Asymmetries in Deep Inelastic Scattering

We review the existing data on asymmetries in deep inelastic muon scattering involving changes in the lepton and quark helicities. The status of the present E.M.C. experiment on spin structure functions is discussed together with future developments. The process of Deep Inelastic Scattering using electron, muon and neutrino probes has been very successful in developing our present understanding of the role of quarks and gluons in strong interactions. This knowledge has been obtained mainly from measurements of the scattering cross-section which determine the structure functions over a Wide kinematic range. Precise measurements of the structure functions using muon beams provided evidence that A related to ô , the strong coupling constant was small (1) and that quarks behaved differently in bound and free nucleons, the E.M.C. effect (2). The extension of these experiments to those where the lepton and/or quark spin is defined in the initial state provide additional new information as highlighted by experiments on weak electromagnetic interference. In this lecture, we shall review the current status of these experiments and indicate what new information can be expected in the future from muon experiments. The kinematics of the deep inelastic scattering process are illustrated in fig. 1. For an incident muon of energy E and scattered energy E' and scattered angle 0, we can define the following variables. Q 2 = 4EE' sin6/ = EE'e (small 6) V = E E' and the scaling variables x = Q/2mv and y = v/E. For a fixed incident energy, low Q 2 corresponds to small scattering angles and as Q 2 increases we move into a region of larger x and larger y in the kinematic plot. The muon beam is naturally polarised since the muons are produced by the weak decay of pions in flight. It is easy to produce a highly polarised beam of 1^ from TT ,s by selecting forward ir + p decays in the pion centreof mass, i.e., muons of energy close to that of the parent pions. The effect of tuning the muon beam momentum to maximise the beam polarisation is illustrated in fig. 2(a) and the corresponding reduction of the muon flux is shown in fig. 2(b). A beam of u^ of approximately half the pion energy can be made from ir ,s by selecting backward TT •»• ]i decays in the pion centre-of-mass. Unfortunately for the same incident pion flux, the vu beam has only 10% of the yL intensity for the same momentum bite, and therefore many more protons are required for a u£ beam. The simplest method of producing a Un beam is to use forward y~ decays from an incident ir~ beam. However we have to consider any additional asymmetry introduced by the change in sign of the charge as well as the helicity. Although it is possible to calculate the polarisation of the muon beam, it Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985214