13,525 research outputs found
A new formulation of compartmental epidemic modelling for arbitrary distributions of incubation and removal times
The paradigm for compartment models in epidemiology assumes exponentially
distributed incubation and removal times, which is not realistic in actual
populations. Commonly used variations with multiple exponentially distributed
variables are more flexible, yet do not allow for arbitrary distributions. We
present a new formulation, focussing on the SEIR concept that allows to include
general distributions of incubation and removal times. We compare the solution
to two types of agent-based model simulations, a spatially homogeneous one
where infection occurs by proximity, and a model on a scale-free network with
varying clustering properties, where the infection between any two agents
occurs via their link if it exists. We find good agreement in both cases.
Furthermore a family of asymptotic solutions of the equations is found in terms
of a logistic curve, which after a non-universal time shift, fits extremely
well all the microdynamical simulations. The formulation allows for a simple
numerical approach; software in Julia and Python is provided.Comment: 21 pages, 11 figures. v2 matches published version: improved
presentation (including title, abstract and references), results and
conclusions unchange
Determination of the weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions
A new method to determine the low-energy couplings of the weak
Hamiltonian is presented. It relies on a matching of the topological poles in
of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the -regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure
Weak low-energy couplings from topological zero-mode wavefunctions
We discuss a new method to determine the low-energy couplings of the weak Hamiltonian in the -regime. It relies on a matching of the
topological poles in of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200
Flavour symmetry restoration and kaon weak matrix elements in quenched twisted mass QCD
We simulate two variants of quenched twisted mass QCD (tmQCD), with
degenerate Wilson quarks of masses equal to or heavier than half the strange
quark mass. We use Ward identities in order to measure the twist angles of the
theory and thus check the quality of the tuning of mass parameters to a physics
condition which stays constant as the lattice spacing is varied. Flavour
symmetry breaking in tmQCD is studied in a framework of two fully twisted and
two standard Wilson quark flavours, tuned to be degenerate in the continuum.
Comparing pseudoscalar masses, obtained from connected quark diagrams made of
tmQCD and/or standard Wilson quark propagators, we confirm that flavour
symmetry breaking effects, which are at most 5%, decrease as we approach the
continuum limit. We also compute the pseudoscalar decay constant in the
continuum limit, with reduced systematics. As a consequence of improved tuning
of the mass parameters at , we reanalyse our previous
results. Our main phenomenological findings are and .Comment: 41 pages, figures included, one reference added. Final version as
accepted for publication on Nucl.Phys.
A precise determination of in quenched QCD
The parameter is computed in quenched lattice QCD with Wilson twisted
mass fermions. Two variants of tmQCD are used; in both of them the relevant
four-fermion operator is renormalised multiplicatively. The
renormalisation adopted is non-perturbative, with a Schroedinger functional
renormalisation condition. Renormalisation group running is also
non-perturbative, up to very high energy scales. In one of the two tmQCD
frameworks the computations have been performed at the physical -meson mass,
thus eliminating the need of mass extrapolations. Simulations have been
performed at several lattice spacings and the continuum limit was reached by
combining results from both tmQCD regularisations. Finite volume effects have
been partially checked and turned out to be small. Exploratory studies have
also been performed with non-degenerate valence flavours. The final result for
the RGI bag parameter, with all sources of uncertainty (except quenching) under
control, is .Comment: 54 pages, 11 figure
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