SUPERSYMMETRY

in particle physics, a symmetry between fermions (subatomic particles with half-integer values of intrinsic angular momentum, or spin) and bosons (particles with integer values of spin). An entity is said to exhibit symmetry when it appears unchanged after undergoing a transformation operation. A square, for example, has a fourfold symmetry by which it appears the same when rotated about its centre through 90, 180, 270, and 360 degrees; four 90-degree rotations bring the square back to its original position. With supersymmetry, fermions can be transformed into bosons without changing the structure of the underlying theory of the particles and their interactions. When a fermion is transformed into a boson and then back again into a fermion, however, it turns out that the particle has moved in space, an effect that is related to special relativity. Thus, supersymmetry relates transformations in an internal property of particles (spin) to transformations in space-time. One attraction of supersymmetry is that it provides a connection between the known elementary particles of matter (quarks and leptons, which are all fermions) and the particles that convey the fundamental forces (all bosons). By showing that one type of particle is in effect a different facet of the other type, supersymmetry reduces the number of basic types of particle from two to one. Supersymmetry also plays an important role in modern theories of particle physics because the new particles that it requires eliminate various infinite quantities that otherwise appear in calculations of particle interactions at high energies, particularly in attempts at unified theories of the fundamental forces. These new particles are the bosons (or fermions) into which the known fermions (or bosons) are transformed by supersymmetry. Thus, supersymmetry implies a doubling of the number of the known particles. For example, fermions such as electrons and quarks should have bosonic supersymmetric partners, which have been given the names of selectrons and squarks. Similarly, known bosons such as the photon and the gluon should have fermionic supersymmetric partners, called the photino and the gluino. There has been no experimental evidence that such superparticles exist. If they do indeed exist, their masses could be in the range of 50 to 1,000 times that of the proton. Christine Sutton