The EMC Effect P.B

europhysics BULLETIN OF THE EUROPEAN PHYSICAL SOCIETY news J.A. Volume 19 Number 11/12 November/December 1988 The EMC Effect P.B. Renton, Oxford (Nuclear Physics Laboratory, University of Oxford) The scattering of high energy muons by nuclei shows an unex pected dependence upon nuclear size, suggesting that the con stituent quarks and gluons may be shared between nucleons. In our attempts to understand the ele ference between the electron and the to describe the many body gas-like mentary structure of matter, the idea of proton or neutron, allows the large dif system of nucleons which make up the unravelling successive layers of sub ference in the scale of the atom ( 10-10 nucleus. structure has been of central impor m) and that of the nucleus (< 10-14 m) to The size of the nucleus (or, more tance. Each new layer corresponds to an be understood. strictly, of its charge distribution) and increasingly smaller distance scale, and Theoretically, electromagnetic inter that of the proton have been determined it is assumed that once the physical pro actions can be calculated, to remarkable by measuring the elastic scattering of perties of the layer in question are under precision, using the relativistic field energetic electrons by them. The me stood then those of the previous layer theory of quantum electrodynamics chanism for this process is shown in Fig. can be accounted for. This article is con (QED). Because a is small, a perturbative 1. The incident electron interacts with cerned with the subtle interplay bet expansion in powers of a can be made the target by the exchange of a photon. ween two layers in this hierarchy: the and only the first few terms need to be The photon in such an interaction is vir nucleon structure of the nucleus and the calculated. Knowledge of the detailed tual (i.e. has non-zero mass). If E and P quark structure of the nucleon. structure of the nucleus is unimportant are the energy and momentum of the The main experimental method which in accounting for most atomic proper photon respectively, then its relativisti- has been used in probing the sub-struc ties. Turning this around, it means that cally invariant mass squared is negative ture of matter is the measurement of the little can be learnt about nuclear sub and is given by E2 - P2 = — Q 2, so that scattering angle distribution of a beam structure from the study of atoms. Q2 is a positive quantity. The distance of particles incident on the target being scale down to which the structure of the investigated. If the incident beam is Quark Structure of Matter target is revealed depends on the value composed of electrons, then (at current Understanding the nucleus in terms of of Q2. From Heisenberg's Uncertainty ly accessible laboratory energies) the its components as against the atom is Principle, the distance scale resolved is scattering is predominantly by the elec much more complicated. The consti roughly given by tromagnetic interaction. This interaction tuent protons and neutrons (generically Δz h /Q2 = 0.2 GeV.fm/ Q2· (1) is well understood and hence the resul referred to as nucleons) have similar ting scattering distributions can be con masses and the inter-nucleon separa Contents verted into a measurement of the charge tion ( 2fm 2x-15 m) is not much density distribution of the target. Histo larger than the radius of the nucleon ( The EMC Effect 129 rically, the idea that an atom of atomic 0.8 fm). The nucleus is confined by the Quark Gluon Plasma number Z is composed of a central strong force, which can be envisaged (at Research with Ultra- nucleus of charge +Ze (where e is the least for some purposes) as being due to relativistic Heavy Ions 133 charge of the proton), surrounded by Z the exchange of pions (and other strong Particle Accelerators in orbiting electrons each of charge -e, ly interacting particles) between the the Future 137 stems from the classical experiments of nucleons. However, in the region of mo Accretion Disks and Rutherford. In the scattering of α-parti- mentum transfers involved in nuclear Magnetic Fields 142 cles (produced by radioactive decays) binding forces, the strong interaction is on thin film targets it was observed that not well understood. The effective Seminar on International many more α-particles were scattered strong interaction coupling constant is Research Facilities 143 through large angles than were expec large in this regime, so that perturbative Nobel Prize 1988 143 ted. The relative weakness of the elec methods cannot be used. Thus we are New Members of EPS 143 tromagnetic coupling constant (α left with empirical models, each with a Index for 1988 