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One PRL a Day
Toward mid-July, the CDF members finally realized they had in their hands a very competitive result. The Z mass was measured to be 90.9 GeV. The statistical uncertainty of the combined fit to di-electron and di-muon Z decay candidates produced by Keutelian was coming out in the ballpark of 300 MeV, and the dominant source of systematic uncertainty was the energy scale, amounting to 200 MeV. Overall, the CDF data allowed a fourfold reduction in the Z mass uncertainty over the current world average! It was time to wrap it up.
The article was written on the night of July 18, just hours before a scheduled collaboration meeting, a two-day event organized every three or four months where CDF members could listen to a detailed overview of the status of the experiment and the analyses that were being carried out. The publication was going to be a "Letter" - a short article to be sent to the prestigious Physical Review Letters (or "PRL" as everybody called it). As short as it needed to be in order to fit within the strict standards of PRL, writing a paper overnight was a challenging task for its editors. It was 6 AM on July 19 when the article was finally finished. Exhausted but happy, Steve went for breakfast, then drove to the Hirise. There the measurement could finally be presented to the collaboration. Sleep-deprived and physically drained, Steve had the task to convince his colleagues, who for the large majority had not had a chance to follow the progress of the measurement, that the result was solid enough to warrant an approval to be published without further scrutiny. The request of an approval without review was an unusual procedure. Normally, an internal review process was required before a publication could be sent out to a scientific journal. Fortunately, the response of the collaborators was very positive: this was indeed a careful, precise measurement deserving to be promptly sent to PRL.
As with any scientific publication, the title and abstract of the article were as important as the article itself and caused a lengthy discussion. On the title Errede managed to have it his way: "Measurement of the mass and width of the Z boson at the Fermilab TEVATRON." The capitalized name of the hadron collider was one of the points on which Errede stood his ground. The other was the explicit mention of the fact that CDF had measured the mass as well as the natural width of the resonance. The latter was something which electron-positron guys expected to be exclusively their own business, thanks to the already mentioned energy scan of the production rate.
The natural width of a particle can be determined by the distribution of observed mass values around the most frequent value. The width is inversely proportional to the particle's lifetime and is directly connected to the intensity of the interactions responsible for the particle decay. The stronger the interactions, the larger is the range of mass values that the particle may take, as the decay proceeds more quickly, giving no time to the particle to "settle" to its nominal mass. Arguably, the natural width of the Z boson is no less important than is the Z mass itself. Compared with model predictions, the measured width allows one to determine whether there may be decay modes of the particle that are not seen experimentally. A larger-than-predicted width implies the existence of unknown particles into which the Z can disintegrate, as mentioned in Chapter 1.
After the meeting Errede, still in a state of sleep deprivation from the past weeks and the frantic final night of work, met Alvin Tollestrup and Barry Wicklund. Together, they took the stairs to the second floor of the Hirise and went straight to John Peoples’ office: they needed something from the Fermilab director. As instructed by Alvin, that morning Errede had sent to Peoples a copy of the article, along with a special request. The laboratory director was asked to undersign the paper before it got sent to PRL. The journal would ordinarily take several weeks to review an article. Suitable reviewers had to be found and be given sufficient time to comment on the results, require clarifications, propose modifications, or reject the article. The exchange between authors and PRL reviewers could easily push the publication date to September or October. Furthermore, the journal editors might choose a member of Mark II as reviewer, bringing in a conflict of interest. A SLAC reviewer might fall in the temptation of purposely slowing down the publication of the CDF paper. However, an article signed by the director of a major American physics laboratory allowed the peer review process to be waived. The article would go to press in the very next issue of the magazine.
The special no-review procedure had been instituted in 1976 by Physical Review Letters in what was called "An Editorial Experiment." The April 26 issue of the magazine featured a letter bearing that title, where the editors explained that the time to publication of even carefully crafted articles could be long, so they had cooked up a special procedure to bypass the regular review process:
"To receive this special treatment, the Letter must be forwarded for publication by a division leader, department chairman, or director at the author's
institution[] This person, who may not be a co-author, must explain why special handling is appropriate. []. The editors reserve the right to evaluate the “extraordinary circumstances” and to refuse the request. [] The published Letter will bear, immediately above the abstract, a note reading, "Published without review at the request of (person forwarding the Letter) under policy announced 26 April 1976.""
