Internal cumulants for femtoscopy with fixed charged multiplicity

A detailed understanding of all effects and influences on higher-order correlations is essential. At low charged multiplicity, the effect of a nonpoissonian multiplicity distribution can significantly distort correlations. Evidently, the reference samples with respect to which correlations are measured should yield a null result in the absence of correlations. We show how the careful specification of desired properties necessarily leads to an average-of-multinomials reference sample. The resulting internal cumulants and their averaging over several multiplicities fulfil all requirements of correctly taking into account nonpoissonian multiplicity distributions as well as yielding a null result for uncorrelated fixed-N samples. Various correction factors are shown to be approximations at best. Careful rederivation of statistical variances and covariances within the frequentist approach yields errors for cumulants that differ from those used so far. We finally briefly discuss the implementation of the analysis through a multiple event buffer algorithm.Comment: 33 pages, 3 figures, 79 reference

Transverse-longitudinal HBT correlations in proton-antiproton collisions at root(s) = 630 GeV

Correlations of like-sign pion pairs emerging from proton-antiproton collisions are analysed in the two-dimensional (q_L,q_T) decomposition of the three-momentum difference q. While the data cannot be adequately represented by gaussian, exponential, power-law or Edgeworth parametrisations, more elaborate ones such as Levy and an exponential with a cross term do better. A two-scale model using a hard cut to separate small and large scales may indicate a core that is more prolate than the halo. Consideration not only of the interference peak at small (q_L,q_T), but also of the shape of the correlation distribution at intermediate momentum differences is crucial to understanding the data.Comment: 12 pages, 5 figures, uses elsart.cl

Integral correlation measures for multiparticle physics

We report on a considerable improvement in the technique of measuring multiparticle correlations via integrals over correlation functions. A modification of measures used in the characterization of chaotic dynamical sytems permits fast and flexible calculation of factorial moments and cumulants as well as their differential versions. Higher order correlation integral measurements even of large multiplicity events such as encountered in heavy ion collisons are now feasible. The change from ``ordinary'' to ``factorial'' powers may have important consequences in other fields such as the study of galaxy correlations and Bose-Einstein interferometry.Comment: 23 pages, 6 tar-compressed uuencoded PostScript figures appended, preprint TPR-92-4

Tuning of crystal structure and magnetic properties by exceptionally large epitaxial strains

Huge deformations of the crystal lattice can be achieved in materials with inherent structural instability by epitaxial straining. By coherent growth on seven different substrates the in-plane lattice constants of 50 nm thick Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches 8,3 % relative to the fcc lattice. The in-plane lattice strain results in a remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. This has dramatic consequences for the magnetic key properties. Magnetometry and X-ray circular dichroism (XMCD) measurements show that Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad ranges.Comment: manuscript, 3 figures, auxiliary materia

A sensitive test for models of Bose-Einstein correlations

Accurate and sensitive measurements of higher order cumulants open up new approaches to pion interferometry. It is now possible to test whether a given theoretical prediction can consistently match cumulants of both second and third order. Our consistency test utilizes a new technique combining theoretically predicted functions with experimentally determined weights in a quasi-Monte Carlo approach. Testing a general quantum statistics-based framework of Bose-Einstein correlations with this technique, we find that predictions for third order cumulants differ significantly from UA1 data. This discrepancy may point the way to more detailed dynamical information.Comment: 5 pages, 2 figures, revte