Coherent versus incoherent quark fragmentation picture

We study the helicity density matrix of vector mesons produced ine ⁺ e ^? interactions at high energies through the two step process \(e^ + e^ - \to q\bar q \to VX\) . Whereas in the usual incoherent fragmentation picture in which each quark decays independently, ρ(V) is predicted to be diagonal, we find that final state interactions (coherent fragmentation) give rise to a non-vanishing ρ_1-1(V). Various corrections to the standard picture such as, e.g., gluon bremsstrahlung, quark masses, transverse momenta, are shown to be small. Therefore, any significant non-zero value of ρ_1-1(V) found experimentally can be considered as a clear measurement of coherence effects.     

Largep cT cross-section differences as tests of quark-gluon scattering

It is shown that certain cross-section differences, such as \(d\sigma (p\bar p \to CX) - d\sigma (p\bar p \to \bar CX)\) or \(d\sigma (pp \to CX) - d\sigma (pp \to \bar CX)\) provide a more direct and stringent test of the QCD partonic picture than do the individual cross-sections. In particular they are very sensitive to quark-gluon scattering and to the three-gluon vertex. They also provide a severe test of the evolution of the distribution functions into a new régime ofQ ². We carry out detailed numerical studies for π andK production, at ISR and collider energies. We find very good agreement with the recent ISR data onpp→π^± x. The cross-section differences for \(p\bar p\) collisions are predicted to be also large and measurable with significant accuracy.     

Spin effects in large angle πp→ρp interactions

Using a model previously introduced to describe spin-spin asymmetries in large angle elastic proton-proton scattering, we have computed the helicity density matrix of ρ vector mesons produced in πp→ρp interactions, at 90° in the c.m. system. Contrary to most other schemes, we obtain some nondiagonal elements sizeably different from zero; when comparing the ρ^? decay angular distribution with experimental data we have an excellent agreement. We also give predictions for the case of ρ⁺ production.     

Lambda polarization in unpolarized hadron reactions

The transverse polarization observed in the inclusive production of Λ hyperons in the high energy collisions of unpolarized hadrons is tackled by considering a new set of spin and k _⊥ dependent quark fragmentation functions. Simple phenomenological expressions for these new “polarizing fragmentation functions” are obtained by a fit of the data on Λ's and \(\bar \Lambda \)'s produced in p?N processes.     

Diquarks in exclusive reactions at large momentum transfer

Following many theoretical and experimental indications we assume that diquarks, in certain intermediate energy regions, can act as quasi-elementary constituents and take part in large momentum transfer reactions. We apply the idea to large angle exclusive interactions and show that diquarks could explain the whole amount of intriguing polarisation data in proton-proton elastic scattering and πp→?p reactions. The behaviour of the electromagnetic form factors of the nucleon is also discussed in the diquark model.