Peoples agreed to sign the paper and write a cover letter to PRL: it was certainly in the interest of the laboratory that CDF proved its worth in the competition with the Stanford experiment. Those were years when SLAC and Fermilab were competing on the amount of funding that they received from the Department of Energy. Richter had the reputation of being the one who won those Washington battles. The perception was that he was more persuasive and more charismatic than Peoples; the Nobel Prize he had won in 1976 also played a role. A joke in fact circulated among American experimental physicists on the matter: Richter and Peoples would both go to Washington to get money for their labs, and Peoples would be the one getting in the revolving doors at the entrance first, but the one who would get out of the doors first would always be Richter! However, the truth was different: Fermilab had actually a larger budget than SLAC at the time, but was starting to suffer from the ramping up of the funding to the Superconducting Supercollider (SSC). The Texas machine was a hadron collider, hence it fished in the same funding pond of the Tevatron.
Peoples (see picture) did more than undersign the paper and the cover letter. That same day, he called the editor-in-chief of PRL David Lazarus, forewarning the arrival of the Z mass paper and asking whether the editor agreed to publish it in the next issue of the magazine. The editor denied the request, on the grounds that it would be difficult to organize the special handling of the article in such a short timescale. As he hung up, Peoples was left with an unpleasant suspicion. After discussing the matter further with his predecessor Lederman and with Tollestrup, Peoples called the editor again. Finally, the situation was clarified: Lazarus explained that SLAC had already preannounced the submission of a Mark II paper on the Z mass, so he had set up an internal review panel to handle it. It was only through the stubborn insistence of Peoples that a similar panel was formed for the CDF paper.
"If You Show Me Yours, I’ll Show You Mine"
While the frenzy at Fermilab was finally subsiding, an epic battle was brewing for Ken Ragan, a post-doctoral scientist at the University of Pennsylvania. Ken was at the time looking for a faculty position, and he needed to embellish his curriculum vitae with a talk at a major conference. A few months earlier he had discussed the matter with Brig Williams, the head of the Pennsylvania group in CDF. Brig had understood that the experiment would soon produce some important new result; perhaps he had surmised that a world's best Z mass was at reach. It was by following Brig's explicit suggestion that Ken applied to give the invited CDF overview talk at the Topical Conference following the 1989 SLAC Summer School. By sheer chance, CDF was going to first present its new Z mass measurement in the lion's lair. Worse still for the Mark II collaborators, the conference organizers had scheduled the CDF talk on Wednesday, two days before the Mark II talk. This had the purpose of closing the conference with the announcement of the new SLAC results, and it also gave Mark II a few additional days for crossing all the t's and dotting all the i's of their own analysis of Z events. Fortunately, this arrangement also worked for CDF, as it gave Ken and his collaborators enough time to review the measurement and approve it. Before traveling to SLAC, Ken had purposely been instructed by the Electroweak group conveners to prepare two different talks. One included the Z mass measurement, and gave it the space it deserved. A second "backup" talk did not mention the Z at all and focused more on the other beautiful physics measurements that the experiment had produced.
The backup talk would be the one to choose, in case some real concern about the soundness of the Z mass measurement arose during Errede's presentation at the collaboration meeting. But nothing like that happened. On the morning of July 19, shortly before the start of the session, Ken was approached by a young Mark II physicist, whom I will call Arthur in the following. He cheered Ragan and soon made a strange request.
"Hey Ken, I hear that you guys in CDF have pulled off a measurement of the Z mass?!"
"Well, I don't know who told you that... But my talk is in two hours, so
come and listen to it if you're interested!"
Sure, I'm definitely coming. But why don't you tell me what is your result? You know, we also have a measurement - We are finalizing it and we'll send it out in just a few days. If you tell me yours, I can tell you ours."
"Hmmm, thanks, but I guess I'll pass. It doesn't matter much, but it would still be a violation of our internal rules - Just come to the talk!"
"Well, okay. It's fine. Look, here is our draft - just so you know that we do have it... You see?"
Arthur opened a folder and showed Ken the first page of a draft paper titled "Initial measurements of Z boson resonance parameters in e+e- collisions." The abstract showed the mass measurement obtained by Mark II, and Arthur made sure that Ken would read the number.
"I see... Well, thanks Arthur. I need to get a seat now."
"Yeah, and good luck with your talk today!"
Despite Ken's refusal to share the CDF measurement, Arthur had reached his covert goal. Ken had read from the paper's abstract the value of Z mass that Mark II was measuring: 92.5 GeV, with a 0.2 GeV uncertainty. This was not incompatible with the old and less precise UA1 and UA2 results, which combined yielded 91.5 ± 1.7 GeV, but it was quite significantly higher than the result Ken was about to present! Was there something wrong with the CDF measurement? A less self-confident physicist would have decided to switch to the backup talk: after all, there were many other results to discuss in detail. But Ken did the right thing: he decided to ignore that extra bit of information. It was certainly strange and worrisome that the Mark II physicists would produce a measurement of the Z mass incompatible with the CDF one: their energy scan could not be affected by large systematic biases, so this implied a possible problem in the CDF measurement. Yet, CDF stood behind the result they had worked so hard to produce, and if their result were to be proved wrong, so be it.
