An Introduction to the Standard Model of Particle Physics

1.9 Particle accelerators

Progress in our understanding of Nature has come through the interplay between theory and experiment. In particle physics, experiment now depends primarily on the great particle accelerators, and ingenious and complex particle detectors, which have been built, beginning in the early 1930s with the Cockroft-Walton linear accelerator at Cambridge, UK, and Lawrence's cyclotron at Berkeley, USA. The Cambridge machine accelerated protons to 0.7 MeV; the first Berkeley cyclotron accelerated protons to 1.2 MeV. For a time after 1945 important results were obtained using the cosmic radiation as a source of high energy particles, events being detected in photographic emulsion, but in the 1950s new accelerators provided beams of particles of increasingly high energies. Some of the machines now in operation, or planned, are listed in Table 1.5. Detailed parameters of these machines, and of others, may be found in Particle Data Group (1996).

**Table 1.5.** Some particle accelerators

Machine	Particles collided	Start date

TEVATRON (Fermilab, Batavia)	p: 900 GeV : 900 GeV	1987
SLC (SLAC, Stanford)	e⁺: 50 GeV e^-: 50 GeV	1989
HERA (DESY, Hamburg)	e: 30 GeV p: 820 GeV	1992
LEP2 (CERN, Geneva)	e⁺: 81 GeV e^-: 81 GeV	1996
PEP-II (SLAC, Stanford)	e^-: 9 GeV e⁺: 3.1 GeV	1999
LHC (CERN, Geneva)	p: 7 TeV p: 7 TeV	2004

The TEVATRON at Fermilab is where the top quark was recently (1995) observed. The physics of the top quark is as yet little explored. It makes only a brief appearance in our text, though it is an essential part of the pattern of the Standard Model. The upgraded LEP2 at CERN is able to create W⁺W^- pairs, and will allow detailed studies of the weak interaction. At Stanford, PEP-II and the associated ``BaBar'' (B) detector is designed to study charge conjugation, parity (CP) violation. The way in which CP violation appears in the Standard Model is discussed in Chapter l8.

The most ambitious machine likely to be built in the immediate future is the Large Hadron Collider (LHC) at CERN. It is expected that with this machine it will be possible to observe the Higgs boson, if such a particle exists. The Higgs boson is an essential component of the Standard Model; we introduce it in Chapter 10. It is also widely believed that the physics of Supersymmetry, which perhaps underlies the Standard Model, will become apparent at the energies, up to 14 TeV, which will be available at the LHC.