544 research outputs found
Discreteness and the origin of probability in quantum mechanics
Attempts to derive the Born rule, either in the Many Worlds or Copenhagen
interpretation, are unsatisfactory for systems with only a finite number of
degrees of freedom. In the case of Many Worlds this is a serious problem, since
its goal is to account for apparent collapse phenomena, including the Born rule
for probabilities, assuming only unitary evolution of the wavefunction. For
finite number of degrees of freedom, observers on the vast majority of branches
would not deduce the Born rule. However, discreteness of the quantum state
space, even if extremely tiny, may restore the validity of the usual arguments.Comment: 5 pages, revtex, 1 figure. Revised version, to appear in Physics
Letters B. (Small clarifcation, references added.
New AdS non Abelian black holes with superconducting horizons
We present arguments for the existence of higher dimensional asymptotically
AdS non Abelian black holes with a Ricci flat event horizon and analyze their
basic properties. Unlike higher dimensional black holes with a curved horizon,
of the usual Einstein-Yang-Mills system, these solutions have finite
mass-energy. Below some non-zero critical temperature, they are
thermodynamically preferred over the Abelian configurations.Comment: 11 pages, 4 figure
The angular distribution of the reaction
The reaction is very important for low-energy
( MeV) antineutrino experiments. In this paper we calculate
the positron angular distribution, which at low energies is slightly backward.
We show that weak magnetism and recoil corrections have a large effect on the
angular distribution, making it isotropic at about 15 MeV and slightly forward
at higher energies. We also show that the behavior of the cross section and the
angular distribution can be well-understood analytically for MeV by calculating to , where is the nucleon mass. The
correct angular distribution is useful for separating events from other reactions and detector backgrounds, as well as for
possible localization of the source (e.g., a supernova) direction. We comment
on how similar corrections appear for the lepton angular distributions in the
deuteron breakup reactions and . Finally, in the reaction , the
angular distribution of the outgoing neutrons is strongly forward-peaked,
leading to a measurable separation in positron and neutron detection points,
also potentially useful for rejecting backgrounds or locating the source
direction.Comment: 10 pages, including 5 figure
Giant Gravitons - with Strings Attached (III)
We develop techniques to compute the one-loop anomalous dimensions of
operators in the super Yang-Mills theory that are dual to open
strings ending on boundstates of sphere giant gravitons. Our results, which are
applicable to excitations involving an arbitrary number of open strings,
generalize the single string results of hep-th/0701067. The open strings we
consider carry angular momentum on an S embedded in the S of the
AdSS background. The problem of computing the one loop anomalous
dimensions is replaced with the problem of diagonalizing an interacting Cuntz
oscillator Hamiltonian. Our Cuntz oscillator dynamics illustrates how the
Chan-Paton factors for open strings propagating on multiple branes can arise
dynamically.Comment: 66 pages; v2: improved presentatio
--Dependence of the Gerasimov-Drell-Hearn Sum Rule
We test the Gerasimov-Drell-Hearn (GDH) sum rule numerically by calculating
the total photon absorption cross sections and on
the nucleon via photon excitation of baryon resonances in the constituent quark
model. A total of seventeen, low-lying, non-strange baryon resonances are
included in this calculation. The transverse and longitudinal interference
cross section, , is found to play an important role in the
study of the variation of the sum rule. The results show that the GDH sum
rule is saturated by these resonances at a confidence level of 94%. In
particular, the excitation largely saturates the sum rule at
, and dominates at small . The GDH integral has a strong
-dependence below and changes its sign around . It becomes weakly -dependent for because of
the quick decline of the resonance contributions. We point out that the
variation of the GDH sum rule is very important for understanding the nucleon
spin structure in the non-perturbative QCD region.Comment: revtex, 17 pages, 3 ps figs include
Nonlinear evolution of the momentum dependent condensates in strong interaction: the ``pseudoscalar laser''
We discuss the relaxation of the scalar and pseudoscalar condensates after a
rapid quench from an initial state with fluctuations. If we include not only
the zero-mode but also higher modes of the condensates in the classical
