Xavier Artru

Spin observables and spin structure functions: inequalities and dynamics

Abstract Model-independent identities and inequalities which relate the various spin observables of collisions in nuclear and particle physics are reviewed in a unified formalism. Their physical interpretation and their implications for dynamical models are also discussed. These constraints between observables can be obtained in several ways: from the explicit expression of the observables in terms of a set of helicity or transversity amplitudes, a non-trivial algebraic exercise which can be preceded by numerical simulation with randomly chosen amplitudes, from anticommutation relations, or from the requirement that any polarisation vector is less than unity. The most powerful tool is the positivity of the density matrices describing the spins in the initial or final state of the reaction or its crossed channels. The inequalities resulting from positivity need to be projected to single out correlations between two or three observables. The quantum aspects of the information carried by spins, in particular entanglement, are considered when deriving and discussing the constraints. Several examples are given, with a comparison with experimental data in some cases. For the exclusive reactions, the cases of the strangeness-exchange proton–antiproton scattering and the photoproduction of pseudoscalar mesons are treated in some detail: all triples of observables are constrained, and new results are presented for the allowed domains. The positivity constraints for total cross sections and for the simplest observables of single-particle inclusive reactions are reviewed. They also apply to spin-dependent structure functions and parton distributions, both integrated or transverse-momentum dependent. The corresponding inequalities are shown to be preserved by the evolution equations of quantum chromodynamics.

Interference and shadow effects in the production of light by charged particles in optical fibers

Abstract A charged particle passing through or near a narrow optical fiber induces, by polarisation, coherent light guided by the fiber. In the limit of zero crossing angle, the radiation tends towards a Cherenkov radiation with a discrete spectrum, studied by different authors. If the particle crosses a bent fiber at regularly spaced points, interference gives quasi-monochromatic lines. If the particle passes near an end of the fiber, light is produced by the capture of virtual photons through the end face. An alternative way consists in sticking a metallic ball to the fiber: the passing particle induces plasmons which are then evacuated as light in the fiber. Interferences can occur between lights from several ends or balls. Applications of these various light signals to beam diagnostics are discussed. The shadow effect, which reduces the photon yield when the particle runs parallel to a row of balls, is pointed out and an upper bound - d E / d z ≤ C ( Ze / b ) 2 for the particle energy loss is conjectured (Ze is the particle charge, b the impact parameter and C a numerical constant). This bound should also apply to other kinds of light sources, in particular to Smith–Purcell radiation.

Classical and quantum phenomenology in radiation by relativistic electrons in matter or in external fields

Abstract Phenomenological aspects of radiation by relativistic electrons in external field, in matter or the vicinity of matter are reviewed, among which: infrared divergence, coherence length effects, shadowing, crystal-assisted radiation, quantum recoil and spin effects, electron side-slipping, photon impact parameter and tunneling in the radiation process.

A positron source using an axially oriented crystal associated to a granular amorphous converter

A non-conventional positron source using the intense γ radiation from an axially oriented monocrystal which materializes into e + e pairs in a granular amorphous converter is described.The enhancement of photon radiation by multi-GeV electrons crossing a tungsten crystal along its axis is repo...

Constraints on spin observables

Positivity constrains the allowed domain for sets of spin observables in exclusive or inclusive reactions. Examples are given for strangeness-echange reactions and photoproduction.

The relevance of positivity in spin physics

Positivity reduces substantially the allowed domain for spin observables. We briefly recall some methods used to determine these domains and give some typical examples for exclusive and inclusive spin‐dependent reactions.

Model-independent constraints on spin observables *

We discuss model-independent constraints on spin observables in exclusive and inclusive reactions, with special attention to the case of photoproduction.