Helicity Density Matrix Research Articles

Top quark production at LEP energies

Top quark pair production is analyzed for e +e − annihilation at LEP energies. γ−Z interference and the heavy mass of the quark cause the non-diagonal helicity density matrix elements of the produced quark to be sizeable. The detection of this feature through the angular distribution of the decay products is discussed. Coherence effects in the fragmentation process are also considered.

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.

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 ρ1-1(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 ρ1-1(V) found experimentally can be considered as a clear measurement of coherence effects.

Two-gluon exchange model for ψ-photoproduction

The spin dependence of ψ-photoproduction is analyzed in the context of a quark-gluon model. The ψ is treated as a nonrelativistic bound state of a c c -pair, with exchange of two colored vector gluons to the spinless target. Effects due to the binding of the c c -pair are ignored, resulting in a simple, parameter-free model for ψ-photoproduction. A detailed calculation of the helicity amplitudes and density matrix elements to leading order in s is given. The scalar gluon exchange case is also analyzed, and the decay angular distribution for ψ → l + l − is discussed in terms of the ψ-density matrix.

Lower bounds on small amplitudes in photoproduction of meson resonances

Lower bounds are set for measurement of corrections to various sets of dynamical assumptions. These bounds are written in terms of the helicity density matrix of the produced resonance, providing a means of model-independent extraction of information from reactions with an unpolarized photon beam.

Discrepancy Between the Vector-Dominance Model and Pion Production by Polarized Photons

The helicity density matrix for the process ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}n$ has been studied as a function of ${M}_{\ensuremath{\pi}\ensuremath{\pi}}$. The ratio $\frac{{{\ensuremath{\rho}}_{1\ensuremath{-}1}}^{\mathrm{hel}}}{{{\ensuremath{\rho}}_{11}}^{\mathrm{hel}}}$ shows no variation with ${M}_{\ensuremath{\pi}\ensuremath{\pi}}$ and it thus seems unlikely that the large discrepancy between the vector-dominance prediction and the observed asymmetry of single pions produced by linearly polarized photons could be the result of density-matrix distortion caused by background interference.

General relations between photoproduction of pions and vector meson production by πN collision based on the photon-vector-meson analogy model

The relations among differential cross section, polarization and helicity density matrix elements in γN → πN and πN → VN processes are given for the photon-vector-meson analogy model. For example, the differential cross sections of γN → πN processes are given in terms of density matrix elements and differential cross sections of πN → ϱN processes, and the relations fit the experimental data fairly well.