144 1/137), together with the large mass dif somewhat limited range of applicability, Published by the European Physical Society: 10 issues per year. © 1988. Reproduction rights reserved. ISSN 0531-7479 129 mass energy of the final state hadrons), can easily be shown that then the interaction probability for the x = Q2/(W2 - M2 + Q2), (3) incoming electron can be expressed (to where MN is the nucleon mass. The a good approximation) as variables x and Q2 can be measured for d2 /dQ2dW = α2(K/Q4) F2 (Q2,W) (2) each event from a knowledge of the where K is a known kinematic function. momentum of the incident and scat A free photon is massless (i.e. has E = P tered electrons alone. The quark density and so Q = 0), and the 1/Q4 term is a distribution q(x) and its momentum dis factor which expresses the suppression tribution xq(x) can thus be built up from which arises when the photon fluctua measurements of many such collisions. tes to a mass value of Q. A further surprise was in store when The construction of the two-mile long the results were analysed. The total Fig. 1 — Principal mechanism in the scat Stanford Linear Accelerator in the USA momentum fraction carried by the pro tering of energetic electrons from nucleons. in the later 1960s extended the upper ton's constituents (the quarks) was range of Q2 which could be explored to found to be only about 0.5. That is, 50% (In eqn. (1) Q2 is measured in GeV2 and about 10 GeV2. From eqn. (1) it can be of the momentum was carried by some units in which h = c = 1 are used.) Thus, seen that this probes the proton down to electrically neutral constituents. We as Q 2 is increased, the structure of mat distances of about 0.1 fm, much less now know that these are the gluons; the ter to smaller and smaller distances is than its radius. A study of the distribu particles which are responsible for the revealed. The theory of QED specifies tions of scattered electrons in these strong force confining the quarks and precisely the scattering distribution ex deep inelastic collisions was carried out. antiquarks inside a hadron. The gluon is pected from a point-like target i.e. one For elastic electron-nucleon scatter analogous to the photon in QED, but with no internal structure. Deviations ing the structure function F2 was found whereas the photon carries no electric from these observations can be con to fall off rapidly with Q2. A similar be charge, the gluon carries the strong verted into a charge density distribution haviour was also found for final states 'charge' (a quantum number arbitrarily for the target. The dependence of the consisting of excited nucleon resonan called colour). This leads to an important nuclear radius on the atomic weight A of ces. The experimental result that, for a difference between the theory of colour the target is found to behave roughly as fixed value of W above the 'resonance charges (Quantum Chromodynamics or R = ROA 1/3, where Ro 1.25 fm. Thus, region' (i.e. W 2 GeV), the structure QCD) and QED. The coupling constant in for a copper nucleus (A = 64), R 5 fm. function F2 was essentially independent QCD decreases (increases) as the sepa The energy of the incident electron of Q2, was thus a great surprise. The ration of the quarks decreases (increa beam used to probe the size of nuclei is discovery of this 'scaling' of the struc ses). Alternatively, in terms of the mo typically 200 MeV, giving Q2 values of ture function is analogous to that of mentum transfer squared (Q2), the about 10-2 GeV2. The value of Q2 need Rutherford more than 50 years earlier; strong coupling constant αs decreases ed to probe the charge radius of a proton only this time it showed that the proton (increases) as Q 2 increases (decreases). target is typically 0.1 GeV2. This mea (and neutron) had point-like consti At large Q2, or small distances, αs surement is performed using the elastic tuents from which the scattering occur becomes small and the quarks become scattering reaction e- +p → e- +p, in red. We now know that these scattering (asymptotically) free. At small Q2, or which the same particles appear in the centres are the quarks which are the large distances, αs becomes large ( 1), initial and final states. building blocks of hadronic matter. and this is believed (although not yet Final states are also formed in which proved) to lead to quark confinement. A schematic diagram of the scattering The interaction mechanisms sketched in an excited state of the target nucleon is process at the nucleon vertex is shown created (e.g. the nucleon resonance A, Fig. 2 can be justified in QCD, at least in Fig.

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