In 1992 the top quark had not been discovered yet, and it did not make much sense for the CDF collaboration to have a full meeting devoted solely to it; rather, analyses targeting the search of the top quark were presented at a meeting which dealt with both bottom and top quarks. This was called back then "Heavy Flavour meeting".
The Heavy Flavour meeting was held every second Thursday afternoon, during busy “on-weeks”, which alternated with “off-weeks” when no physics meetings were scheduled. I was back then a summer student, and together with my buddy Esov I had been instructed by our supervisor Luca Stanco to attend the works of the group and start building an understanding of how top physics data analyses were performed, and to get familiar with the tools and the experimental techniques which we would be using.
To me, attending a meeting of the Heavy Flavour group was a real event. I remember very well my first time. I got to the Hirise with Esov in his Camaro 25th anniversary, we parked illegally, walked to the front door of the Hirise, went to the elevator on the right of the entrance, and got in with a half dozen people who told jokes to each other. I stood little chance of understanding what they were saying: my English back then was horrible. Similar occasions were a supplement of embarassment and frustration to the other usual sources for a newbye in a particle physics laboratory.
Esov and I got off at the 12th floor where, on the north corner of the building, stood a square conference room equipped with a very large oval table in the middle, and chairs everywhere else. The wall opposite to the entrance was lined with blackboards and a large square screen; in front of the latter, a plastic transparency projector stood on top of the table. On the left of the screen two large television sets were placed side to side, with a video camera on top: it was the videoconferencing system which allowed remote sites to follow the meeting. The left wall of the room was all glass, with a stunning view of the woods and hills of the Fox River valley.
As we got in we realized we had arrived a good ten minutes earlier than the start time: the room was still empty. Around the oval table stood nice soft-padded rotating chairs which looked a way better sitting option than the cheap ones lined along the walls. Only half-conscious of the assertive power of our action, we chose two with a nice view of the screen and sat down.
A few minutes later a tall, lean guy in his mid-thirties, better dressed than the average physicist, arrived and sat down on the opposite side of the table, next to the remote controls of the videoconferencing screens and the slide projector. Turning on the revolving chairs with curious eyes, leaning back and jolly, we certainly looked more like tourists than physics students. The guy stared at us with a perplexed look behind his round glasses. He opened a logbook, wrote something in it; then he addressed us with the best smile he could put together.
“Good afternoon... I do not think I have seen you at this meeting before. May I ask you to introduce yourself ? Are you students ?”
My English was trembling and broken, but I managed to put together an answer of sorts.
“Er... I am Tommaso Dorigo. I am undergraduate student in the University of Padova. I work with Luca Stanco.”
“And my name is Esov Velazquez. I am also an undergraduate, from the University of Puerto Rico. I am here as a Summer Student, working under the advisorship of Dr. Stanco.”
“I see. Luca Stanco...Yes, I know him. Nice guy. What are you working on, then, top physics I guess?
Esov’s English was on another league, and I was happy to let him answer first:
“Yes, we are putting together a search for all-hadronic decays of the top quark pairs, in the multijet dataset.”
Liss nodded, and he went back to scribbling in a logbook. That would have been the end of it for a less naive person than me, but I feld obliged to continue the conversation:
“...And who are you?”
Tony’s perplexed look at seeing two undergraduates sitting in the places that full professors would usually occupy turned now slightly hopeless, but he kept his cool. His tone was however slightly stiffer as he explained:
“My name is Tony Liss. I am professor of Physics at the University of Urbana-Campaign, and I chair this meeting with Claudio Campagnari.”
So that was the CDF debut of my utter incapability of keeping silent when advisable; the first in a long row of verbal hemorrhages during my long career in the experiment. On that occasion, I decided that was enough self-embarrassment for the afternoon, and I made my best effort to shut up for the rest of the day. So I acted nonchalantly as dozens of other physicists started to swarm in, taking chairs along the walls and leaving the seats next to ours free for the most authoritative members of the collaboration. The feeling of their eyes pointing at my back slowly faded away, as all the chairs were eventually occupied, and the meeting started.
I was a total newcomer, but luck was on my side that time. Physicists are by and large not really attached to formal issues; on that occasion, two summer students sitting in the front row did not raise more than a few ounces of eyebrows. At the meetings of the experiment, but also during the review process of analyses, the drafting of publications, the exchange of public e-mail messages, and all the other situations when CDF members had to interact and bounce opinions off each other, there was a total disregard of academic titles. It was not abnormal for the undergraduate to sit next to the full professor; it was just as normal for the former to interrupt the latter during a debate, showing no reverence if a mistake of the full professor needed to be exposed. This was a real point of strength for the experiment. One which, I believe, was totally American in spirit. There just existed two categories of persons: collaborators, and the rest of the World.
