6,227 research outputs found
gauge vector field on a codimension-2 brane
In this paper, we obtain a gauge invariant effective action for a bulk
massless gauge vector field on a brane with codimension two by using a
general Kaluza-Klein (KK) decomposition for the field. It suggests that there
exist two types of scalar KK modes to keep the gauge invariance of the action
for the massive vector KK modes. Both the vector and scalar KK modes can be
massive. The masses of the vector KK modes contain two parts,
and , due to the existence of the two extra
dimensions. The masses of the two types of scalar KK modes and
are related to the vector ones, i.e.,
and . Moreover, we
derive two Schr\"{o}dinger-like equations for the vector KK modes, for which
the effective potentials are just the functions of the warp factor.Comment: 15 pages,no figures, accepted by JHE
Cooling mechanical resonators to quantum ground state from room temperature
Ground-state cooling of mesoscopic mechanical resonators is a fundamental
requirement for test of quantum theory and for implementation of quantum
information. We analyze the cavity optomechanical cooling limits in the
intermediate coupling regime, where the light-enhanced optomechanical coupling
strength is comparable with the cavity decay rate. It is found that in this
regime the cooling breaks through the limits in both the strong and weak
coupling regimes. The lowest cooling limit is derived analytically at the
optimal conditions of cavity decay rate and coupling strength. In essence,
cooling to the quantum ground state requires , with being the mechanical quality factor and
being the thermal phonon number. Remarkably, ground-state
cooling is achievable starting from room temperature, when mechanical
-frequency product , and both of the
cavity decay rate and the coupling strength exceed the thermal decoherence
rate. Our study provides a general framework for optimizing the backaction
cooling of mesoscopic mechanical resonators
Null geodesics and gravitational lensing in a nonsingular spacetime
In this paper, the null geodesics and gravitational lensing in a nonsingular
spacetime are investigated. According to the nature of the null geodesics, the
spacetime is divided into several cases. In the weak deflection limit, we find
the influence of the nonsingularity parameter on the positions and
magnifications of the images is negligible. In the strong deflection limit, the
coefficients and observables for the gravitational lensing in a nonsingular
black hole background and a weakly nonsingular spacetime are obtained.
Comparing these results, we find that, in a weakly nonsingular spacetime, the
relativistic images have smaller angular position and relative magnification,
but larger angular separation than that of a nonsingular black hole. These
results might offer a way to probe the spacetime nonsingularity parameter and
put a bound on it by the astronomical instruments in the near future.Comment: 15 pages, 5 figures, 1 tabl
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