C.H. Llewellyn Smith

High energy behaviour and gauge symmetry

The imposition of unitarity bounds is shown to lead to a Yang-Mills structure in a wide class of theories involving vector mesons. Scalar fields are needed and, at least in simple cases, the unique unitary theory is of the Higgs type.

A possible explanation of the difference between the structure functions of iron and deuterium

Abstract It is shown that the difference between F Fe 2 and F D 2 can be accounted for by a simplistic model in which iron contains 6–12 more pions than deuterium and these pions carry ∼ 5% of the momentum. This requires the one-pion exchange contribution to be enhanced in heavy nuclei. A nuclear enhancement with the required characteristics has long been predicted.

QCD predictions for processes involving real photons

Abstract We show that the point-like nature of the photon leads to precise predictions for deep inelastic scattering on a photon (as discussed by Witten), γ N → μ + μ − X, γγ → large- p T jets, γ N → large- p T jets, the production of energetic photons in e + e − and hadronic collisions, and for dσ ( e − p → e − γ X) − dσ ( e + p → e + γ X).

A relativistic formulation of the quark model for mesons

Mesons are treated as bound states in quark-antiquark scattering using the Bethe-Salpeter equation (this does not necessarily imply that the properties of mesons are dominated by the Q-Q state). It is shown that the existence of very tightly bound states with a nonrelativistic spinor structure (such as are considered in the usual norelativistic quark model) requires a Dirac scalar potential (or sum of irreducible graphs) as well as small internal three momentum (q2 ⪡ MQ2). The assumption that the pseudoscalar mesons have a particular simple relativistic spinor structure leads naturally to the correct value of Γ(π→μν)Γ(K→μν) (in contrast to the usual model) and gives the prediction γρ2:γφ2:γω2=Mρ2:3Mφ2cos2θ:3Mω2sin2θ (where γV are the usual V − γ coupling constants and θ is the ω − φ mixing angle) which is compatible with the data. The assumption that the internal momentum is small and the potential is spin-independent can lead to approximate SU(6) symmetry, as in the usual model, although the spinor structure is relativistic. Using the value of the weak axial vector coupling constant for quarks given by the nonrelativistic quark model for baryons, these assumptions lead to the Kawarabayashi-Suzuki relation 2γϱfπ = Mϱ which agrees with experiment. The states which have L = 1 in the usual model are discussed. The model leads naturally to quadratic mass formulae for mesons. The processes π0→γγ and η0→γγ are discussed using the relativistic formalism.

The applicability of perturbative QCD to exclusive processes

Abstract We re-examine our arguments against the dominance of perturbative QCD over soft, non-perturbative effects in exclusive processes at currently available Q 2 . We continue to find that perturbative contributions are much smaller than soft contributions (which are capable of explaining the data) in the simple test cases of the pion and nucleon electromagnetic form factors. We see no reason why our results should not generalize, and conclude that there is no justification for the continued application of perturbative QCD to exclusive processes.

Leptoproduction of Supersymmetric Particles

Abstract We study the production of scalar leptons ( l ) , scalar quarks ( q ) and gluinos ( g ) in ep → ν X , and ep → e X and the associated production of q and g in ep → eX and ep → νX. We find that v q and e q production is probably measurable at HERA for masses of order 40 GeV with exchanged ticle masses of order 80 GeV. We derive and solve the asymptotic supersymmetric evolution equations for quark, gluon, scalar quark and gluino distributions. We find sum rules for the asymptotic momentum fractions and for the partition of the net quark number between quarks and scalar quarks. We use a fusion model to study associated leptoproduction in the threshold region. The only appreciable contribution is from small Q2 electroproduction but the rates are low even for scalar quarks and gluinos with masses close to the present limits.

Current-Algebra sum rules suggested by the parton model

Abstract Those results which depend only on the weak and electromagnetic currents being carried by quarks are extracted from the parton model. We show that the most directly tested consequences of these results can be derived without the parton model if certain quark Hamiltonians are assumed and certain formal manipulations of operators are valid. Bounds and estimates are given for high-energy neutrino and anti-neutrino cross sections.

Possible pattern of scaling violations in the production of W'S, Z'S and μ-pairs

Scaling violations which may be implied by asymptotic freedom for pp and p p production of Ws, Zs and μ-pairs are investigated. An explanation of the observed growth of /tpinT2〉μμ with Mμμ2 is presented and scaling violations are found to have a substantial effect for large masses.

On the determination of sin2θw in semileptonic neutrino interactions

Abstract It is shown that if sin 2 θ w is measured in semileptonic neutrino interactions then, contrary to a claim in the literature, the error due to unknown dynamical (higher-twist) corrections to the QCD parton model is small provided an isoscalar target is used. The largest contributions to σ nc and σ cc are related by isospin invariance alone. Neglecting heavy quarks and Kobayashi-Maskawa (KM) mixing, the parton model is only needed for very small terms and introduces an uncertainty in sin 2 θ w which is probably less than 1%. There is a much larger theoretical error due to uncertainties in the element U cs of the KM matrix and in the strange quark distribution. With the full range 0.80 ⩽ | U cs | 0.98 which is allowed phenomenologically, these uncetainties give δ sin 2 θ w = ± 0.008. There is also an error of ±0.004 due to uncertainties in | U dc | and | U du |.

Detailed treatment of scaling violations in asymptotically free gauge theories

Abstract Scaling violations in lepto-production are discussed on the basis of asymptotically free gauge theories. Detailed attention is given to the problems of operator mixing and data parametrisation. All the electro-/muo-production data for F2 can be accommodated. The calculated values for FL are also compatible with the data in the region where the theory may be trusted. It is shown that the FNAL data for σ ν /σ ν and 〈y〉 ν can be explained if the freedom to input rather large amounts of antiquarks is exploited. It is therefore premature to conclude that new flavours are required. Predictions are given for very high energies which are relevant for possible new experimental facilities. The consequences of a conjecture about the possible pattern of scaling violations in the production of Ws, Zs and μ-pairs are explored. Some theoretical problems and uncertainties in testing asymptotic freedom are discussed.