Spin correlations: Tevatron vs. LHC

We compare theoretical expectations for the observation of spin correlations in top quark pair production and decay at the Fermilab Tevatron and the CERN Large Hadron Collider (LHC). In particular, we note that the differing top quark pair production mechanisms in the two environments test different aspects of the Standard Model and require different strategies to observe the correlations. At the Tevatron, production is dominated by q qbar --> t tbar and the strategy is to construct a double-decay angle distribution where one decay angle is measured in the t rest frame and the other in the tbar rest frame. The dominant process at the LHC is gg --> t tbar, with a rich spin structure that allows for a second option in observing spin correlations. Here the strategy is to select events where the t tbar pair is produced at relatively low velocity in the zero momentum frame (ZMF). For these events, there are strong azimuthal correlations between t and tbar decay products present. This measurement enjoys the advantage that it can be carried out in the laboratory frame.Comment: 8 pages, talk presented at top2010, Bruges, Belgiu

Gluons in a Color-Neutral Nucleus

We improve the McLerran-Venugopalan model by introducing a charge-density correlation function which is consistent with the observation that nucleons carry no net color charge. The infrared divergence in the transverse coordinates that was present in the McLerran-Venugoplan model is eliminated by the enforcement of color neutrality.Comment: 8 pages with 6 imbedded figures. Talk presented at MRST99: High Energy Physics at the Millennium, Carleton University, Ottawa, Ontario, Canada, May 10-12, 199

From Crepes to Pancakes in the MV Model

The McLerran-Venugopalan model provides a framework which allows one to compute the gluon distribution function of a very large nucleus from the equations of QCD, provided that the longitudinal momentum fraction, xF, is sufficiently small. The source of color charge for this computation may be thought of as a crepe moving along the z axis at the speed of light. We refine the MV model by allowing for the presence of non-trivial longitudinal correlations between the color charges that comprise the nucleons. We find that a consistent treatment forces us to consider a pancake-like source which moves at slightly less than the speed of light. Our calculation allows us to consider larger values of xF than were allowed in the original MV model.Comment: 6 pages with 4 figures (aipproc

Single Top Quark Production at the LHC: Understanding Spin

We show that the single top quarks produced in the Wg-fusion channel at a proton-proton collider at a center-of-mass energy sqrt{s}=14 TeV posses a high degree of polarization in terms of a spin basis which decomposes the top quark spin in its rest frame along the direction of the spectator jet. A second useful spin basis is the eta-beamline basis, which decomposes the top quark spin along one of the two beam directions, depending on which hemisphere contains the spectator jet. We elucidate the interplay between the two- and three-body final states contributing to this production cross section in the context of determining the spin decomposition of the top quarks, and argue that the zero momentum frame helicity is undefined. We show that the usefulness of the spectator and eta-beamline spin bases is not adversely affected by the cuts required to separate the Wg-fusion signal from the background.Comment: 12 pages with 3 figure

Longitudinal Resolution in a Large Relativistic Nucleus: Adding a Dimension to the McLerran-Venugopalan Model

We extend the McLerran-Venugopalan model for the gluon distribution functions of very large nuclei to larger values of the longitudinal momentum fraction xF. Because gluons with larger values of xF begin to resolve the longitudinal structure of the nucleus, we find that it is necessary to set up a fully three-dimensional formalism for performing the calculation. We obtain a relatively compact expression for the gluon number density provided that the nucleus is sufficiently large and consists of color-neutral nucleons. Our expressions for the gluon number density saturate at small transverse momenta. The nuclear dependence we obtain is such that the number of gluons increases more slowly than the number of nucleons is increased.Comment: 45 pages with 11 figures (revtex). Expanded discussion of which features of our result are generic. Final version, to appear in PR