If this pans out, it means new physics…
Scientists have been searching hard for flaws in the standard model, only to be disappointed time and again- but maybe this is it?
To measure the mass of a W boson, one must first build a particle collider. The Tevatron, which ran from 1983 to 2011, was a 3.9-mile (6.3-kilometer) loop where protons crashed into antiprotons at up to about two tera-electron-volts (TeV)—some 25 times the mass of a W boson. The CDF experiment, located along the loop, sought signs of W bosons in these collisions from 2002 until the Tevatron shut down.
But one cannot simply observe a W boson; it decays into other particles far too quickly to register in any detector. Instead physicists must infer its presence and properties by studying those decay products—chiefly electrons and muons. Counting carefully, the CDF team found about four million events in the experiment’s data attributable to a W boson decay. By measuring the energy deposited in the CDF detector by those events’ electrons and muons, the physicists worked backward to figure out how much energy—or mass—the W boson originally had.
This work took a decade because of the numerous uncertainties in the data, Kotwal says. To reach its unprecedented level of precision—twice as precise as the previous best single experiment measurement of the W boson mass, which was made by the ATLAS collaboration——the CDF team quadrupled their dataset and also used new techniques. These included modeling proton and antiproton collisions and conducting a new, more thorough examination of the decommissioned detector’s operational quirks—even using old cosmic-ray data to map its layout down to the micron.
That was enough to elevate the researchers’ anomalous result to remarkable heights of statistical significance: nearly seven sigma, in the parlance of statistics. Seven sigma means that if no new physics affected the W boson, discrepancies at least as large as the one observed would still arise from pure chance once every 800 billion times the experiment was run. Even in the world of particle physics, where astronomical numbers are the norm, this almost seems like overkill: the field’s “gold standard” threshold for statistical significance is only five sigma, which corresponds to a given effect appearing through chance once every 3.5 million runs. Crucially, the seven-sigma value of the CDF team’s new measurement does not mean that result has a 99.999999999 percent chance of being new physics. It does not even mean other measurements of the W mass are wrong. Rather a seven-sigma result means that whatever the CDF collaboration is seeing is not by chance. It is a call to further inquiry, not a conclusion.