A Quantum Diaries Survivor

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.

Statistics data analysis is one of those things that experimental physicists learn along the way. It is not a topic usually included in the curriculum studiorum of physics students at Universities: only few basic ingredients are taught during laboratory courses, and not much is added to that during a typical Ph.D. program. One usually learns the most common tools to fit histograms, combine measurements, estimate uncertainties on the field, as these things are always needed to produce publishable physics results. But several key statistical concepts often remain fuzzy and obscure in the mind of a large fraction of experimental physicists throughout their career. I know this because this happened to me, too - for quite a few years after my graduation.

[The title of this article comes from a T-shirt with ten advices on what to do when everything else fails]It has always surprised me to realize how confident we physicists are of the good faith of our colleagues. We may argue endlessly over one graph or result, getting to the point of publically casting doubts on the dexterity or intelligence of our peers (yes, I've seen that), but we never seem to doubt -privately or otherwise- their scientific integrity.

"Oh Wind, if Winter comes, can Spring be far behind ?"Good old Shelley inspired me to start today's article with the above verse, taken from his magnificent "Ode to the West Wind". With the weather we are experiencing these days in Geneva and northern Italy, I found it a relieving thought...So, winter conferences are over, and summer ones are still far away. This is therefore a nice moment to try an assessment on the quality of the results that the two competing CERN experiments have produced on the study of the Higgs boson. Why ? Because we are not going to have to change our conclusions in a short time scale caused by a result about to be published. How to compare the results

The results of a third-party investigation of Rossi's E-CAT reactor have appeared on the Cornell arxiv, and the conclusions of the tests are at the very least startling:

Finally the decay of Higgs bosons to b-quark pairs is emerging from LHC data, too.

Supersymmetry, the extension of the Standard Model of particle physics that was once sold as an almost certain discovery that the LHC experiments would bump into upon starting to collect proton-proton collisions, is not in a very healthy situation these days.

In 1992 the top quark had not been discovered yet, and it did not make much sense for the CDF collaboration to have a full meeting devoted solely to it; rather, analyses targeting the search of the top quark were presented at a meeting which dealt with both bottom and top quarks. This was called back then "Heavy Flavour meeting".

Two days ago I wrote a quick post to stimulate non-flat-EEG readers to consider an apparently trivial question, which in fact hid many subtleties. The general question I wanted to address was whether an estimate missing an uncertainty was more or less useful than a quoted uncertainty on the same parameter when the estimate itself was missing.

With the Higgs boson in the bag, the game called "global fit" that particle physicists have been playing for a couple of decades has changed significantly. The knowledge of the Higgs boson mass provided by the measurements obtained by the ATLAS and CMS experiments, added to dozens of  other measurements of critical observable properties of subatomic particles that have been measured at LEP/SLC, LEPII, the Tevatron, and the LHC itself, allow us to constrain some of the fundamental parameters of the Standard Model more than direct experimental determinations do.But what the heck is a global fit ?

ATLAS has just produced a very nice new study of jet production in Z-boson events. I will describe a sample graph below, but before I do I find it useful to explain to the less knowledgeable among you what a hadronic jet is, just in case you've been away during the last forty years.Hadronic Jets: what are they ?

One of the most intriguing effects in subatomic physics is the phenomenon of violation of the discrete symmetry called "CP". It is intriguing at various levels. First of all, CP violation is intriguing because of the depth of the concept: proof of that be that it is not at all easy to explain it to outsiders (I will make an attempt below, but I am likely to fail!). Second, its elusive nature makes it even more mysterious and difficult to study: only a few subatomic physical systems exhibit it, and the effect is visible only as a modification of measurable quantities at the level of a few parts in a thousand.