The heavy photon

If the weak and electromagnetic interactions are different manifestations of the same basic law of nature, there must be a particle which acts as a carrier of the weak force, just as the photon carries the electromagnetic force. The electroweak theory made specific predictions about this particle. First of all there should be not one but three particles: one with positive charge, one with negative charge and one neutral. Secondly, unlike the 'ordinary' photon, they would have mass. Not only that, but they would be heavier than any other fundamental particle — almost 100 times heavier then the proton. In fact, the electroweak force carriers had accurately predicted masses and even names before they had ever been observed.

Two examples of interactions mediated by a heavy photon are given below:

Neutrino-neutron interaction

A neutrino interacts with a neutron by exchanging a virtual W- particle. The neutron changes into a proton while the neutrino changes into an electron.

To make a real W particle an amount of energy equal to its mass energy has to be supplied. When acting as the communicator of the weak nuclear force the W- is said to be in a virtual state, because it has to 'borrow' this energy courtesy of the

Heisenberg uncertainty principle DED t = h/2p . In the case of the

W particle, the amount of borrowed energy (AE) is very large, so the 'borrowed' time (At) is very short. As a consequence the range of the weak interaction is short even on a nuclear scale.

Neutron beta decay e A free neutron decays by emitting a W which in turn decays into an electron and an antineutrino.

The Nobel committee showed great confidence in the electroweak theory by awarding the 1979 Nobel Prize to Glashow, Salaam and Weinberg, without waiting for proof that the W and Z particles really existed. Perhaps they felt that it would not be possible to create such heavy particles in the foreseeable future since no accelerators at that time could produce enough energy. There was one possibility, but it appeared technically almost impossible to realise. If, instead of accelerated protons colliding with stationary protons in a fixed target, they could be made to move in opposite directions and crash into one another, the available energy would be much larger.

The difficulty of crashing nuclear size particles into one another head-on can be illustrated by the following analogy. Let us represent the target proton by a disc of diameter ~10-15 m and make the reasonable assumption that we can focus protons from an accelerator into an area of 1 mm2. Then the chance of hitting the target proton is only about 1 in 1011. This is roughly the same as the chance of hitting a grain of sand with a second grain of sand in an area equal to the size of France!

Despite such apparent difficulties the technology for building colliding electron and positron beams had already been developed in Frascati in Italy and at Stanford in the US. At CERN, intersecting storage rings had been built to give proton-proton collisions. However, the energy of the beams in all these machines was much too low to produce either a W or a Z particle.

In 1978, the scientific policy committee at CERN showed the same confidence as the Nobel committee by deciding to adapt the giant 7 km ring at the 400 GeV Super Proton Synchrotron (SPS) to accommodate antiprotons circulating in the opposite direction to the protons. The project was quite overwhelming in its complexity.

Anti-protons had first to be created at the target of the 28 GeV Proton Synchrotron (PS). This was the original accelerator around which the CERN laboratory had grown from the 1950s. Now CERN's original prima ballerina became simply a member of the supporting cast. The antiprotons were then sent to another ring, the Anti-proton Accumulator (AA), in which they circled repeatedly to build up energy.

Forty hours and 30,000 pulses later, when about 6 x 1011 protons had been accumulated, they were sent the 'wrong way' into the one-way traffic of protons in the SPS. The two beams were very delicately focused to ensure that the particles collided in designated cross-over areas where complex detector systems were set up to detect and measure the products of the very high energy proton/antiproton collisions. The analysis of the enormous amount of debris created by the collision of 3 quarks with 3 antiquarks was in itself a monumental task faced by two collaborations, composed of over 20 research teams from Europe and the US.

There is no room in this epilogue to do justice to the full story of the experiment. Suffice it to say that in 1982 the first W particle was observed, followed by the Z0 in 1983. Carlo Rubbia (1934-), the driving force behind the experiment, and Simon van der Meer (1925-), the architect of the beam focusing which made it possible, received the Nobel Prize in 1984. Glashow, Salaam and Weinberg, from 'the class of 1979', were special guests at the ceremony as the King of Sweden presented the prize 'for the discovery of the massive short-lived W particle and Z particle'.

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