1,786 research outputs found
Phase structure of a generalized Nambu Jona-Lasinio model with Wilson fermions in the mean field or large -expansion
We analyze the vacuum structure of a generalized lattice Nambu--Jona-Lasinio
model with two flavors of Wilson fermions, such that its continuum action is
the most general four-fermion action with 'trivial' color interactions, and
having a symmetry in the chiral limit. The phase structure
of this model in the space of the two four-fermion couplings shows, in addition
to the standard Aoki phases, new phases with , in
close analogy to similar results recently suggested by some of us for lattice
QCD with two degenerate Wilson fermions. This result shows how the phase
structure of an effective model for low energy QCD cannot be entirely
understood from Wilson Chiral Perturbation Theory, based on the standard QCD
chiral effective Lagrangian approach.Comment: 24 pages, 3 figures, sent to Nuclear Physics
Level and length of cyclic solar activity during the Maunder minimum as deduced from the active day statistics
The Maunder minimum (MM) of greatly reduced solar activity took place in
1645-1715, but the exact level of sunspot activity is uncertain as based, to a
large extent, on historical generic statements of the absence of spots on the
Sun. Here we aim, using a conservative approach, to assess the level and length
of solar cycle during the Maunder minimum, on the basis of direct historical
records by astronomers of that time. A database of the active and inactive days
(days with and without recorded sunspots on the solar disc respectively) is
constructed for three models of different levels of conservatism (loose ML,
optimum MO and strict MS models) regarding generic no-spot records. We have
used the active day fraction to estimate the group sunspot number during the
MM. A clear cyclic variability is found throughout the MM with peaks at around
1655--1657, 1675, 1684 and 1705, and possibly 1666, with the active day
fraction not exceeding 0.2, 0.3 or 0.4 during the core MM, for the three
models. Estimated sunspot numbers are found very low in accordance with a grand
minimum of solar activity.
We have found, for the core MM (1650-1700), that: (1) A large fraction of
no-spot records, corresponding to the solar meridian observations, may be
unreliable in the conventional database. (2) The active day fraction remained
low (below 0.3-0.4) throughout the MM, indicating the low level of sunspot
activity. (3) The solar cycle appears clearly during the core MM. (4) The
length of the solar cycle during the core MM appears years, but there
is an uncertainty in that. (5) The magnitude of the sunspot cycle during MM is
assessed to be below 5-10 in sunspot numbers;
A hypothesis of the high solar cycles during the MM is not confirmed.Comment: Accepted to Astron. Astrophy
Performance Portability Strategies for Grid C++ Expression Templates
One of the key requirements for the Lattice QCD Application Development as
part of the US Exascale Computing Project is performance portability across
multiple architectures. Using the Grid C++ expression template as a starting
point, we report on the progress made with regards to the Grid GPU offloading
strategies. We present both the successes and issues encountered in using CUDA,
OpenACC and Just-In-Time compilation. Experimentation and performance on GPUs
with a SU(3)SU(3) streaming test will be reported. We will also report
on the challenges of using current OpenMP 4.x for GPU offloading in the same
code.Comment: 8 pages, 4 figures. Talk presented at the 35th International
Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai
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