W Boson: Is About to Break Physics?

The mass of the W boson, a subatomic particle, appears to be wrong. What could this mean for the Standard Model of particle physics?

- By Prof Jon Butterworth. Photographs by Getty Images and Science Photo Library

A new measurement from an old experiment may have just given us a huge clue to some big unanswered questions in physics.

The Collider Detector at Fermilab (CDF), a particle accelerator experiment which operated until 2011, recently caused a stir by re-measuring the mass of a particle known as the 'W boson'.

Each of the four fundamental forces (the strong force, the weak force, electromagnetism and gravity) has associated particles which 'carry' the force: for example, the photon - a particle of light - is a carrier of the electromagnetic force. The W boson is one of the carriers of the weak force.

It is unusual for an experiment which stopped taking data more than a decade ago to rouse such interest. The reasons are subtle, but compelling. To see why, let's step back and see where our knowledge of the fundamental forces and constituents of matter - expressed in the so-called 'Standard Model' stands at present.