Leggi in italiano

The Collider Detector at Fermilab recorded high-energy particle collisions produced by the Tevatron collider from 1985 to 2011. About 400 scientists at 54 institutions across 23 countries are still analyzing the wealth of data collected by the experiment. Photo: Fermilab.

The CDF II collaboration recently published1 the most precise measure of the W boson mass ever achieved1, more than 10 years after the detector witnessed the last crashes between protons and antiprotons produced by the Tevatron collider installed at the Fermilab in the outskirts of Chicago.

The W boson is a fundamental particle that, together with its neutral companion the Z boson, mediates the weak interaction, the force responsible for radioactive decay. It turned out to be 0.1% heavier than predicted by the Standard Model of particle physics, and the statistical strength of the discrepancy is notable. While the standard model has its limits — most notably, its failure to incorporate gravity and explain dark matter — it had successfully withstood decades of tests, until now.

Nature Italy talked to Giorgio Chiarelli, senior researcher at the Italian Institute for Nuclear Physics and co-spokesperson of the CDF collaboration, about the Italian role in this discovery.

Why did the measure of the W boson mass take so long?

It is an exquisitely difficult measure and the last one on which CDF focused on. Before that, the experiment had discovered the top quark in 1995, studied the physics of B mesons, and hunted for the Higgs boson. The W boson decays in an electron or a muon and a neutrino, a neutral and nearly massless particle which escapes the detector without leaving any clue of its passage. In 2012, analysing about one quarter of the data from 2002 to 2011, we produced the first measurement of the W boson mass: it was higher than predicted by the standard model, but still compatible with it once error margins were considered. We then went after every single source of uncertainty. We gathered much more data, 4 million W boson candidates. And we studied every detail of the detector, in particular exploiting the large amounts of cosmic rays data collected during the years to calibrate the central outer tracker, the true star of this measure. This helped increase the precision with which we reconstruct the trajectory of leptons originating from the W boson.

How did you ensure the integrity of the scientific process on such a sensitive measure?

We set up separate analysis teams and the internal referees did a great job. The result was kept encrypted until the end to avoid biases, precisely because a discrepancy with the theoretical prediction would have such big implications. Moreover, the collaboration went through many changes since it was established. Some people left to join other experiments, others switched to industry: it was not at all obvious that we would succeed. When we finally got the result, we were relieved.

Carlo Rubbia, together with Simon van der Meer, earned a Nobel prize for the discovery of the W and Z bosons in 1984. How did that discovery lead to the latest result by CDF?

The Italian contribution to this scientific enterprise and to CDF in general started even before the discovery of the W and Z bosons at the CERN’s Super Proton-Antiproton Synchrotron. The Pisa-Stony Brook collaboration carried out one of the experiments at the Intersecting Storage Ring at CERN, the first hadron collider ever built. The Italian side of the group was coordinated by Giorgio Bellettini, who was invited in 1979 at the very first meeting at the Fermilab where the Tevatron was conceived. Bellettini, together with other Italian researchers from Pisa and Frascati, was responsible for the first technical design of the CDF detector in early 1980. Later the Italian group, and in particular the late Aldo Menzione, led the effort of designing and building the Silicon Vertex Detector, which was key to the discovery of the top quark, one of the main reasons for which CDF was built.

What are the next steps?

First, the discrepancy with the standard model prediction needs to be confirmed by other experiments. These measures will probably arrive from the Large Hadron Collider at CERN. CMS and ATLAS, as well as LHCb. They will ramp up the teams dedicated to the W boson mass analysis to increase the precision with respect to their previous measurements. In the meantime, theoreticians are looking at possible models beyond the standard one to see which ones can account for a heavier W boson. Less than an hour after our paper appeared online, a theoretical work was uploaded on the arXiv proposing a model with an additional Higgs field to explain our result. Today, there are nearly 70 theoretical papers. It is an extremely exciting time for particle physicists, both theoreticians and experimentalists.