Complete experiments in pseudoscalar photoproduction

Necessary and sufficient conditions are derived for three double polarisation measurements to complement cross section and single polarisation measurements in pseudoscalar meson photoproduction to enable amplitudes to be determined up to discrete ambiguities. Rules for choosing two further measurements to resolve the discrete ambiguities are given and practical applications of the rules to particular reactions are discussed.     

Ambiguities in Regge analyses of backward scattering

Ambiguities in Regge analyses of backward scattering due to the possibility of $\text{u}$ dependence in trajectory and residue functions of fermion Regge poles are investigated. Experiments to clarify the ambiguities are suggested. Also an intrinsic difficulty in separating exchanges of opposite parity is discussed.     

A phenomenological analysis of the photon structure functionF 2 γ (x,Q 2 )

A new method of analysis of the photon structure functionF ₂ (x, Q ² ) is presented. It is a generalisation of Rossi's ansatz for generating regularised solutions of the inhomogeneous Altarelli-Parisi equations which enables initial conditions in theQ ² evolution to be incorporated. Its equivalence to the normal solution is demonstrated. It is applied in a phenomenological analysis of the data on the structure functionF ₂ . It is found that the recent TPCF ₂ data in the range 0.3Q ²<1.6>² cannot be used as a starting point for a perturbative QCD analysis, despite being well described by the vector meson dominance model (VMD) and exhibiting scaling. This conclusion invalidates previous discussions of the effect of the regularisation of the asymptotic point-like QCD prediction on the sensitivity ofF ₂ to the QCD mass scale , because they incorporated VMD assumptions in the perturbative region. A perturbative QCD analysis using higherQ ² data as a starting point is presented but here the initial conditions necessarily involve more assumptions, and starting the evolution at a higherQ ² involves an inevitable decrease in the sensitivity to .     

LargeP T photoproduction in the hard scattering expansion

Largep _T photoproduction is studied in the hard scattering expansion, including both three- and four-jet processes. The higher twist corrections (∝p _T ^?6 andp _T ^?8 ) to the basic QCD hard scattering process (∝p _T ^?4 ) are estimated using the constituent interchange model and the various terms compared. Both single jet inclusive cross sections and single particle inclusive π⁰ cross sections are evaluated, the latter using scale violating fragmentation functions. We find that the higher twist terms are significant at present energies, particularly for three-jet events, though the four-jet anomalous photon contribution can be measured. Three-jet QCD processes require considerably higher energies to provide a significant contribution to the cross section. Methods of separating the different processes experimentally are discussed.     

Baryon trajectories revisited

Various properties of baryon trajectories are re-examined in the light of the latest data: in particular we study trajectory assignments of the 1982 baryon resonance data and evaluate effective trajectories for backward πN scattering incorporating FNAL data. We find that all of the information is consistent with trajectories linear inu, withF/D ratios constant along the trajectories and parity doublets absent. We find that exchange degeneracy (EXD) is good for Λ and Σ trajectories and residues and shed some qualitative light on the systematics of EXD breaking.     

On the elimination of parity doublets

In the theory of Reggeized baryon exchange with linear trajectories residue functions which eliminate all parity doublets are constructed. The simplest such residue function is shoen to be unsatisfactory and modifications are given and discussed.     

Analysis of high-energy backward πN scattering

We analyse the current data on backward πN scattering cross sections and elastic polarizations. We show that the conventional Regge-pole model with N_α, Δ_δ and N_γ trajectories cannot fit the polarization data, and argue that none of the conventional cut models can do so either. However a pole model with a double zero in N_α exchange at $\text{α}{}_{\mathrm{<mtext>N</mtext>}}{}_{\mathrm{\alpha }}\text{= ?}\text{1}\text{2}$ cannot quite be ruled out at present. We also consider a picture in which $\text{I =}\text{1}\text{2}$ exchange is then determined to be very far from a simple Δ_δ pole. There is only slight evidence for a “geometrical zero” in the spin-no-flip $\text{I =}\text{3}\text{2}$ amplitude and no evidence exists for or against such a zero in the spin-flip $\text{I =}\text{3}\text{2}$ amplitude. We also discuss systematic errors in the data and suggest future experiments.     

Exchange ofA 1 andZ 1 (J PC=2−−) mesons in charged pion photoproduction

Finite energy sum rules are evaluated for the amplitudes corresponding to the exchange of theA ₁ and its exchange degenerate partner, theZ ₁ (J ^PC=2^??), in charged pion photoproduction. Both sum rules are roughly compatible with theoretical estimates of these exchanges, and a definiteA ₁ signal found.     

Baryon exchange processes

The theory and phenomenology of baryon exchange processes are critically reviewed. As regards theory the main topics are the parity doublet problem and the patterns of exchange degenerate baryon trajectories implied by duality. In the phenomenological part, all baryon exchange reactions are considered, with the emphasis being on seeing how far simple ideas of pole exchange with the constraints implied by duality and broken SU(3) can be taken in forming an understanding of the data.