A systematic quantum-statistical (density matrix) study of Bose-Einstein correlations (BEC’s) is performed. The minimum number of independent parameters that enter into BEC’s is determined. Expressions for the higher order correlation functions are derived, and the relation between BEC’s and multiplicity distributions is clarified. New results concerning the interplay between the chaotic and the coherent component are presented. The investigation of the isospin dependence of BEC’s leads to the prediction of quantum-statistical π+π− correlations and, related with it, the appearance of squeezed states in particle physics. Soft pions are found to play a specially important part in BEC’s, both with respect to the experimental detection of the new correlation and in the clarification of the role of coherence. A significant increase of the available statistics in BEC’s is necessary in order to test experimentally the new theoretical predictions. Two model sources are investigated: a “static” source and an expanding one. A comparison with a model proposed by Gyulassy, Kauffmann and Wilson is performed.

An investigation of the space-time extension of particle sources in the current-ensemble formalism shows that under quite general assumptions there exist a quantum-statistical {n^n~) correlation and a difference between {n^^n^) and {K~K~)Bose-Einstein correlations. These effects are found to be enhanced for small momenta and radii.PACS numbers: 13.85. Hd, 05.30.Jp, l2.40.Ee, 25.70.Np Identical particles produced in high-energy collisions are correlated in their momenta due to quantum statistics (QS). This efTect is used to determine the space-time form and the coherence properties of the source (for recent reviews, cf. [1][2][3]). Let us consider in particular the normalized two-and three-particle distributions of n mesons, C2(k,,k2) =Pi^kxMi) C3(k,,k2,k3) = Pi(k|)pi(k2) ' P3(ki,k2,k3)

Microwave transmission of a coplanar microresonator deposited on a sample surface over a two-dimensional electron system has been studied. The transmission signal reveals a series of resonances corresponding to the excitation of hybrid cavity plasmon-photon modes, and ultrastrong plasmon-photon coupling has been realized. The hybridization frequency (Rabi frequency) is shown to be anomalously larger than the frequencies of unperturbed modes and it reaches values of up to 25 GHz. The effect of electron density and magnetic field on the excitation spectrum of cavity polariton has been investigated.

The microwave response of a two-dimensional electron system (2DES) covered by a conducting top gate is investigated in the relativistic regime for which the 2D conductivity σ_{2D}>c/2π. Weakly damped plasma waves are excited in the gated region of the 2DES. The frequency and amplitude of the resulting plasma excitations show a very unusual dependence on the magnetic field, conductivity, gate geometry, and separation from the 2DES. We show that such relativistic plasmons survive for temperatures up to 300 K, allowing for new room-temperature microwave and terahertz applications.