Jan. 25, birthday of the W boson, charged carrier of the weak nuclear force

25 January 2023

Forty years ago today, physicists at CERN announced the discovery of the electrically charged particle that transmits the weak force, one of the four fundamental forces in nature.

On Jan. 25, 1983, the spokesman for the UA1 experiment at CERN presented the first six observed particles that resembled the predicted W boson. Rubbia played a key role in the discovery. In the 1970s, he proposed converting the existing SPS proton accelerator at CERN into a collision machine for protons and antiprotons.

With that, the predicted force particles were to be found, it was expected. The conversion was completed in 1981, for which Dutch engineer Simon van der Meer invented a crucial technique to get the beams stable enough.

Six candidates

Two years later, Rubbia’s UA1 and competing UA2 had six candidates for a charged boson, the W particle. Enough to claim a discovery. A few months later, the neutral Z boson was also discovered. Rubbia and Van der Meer were awarded the 1984 Nobel Prize in physics for the discovery.

Rubbia and Van der Meer in 1983, at the announcement of their Nobel Prize in physics. PHOTO CERN

Rubbia and Van der Meer in 1983, at the announcement of their Nobel Prize in Physics. PHOTO CERN

Finding the W and Z particles, with their specific masses, also constituted the preliminary confirmation of the Standard Model of particle physics, for which Martinus Veltman and Gerard ‘t Hooft, among others, laid the theoretical foundation in the 1970s. The Dutch received a Nobel Prize in 1999.

The weak nuclear force is the force in the atomic nucleus that occasionally turns neutrons into protons, the so-called radioactive beta decay. The force can be imagined as the jumping of a W or Z particle. The process changes the nucleus, releasing an electron that is measured as radiation.

Closely related

According to the Standard Model, the weak nuclear force and electromagnetism are closely related, only one has force carriers with mass and the other has only massless photons. For the electroweak force, the Higgs particle finally discovered in 2012 is crucial.

The strong nuclear force, which holds nuclear particles together, is also related in theory, but it is only active at much smaller distances.

W and Z particles, about 80 times heavier than a proton, are still important parts of the building block of matter 40 years later. In experiments at CERN, not only are they themselves still being studied, but they also provide important signals about other particles.

The CERN Courier published an article on the discovery of W and Z in 1983, the carriers of the weak nuclear force.