In September 2006 I was in Ponta Delgada, the main town of the island of San Miguel in the Azores, for a physics conference where I was presenting results of the CDF experiment.

I remember listening to a very nice talk by Guido Martinelli, who was discussing the status of flavour physics, and getting rather depressed at the view of a very consistent picture of agreement between B physics observables and Standard Model predictions. This came at a moment when the CDF experiment had been probing the high-energy frontier with very detailed measurements, none of which appeared to show even the smallest glimpse of a departure from model predictions.

A thick paper by the ATLAS collaboration has been published by the Cornell Arxiv today. It is going to become a reference to all ATLAS analyses searching for new phenomena at high energy, or studies of boosted top quarks or vector bosons; and a good example of the new techniques that make sense of the energy distribution inside high-momentum jets.

The problem of classifying elements of a data set as belonging to one class or another, depending on their characteristics, is a very, very well-studied one, and one which is particularly important in particle physics.

Imagine, for instance, that you collect events with four high-transverse-momentum leptons (electrons or muons) with the ATLAS or CMS detector, and you wish to sort out which of these fit better to the hypothesis of being originated by Higgs boson decay into two Z bosons (with each Z boson in turn producing a lepton pair) rather than to the alternative hypothesis of being due to the incoherent production of a pair of Z bosons -a process that has nothing to do with Higgs bosons. This means you need to classify the data events using their observed features.