis approach he had dubbed "dynamical likelihood" which would become a sophisticated standard only a decade later, but which was taken with quite a bit of scepticism at the time; in private, quite a few of his American and Italian colleagues would even make silly jokes on it. The method consisted in constructing probability distributions for the observed kinematics of the events, which could then be used to derive the likelihood that the events were more signal-like or background-like.
It is ironic to think that nowadays all the most precise measurements of the mass of the top quark rely on the method called "matrix element", which is nothing but Kondo's original idea recast in the context of a measurement of the mass rather than the discrimination of a top signal. Kondo was way ahead of his time, and like most pioneers in science he did not have an easy life getting his work appreciated and accepted, in a situation dominated by a conservative mainstream.
It is by now four in the afternoon, and Kondo finally gives a full status report of his analysis. His presentation is thorough and yet almost unintelligible by a good half of his listeners; his analysis includes highly unorthodox and yet brilliant tricks, like taking a jet from one event and mixing it in with other jets in a different event to study the behaviour of some of his selection variables for background events. His colleagues listen in an atmosphere of disbelief mixed with awe. Despite the complexity of the material and the possibility to object on a hundred of details, no questions are asked. As Kondo reaches the end of his talk, he concludes with a tone of voice just a milli-decibel higher than the rest of his speech:
"And therefore", a pause, and then "I thinkwe have discovered the top quark".
The audience remains silent. The convener is a tall, lean guy with a sharp nose and a penetrating stare; he looks like an English gentleman from a XIXth century novel, especially thanks to his considerable aplomb. He is not impressed, and that much does show.
"Thank you very much Kuni. Is there any question ?", one, two, three, four, "...No questions. Okay, thanks again Kuni. The next speaker is...".
In retrospect the convener's attitude and lack of consideration toward an esteemed colleague and a visitor from another country, who had brought to the experiment lots of resources and had contributed significantly to the detector construction, sounds at least rude and unjustified. Still, back then CDF was not a place where people would exchange courtesies and compliments (it never was, in truth): there everybody had to work hard and the only way to earn the respect of colleagues was through the good physics output of one's analysis results. If your analysis methods were not considered publishable or your results were thought fallacious, you would be considered a potential threat to the good name of the experiment, and you would suffer little short than boycott. But the way Kondo was treated was all flowers in comparison to what other physicists would experience, along the way to the top discovery.