The physicist directing the Heavy Flavour meeting with Tony in 1992 was a tall, handsome young man with dark hair, a big nose, and a clean-shaven face. He spoke good English but he could not hide a certain Italian accent. Everybody called him Claudio, and I stared at him in admiration: at less than 35 years of age being at the centre of the attention, in the heart of one of the most important particle physics experiments in the world, looked to me like the crowning of a dazzling career, the one of a surely brilliant mind. My opinion on the deductions that could be made on the scientific merits of a physicist by observing his or her chairing an analysis meeting would change in the forthcoming years, but my esteem of Claudio Campagnari, who appeared in full command of the scientific output of CDF on heavy flavour physics, would stay high for the years to come. I would later discover he was the co-author of the algorithm which identified b-quark jets using soft leptons: "heavy machinery unsuitable to be used by monkeys", as described by the other author of the code, Avi Yagil.
After an introduction by the chairmen, there were short update reports from the subgroup leaders. In rapid succession Esov and I could hear in several "sub-group reports" the status of searches for top quarks in the their various characteristic signatures, as well as other analyses of high interest and relevance for the top hunt, which was back then in full swing. (In June 1992 the CDF detector had just started to collect data after a shutdown during which it had been endowed with a new silicon microvertex detector, which would be instrumental in finding evidence of top quark pairs.) After those summaries, a graduate student went to the screen to present the slides of his work. It was the first real talk of the meeting, where he was to show the results of a new kinematical fitter.
A kinematical fitter is an algorithm which attempts to tailor the measured energy and direction of all particles observed in an event to a given hypothesis. If the hypothesis matches the event characteristics, or in other words if this "tailoring" succeeds, the fit yields an estimate of a free parameter of interest. The algorithm that the student was presenting used a sample of Monte Carlo events, which simulated the production of top-quark pairs followed by their decay into the single-lepton topology: two b-quark jets, two additional jets from a W boson, and a high-momentum lepton and a neutrino from the other W. The program was designed to determine the most probable mass of the top quark; this was of course known from the start in the used simulated sample, which allowed one to test and tune the fit.
The first introductory slides explained the algorithm and received no comment from the audience, but once the speaker landed on the slide projector a transparency displaying a histogram of the top mass reconstructed by the algorithm, the guy sitting to my left started complaining. It was Avi Yagil, a tall, lean, neatly dressed guy with dark hair, olive skin, and quickly flying hands which accompanied his speech everywhere it went –very far, sometimes. Handsome, in his late thirties, he spoke with a strange tune, which I could describe as the one of Yogi the bear, if you are familiar with Hanna and Barbera’s cartoons.
“Wait a minute, what’s that chickenshit scattered around at random masses ? Your algorithm is failing big time on those events!”
With his funny expression Avi was pointing at several bins of the histogram displayed on the screen, each containing very few entries. Those bins were separated from the core of the distribution, where bins contained tens or even hundreds of entries each. Those non-empty outliers were indeed exposing some pathology of the reconstruction algorithm, and Avi was right in his assessment, although one could have argued that his question could have been formulated in a more polite way. The speaker hesitated, as the questioner continued his diatribe, using the well-known tactical trick of cutting off the person he was questioning, addressing someone else.
“You see, Claudio? I am telling you, that is exactly what we should avoid: methods that lose efficiency by attempting a full-fledged reconstruction, and cannot be kept under control. Only a small part of those top events are clustering at the right mass! The inefficiency is huge! We need to avoid unstable fitters like this one...” And on, and on. He was like a flood.
Claudio was a good friend of Avi –they had developed the soft-lepton tagging algorithm together-, and he knew he sometimes needed to restrain him.
“Wait a minute, Avi, let him speak” he interrupted. The graduate student took courage:
“Yeah, I guess the algorithm still requires some tuning... I will have a look at those events. But I’d like to stress the fact that here I have ran it over the whole sample containing a lepton and at least three jets, without applying the tight cuts we usually make to select the cleanest topologies...”
Avi did not appear placated, and the discussion went on for a while. A similar fate awaited the other talks in the line, so that by the time the meeting ended, three hours later, Avi had questioned with similar verve and vis polemica all of the speakers in at least a couple of occasions each. After observing this performance, my first impression of the guy was thus that of a really smart person, who seemed to know everything way better than those who were doing the work; but also of some kind of a nagger, one of those sorry souls with a career who like to use their position and their experience to please themselves by shooting at sitting ducks, every time the chance arises –always, that is. Despite that, I could not hide to myself that I found Avi Yagil a quite fascinating person, in a peculiar way.
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