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On April 7, this year, in a paper in Science, researchers from Collider Detector at Fermilab (CDF) Collaboration analysed ten years of data and announced that they have made a precise detection of the mass of the so-called W boson and that it does not match with value expected from estimates using the standard model and previous measurements. If confirmed by other experiments this would be a sign of the incompleteness, or even incorrectness, of the Standard Model of particle physics.
What is the W boson and why is a discrepancy in its mass so important?
The W boson is an elementary particle that plays an important role in mediating weak nuclear interactions. The weak nuclear force is one of the four fundamental interactions between matter particles in physics, the others being electromagnetic interaction, strong nuclear interaction and gravitational interactions. In quantum electrodynamics, the theory that describes electromagnetic interactions, the photon is the particle that mediates the interaction – for example, charged particles exchange a photon when they interact. In the case of weak interactions there are three such ‘gauge bosons’ – the W+ (W-plus), W- (W-minus) and Z particles. Unlike the photon, the W-plus and W-minus are charged and by exchanging such bosons, a neutron can change into a proton, for example. This helps in the transmutation of elements. The W boson helps the interactions that make the Sun burn and produce energy.
Inspired by the success of quantum electrodynamics, Sheldon Glashow, Abdus Salam and Steven Weinberg developed the similar but more general ‘electroweak’ theory in which they predicted these three particles and how they mediated the weak interactions. They were given the Nobel prize for their efforts in 1979. The W boson was first discovered at CERN, located in the Franco-Swiss border. Unlike the photon, which is massless, the W bosons are quite massive, which results in the force they meaidate – the weak force – being very short ranged.
The Standard Model of particle physics has been very successful in predicting the behaviour of elementary particles, for about 60 years now, since its inception. It predicted the existence of the Higgs boson which was discovered, also at CERN in 2012. However, there are glaring gaps – the SM does not describe gravity, and has no room to include dark matter particles. The latest discovery that the W boson mass is not agreeing with the value allowed by the standard model, would be another crack in the theory. This finding by Fermilab awaits being confirmed by other major experiments.
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