Particularly at large momentum fractions (x), there appears to be a systematic difference between the nucleon's quark structure (structure functions) for nucleons bound in deuterium and those in heavier nuclei. The effect first noticed in by the European Muon Collaboration (EMC) at CERN, is now underlined by SLAC data on steel and aluminium.
crystal near one of the principal axes fly 3.4 m before being picked up in a detector consisting of multiwire pro portional chambers and scintillators. Comparing the recorded angular dis tribution with an X-ray diffraction pattern shows that positrons are blocked by certain crystal planes, with a central minimum caused by axial blocking. This provides valuable information to help locate the interstitial site where the muon comes to rest. Channelling effects have also been seen in the particles from pion decay in an experiment by a Stuttgart group at SIN.
These nuclear shadowing effects, which are presumably nuclear higher SIN twist effects in the language of Quan Channelled muon tum Chromodynamics (QCD), are ex pected, like higher twist effects in the decay nucleon, to be small at large values of Q2. Therefore, the extraction of the One of the results reported at the strong QCD coupling parameter from recent Yamada meeting on muon structure function data taken with spin rotation (see page 265) was the nuclear targets at high values Q2 may observation at SIN of the 'channel not be affected by these terms. ling' of positrons coming from the Comparison of the SLAC electron decay of muons implanted in a silicon and CERN muon data indicate that crystal. the nuclear distortions in steel and Channelling is the delicate steering aluminium are similar. of particle trajectories by the axes The understanding of the mechan and planes inside crystals, and could isms responsible for the distortion of offer new possibilities for studying the structure functions is still in a stopped muons. very qualitative state. Several recent In the experiment by a Zurich / theoretical papers have attempted to Konstanz group at SIN, positive explain these observations using muons are stopped in a silicon wafer ideas such as six quark bags, pions bent into a spherical shell. (This ar and quasipions in nuclei, delta reso rangement allows a crystal axis at nances in nuclei, diquark states, and any point in the wafer to be brought percolation of quarks from nucleon to a unique focus at the centre of the to nucleon in a large nucleus. New sphere, thus allowing a large surface studies will surely shed further light area to be studied.) on this interesting new effect. The decay positrons leaving the
262 CERN Courier, September 1983