Anomaly! Collider Physics and the Quest for New Phenomena at Fermilab

The following sections are included:

• Dedication
• A Word from the Author
• Prologue
• Contents

A famous quote attributed to Albert Einstein says “The most incomprehensible thing about the world is that it is comprehensible.” If one looks at the depth of our current understanding of subnuclear physics, one has to agree with the great Einstein. Humans have succeeded in explaining natural phenomena down to mind-bogglingly small distance scales, and such is an extraordinary accomplishment; yet, even more surprising is the fact that a theoretical explanation was there to be unearthed in the first place. The standard model is that theory. It results from unifying together into a single elegant construction the mathematical descriptions of phenomena involving three apparently unrelated natural forces: the electromagnetic interaction, which holds atoms together and describes the forces acting on electrically charged bodies; the weak interaction, which helps make stars burn and causes the radioactive decay of heavy elements; and the strong interaction, to which we owe the stability of atomic nuclei. These forces act on matter particles, so it is useful to introduce the latter before we explain how forces work.

In this chapter we focus on the physics of the hadron collisions of the Tevatron collider at Fermilab, and on the CDF experiment. In order to understand the design choices that were made for the CDF detector, a few key concepts for the complicated physics of hadron collisions must be briefly described. Among these are the cross section of subnuclear processes and the luminosity of particle accelerators.

In late August 1985 a fixed-target run of the newly commissioned Tevatron was terminated ahead of schedule due to the failure of a magnet. A few days later, the 5000-ton central detector was rolled into the collision hall: an engineering run in collider mode could finally start. For the first time, the Tevatron began circulating protons and antiprotons at 800 GeV per beam, attempting to create collisions in the core of CDF. This was little more than a first test, meant to verify the operational status of the accelerator. It was not expected to deliver a significant amount of data to the still incomplete detector. Hence, there was no real chance to produce physics results yet…

By the late 1980s, the main target of the CDF experiment had declaredly become the observation of the last quark still on the loose, the top. Yet, the road to the discovery of the top quark proved much longer and more difficult than anticipated, because of the unexpectedly large mass of that particle. The production of a heavy top quark was a quite rare outcome even for the very high energy of the Tevatron collisions; as a result, the collection of a significant number of signal events took longer than anticipated. In addition, the tools and the analysis strategies deployed by the early searches were inadequate, since they targeted a quark lighter than the W boson, which would display a phenomenology quite different from that of a heavier object. Those studies yielded a string of negative results. Hence, in spite of the great successes that the CDF experiment was reaping with the study of other physics processes, the general feeling was a mix of disappointment and frustration. The table was set and dinner was ready, but the VIP was not showing up!

The air in CDF was electric during the hot summer of 1992. The chance to make history by finding the sixth quark inspired and excited old and young researchers alike. The old leaders saw in the top-quark discovery their last chance of catching a Nobel Prize. The younger post-docs and tenure-track professors wanted to show their worth in the challenge, paving the way to better academic positions. Tom Ferbel, a professor at Rochester, recalls a conversation with Paul Tipton, who had just been hired as an assistant professor there. After explaining to Paul that he had overheard his University of Michigan colleague Dante Amidei say he, Dante, would discover the top quark, Tom was staggered by Paul's reply:

“Oh, yeah? Let's see who will be first. I will find top, even if it's not there!”…

As Run 1 data started to fill the Exabyte storage tapes in the vault of the Feynman center, the CDF collaboration displayed the first symptoms of a sort of superiority complex with respect to the rest of the world of high-energy physics. Surely, the LEP accelerator was a giant marvel, and the exquisite measurements of the properties of the Z boson that the four CERN experiments had been producing in e⁺e⁻ collisions since 1989 were an impressive achievement. Yet, the Tevatron was the highest energy collider: here laid the forefront of research in particle physics. And it was CDF who would tell the world whether there was a sixth quark; CDF would discover whatever new physics was waiting at the high-energy frontier…

Toward the end of the summer 1993 CDF went undercover. The spokespersons Shochet and Carithers decided that the evidence for top production in the data was mounting; It looked likely that soon they would be able to make the public announcement of seeing the long-sought sixth quark. Results were feverishly expected from the analyses of the full dataset of 20 inverse picobarns of collisions that CDF had collected in the just ended Run 1A. It was considered dangerous for the experiment to produce interim, inconclusive results which could backfire if unsupported by future data.

Suddenly an uncommon reaction takes place in the detector. It happens on April 28, 1995, in the middle of an otherwise anonymous store. CDF is collecting good data, and the shift crew in the control room takes care of the usual business: keeping an eye on the colorful monitors that plaster the walls, checking trigger rates, logging the warnings issued by the data acquisition system, and answering e-mails.

In this chapter we deal with one of the most intriguing and exciting among the several anomalies that arose in the analysis of Run 1 data. One which lent itself to an immediate, startling interpretation: maybe quarks are themselves composite objects after all! The possibility of such a revolutionary discovery sucked the CDF collaboration into the center of the first media storm of its career. The coverage of the story by the press was comparable in intensity to the news on the top discovery, which had been announced only a few months earlier. But, in this case, the management of the experiment was caught entirely unprepared. The unanticipated and uncoordinated interviews that science reporters obtained directly from members of the collaboration, as well as the uncontrolled claims artfully inserted in some of the resulting articles, generated a good deal of friction within the experiment.

Participating in an experiment that counts half a thousand collaborators means you will never get to know all of them, not even if you spend all your life at the lab. During Run 1, only a minority of the members of the CDF collaboration were based at Fermilab. Most of the others were “irregulars” who visited infrequently. One such occasion was to participate in collaboration meetings. Another was to attend to their data-taking shifts duty, a necessary requirement they had to fulfil in order to be granted authorship rights…

In 1995 a group of physicists from Pisa and Frascati led by Paolo Giromini finally completed a measurement which had kept them busy for several years: the determination of the total cross section of 1.8 TeV proton–antiproton collisions, a number which is a fundamental input to any determination of the luminosity of a data sample. In the early 1980s, the Italians had built a set of detectors specifically designed to track small-angle particles emitted in the very forward direction. Those detectors, called “roman pots,” provide an accurate measurement of the rate of interactions that do not break up the protons. A detailed study of the resulting data allowed a threefold decrease in the systematic uncertainties on the total cross section. This translated directly into a reduction from 15 to 5% of the uncertainty in the integrated luminosity of all CDF datasets. It was an important contribution to the scientific output of the experiment, as the precision of the cross section of all rare phenomena measured by the experiment improved by the same factor…

The 13 superjet events had only started to stir up trouble in CDF when Giromini came up with something even harder to deal with. There had, in truth, been warnings that their anomalous observation would be the beginning of a bigger story. A significant portion of Frascati's CDF note 4348, the main documentation of the superjet analysis, dealt with the description of a new physics mechanism which could be the source of the observed signature. The note also included some preliminary tests of how that hypothesis could fit the numerical excess of W+2 or 3 jet events as well as explain their kinematics. The Superjet godparent committee had been specifically asked to stay clear of that part of the documentation, at least in a first phase of the review, and only focus on ensuring the correctness of the analysis.

The following sections are included:

• Epilogue
• Acknowledgments
• Name Index
• Subject Index