351 research outputs found
A review of the decoherent histories approach to the arrival time problem in quantum theory
We review recent progress in understanding the arrival time problem in
quantum mechanics, from the point of view of the decoherent histories approach
to quantum theory. We begin by discussing the arrival time problem, focussing
in particular on the role of the probability current in the expected classical
solution. After a brief introduction to decoherent histories we review the use
of complex potentials in the construction of appropriate class operators. We
then discuss the arrival time problem for a particle coupled to an environment,
and review how the arrival time probability can be expressed in terms of a POVM
in this case. We turn finally to the question of decoherence of the
corresponding histories, and we show that this can be achieved for simple
states in the case of a free particle, and for general states for a particle
coupled to an environment.Comment: 10 pages. To appear in DICE 2010 conference proceeding
The stochastic gravitational wave background from turbulence and magnetic fields generated by a first-order phase transition
We analytically derive the spectrum of gravitational waves due to
magneto-hydrodynamical turbulence generated by bubble collisions in a
first-order phase transition. In contrast to previous studies, we take into
account the fact that turbulence and magnetic fields act as sources of
gravitational waves for many Hubble times after the phase transition is
completed. This modifies the gravitational wave spectrum at large scales. We
also model the initial stirring phase preceding the Kolmogorov cascade, while
earlier works assume that the Kolmogorov spectrum sets in instantaneously. The
continuity in time of the source is relevant for a correct determination of the
peak position of the gravitational wave spectrum. We discuss how the results
depend on assumptions about the unequal-time correlation of the source and
motivate a realistic choice for it. Our treatment gives a similar peak
frequency as previous analyses but the amplitude of the signal is reduced due
to the use of a more realistic power spectrum for the magneto-hydrodynamical
turbulence. For a strongly first-order electroweak phase transition, the signal
is observable with the space interferometer LISA.Comment: 46 pages, 17 figures. Replaced with revised version accepted for
publication in JCA
Symmetric coupling of four spin-1/2 systems
We address the non-binary coupling of identical angular momenta based upon
the representation theory for the symmetric group. A correspondence is pointed
out between the complete set of commuting operators and the
reference-frame-free subsystems. We provide a detailed analysis of the coupling
of three and four spin-1/2 systems and discuss a symmetric coupling of four
spin-1/2 systems.Comment: 20 pages, no figure
Stochastic String Motion Above and Below the World Sheet Horizon
We study the stochastic motion of a relativistic trailing string in black
hole AdS_5. The classical string solution develops a world-sheet horizon and we
determine the associated Hawking radiation spectrum. The emitted radiation
causes fluctuations on the string both above and below the world-sheet horizon.
In contrast to standard black hole physics, the fluctuations below the horizon
are causally connected with the boundary of AdS. We derive a bulk stochastic
equation of motion for the dual string and use the AdS/CFT correspondence to
determine the evolution a fast heavy quark in the strongly coupled
plasma. We find that the kinetic mass of the quark decreases by while the correlation time of world sheet
fluctuations increases by .Comment: 27 pages, 5 figures; v2 final version, small changes, references
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Supersymmetric Axion-Neutrino Merger
The recently proposed supersymmetric model of the neutrino mass matrix
is modified to merge with a previously proposed axionic solution of the strong
CP problem. The resulting model has only one input scale, i.e. that of
symmetry breaking, which determines both the seesaw neutrino mass scale and the
axion decay constant. It also solves the problem and conserves R parity
automatically.Comment: 7 pages, no figur
The Neutrino Mass Matrix - New Developments
With the recent experimental advance in our precise knowledge of the neutrino
oscillation parameters, the correct form of the 3 X 3 neutrino mass matrix is
now approximately known. I discuss how this may be obtained from symmetry
principles, using as examples the finite groups A_4 and Z_4, predicting as a
result three nearly degenerate Majorana neutrino masses in the 0.2 eV range.Comment: 14 pages, talk at BEYOND 200
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