Ken Ragan's talk was memorable. The measurement of the Z mass he showed was a little jewel. Many of his listeners did not know the first thing about the cunning methods that had been used to calibrate lepton energy and momentum. They listened in shock and awe. And the numerical result itself was shocking to the SLAC physicists. CDF had measured the mass with a total precision of 360 MeV, a fivefold reduction from the combined result of the two previous CERN experiments, a result competitive even with the systematics-free, lineshape-driven measurement that Mark II itself was about to produce! That was as close to a punch below the belt as an experimental physics result could be. It showed that hadron colliders could deliver competitive precision physics measurements despite the
"dirty" collisions they were studying.
The intricacies of the Z measurement would have been enough for an hour-long seminar, but CDF had more to show. Using quite similar techniques, the W mass measurement was another world’s best result. Further, measuring the two boson masses together allowed an important check of the standard model prediction for their ratio. The top-quark searches yielded an upper limit on its mass at 77 GeV, a result which again beat those that the CERN UA1 and UA2 experiments presented at the same conference. Furthermore, Ken's talk included a new, high-quality measurement of the cross-section of QCD processes and a first indication of the B-physics potential of the detector.
While the above events unfolded in the main auditorium at Stanford, Barry Wicklund was not in a jovial mood: Tollestrup had asked him to prepare a late afternoon "Wine and Cheese" seminar for the Fermilab physicists. Barry did not really want to give that seminar: it was a lot of work to put it together in the matter of one day. Barry was a power machine who would work like a truck when you gave him a difficult physics problem to solve, but he much less liked to spend his time on presentations. He had tried to redirect the task to Errede’s student Keutelian, but Tollestrup would have none of that. Melissa Franklin finally convinced Barry by pointing out that he had been the source of the critical ingredient in the measurement, the electron energy scale, and that CDF wanted to acknowledge his contribution. So Barry spent the night of July 21 preparing his talk. When he finally drove to the Hirise he was caught by heavy rain and got drenched as he ran from the parking lot to the Ramsey auditorium. There Tollestrup introduced Barry as the star of the measurement team, but at that point Barry could not care less: soaked wet, he was feeling dead tired and demotivated. Yet he was an excellent speaker, and as he delivered his talk he did raise the interest of all the Fermilab audience. The lab theorists as well as the experimentalists from the other Fermilab experiments were all strongly impressed and almost in denial. Why, lepton collider specialists had been repeating to them that you could not do precision measurements at a hadron collider. And yet there it was, a 0.4% measurement of the Z mass, and a nice determination of the Z width to boot!
"If You Show Me Yours, I’ll Show You Mine"
While the frenzy at Fermilab was finally subsiding, an epic battle was brewing for Ken Ragan, a post-doctoral scientist at the University of Pennsylvania. Ken was at the time looking for a faculty position, and he needed to embellish his curriculum vitae with a talk at a major conference. A few months earlier he had discussed the matter with Brig Williams, the head of the Pennsylvania group in CDF. Brig had understood that the experiment would soon produce some important new result; perhaps he had surmised that a world's best Z mass was at reach. It was by following Brig's explicit suggestion that Ken applied to give the invited CDF overview talk at the Topical Conference following the 1989 SLAC Summer School. By sheer chance, CDF was going to first present its new Z mass measurement in the lion's lair. Worse still for the Mark II collaborators, the conference organizers had scheduled the CDF talk on Wednesday, two days before the Mark II talk. This had the purpose of closing the conference with the announcement of the new SLAC results, and it also gave Mark II a few additional days for crossing all the t's and dotting all the i's of their own analysis of Z events. Fortunately, this arrangement also worked for CDF, as it gave Ken and his collaborators enough time to review the measurement and approve it. Before traveling to SLAC, Ken had purposely been instructed by the Electroweak group conveners to prepare two different talks. One included the Z mass measurement, and gave it the space it deserved. A second "backup" talk did not mention the Z at all and focused more on the other beautiful physics measurements that the experiment had produced.
The backup talk would be the one to choose, in case some real concern about the soundness of the Z mass measurement arose during Errede's presentation at the collaboration meeting. But nothing like that happened. On the morning of July 19, shortly before the start of the session, Ken was approached by a young Mark II physicist, whom I will call Arthur in the following. He cheered Ragan and soon made a strange request.
