Torsion algebraic cycles and complex cobordism

We show that the cycle map on a variety X, from algebraic cycles modulo algebraic equivalence to integer cohomology, lifts canonically to a topologically defined quotient of the complex cobordism ring of X. This more refined cycle map gives a topological proof that the Griffiths group is nonzero for some varieties X, without any use of Hodge theory. We also use this more refined cycle map to give examples of torsion algebraic cycles which map to 0 in Deligne cohomology but are not algebraically equivalent to 0, thus answering some questions by Colliot-Thelene and Schoen.Comment: 20 page

Music on the margins : fiddle music in Cape Breton

Publisher PD

Systematic Study of Au-Au Collisions with AGS Experiment E917

The systematics of baryon stopping and strange particle production have been studied in Au+Au collisions at 6, 8, and 10.8 GeV per nucleon with AGS experiment E917. In particular, the systematic behavior of the proton yields, phi meson production, and the yields of antilambdas and antiprotons are examined as functions of beam energy and centrality.Comment: 4 pages, 2 figures; Proceedings of Quark Matter 200

Pseudo-abelian varieties

Chevalley's theorem states that every smooth connected algebraic group over a perfect field is an extension of an abelian variety by a smooth connected affine group. That fails when the base field is not perfect. We define a pseudo-abelian variety over an arbitrary field k to be a smooth connected k-group in which every smooth connected affine normal k-subgroup is trivial. This gives a new point of view on the classification of algebraic groups: every smooth connected group over a field is an extension of a pseudo-abelian variety by a smooth connected affine group, in a unique way. We work out much of the structure of pseudo-abelian varieties. These groups are closely related to unipotent groups in characteristic p and to pseudo-reductive groups as studied by Tits and Conrad-Gabber-Prasad. Many properties of abelian varieties such as the Mordell-Weil theorem extend to pseudo-abelian varieties. Finally, we conjecture a description of Ext^2(G_a,G_m) over any field by generators and relations, in the spirit of the Milnor conjecture.Comment: 28 pages; v3: some proofs simplified, some questions answered. To appear in Ann. Sci. EN