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# ANNIHILATION AND CREATION

In Newtonian mechanics mass is conserved—it can neither be created nor destroyed. Energy is also conserved. In Einstein's relativistic mechanics, however, these two conservation laws are replaced by one law only: mass-energy is conserved. It is possible, in Einstein's theory, for mass to be changed into energy and vice versa; the formula giving the equivalence between them is of course E= mc2 (c is the speed of light). Thus relativity sets the scene for creation of particles of matter from energy alone, and their annihilation into energy alone.

Modern particle physics relies heavily on this phenomenon; in fact, virtually every reaction in particle physics involves the conversion of mass into energy or energy into mass. For example, an electron e- and its antiparticle the positron e+ can annihilate each other into photons—quanta of electromagnetic radiation

The restmass me of an electron has an energy equivalent mec2 = 0.5 MeV. So, if in the above reaction e- and e+ have very little kinetic energy (that is, if they are moving very slowly), then the total energy available is about 1 MeV, which the photons divide between themselves, moving off in opposite directions in the center-of-mass frame.

An important series of experiments involving e+e- annihilation was undertaken at the Stanford Linear Accelerator Center (SLAC) from the late 1960s, in which electrons and positrons were accelerated to very high energies—5 GeV (= 5,000 MeV) and beyond. At such energies new, heavy particles may be produced, a typical example being the J/ψ particle:

J/ψ has a mass of 3.1 GeV/c2, and its significance lies in the fact that it is a bound state of the charm quark c and its antiparticle . (In the above reaction, Χ simply stands for any other particle or particles which may be produced; for the purposes of the experiment they are not of interest.) The annihilation process (1) is an example of mass being converted into energy, whereas (2) exemplifies energy being converted into mass—the creation of new particles from energy alone (in this case, the kinetic energy of e- and e+ ). Relativity allows reactions like these to happen, whereas Newtonian mechanics does not.

A final example of historical importance is the discovery of the W boson, the field quantum of the weak field. It was first found in the reactionin which proton and antiproton, at very high energy, annihilate and produce a W (and other stuff X ). The proton restmass is mpc2 = 0.98 GeV, and the W mass mWc2 = 80.6 GeV—almost all the mass of the W comes from the p and kinetic energy. The W particle, predicted by the electroweak theory of Sheldon Lee Glashow, Steven Weinberg, and Abdus Salam, was discovered in the above reaction at the European Laboratory for Particle Physics (CERN) in 1983.

## Bibliography

Alldar, J. Quarks, Leptons, and the Big Bang (IoP Publishing, Philadelphia, 1998).

Brehm, J. J., and Mullin, W. J. Introduction to the Structure of Matter (Wiley, New York, 1989).

Okun, L. B. α, β, γ… Z; A Primer in Particle Physics (Harwood Academic Publishers, Chur, Switzerland, 1987).

Taylor, J. C. Hidden Unity in Nature's Laws (Cambridge University Press, Cambridge, UK, 2001).

Tipler, P. A. Physics for Scientists and Engineers, 4th ed. (W. H. Freeman, New York, 1999).

Lewis Ryder