SPIN EFFECTS FROM QUARK AND LEPTON SUBSTRUCTURE AT FUTURE MACHINES

Abstract

If quarks and leptons are composite on a distance scale A the physics at energies larger than A will provide plenty of evidence for the new level of substructure. However, already at energies below A compositeness should become manifest in deviations from the standard model due to form factors, residual interactions and, possibly, new light states. I discuss the virtue of polarized lepton and hadron beams in searching for new interactions and exemplify the production of excited fermions and bosons focussing on spin properties. Present-day experiments provide strong evidence that leptons and quarks are structureless at distance scales as small as 10~'°cm, and that their interactions are correctly described by the standard SU(3) x SU(2)L X U(l) gauge model up to energies in the few hundred GeV range. Given this fact, it may appear somewhat premature to speculate about effects from lepton and quark compositeness. On the other hand, it is quite conceivable /I/ that the key to some of the miracles of the standard theory, such as the complicated fermion mass spectrum, the existence of families, and the scale of the electroweak symmetry breakdown, is hidden at a deeper level of substructure: leptons and quarks (and some of the standard gauge bosons) may not be elementary but composite of more fundamental constituents (preons) bound together by a new strong force (hypercolour). The strength of this new (probably confining) force can be characterized by the A which is necessary to break up leptons and quarks or, equivalently, by their inverse radius r' ~ (J (A). The physics at scales larger than A would then be described by a new fundamental theory, while the presently known physics should emerge from the energy limit of this ultimate theory. Clearly, the impressive success of the standard model implies that the latter must be a good approximation to the true effective low theory. It is not an easy task to construct interesting models which have this property. Quite obviously, a minimal requirement is A > 0(G]r'2~ 300 GeV) that is r~'~0lA) >>m, where Gp is the Fermi constant and m is a lepton or quark mass. Present address : Sektion Physik, Universitat Miinchen, F.R.G. and Max-Planck-Institut fur Physik, Miinchen, F.R.G. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985206 C2-56 JOURNAL DE PHYSIQUE The only known way to obtain almost massless and, at the same time, almost pointlike fermionic bound states is by requiring an unbroken symmetry /2/ (chiral, SUSY). This point is common to all preon models. As far as features are concerned such as the value of A , the spin nature and multiplet structure of the preons, the fundamental gauge fields (hypergluon, gluon, photon, weak bosons) etc, there is room for quite different scenarios / 3 / . Despite numerous interesting ideas, a fully satisfactory and compelling theoretical framework has still to be developed. In particular, the mysteries of the standard theory mentioned above have, at least so far, not found a natural explanation. Nevertheless, compositeness may be the correct route to go beyond the standard theory. In this state of ignorance, clues from experiment are particularly important. Recent phenomenological.studies /4-111 have demonstrated the great virtues of the future e+e-, 'pb and ep colliders in the search for signals of compositeness. Figure 1 illustrates pictorially ep-scattering at various momentum transfers Q