19,569 research outputs found
Measurements of from event shapes and the four-jet rate
New results from measurements of the strong coupling constant
at LEP are presented. In particular, a new LEP combination of results based on
event-shape variables has become available, where a new method for the
estimation of the theoretical uncertainty has been implemented. Furthermore,
two other analyses are quoted, based on power corrections and the four-jet
rate.Comment: Talk presented at the 31st International Conference on High Energy
Physics, Amsterdam, 24-31 July 200
Systems of symplectic forms on four-manifolds
We study almost Hermitian 4-manifolds with holonomy algebra, for the
canonical Hermitian connection, of dimension at most one. We show how
Riemannian 4-manifolds admitting five orthonormal symplectic forms fit therein
and classify them. In this set-up we also fully describe almost Kaehler
4-manifolds.Comment: Final version, to appear in Annali di SNS.added thm. 5.
Complex homothetic foliations on K\"ahler manifolds
We obtain a local classification of complex homothetic foliations on Kaehler
manifolds by complex curves. This is used to construct almost Kaehler,
Ricci-flat metrics subject to additional curvature properties.Comment: Final version, up to typos. To appear in BLM
The Dicke model phase transition in the quantum motion of a Bose-Einstein condensate in an optical cavity
We show that the motion of a laser-driven Bose-Einstein condensate in a
high-finesse optical cavity realizes the spin-boson Dicke-model. The quantum
phase transition of the Dicke-model from the normal to the superradiant phase
corresponds to the self-organization of atoms from the homogeneous into a
periodically patterned distribution above a critical driving strength. The
fragility of the ground state due to photon measurement induced back action is
calculated.Comment: 5 pages, 2 figure
Photonic tuning of quasi-particle decay in a superfluid
We show that the damping rate of elementary excitations of hybrid systems
close to a phase transition can undergo a remarkable resonance like enhancement
before mode softening takes place. In particular, we consider the friction of a
collective density wave in a homogeneous superfluid of weakly interacting
bosonic atoms coupled to the electromagnetic field of a single mode optical
resonator. Here the Beliaev damping can thus be controlled by an external laser
drive and be enhanced by several orders of magnitude
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