evolution, we observe parametric amplification of those ``hard'' modes. Thus,
they couple nonlinearly to the ``soft'' modes. As a consequence, domains of
coherent pi-field emerge long after the initial spinodal decomposition. The
momentum-space distribution of pions emerging from the decay of that
momentum-dependent condensate is discussed.Comment: 6 Pages, REVTEX, 8 Figures; one reference and one figure adde
The Proton Spin and Flavor Structure in the Chiral Quark Model
After a pedagogical review of the simple constituent quark model and deep
inelastic sum rules, we describe how a quark sea as produced by the emission of
internal Goldstone bosons by the valence quarks can account for the observed
features of proton spin and flavor structures. Some issues concerning the
strange quark content of the nucleon are also discussed.Comment: 59 pages with table of contents, Lecture delivered at the Schladming
Winter School (March 1997), to be published by Springer-Verlag under the
title "Computing Particle Properties" (eds. C. B. Lang and H. Gausterer
Challenges and Obstacles for a Bouncing Universe in Brane Models
A Brane evolving in the background of a charged AdS black-hole displays in
general a bouncing behaviour with a smooth transition from a contracting to an
expanding phase. We examine in detail the conditions and consequences of this
behaviour in various cases. For a cosmological-constant-dominated Brane, we
obtain a singularity-free, inflationary era which is shown to be compatible
only with an intermediate-scale fundamental Planck mass. For a
radiation-dominated Brane, the bouncing behaviour can occur only for
background-charge values exceeding those allowed for non-extremal black holes.
For a matter-dominated Brane, the black-hole mass affects the proper volume or
the expansion rate of the Brane. We also consider the Brane evolving in an
asymmetric background of two distinct charged AdS black hole spacetimes being
bounded by the Brane and find that, in the case of an empty critical Brane,
bouncing behaviour occurs only if the black-hole mass difference is smaller
than a certain value. The effects of a Brane curvature term on the bounce at
early and late times are also investigated.Comment: 23 pages, Latex file, comments and references added, version to
appear in Phys. Rev.
Finite size corrections for open strings/open chains in planar AdS/CFT
We identify the leading finite-size (Luscher-type) correction to the energy
of open strings ending on maximal giant gravitons. In particular we obtain the
leading finite size correction at weak 't Hooft coupling and in the planar
limit to the energy of very short vacuum states. These results are shown to
agree with certain 1, 2, 3 and 4-loop dual gauge theory perturbative
calculations, which we also perform.Comment: 31 pages; v2: comments and references added; v3: clarifications and
references adde
Quantum dynamics and thermalization for out-of-equilibrium phi^4-theory
The quantum time evolution of \phi^4-field theory for a spatially homogeneous
system in 2+1 space-time dimensions is investigated numerically for
out-of-equilibrium initial conditions on the basis of the Kadanoff-Baym
equations including the tadpole and sunset self-energies. Whereas the tadpole
self-energy yields a dynamical mass, the sunset self-energy is responsible for
dissipation and an equilibration of the system. In particular we address the
dynamics of the spectral (`off-shell') distributions of the excited quantum
modes and the different phases in the approach to equilibrium described by
Kubo-Martin-Schwinger relations for thermal equilibrium states. The
investigation explicitly demonstrates that the only translation invariant
solutions representing the stationary fixed points of the coupled equation of
motions are those of full thermal equilibrium. They agree with those extracted
from the time integration of the Kadanoff-Baym equations in the long time
limit. Furthermore, a detailed comparison of the full quantum dynamics to more
approximate and simple schemes like that of a standard kinetic (on-shell)
Boltzmann equation is performed. Our analysis shows that the consistent
inclusion of the dynamical spectral function has a significant impact on
relaxation phenomena. The different time scales, that are involved in the
dynamical quantum evolution towards a complete thermalized state, are discussed
in detail. We find that far off-shell 1 3 processes are responsible for
chemical equilibration, which is missed in the Boltzmann limit. Finally, we
address briefly the case of (bare) massless fields. For sufficiently large
couplings we observe the onset of Bose condensation, where our scheme
within symmetric \phi^4-theory breaks down.Comment: 77 pages, 26 figure
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