Anomaly!

Tommaso Dorigo’s new book Anomaly! Collider Physics and the Quest for New Phenomena at Fermilab has just become available. I highly recommend it to anyone with an interest in high energy physics who wants some insight into how collider experiments are done. Dorigo is a well-known blogger, with the best (as well as most entertaining) blog there is about the experimental side of high energy physics. If you’re not following his blogging, you should be.

The nominal topic of the book is research conducted at the Tevatron by the CDF experiment during the 80s and 90s, into the early 2000s. Some of the specific well-known CDF results discussed in detail include measurement of the Z mass and the discovery of the top quark. The material about the top quark discovery comes closest to the kind of thing you find in other books. It’s a very well-done insider’s account of the story of an important discovery. I don’t know of a better place to read about the top quark search and how it finally succeeded.

One of the most unusual aspects of the book, in evidence in the top quark section and throughout the rest, is that it goes much deeper into an explanation of how collider experiment physics analyses are actually done than is usual in a popular or semi-popular book. Besides the insider’s local color and insights into the personalities involved, there’s quite a bit of discussion of the technical issues of the subject. This is a subject involving thorny issues of how best to reconstruct the properties of particles coming out of collisions, and finding clever new ways to deal with these while avoiding subtle pitfalls is a central problem, one outsiders normally don’t get to hear about.

The other unusual aspect of the book is that it doesn’t just discuss success stories of striking discoveries (of which there hasn’t been much between the top and the much later Higgs discovery at the LHC). One of the main activities of the field has been the search for “anomalies”, experimental results that disagree with the Standard Model and point to new physics. The problem is that finding anomalies is common, but they almost certainly will turn out to be due to some problem with the experiment or its analysis (such problems are always much more likely than revolutionary new physics).

One of the stories of an anomaly described extensively is that of an excess of high transverse energy jets, something that one might expect to see if quarks had some substructure (“preons”). Here the problem turned out to have a lot to do not with the experimental result, but with the theoretical modeling of what to expect from QCD. Another example is the story of “superjets”, events involving a W and 2 or 3 jets, with unusual properties.

For the “superjets” and for other anomalies, a favorite explanation was to invoke supersymmetry, since supersymmetric models predict a large range of different kinds of new particles, and one might hope that any anomaly is due to one of them. Dorigo has a few stories about theorists I hadn’t heard, in particular that of a fall 1995 letter signed by many prominent theorists (except Howard Georgi, who refused to sign). Despite ongoing efforts to look for superpartners, all of which had been unsuccessful, the feeling of the theorists was that the Fermilab experimenters weren’t trying hard enough. The letter was sent to the Fermilab director as well as the CDF and DZERO spokespersons. It explained what a great idea supersymmetry was, and ended with

We, the undersigned, believe that Fermilab has unique detection possibilites for supersymmetry, and urge you to direct your laboratory’s efforts in that direction, and ask the leaders of the collider detector collaborations to intensify their search for massive superpartners.

For more than two decades now, such searches for superpartners have been one of the dominant activities of collider experiments, with well-known negative results.

Finally, another important and unusual theme that runs through the book is the question of how to statistically characterize the significance of an anomaly. This is a topic where Dorigo is very much an expert, recently heading the CMS Statistics Committee.

Anomaly! is both a great tale of how science is really done, and an unusually insightful exploration of the crucial question of how one evaluates the significance of hints of new experimental discoveries. This question is of central importance now as the LHC gathers and analyzes data in a previously unexplored energy range. There are undoubtedly anomalies galore being studied by the LHC scientists, almost all of which will never be heard of by the public, as the experiments cautiously work to eliminate every possible conventional physics explanation for the anomaly. The 750 GeV diphoton bump of the past year is one example of an anomaly that made it out to a public announcement, with its disappearance in this year’s data making clear why the experiments are so cautious. You can’t find out about these ongoing stories, perhaps for good reason, due to the policies of how collider experiments are run, but now you can buy a copy of Anomaly! and at least read about how things played out in the previous generation of such experiments. This background should be helpful to make sense of what is going on if and when (March?), the next LHC anomaly gets reported.