Fifteen years of W particles

28 January 1998

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On 21 and 22 January 1983, packed houses in the Main Auditorium at CERN, the European Laboratory for Particle Physics, heard physicists describe the first observations of W bosons - the long-sought carriers of the weak force. Two detectors, known as UA1 and UA2, had recorded thousands of collisions at CERN between high energy protons and antiprotons. On a few occasions, the proton and antiproton had interacted in just the right way to form a W particle - some 90 times heavier than a proton - and almost immediately the W had decayed to leave its signature in the detector. This image shows one historic example, caught by UA1 and reproduced in the paper received by Physics Letters* the day after the presentations. Here the central part of the detector makes visible the tracks of as many as 65 charged particles thrown out from the proton-antiproton collision. One track (arrowed, towards bottom right) holds the key to the W's decay. It's the track of a high-energy electron, balanced by an invisible neutrino; together they carry the mass that once was the W, but converted now to energy, according to E=mc2.

(*see G.Arnison et al, Physics Letters 122B 103 (1983) )

Credit: CERN Photo

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