1,830 research outputs found
Top Spin and Experimental Tests
We examine pair mass dependence near threshold as a means to measure the spin
of the top quark in hadron collisions, and we discuss the possibility that a
top squark signal could be hidden among the top events.Comment: 9 pages, RevTeX, 3 figures. Contribution to the Thinkshop on
Top-Quark Physics for the Tevatron Run II, Fermilab, October 16 - 18, 199
Isolated leptons from heavy flavor decays: Theory and data
Events with isolated leptons play a prominent role in signatures of new
physics phenomena at high energy collider physics facilities. In earlier
publications, we examine the standard model contribution to isolated lepton
production from bottom and charm mesons and baryons through their semileptonic
decays (b, c -> l + X), showing that this source can overwhelm the effects of
other standard model processes in some kinematic domains. In this paper, we
show that we obtain good agreement with recent Tevatron collider data, both
validating our simulations and showing that we underestimate the magnitude of
the heavy-flavor contribution to the isolated lepton yields. We also show that
the isolation requirement acts as a narrow bandpass filter on the momentum of
the isolated lepton, and we illustrate the effect of this filter on the
background to Higgs boson observation in the dilepton mode. We introduce and
justify a new rule of thumb: isolated electrons and muons from heavy flavor
decay are produced with roughly the same distributions as b and c quarks, but
with 1/200 times the rates of b and c production, respectively.Comment: 12 pg, revtex, 5 fig, corrected typo
Predictions for associated production of gauginos and gluinos at NLO in SUSY-QCD
NLO SUSY-QCD contributions to associated production of gluinos and gauginos
are shown to enhance the cross sections by about 10% at the Tevatron and by as
much as a factor of two at the LHC. They shift the mass determinations or
discovery limits, soften the spectra, and stabilize the predictions
against variations of the renormalization and factorization scales.Comment: 4 pages, Latex (uses Moriond style file), 8 eps-figures. Talk
presented at the XXXVth Rencontres de Moriond, QCD and High Energy Hadronic
Interactions, Les Arcs, France, March 18-25, 200
Missing heavy flavor backgrounds to Higgs boson production
We investigate characteristics of the signal and backgrounds for Higgs boson
decay into WW at the Fermilab Tevatron and CERN Large Hadron Collider. In the
the lepton-pair-plus-missing-energy final state, we show that the background
receives an important contribution from semileptonic decays of heavy flavors.
Lepton isolation cuts provide too little suppression of these heavy flavor
contributions, and an additional 4 to 8 orders-of-magnitude suppression must
come from physics cuts. We demonstrate that an increase of the minimum
transverse momentum of nonleading leptons in multilepton events is one
effective way to achieve the needed suppression, without appreciable loss of
the Higgs boson signal. Such a cut would impact the efficiency of searches for
supersymmetry as well. We emphasize the importance of direct measurement of the
lepton background from heavy flavor production.Comment: 23 pgs., 10 figs, revtex4, 1 Ref. added, minor typos corrected, to
appear in Phys. Rev.
Next-to-Leading Order SUSY-QCD Predictions for Associated Production of Gauginos and Gluinos
We present complete results of a next-to-leading order calculation of the
production of gaugino-like charginos (\chargino) and neutralinos
(\neutralino) in association with gluinos (\gluino) at hadron colliders,
including the strong corrections from the exchange of colored particles and
sparticles. Adopting a variety of models for the sparticle mass spectrum,
including typical supergravity (SUGRA) models and a light gluino model, we
provide predictions for total and differential cross sections at the energies
of the Fermilab Tevatron and CERN Large Hadron Collider (LHC).Comment: 91 pages, Latex, including 26 figure
Analytic Expression for the Joint x and Q^2 Dependences of the Structure Functions of Deep Inelastic Scattering
We obtain a good analytic fit to the joint Bjorken-x and Q^2 dependences of
ZEUS data on the deep inelastic structure function F_2(x, Q^2). At fixed
virtuality Q^2, as we showed previously, our expression is an expansion in
powers of log (1/x) that satisfies the Froissart bound. Here we show that for
each x, the Q^2 dependence of the data is well described by an expansion in
powers of log Q^2. The resulting analytic expression allows us to predict the
logarithmic derivatives {({\partial}^n F_2^p/{{(\partial\ln Q^2}})^n)}_x for n
= 1,2 and to compare the results successfully with other data. We extrapolate
the proton structure function F_2^p(x,Q^2) to the very large Q^2 and the very
small x regions that are inaccessible to present day experiments and contrast
our expectations with those of conventional global fits of parton distribution
functions.Comment: 4 pages, 3 figures, a few changes in the text. Version to be
published in Physical Review Letter
Erratum: Next-to-leading order supersymmetric QCD predictions for associated production of gauginos and gluinos [Phys. Rev. D 62, 095014 (2000)]
Errors in the published version of the paper are corrected, and new figures
are provided.Comment: 3 pages, latex, 4 figure
Small x Behavior of Parton Distributions from the Observed Froissart Energy Dependence of the Deep Inelastic Scattering Cross Section
We fit the reduced cross section for deep-inelastic electron scattering data
to a three parameter ln^2 s fit, A + beta ln^2 (s/s_0), where s= [Q^2/x] (1-x)
+ m^2, and Q^2 is the virtuality of the exchanged photon. Over a wide range in
Q^2 (0.11 < Q^2 < 1200 GeV^2) all of the fits satisfy the logarithmic energy
dependence of the Froissart bound. We can use these results to extrapolate to
very large energies and hence to very small values of Bjorken x -- well beyond
the range accessible experimentally. As Q^2 --> infinity, the structure
function F_2^p(x, Q^2) exhibits Bjorken scaling, within experimental errors. We
obtain new constraints on the behavior of quark and antiquark distribution
functions at small x.Comment: 10 pages, 2 figure
Quantum dynamics of a binary mixture of BECs in a double well potential: an Holstein-Primakoff approach
We study the quantum dynamics of a binary mixture of Bose-Einstein
condensates (BEC) in a double-well potential starting from a two-mode
Bose-Hubbard Hamiltonian. Focussing on the regime where the number of atoms is
very large, a mapping onto a SU(2) spin problem together with a
Holstein-Primakoff transformation is performed. The quantum evolution of the
number difference of bosons between the two wells is investigated for different
initial conditions, which range from the case of a small imbalance between the
two wells to a coherent spin state. The results show an instability towards a
phase-separation above a critical positive value of the interspecies
interaction while the system evolves towards a coherent tunneling regime for
negative interspecies interactions. A comparison with a semiclassical approach
is discussed together with some implications on the experimental realization of
phase separation with cold atoms.Comment: 12 pages, 7 figures, accepted for publication in J. Phys.
Ultra-high energy neutrino scattering
Estimates are made of the ultra-high energy neutrino cross sections based on
an extrapolation to very small Bjorken x of the logarithmic Froissart
dependence in x shown previously to provide an excellent fit to the measured
proton structure function F_2^p(x,Q^2) over a broad range of the virtuality
Q^2. Expressions are obtained for both the neutral current and the charged
current cross sections. Comparison with an extrapolation based on perturbative
QCD shows good agreement for energies where both fit data, but our rates are as
much as a factor of 10 smaller for neutrino energies above 10^9 GeV, with
important implications for experiments searching for extra-galactic neutrinos.Comment: 4 pages, 1 figure, 1 table; Title, abstract and text changed,
conclusions unchanged. Version accepted for publication in Physical Review
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