UA1

The detector at Underground Area 1 scoured a billion proton-antiproton collisions from the Super Proton Synchrotron for traces of W and Z particles

UA1 (Underground Area 1) was a particle detector at the Super Proton Synchrotron (SPS). It ran from 1981 until 1990, when the SPS was used as a proton-antiproton collider, searching for traces of W and Z particles in collisions.

Two moveable detectors – UA1 and UA2 – were custom built around the SPS beam pipe for use during proton-antiproton running. They could be rolled back after periods of collision data taking, so that the SPS could revert to fixed-target operation.

The central detector of UA1 was a six-chambered cylinder, 5.8 metres long and 2.3 metres in diameter. It was the largest imaging drift chamber of its day. Charged particles passing through the detector would ionise molecules in the argon-ethane gas mixture inside, releasing electrons. The electrons drifted along an electric field shaped by 170,00 field wires and were collected on 6125 sense wires. The geometric arrangement of these wires allowed UA1 physicists to reconstruct collision events in three dimensions.

UA1 recorded the tracks of charged particles curving in a 0.7 Tesla magnetic field, measuring their momentum, the sign of their electric charge and their rate of energy loss. An 800-tonne conventional electromagnet provided the field using thin aluminium coils enclosing a region of 85 cubic metres. Around the central detector and inside the magnet were 48 electromagnetic subdetectors. Made from layers of lead, their boat-like shape earned them the nickname "gondolas".

Outside the magnet were the calorimeters – detectors that measured the energy particles lost as they passed through. The electromagnetic calorimeter measured the energy of electrons and photons while the hadronic calorimeter sampled the energy of hadrons (particles containing quarks, such as protons and neutrons).

One type of particle, the muon, interacts very little with matter – it can travel through metres of dense material before it is stopped. For this reason, muon chambers – tracking devices specialized for detecting muons – usually make up the outermost layer of a detector. The UA1 detector was no exception: The slab-like arrays of muon chambers that covered the outside of UA1 required some 30 kilometres of extruded aluminium.

The discovery of the W boson in January 1983 was a highlight of the UA1 detector's life. UA1 later informed the design of the multipurpose, “hermetic” detectors developed for the Large Electron Positron Collider and the Large Hadron Collider.

You can see UA1's central detector at the Microcosm exhibit at CERN

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