An Effective pTp_T Cutoff for the Isolalated Lepton Background from Bottom Decay --

There is a strong correlation between the $p_T$ and isolation of the lepton coming from $B$ decay. Consequently the isolated lepton background from $B$ decay goes down rapidly with increasing lepton $p_T$; and there is a $p_T$ cutoff beyond which it effectively vanishes. For the isolation cut of $E^{AC}_T < 10$ GeV, appropriate for LHC, the lepton $p_T$ cutoff is 80 GeV. This can be exploited to effectively eliminate the $B$ background from the like sign dilepton channel apropriate for Majorana particle searches, as well as the unlike sign dilepton and the single lepton channels appropriate for the top quark search. We illustrate this with a detailed analysis of the $B$ background in these channels along with the signals at LHC energy using both parton level MC and ISAJET programs.Comment: TIFR/TH/93-23 (LATEX, 20 pages, 7 figures available on request

Non-equilibrium dynamics of quantum systems: order parameter evolution, defect generation, and qubit transfer

In this review, we study some aspects of the non-equilibrium dynamics of quantum systems. In particular, we consider the effect of varying a parameter in the Hamiltonian of a quantum system which takes it across a quantum critical point or line. We study both sudden and slow quenches in a variety of systems including one-dimensional ultracold atoms in an optical lattice, an infinite range ferromagnetic Ising model, and some exactly solvable spin models in one and two dimensions (such as the Kitaev model). We show that quenching leads to the formation of defects whose density has a power-law dependence on the quenching rate; the power depends on the dimensionalities of the system and of the critical surface and on some of the exponents associated with the critical point which is being crossed. We also study the effect of non-linear quenching; the power law of the defects then depends on the degree of non-linearity. Finally, we study some spin-1/2 models to discuss how a qubit can be transferred across a system.Comment: 36 pages, 14 figures; an updated version will be published in "Quantum Quenching, Annealing and Computation", Eds. A. Das, A. Chandra and B. K. Chakrabarti, Lect. Notes in Phys., Springer, Heidelberg (2009, to be published