60,032 research outputs found
Does a given vector-matrix pair correspond to a PH distribution?
The analysis of practical queueing problems benefits if realistic distributions can be used as parameters. Phase type (PH) distributions can approximate many distributions arising in practice, but their practical applicability has always been limited when they are described by a non-Markovian vector–matrix pair. In this case it is hard to check whether the non-Markovian vector–matrix pair defines a non-negative matrix-exponential function or not. In this paper we propose a numerical procedure for checking if the matrix-exponential function defined by a non-Markovian vector–matrix pair can be represented by a Markovian vector–matrix pair with potentially larger size. If so, then the matrix-exponential function is non-negative. The proposed procedure is based on O’Cinneide’s characterization result, which says that a non-Markovian vector–matrix pair with strictly positive density on and with a real dominant eigenvalue has a Markovian representation. Our method checks the existence of a potential Markovian representation in a computationally efficient way utilizing the structural properties of the applied representation transformation procedure
LUSIFER: a LUcid approach to SIx-FERmion production
LUSIFER is a Monte Carlo event generator for all processes e+e-->6fermions,
which is based on the multi-channel Monte Carlo integration technique and
employs the full set of tree-level diagrams. External fermions are taken to be
massless, but can be arbitrarily polarized. The calculation of the helicity
amplitudes and of the squared matrix elements is presented in a compact way.
Initial-state radiation is included at the leading logarithmic level using the
structure-function approach. The discussion of numerical results contains a
comprehensive list of cross sections relevant for a 500GeV collider, including
a tuned comparison to results obtained with the combination of the WHIZARD and
MADGRAPH packages as far as possible. Moreover, for off-shell top-quark pair
production and the production of a Higgs boson in the intermediate mass range
we additionally discuss some phenomenologically interesting distributions.
Finally, we numerically analyze the effects of gauge-invariance violation by
comparing various ways of introducing decay widths of intermediate top quarks,
gauge and Higgs bosons.Comment: 39 pages, latex, 14 postscript files, some minor misprints corrected,
version to appear in Nucl.Phys.
Discrete phase space based on finite fields
The original Wigner function provides a way of representing in phase space
the quantum states of systems with continuous degrees of freedom. Wigner
functions have also been developed for discrete quantum systems, one popular
version being defined on a 2N x 2N discrete phase space for a system with N
orthogonal states. Here we investigate an alternative class of discrete Wigner
functions, in which the field of real numbers that labels the axes of
continuous phase space is replaced by a finite field having N elements. There
exists such a field if and only if N is a power of a prime; so our formulation
can be applied directly only to systems for which the state-space dimension
takes such a value. Though this condition may seem limiting, we note that any
quantum computer based on qubits meets the condition and can thus be
accommodated within our scheme. The geometry of our N x N phase space also
leads naturally to a method of constructing a complete set of N+1 mutually
unbiased bases for the state space.Comment: 60 pages; minor corrections and additional references in v2 and v3;
improved historical introduction in v4; references to quantum error
correction in v5; v6 corrects the value quoted for the number of similarity
classes for N=
The Leptoquark Hunter's Guide: Pair Production
Leptoquarks occur in many new physics scenarios and could be the next big
discovery at the LHC. The purpose of this paper is to point out that a
model-independent search strategy covering all possible leptoquarks is possible
and has not yet been fully exploited. To be systematic we organize the possible
leptoquark final states according to a leptoquark matrix with entries
corresponding to nine experimentally distinguishable leptoquark decays: any of
{light-jet, b-jet, top} with any of {neutrino, , }. The 9
possibilities can be explored in a largely model-independent fashion with
pair-production of leptoquarks at the LHC. We review the status of experimental
searches for the 9 components of the leptoquark matrix, pointing out which 3
have not been adequately covered. We plead that experimenters publish bounds on
leptoquark cross sections as functions of mass for as wide a range of
leptoquark masses as possible. Such bounds are essential for reliable recasts
to general leptoquark models. To demonstrate the utility of the leptoquark
matrix approach we collect and summarize searches with the same final states as
leptoquark pair production and use them to derive bounds on a complete set of
Minimal Leptoquark models which span all possible flavor and gauge
representations for scalar and vector leptoquarks.Comment: 19 pages + references and appendices, 18 figures, 15 tables. Added
references, fixed typo
Geometric Quantum Mechanics
The manifold of pure quantum states is a complex projective space endowed
with the unitary-invariant geometry of Fubini and Study. According to the
principles of geometric quantum mechanics, the detailed physical
characteristics of a given quantum system can be represented by specific
geometrical features that are selected and preferentially identified in this
complex manifold. Here we construct a number of examples of such geometrical
features as they arise in the state spaces for spin-1/2, spin-1, and spin-3/2
systems, and for pairs of spin-1/2 systems. A study is undertaken on the
geometry of entangled states, and a natural measure is assigned to the degree
of entanglement of a given state for a general multi-particle system. The
properties of this measure are analysed for the entangled states of a pair of
spin-1/2 particles. With the specification of a quantum Hamiltonian, the
resulting Schrodinger trajectory induces a Killing field, which is quasiergodic
on a toroidal subspace of the energy surface. When the dynamical trajectory is
lifted orthogonally to Hilbert space, it induces a geometric phase shift on the
wave function. The uncertainty of an observable in a given state is the length
of the gradient vector of the level surface of the expectation of the
observable in that state, a fact that allows us to calculate higher order
corrections to the Heisenberg relations. A general mixed state is determined by
a probability density function on the state space, for which the associated
first moment is the density matrix. The advantage of a general state is in its
applicability in various attempts to go beyond the standard quantum theory.Comment: 27 pages. Extended with additional materia
On the angular distribution of decay
We present a full angular distribution of the four body
decay where the leptons are massive
and the is unpolarized, in an operator basis which includes the
Standard Model operators, new vector and axial-vector operators, and scalar and
pseudo-scalar operators. The angular coefficients are expressed in terms of
transversity amplitudes. We study several observables in the Standard Model and in the presence of the
new operators. For our numerical analysis, we use the form factors from lattice
QCD calculations.Comment: accepted by JHEP (charmonium and duality contributions included in
this final version
Heavy Vector-like Top Partners at the LHC and flavour constraints
We consider the phenomenology at the Large Hadron Collider of new heavy
vector-like quarks which couple mainly to the third generation quarks via
Yukawa interactions, with special emphasis on non-standard doublet
representations which are less constrained from present data. We also discuss
in detail the flavour limits at tree level and loop level and implications of a
generalised CKM mixing matrix to these cases.Comment: 45 pages, 20 figures, 8 tables. Updated limits in the B-physics part,
typos corrected, minor modifications in the LHC par
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