184 research outputs found
Use of W-Boson Longitudinal-Transverse Interference in Top Quark Spin-Correlation Functions: II
This continuation of the derivation of general beam-referenced stage-two
spin-correlation functions is for the analysis of top-antitop pair-production
at the Tevatron and at the Large Hadron Collider. Both the gluon-production and
the quark-production contributions are included for the
charged-lepton-plus-jets reaction p p or p bar{p} --> t bar{t} --> (W^+ b)(W^-
bar{b}) --> (l^{+} nu b)(W^- bar{b}). There is a simple 4-angle beam-referenced
spin-correlation function for determination of the relative sign of, or for
measurement of a possible non-trivial phase between the two dominant helicity
amplitudes for t --> W^{+} b decay. There is an analogous function and tests
for bar{t} --> W^{-} bar{b} decay. This signature requires use of the (t
bar{t}) c.m.-energy of the hadronically decaying W-boson, or the kinematically
equivalent cosine of the polar-angle of W-boson emission in the anti-top (top)
decay frame. Spinors and their outer-products are constructed so that the
helicity-amplitude phase convention of Jacob & Wick can be used throughout for
the fixing of the signs associated with this large W-boson
longitudinal-transverse interference effect.Comment: Continuation of hep-ph/0506240 to include gluon-production
contribution; 3 "postscript" figures. Equation numbers as in
published-on-line EPJ
Lyashko-Looijenga morphisms and submaximal factorisations of a Coxeter element
When W is a finite reflection group, the noncrossing partition lattice NCP_W
of type W is a rich combinatorial object, extending the notion of noncrossing
partitions of an n-gon. A formula (for which the only known proofs are
case-by-case) expresses the number of multichains of a given length in NCP_W as
a generalised Fuss-Catalan number, depending on the invariant degrees of W. We
describe how to understand some specifications of this formula in a case-free
way, using an interpretation of the chains of NCP_W as fibers of a
Lyashko-Looijenga covering (LL), constructed from the geometry of the
discriminant hypersurface of W. We study algebraically the map LL, describing
the factorisations of its discriminant and its Jacobian. As byproducts, we
generalise a formula stated by K. Saito for real reflection groups, and we
deduce new enumeration formulas for certain factorisations of a Coxeter element
of W.Comment: 18 pages. Version 2 : corrected typos and improved presentation.
Version 3 : corrected typos, added illustrated example. To appear in Journal
of Algebraic Combinatoric
Effective action and interaction energy of coupled quantum dots
We obtain the effective action of tunnel-coupled quantum dots, by modeling
the system as a Luttinger liquid with multiple barriers. For a double dot
system, we find that the resonance conditions for perfect conductance form a
hexagon in the plane of the two gate voltages controlling the density of
electrons in each dot. We also explicitly obtain the functional dependence of
the interaction energy and peak-splitting on the gate voltage controlling
tunneling between the dots and their charging energies. Our results are in good
agreement with recent experimental results, from which we obtain the Luttinger
interaction parameter .Comment: 5 pgs,latex,3 figs,revised version to be publshed in Phys.Rev.
Effect of an inhomogeneous external magnetic field on a quantum dot quantum computer
We calculate the effect of an inhomogeneous magnetic field, which is
invariably present in an experimental environment, on the exchange energy of a
double quantum dot artificial molecule, projected to be used as a 2-qubit
quantum gate in the proposed quantum dot quantum computer. We use two different
theoretical methods to calculate the Hilbert space structure in the presence of
the inhomogeneous field: the Heitler-London method which is carried out
analytically and the molecular orbital method which is done computationally.
Within these approximations we show that the exchange energy J changes slowly
when the coupled dots are subject to a magnetic field with a wide range of
inhomogeneity, suggesting swap operations can be performed in such an
environment as long as quantum error correction is applied to account for the
Zeeman term. We also point out the quantum interference nature of this slow
variation in exchange.Comment: 12 pages, 4 figures embedded in tex
Angular Correlations in Top Quark Pair Production and Decay at Hadron Colliders
We show how to observe sizable angular correlations between the decay
products of the top quark and those of the anti-top quark in top quark pair
production and decay at hadron colliders. These correlations result from the
large asymmetry in the rate for producing like-spin versus unlike-spin top
quark pairs provided the appropriate spin axes are used. The effects of new
physics at production or decay on these correlations are briefly discussed.Comment: 34 pages, revtex, including 12 uuencoded postscript figure
Intersubband spin-density excitations in quantum wells with Rashba spin splitting
In inversion-asymmetric semiconductors, spin-orbit coupling induces a
k-dependent spin splitting of valence and conduction bands, which is a
well-known cause for spin decoherence in bulk and heterostructures.
