The W boson is the carrier particle of the weak nuclear force and is therefore involved wherever atomic nuclei disintegrate or fuse. The standard model of physics predicts the mass of this elementary particle. But now the most accurate measurement of the W boson mass to date has revealed significant deviations from the theoretical value – the W boson is heavier than it should be. According to the international research team, this discrepancy could indicate that the standard physical model is incomplete and that forces or particles beyond known physics still exist.
The W boson, discovered in 1983, is one of the fundamental elementary particles in the standard model of physics. Because it is one of the carrier particles of the basic physical forces and, together with the Z boson, mediates the weak nuclear force. This always works when atoms decay or fuse with one another – for example in radioactive beta decay or the fusion of hydrogen nuclei in the sun. While other gauge bosons such as the gluons of the strong nuclear force or the photons of the electromagnetic interactions are massless, the W bosons are real heavyweights: according to the standard model of physics, their mass is about 80 times higher than that of the proton. This follows from theoretical calculations on its interaction with other particles such as the Higgs boson, the top quark and also the charge of the electron. If these assumptions are correct, the W boson should have a mass of 80.357 megaelectronvolts – according to the theory.
Particle decays suggest higher mass of the W boson
Physicists from the CDF collaboration at the Fermi National Accelerator Laboratory in the USA have now checked whether this is true. Their measurement is based on data from the Tevatron particle accelerator, in which protons and antiprotons were brought to collision at high speed. These collisions produce W bosons, which after a short time either decay into an electron and neutrino or a muon and neutrino. The direction of flight and the energy of these decay products can be used to determine how heavy the W boson must have been. For their study, the scientists evaluated more than 4.2 million such decays that were registered in the Tevatron between 1985 and 2011. “This dataset is four times larger than previous measurements and has twice the statistical accuracy,” explains CDF physicist Ashutosh Kotwal of Duke University. “We subjected our result to an enormous amount of review and testing.”
Analysis of the Tevatron data revealed that the W boson has a mass of 80.4335 megaelectronvolts. This makes it much more difficult than the current models predict. As the physicists report, the discrepancy between the new, most precise measurement to date and the mass predicted by the standard model corresponds to a significance of seven sigma. It is thus far above the five sigma required for a discovery in particle physics. The new, most precise value to date for the W boson mass thus challenges the Standard Model, but at the same time confirms some previous measurements that also found higher masses for the W boson than the values predicted by the Standard Model. However, there were also measurements, including one from the ATLAS detector at CERN’s Large Hadron Collider (LHC), which did not find any significant deviations.
Is the standard model incomplete?
The discrepancies in the W boson mass that are now appearing again on the basis of a relatively precise measurement based on a large amount of data are therefore at least cause for further consideration. “The surprisingly high value for the W boson mass contradicts a fundamental element at the heart of the Standard Model,” write Claudio Campagnari of the University of California and Martijn Mulders of CERN in an accompanying commentary in Science. “It affects theoretical predictions and experimentally observed data that were previously considered established and well understood.” This could mean that the Standard Model is incomplete and needs to be corrected. There may still be particles or forces in the universe that have not yet been included in our catalog of knowledge. “Now the theoretical physicists, as well as other experiments, need to follow this up and shed light on this mystery,” says CDF spokesman David Toback of Texas A&M University.
Source: CDF Collaboration, Science, doi: 10.1126/science.abk1781