1,595 research outputs found
A geometric approach to time evolution operators of Lie quantum systems
Lie systems in Quantum Mechanics are studied from a geometric point of view.
In particular, we develop methods to obtain time evolution operators of
time-dependent Schrodinger equations of Lie type and we show how these methods
explain certain ad hoc methods used in previous papers in order to obtain exact
solutions. Finally, several instances of time-dependent quadratic Hamiltonian
are solved.Comment: Accepted for publication in the International Journal of Theoretical
Physic
CP--violating Chargino Contributions to the Higgs Coupling to Photon Pairs in the Decoupling Regime of Higgs Sector
In most supersymmetric theories, charginos belong to
the class of the lightest supersymmetric particles and the couplings of Higgs
bosons to charginos are in general complex so that the CP--violating chargino
contributions to the loop--induced coupling of the lightest Higgs boson to
photon pairs can be sizable even in the decoupling limit of large pseudoscalar
mass with only the lightest Higgs boson kinematically accessible at
future high energy colliders. We introduce a specific benchmark scenario of CP
violation consistent with the electric dipole moment constraints and with a
commonly accepted baryogenesis mechanism in the minimal supersymmetric Standard
Model. Based on the benchmark scenario of CP violation, we demonstrate that the
fusion of the lightest Higgs boson in linearly polarized photon--photon
collisions can allow us to confirm the existence of the CP--violating chargino
contributions {\it even in the decoupling regime of the Higgs sector} for
nearly degenerate SU(2) gaugino and higgsino mass parameters of about the
electroweak scale.Comment: 1+13 pages, 3 eps figure
Supersymmetry and the positron excess in cosmic rays
Recently the HEAT balloon experiment has confirmed an excess of high-energy
positrons in cosmic rays. They could come from annihilation of dark matter in
the galactic halo. We discuss expectations for the positron signal in cosmic
rays from the lightest superpartner. The simplest interpretations are
incompatible with the size and shape of the excess if the relic LSPs evolved
from thermal equilbrium. Non-thermal histories can describe a sufficient
positron rate. Reproducing the energy spectrum is more challenging, but perhaps
possible. The resulting light superpartner spectrum is compatible with collider
physics, the muon anomalous magnetic moment, Z-pole electroweak data, and other
dark matter searches.Comment: 4 pages, 2 figures, references added, minor wording change
Dark matter annihilation and non-thermal Sunyaev-Zel'dovich effect: II. dwarf spheroidal galaxy
We calculate the CMB temperature distortion due to the energetic electrons
and positrons produced by dark matter annihilation (Sunyaev-Zel'dovich effect),
in dwarf spheroidal galaxies (dSphs). In the calculation we have included two
important effects which were previously ignored. First we show that the
electron-positron pairs with energy less than GeV, which were neglected in
previous calculation, could contribute a significant fraction of the total
signal. Secondly we also consider the full effects of diffusion loss, which
could significantly reduce the density of electron-positron pairs at the center
of cuspy halos. For neutralinos, we confirm that detecting such kind of SZ
effect is beyond the capability of the current or even the next generation
experiments. In the case of light dark matter (LDM) the signal is much larger,
but even in this case it is only marginally detectable with the next generation
of experiment such as ALMA. We conclude that similar to the case of galaxy
clusters, in the dwarf galaxies the SZ_2DM} effect is not a strong probe of
DM annihilations.Comment: 22 pages, 9 figures, version accepted by JCA
Experimental preparation of Werner state via spontaneous parametric down-conversion
We present an experiment of preparing Werner state via spontaneous parametric
down-conversion and controlled decoherence of photons in this paper. In this
experiment two independent BBO (beta-barium borate) crystals are used to
produce down-conversion light beams, which are mixed to prepare Werner state.Comment: 6 pages, 4 figures and 2 table
Efficient quantum key distribution scheme with nonmaximally entangled states
We propose an efficient quantum key distribution scheme based on
entanglement. The sender chooses pairs of photons in one of the two equivalent
nonmaximally entangled states randomly, and sends a sequence of photons from
each pair to the receiver. They choose from the various bases independently but
with substantially different probabilities, thus reducing the fraction of
discarded data, and a significant gain in efficiency is achieved. We then show
that such a refined data analysis guarantees the security of our scheme against
a biased eavesdropping strategy.Comment: 5 Pages, No Figur
Quantum computing with four-particle decoherence-free states in ion trap
Quantum computing gates are proposed to apply on trapped ions in
decoherence-free states. As phase changes due to time evolution of components
with different eigenenergies of quantum superposition are completely frozen,
quantum computing based on this model would be perfect. Possible application of
our scheme in future ion-trap quantum computer is discussed.Comment: 10 pages, no figures. Comments are welcom
Raman study of carrier-overdoping effects on the gap in high-Tc superconducting cuprates
Raman scattering in the heavily overdoped (Y,Ca)Ba_2Cu_3O_{7-d} (T_c = 65 K)
and Bi_2Sr_2CaCu_2O_{8+d} (T_c = 55 K) crystals has been investigated. For the
both crystals, the electronic pair-breaking peaks in the A_{1g} and B_{1g}
polarizations were largely shifted to the low energies close to a half of
2Delta_0, Delta_0 being the maximum gap. It strongly suggests s-wave mixing
into the d-wave superconducting order parameter and the consequent
manifestation of the Coulomb screening effect in the B_{1g}-channel. Gradual
mixing of s-wave component with overdoping is not due to the change of crystal
structure symmetry but a generic feature in all high-T_c superconducting
cuprates.Comment: 5 pages, 4 figures, to be published in Phys. Rev. B, Rapid
communicaito
Reducing the communication complexity with quantum entanglement
We propose a probabilistic two-party communication complexity scenario with a
prior nonmaximally entangled state, which results in less communication than
that is required with only classical random correlations. A simple all-optical
implementation of this protocol is presented and demonstrates our conclusion.Comment: 4 Pages, 2 Figure
Field theory models for variable cosmological constant
Anthropic solutions to the cosmological constant problem require seemingly
unnatural scalar field potentials with a very small slope or domain walls
(branes) with a very small coupling to a four-form field. Here we introduce a
class of models in which the smallness of the corresponding parameters can be
attributed to a spontaneously broken discrete symmetry. We also demonstrate the
equivalence of scalar field and four-form models. Finally, we show how our
models can be naturally embedded into a left-right extension of the standard
model.Comment: A reference adde
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