Weighing the W particle

4 February 1998

Last week's Picture of the Week showed one of the earliest observations, from 15 years ago, of the decay of a W boson, carrier of the weak force. Since then, the decays of many thousands of W particles have been observed, in particular at the Fermi National Accelerator Laboratory in Illinois, where two giant detectors have analysed the decays of more than 160,000 Ws. The particles have been made in the collisions of protons and antiprotons at the machine known as the Tevatron. With this many examples to study, the two experimental teams have been able to make the world's best measurements of the mass of the W particle*. This image, from the D0 detector, shows an "end view" of energy deposited in the detector, after a proton and an antiproton (both unseen) have collided at the centre. In a fairly rare occurrence, the W particle has recoiled against an energetic "jet" of hadrons (particles made from quarks). The particle "jet" points down and to the left, and the W would have set off in the opposite direction. However, it almost immediately decayed into an electron, leaving the block of energy to the right, and a neutrino. The neutrino is not seen directly, but the imbalance in energy in the detector shows where the missing energy went, towards the top. The total energy of the electron and the neutrino give the mass of the W particle.

(* See the 12 December 1997 edition of FermiNews.)

Credit: D0 collaboration, Fermi National Accelerator Laboratory

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