5,853 research outputs found
Particle Physics on Ice: Constraints on Neutrino Interactions Far Above the Weak Scale
Ultra-high energy cosmic rays and neutrinos probe energies far above the weak
scale. Their usefulness might appear to be limited by astrophysical
uncertainties; however, by simultaneously considering up- and down-going
events, one may disentangle particle physics from astrophysics. We show that
present data from the AMANDA experiment in the South Pole ice already imply an
upper bound on neutrino cross sections at energy scales that will likely never
be probed at man-made accelerators. The existing data also place an upper limit
on the neutrino flux valid for any neutrino cross section. In the future,
similar analyses of IceCube data will constrain neutrino properties and fluxes
at the O(10%) level.Comment: 4 pages, 1 figure, published versio
Short-Term Memory in Orthogonal Neural Networks
We study the ability of linear recurrent networks obeying discrete time
dynamics to store long temporal sequences that are retrievable from the
instantaneous state of the network. We calculate this temporal memory capacity
for both distributed shift register and random orthogonal connectivity
matrices. We show that the memory capacity of these networks scales with system
size.Comment: 4 pages, 4 figures, to be published in Phys. Rev. Let
Robustness against Power is PSPACE-complete
Power is a RISC architecture developed by IBM, Freescale, and several other
companies and implemented in a series of POWER processors. The architecture
features a relaxed memory model providing very weak guarantees with respect to
the ordering and atomicity of memory accesses.
Due to these weaknesses, some programs that are correct under sequential
consistency (SC) show undesirable effects when run under Power. We call these
programs not robust against the Power memory model. Formally, a program is
robust if every computation under Power has the same data and control
dependencies as some SC computation.
Our contribution is a decision procedure for robustness of concurrent
programs against the Power memory model. It is based on three ideas. First, we
reformulate robustness in terms of the acyclicity of a happens-before relation.
Second, we prove that among the computations with cyclic happens-before
relation there is one in a certain normal form. Finally, we reduce the
existence of such a normal-form computation to a language emptiness problem.
Altogether, this yields a PSPACE algorithm for checking robustness against
Power. We complement it by a matching lower bound to show PSPACE-completeness
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