The clip below, together with the following few which will be published every few days in the coming weeks, is extracted from the third chapter of my book "Anomaly! Collider Physics and the Quest for New Phenomena at Fermilab". It recounts the pioneering measurement of the Z mass by the CDF detector, and the competition with SLAC during the summer of 1989. The title of the post is the same as the one of chapter 3, and it refers to the way some SLAC physicists called their Fermilab colleagues, whose hadron collider was to their eyes obviously inferior to the electron-positron linear collider. I plan to follow it with a few clips that were removed from the book, but which contain interesting related anecdotes.
The years 1986 and 1987 saw the completion of the CDF detector as it had been envisioned in the 1981 technical design report. Finally, hundreds of millions of collisions could be collected in 1988–1989, a data-taking period which came to be called “Run 0” only after 1992, when the following “Run 1” commenced. The acquired data raised huge interest due to the unprecedented collision energy (now 1.8 TeV). This allowed for the production of very energetic hadronic jets, as well as a significant number of W and Z boson decays. And many CDF collaborators were now confident that a top-quark signal would also eventually show up.
Despite the general optimism, the fruition of the data proved to be a long and grievous process. First of all, one needed to understand in detail the effect that the online trigger selections had on the acquired data, a task which by itself required several man-years of work. Then a precise reconstruction of particle trajectories had to be produced from the thousands of position measurements determined by the tracking chamber.
The unprecedented number of charged particles produced by each high-energy Tevatron collision made this a novel challenge. The identification of hadronic jets and the measurement and calibration of their energy also required careful, detailed procedures that had to be designed from scratch. Furthermore, the definition of suitable recipes for the identification of electrons and muons called for a deep study of all their characteristics. Because of those difficulties, it took time before the experiment could start to produce significant physics results.
One of the first occasions for the CDF collaborators to prove their worth to the scientific community occurred in 1989, as they realized that they could achieve a “world’s best” measurement of the Z boson mass, just as the Mark II experiment was about to do the same using the electron–positron collisions that the brand new SLC collider had started to provide.
The competition between the Stanford Linear Accelerator Center (SLAC) and Fermilab was in those years quite fierce: both laboratories wanted to be recognized as the most important particle physics center of the United States. It was a matter of prestige as well as of access to research funds. In comparison, even the long-time competition between US laboratories and CERN remained in the background.
SLAC’s director Burton Richter (left), who together with Samuel Ting had been awarded a Nobel Prize in 1976 for the J/ψ discovery, was one of the most notable advocates of electron–positron colliders, and he appeared critical of the hadron collider program of Fermilab. In 1988, Richter and Lederman (then at his last year as director of Fermilab) were both interviewed by Malcolm Browne for the New York Times. Browne’s article, titled “Search Quickens for Ultimate Particles,” featured Richter explaining how the SLC would soon measure the Z boson mass with a much better precision than what the UA1 and UA2 experiments had achieved a few years before at CERN. According to Richter, the large number of Z bosons produced by SLC would yield a deeper understanding on fundamental physics, answering questions on the existence of new generations of matter, on the Higgs boson, and a lot more.
Although the Times piece did not cast the Tevatron collider in a bad light, it did give the clear impression that SLAC was the real center of the action for US particle physics. That was already a bit annoying to CDF members. A still harder pill for them to swallow was hearing the random nasty comments that some SLAC scientists used to offer at conferences and in other public venues: they pictured hadron colliders as incapable of doing precision physics and as lacking sufficient resolution to measure the Z boson mass.
Of course, there was a bit of truth in those statements. The cleanest and most precise way to measure the mass of a resonance is not to compute it from the momenta of its decay products, but rather to perform a formation experiment where the energy of the collision is gradually stepped up in suitably spaced intervals in the vicinity of the mass of the particle. At each energy point, the rate of creation of the resonance is determined; the peak rate is achieved when the collision energy equals the particle mass. As the energy of electrons and positrons in the beam is very precisely known, the mass of the resonance can be measured with high accuracy. Such an energy scan may only be performed using point-like projectiles as electrons and positrons, and not with protons, as the latter are composite objects which yield an undetermined fraction of their kinetic energy to the collision.
But the truth stopped there: even if hadron collisions could not scan the resonance shape, a well-designed detector could still measure the decay products of the studied particle with sufficient precision to produce a competitive measurement. Given a large enough number of observed decays, the issue was whether systematic uncertainties could be reduced as much as needed.
A crucial input on the feasibility of a precise Z mass measurement at CDF came in the late spring of 1989. Barry Wicklund, an Argonne physicist and one of the deepest thinkers in the collaboration, demonstrated during a talk at an Electroweak group meeting how a calibration of the energy measurement produced by the electromagnetic calorimeter could be achieved with a precision 10 times better than what had been until then thought possible.
According to Brig Williams, Steve Hahn, and the other calorimeter guys, the electron energy calibration had to rely on test beam measurements. Data had been collected for that very purpose by directing a beam of 50-GeV electrons to the 15° wedges of the calorimeter before assembling them into the wheels that made up the central detector. The test beam data enabled a neat cross-calibration of the detector modules: this resulted in a 2% precision of the energy measurement. Such was a reasonable number for most measurements, but it was not nearly precise enough to allow a competitive measurement of the Z boson mass from the decay electrons.
Barry, who had worked at the problem with his Pennsylvania University colleague Fumihiko Ukegawa, demonstrated how it was possible to inter-calibrate the detector modules more effectively by using a sample of 17,000 electron candidates that he had isolated in the Run 0 dataset. Those were electrons produced by proton–antiproton collisions rather than by a dedicated test beam, but they were still a pure sample. They came from well-reconstructed decays of heavy-flavored B or D hadrons, or conversions of energetic photons.
Those 17,000 electrons were enough to study the so-called E/p ratio in each of the forty-eight 15° wedges that made up the two “wheels” of the central calorimeter (see right). The E/p ratio is computed as the energy E of the electromagnetic shower initiated by the electron in the calorimeter, divided by the electron momentum p determined from the curvature of its trajectory in the tracking chamber.
The ratio equals 1.0 for highly relativistic particles, but it is often measured to be a bit larger than that for electrons. Electrons emit photons in a magnetic field, so their momentum can be underestimated. The measured energy is instead unaffected by the photon emissions, since the photons land close to the electron shower in the calorimeter and their energy is automatically accounted for.
A careful modeling of the detector material in the tracker, whose amount determines the energy that electrons lose by photon radiation, allows one to connect the energy measurement to the momentum measurement using the E/p distribution. Thus, Wicklund had a way to calibrate the wedges of the electromagnetic calorimeter quite effectively, with uncertainties one order of magnitude smaller than those achievable with test beam data. Alvin Tollestrup, the spokesperson of CDF, was deeply impressed. After the meeting he approached Wicklund, insisting that the data-driven calibration be carried out in full: the E/p method could enable precision measurements of W and Z bosons.
(To be continued)
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.