"Hey Ken, I hear that you guys in CDF have pulled off a measurement of the Z mass?!"
"Well, I don't know who told you that... But my talk is in two hours, so
come and listen to it if you're interested!"
Sure, I'm definitely coming. But why don't you tell me what is your result? You know, we also have a measurement - We are finalizing it and we'll send it out in just a few days. If you tell me yours, I can tell you ours."
"Hmmm, thanks, but I guess I'll pass. It doesn't matter much, but it would still be a violation of our internal rules - Just come to the talk!"
"Well, okay. It's fine. Look, here is our draft - just so you know that we do have it... You see?"
Arthur opened a folder and showed Ken the first page of a draft paper titled "Initial measurements of Z boson resonance parameters in e+e- collisions." The abstract showed the mass measurement obtained by Mark II, and Arthur made sure that Ken would read the number.
"I see... Well, thanks Arthur. I need to get a seat now."
"Yeah, and good luck with your talk today!"
Despite Ken's refusal to share the CDF measurement, Arthur had reached his covert goal. Ken had read from the paper's abstract the value of Z mass that Mark II was measuring: 92.5 GeV, with a 0.2 GeV uncertainty. This was not incompatible with the old and less precise UA1 and UA2 results, which combined yielded 91.5 ± 1.7 GeV, but it was quite significantly higher than the result Ken was about to present! Was there something wrong with the CDF measurement? A less self-confident physicist would have decided to switch to the backup talk: after all, there were many other results to discuss in detail. But Ken did the right thing: he decided to ignore that extra bit of information. It was certainly strange and worrisome that the Mark II physicists would produce a measurement of the Z mass incompatible with the CDF one: their energy scan could not be affected by large systematic biases, so this implied a possible problem in the CDF measurement. Yet, CDF stood behind the result they had worked so hard to produce, and if their result were to be proved wrong, so be it.
Ken Ragan's talk was memorable. The measurement of the Z mass he showed was a little jewel. Many of his listeners did not know the first thing about the cunning methods that had been used to calibrate lepton energy and momentum. They listened in shock and awe. And the numerical result itself was shocking to the SLAC physicists. CDF had measured the mass with a total precision of 360 MeV, a fivefold reduction from the combined result of the two previous CERN experiments, a result competitive even with the systematics-free, lineshape-driven measurement that Mark II itself was about to produce! That was as close to a punch below the belt as an experimental physics result could be. It showed that hadron colliders could deliver competitive precision physics measurements despite the
"dirty" collisions they were studying.
The intricacies of the Z measurement would have been enough for an hour-long seminar, but CDF had more to show. Using quite similar techniques, the W mass measurement was another world’s best result. Further, measuring the two boson masses together allowed an important check of the standard model prediction for their ratio. The top-quark searches yielded an upper limit on its mass at 77 GeV, a result which again beat those that the CERN UA1 and UA2 experiments presented at the same conference. Furthermore, Ken's talk included a new, high-quality measurement of the cross-section of QCD processes and a first indication of the B-physics potential of the detector.
While the above events unfolded in the main auditorium at Stanford, Barry Wicklund was not in a jovial mood: Tollestrup had asked him to prepare a late afternoon "Wine and Cheese" seminar for the Fermilab physicists. Barry did not really want to give that seminar: it was a lot of work to put it together in the matter of one day. Barry was a power machine who would work like a truck when you gave him a difficult physics problem to solve, but he much less liked to spend his time on presentations. He had tried to redirect the task to Errede’s student Keutelian, but Tollestrup would have none of that. Melissa Franklin finally convinced Barry by pointing out that he had been the source of the critical ingredient in the measurement, the electron energy scale, and that CDF wanted to acknowledge his contribution. So Barry spent the night of July 21 preparing his talk. When he finally drove to the Hirise he was caught by heavy rain and got drenched as he ran from the parking lot to the Ramsey auditorium. There Tollestrup introduced Barry as the star of the measurement team, but at that point Barry could not care less: soaked wet, he was feeling dead tired and demotivated. Yet he was an excellent speaker, and as he delivered his talk he did raise the interest of all the Fermilab audience. The lab theorists as well as the experimentalists from the other Fermilab experiments were all strongly impressed and almost in denial. Why, lepton collider specialists had been repeating to them that you could not do precision measurements at a hadron collider. And yet there it was, a 0.4% measurement of the Z mass, and a nice determination of the Z width to boot!
(to be continued in part 5)
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Tommaso Dorigo is an experimental particle physicist, who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”. You can purchase a copy of the book by clicking on the book cover in the column on the right.
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