Elementary Particles

10. DETECTION OF W AND Z

In this section we will talk about how the W and Z particles were first detected at the CERN using the Super proton-antiproton Synchrotron (SPS) in the 1980s.SPS is a particle accelerator and the predecessor of the famous LHC. A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. Accelerator technology was invented in the 1930s to investigate the structure of the atomic nucleus using highly energetic particles. Since then, they have become indispensable tools for investigating many aspects of particle physics. Accelerators generate very high energy beams of particles using electric fields to accelerate particles, and magnetic fields to steer them into focus. The high energy particles are made to collide with some targets.

Accelerator technology has evolved over the years. With each new generation of accelerators, higher beam energies are available and in doing so we are really approaching the conditions just after the big bang so that we can see what it was like when the universe just started. In the process we hope to understand the basic laws of nature. We study particles coming out of these collisions, because just after the big bang, all was there were particles. These particles carry the information of how our universe started and how it got to be the way it is now and what will happen to it in the future.

In 1976, the SPS was switched on at CERN. It made use of 1317 conventional (room-temperature) electromagnets, including 744 dipoles to bend the beams round the ring. Originally designed to accelerate particles up to an energy of 300 GeV, the SPS was actually built to support a beam energy of 400 GeV, a feat that was achieved on 17 June 1976.

The same year, Carlo Rubbia, David Cline, and Peter McIntyre presented an idea to convert the existing accelerator into a storage ring for protons and antiprotons as this would yield center-of-mass energies in the 500-700 GeV range. The W and Z particles could then be produced in head-on collisions between the stored particles, provided an abundant supply of antiprotons (antimatter) was available. Colliding proton and antiproton beams had the advantage that it required only one ring where particles of opposite signs could circulate in opposite directions guided by the same magnetic fields.

Without an intense beam, the collision probability of the particles in the beam was too low for the physicists to see anything of interest. It was the ingenuity of Simon van der Meer who developed a method called stochastic cooling, to accumulate a large number of energetic antiprotons in the accelerator ring. It was really a combination of these two revolutionary ideas that allowed the first collisions of 270 GeV protons and antiprotons possible in 1981.

Two moveable detectors, UA1 and UA2, were custom built around the SPS beam pipe to search proton-antiproton collisions for signatures of the W and Z particles.

The central detector of UA1 (Underground Area 1) consisted of an imaging drift chamber, the largest of its day. Charged particles ionized molecules of the argon-ethane gas mixture inside the detector, releasing electrons. The electrons drifted along an electric field shaped and the topology allowed physicists to reconstruct collision events in three dimensions. A 0.7 Tesla magnetic field was applied for momentum and charge measurements of the particles. Calorimeters were positioned outside the magnet to measure the energy loss of the particles during their flight. The electromagnet calorimeter measured the energy of electrons and photons while hadronic calorimeter sampled the energy of hadrons (particles containing quarks, such as protons and neutrons). Muons have very little with interaction with matter and need specialized tracking devices called muon chambers. The outer layer of the UA1 detector consisted of slab-like arrays of muon chambers.

The UA2 experiment (Underground Area 2) was approved in December 1978. Unlike UA1, UA2 was not a multipurpose detector in its scope; its focus was the calorimeters. Its electromagnetic and hadronic calorimeters were designed to detect electrons and hadrons, but could not measure particle charges except for limited regions where the W decay asymmetry was maximal. There was no muon detector. However, UA2 provided the more accurate measurements of the W and Z masses and it was invaluable in detecting particle jets—sprays of hadrons and other particles that are formed when a quark, gluon or antiquark is ejected from the collision.

Figure 10-1
Figure 10-1: A PICTURE OF FIRST W BOSON PRODUCTION AS DETECTED BY THE UA1 DETECTOR.
Figure 10-2
Figure 10-2: FIRST DETECTION OF Z PARTICLE. EXTRACTED FROM CERN COURIER NOVEMBER 1982 PP360–361.

The detectors started collecting collision data in 1981. But how will the physicists recognize signatures of the decay of W and Z bosons in the debris of other particles coming out of the interaction vertex? The answer depends on the decay channels of W and Z, shown in Table 4. The W particle decays into an energetic lepton and a neutrino that carries energy but is invisible. The energetic electron (or muon) from the decay of a W will leave a straight track in the detector (see the arrowed track in Figure 10-1). On the other hand, the signature of a Z^0 decay was expected to be cleaner as there would be no thorny missing energy to deal with. The experiments would be in the lookout for clean lepton-antilepton pairs, carrying more energy than had ever been seen before. Figure 10-2 shows a typical Z^0 decay event.

Table 4: W AND Z BOSONS
W and Z bosons

Table 4 shows the mass of the W and Z bosons as measured by UA1 and UA2 in 1983. The number of events from both experiments are also listed. The independent mass measurements were in very good agreement, so the weighted average are shown in Table 4. The decision to construct two independent experiments proved to be a wise choice because they provided totally independent measurements that eliminated detector biases. The same philosophy was used during the Higgs boson detection thirty years later.

In 1984, Rubbia and van der Meer shared the Nobel Prize for Physics for “their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction”.

Today, the SPS operates at up to 450 GeV. It is the second-largest machine in CERN’s accelerator complex, measuring nearly 7 kilometers in circumference, it accelerates proton beams and delivers them to the LHC (see Figure 11-1).

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