Manipulating nonequilibrium spin coherence in device applications thus requires
understanding how valence and conduction band spin splitting affects carrier
spin dynamics. This paper studies the relevance of this decoherence mechanism
for collective intersubband spin-density excitations (SDEs) in quantum wells. A
density-functional formalism for the linear spin-density matrix response is
presented that describes SDEs in the conduction band of quantum wells with
subbands that may be non-parabolic and spin-split due to bulk or structural
inversion asymmetry (Rashba effect). As an example, we consider a 40 nm
GaAs/AlGaAs quantum well, including Rashba spin splitting of the conduction
subbands. We find a coupling and wavevector-dependent splitting of the
longitudinal and transverse SDEs. However, decoherence of the SDEs is not
determined by subband spin splitting, due to collective effects arising from
dynamical exchange and correlation.Comment: 10 pages, 4 figure
Spin-dependent thermoelectric transport coefficients in near-perfect quantum wires
Thermoelectric transport coefficients are determined for semiconductor
quantum wires with weak thickness fluctuations. Such systems exhibit anomalies
in conductance near 1/4 and 3/4 of 2e^2/h on the rising edge to the first
conductance plateau, explained by singlet and triplet resonances of conducting
electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak,
and J.H. Jefferson, Phys. Rev. B 62, 12985 (2000)]. We extend this work to
study the Seebeck thermopower coefficient and linear thermal conductance within
the framework of the Landauer-Buettiker formalism, which also exhibit anomalous
structures. These features are generic and robust, surviving to temperatures of
a few degrees. It is shown quantitatively how at elevated temperatures thermal
conductance progressively deviates from the Wiedemann-Franz law.Comment: To appear in Phys. Rev. B 2002; 3 figure
CP--odd Correlation in the Decay of Neutral Higgs Boson into , , or
We investigate the possibility of detecting CP--odd angular correlations in
the various decay modes of the neutral Higgs boson including the modes of a
pair, a pair, or a heavy quark pair. It is a natural way to probe
the CP character of the Higgs boson once it is identified. Final state
interactions (i.e. the absorptive decay amplitude) is not required in such
correlations. As an illustrative example we take the fundamental source of the
CP nonconservation to be in the Yukawa couplings of the Higgs boson to the
heavy fermions. A similar correlation in the process is
also proposed. Our analysis of these correlations will be useful for
experiments in future colliders such as LEP II, SSC, LHC or NLC.Comment: 16 pages, plus 8 postscript graphs not posted befor
W=0 Pairing in Carbon Nanotubes away from Half Filling
We use the Hubbard Hamiltonian on the honeycomb lattice to represent the
valence bands of carbon single-wall nanotubes. A detailed symmetry
analysis shows that the model allows W=0 pairs which we define as two-body
singlet eigenstates of with vanishing on-site repulsion. By means of a
non-perturbative canonical transformation we calculate the effective
interaction between the electrons of a W=0 pair added to the interacting ground
state. We show that the dressed W=0 pair is a bound state for resonable
parameter values away from half filling. Exact diagonalization results for the
(1,1) nanotube confirm the expectations. For nanotubes of length ,
the binding energy of the pair depends strongly on the filling and decreases
towards a small but nonzero value as . We observe the existence
of an optimal doping when the number of electrons per C atom is in the range
1.21.3, and the binding energy is of the order of 0.1 1 meV.Comment: 16 pages, 6 figure
Supersymmetric Models with a Gauge Singlet and Non-Universal Soft Terms from Orbifold String Theory
The particle spectrum of the supersymmetric extension of the standard model
with a gauge singlet is studied. Soft supersymmetry breaking terms are
explicitly chosen to be non-universal according to orbifold string theory. they
depend on modular weights of chiral fields and on an angle specifying
the supersymmetry breaking sector. Imposing radiative weak symmetry breaking
and requiring that standard model Yukawa couplings should be allowed yield
constraints on modular weights and almost specifies the angle . We then
perform a numerical anlysis of the low energy spectrum. It turns out that the
spectrum is very constrained, revealing salient features such as light Higgses
and neutralinos. The latter turns out to be essentially gaugino-like.Comment: 13 pages, uuencoded ps-fil
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