Quantum Gravity Vacuum and Invariants of Embedded Spin Networks

We show that the path integral for the three-dimensional SU(2) BF theory with a Wilson loop or a spin network function inserted can be understood as the Rovelli-Smolin loop transform of a wavefunction in the Ashtekar connection representation, where the wavefunction satisfies the constraints of quantum general relativity with zero cosmological constant. This wavefunction is given as a product of the delta functions of the SU(2) field strength and therefore it can be naturally associated to a flat connection spacetime. The loop transform can be defined rigorously via the quantum SU(2) group, as a spin foam state sum model, so that one obtains invariants of spin networks embedded in a three-manifold. These invariants define a flat connection vacuum state in the q-deformed spin network basis. We then propose a modification of this construction in order to obtain a vacuum state corresponding to the flat metric spacetime.Comment: 15 pages, revised version to appear in Class. Quant. Gra

Measurement of the ttˉ production cross section in pp collisions at √s=7 TeV using the kinematic properties of events with leptons and jets

A measurement of the t (t) over bar production cross section in proton-proton collisions at a centre-of-mass energy of 7 TeV has been performed at the LHC with the CMS detector. The analysis uses a data sample corresponding to an integrated luminosity of 36 pb(-1) and is based on the reconstruction of the final state with one isolated, high transverse-momentum electron or muon and three or more hadronic jets. The kinematic properties of the events are used to separate the t (t) over bar signal from W+jets and QCD multijet background events. The measured cross section is 173(-32)(+39) (stat. + syst.) pb, consistent with standard model expectations