One of the fundamental particles of physics may be significantly heavier than the conventional laws of physics allow. The measurement could therefore be a sign of the longed-for “new physics” that goes beyond what was previously known. But independent confirmation is still pending.
In the renowned trade journal Science has an international team on Thursday reported on an analysis of the data, collected over a period of almost ten years at the Tevatron accelerator’s CDF detector. The giant machine was operated at Fermilab in the US state of Illinois until 2011, but researchers around the world are still analyzing the data from the collisions of protons and anti-protons. If these particles collide, a large number of other particles are created like debris from the collision, which are then recorded by the detectors. These can also be much heavier than the original protons because the energy of the collision can be converted into mass.
The current publication deals with the W boson, a little-known but absolutely fundamental particle. Just as the light particle – the photon – is responsible for transmitting the electromagnetic force, the W boson, together with the Z boson, is the carrier of the so-called weak nuclear force, one of the four basic forces of physics, along with electromagnetism, the strong nuclear force and gravitation. Unlike the massless photon, however, the W boson is quite heavy. It weighs about as much as 80 hydrogen nuclei – that’s still not even a trillionth of a gram, but for an elementary particle at the lush end.
The measurement caused some excitement in professional circles
Although the “Standard Model of Particle Physics” does not make a direct prediction for the mass of the W boson, it sets narrow limits, which in turn depend on other measured masses, for example that of the top quark or the Higgs particle. According to previous measurements, the mass of the W boson should correspond to about 80.37 gigaelectronvolts in the usual unit of particle physics. However, the CDF researchers now arrive at 80.43 gigaelectronvolts, with a very low uncertainty of just 0.01 percent. That may not sound like much, but it is not compatible with the previous measurement. Most importantly, it’s significantly harder than the Standard Model allows for, by a whopping seven “standard deviations” – a measure of how much a measurement differs from the expected value given the spread of the data, and seven standard deviations is a lot.
The measurement caused some excitement in professional circles. After all, physicists have long cursed the Standard Model for being both perfect and incomplete. On the one hand, it cannot explain what the mysterious dark matter that fills space is, or why some particle masses are so small and others so large. On the other hand, it has not been possible to prove a mistake so far. All experiments only confirm the predictions of the model in a frustrating way – apart from recent results on the magnetic moment of the muon or indications of so-called leptoquarks, which could also point to new physics. That’s why it’s always exciting when a measurement doesn’t match the prediction. It could be an indication of still unknown particles or new interactions.
But will the current result last? Matthias Schott from the University of Mainz, who conducts research at the LHC particle accelerator at CERN near Geneva, is skeptical. “It’s a shame that the work wasn’t published on a preprint server, as is usually the case,” says Schott, who has also been working on W bosons for a long time. “These measurements are extremely complex and one could have discussed in advance how different sources of error were taken into account.” For this is in Science-Article but not much to read. Schott believes that before you start speculating as to why the measurement doesn’t fit the Standard Model, you need to understand why it doesn’t match the previous results. Because so far all measurements agree to some extent within the framework of the uncertainties, only the new one stands out. First of all, this is suspicious.
But does that mean the work is probably fake and worthless? Not for a long time. “The researchers in the CDF team are all excellent physicists and the experimental part of the work is really fantastic,” says Schott. It may turn out that the new measured value is a bit too high and the measurement uncertainty is a bit larger, so that the contradiction to previous results is resolved. Then one could combine the measurement with the earlier data and would have a W boson mass on average over all measurements that is no longer quite as extreme, but still higher than the standard model allows. “So it’s getting more exciting again,” says Schott. “It’s definitely worth looking at the W boson further.”
This is exactly what many researchers are currently planning: while the Tevatron has long since been switched off, the particle accelerator LHC is still active, and all the data from the last round of measurements are far from being evaluated. W boson measurements can also be expected from there in the future.