Narrow Results

We discuss the gravitomagnetic time delay and the Lense–Thirring effect in the context of Brans–Dicke theory of gravity. We compare the theoretical results obtained with those predicted by general relativity. We show that within the accuracy of experiments designed to measure these effects both theories predict essentially the same results.

Motivated by the recent experimental evidence of the exotic B=S=+1 baryonic state Θ (1540), we examine the older existing data on K⁺N elastic scattering through the time delay method. We find positive peaks in time delay around 1.545 and 1.6 GeV in the D₀₃ and P₀₁ partial waves of K⁺N scattering respectively, in agreement with experiments. We also find an indication of the J=3/2 Θ* spin-orbit partner to the Θ, in the P₀₃ partial wave at 1.6 GeV. We discuss the pros and cons of these findings in support of the interpretation of these peaks as possible exotics.

We manipulated and adjusted the absorption, dispersion, group index, and group velocity in a sodium atomic medium by controlling the strength and collective phase of the driving fields. The group index in these regions is calculated to be $-$250,000. The group velocity $v_g$ in these locations is measured to be $-0.12\times 10^{-4}$m/s. An investigation is conducted on a negative delay of $-50\mu$s in the sodium medium. The presence of negative delay and negative group velocity indicates the occurrence of superluminal propagation of light pulses within the medium. The negative group index is increased from $-1\times 10^5$ to $-4\times 10^5$, and the negative delay time is increased from $-20\mu$s to $-70\mu$s, with the driving field $E_2$ having a Rabi frequency. The work presented in this paper has substantial implications for the fields of gravitational wave detection, radar technology, and temporal cloaks.

We review the analytic results for the phase shifts δ_l(k) in nonrelativistic scattering from a spherical well. The conditions for the existence of resonances are established in terms of time-delays. Resonances are shown to exist for p-waves (and higher angular momenta) but not for s-waves. These resonances occur when the potential is not quite strong enough to support a bound p-wave of zero energy. We then examine relativistic scattering by spherical wells and barriers in the Dirac equation. In contrast to the nonrelativistic situation, s-waves are now seen to possess resonances in scattering from both wells and barriers. When s-wave resonances occur for scattering from a well, the potential is not quite strong enough to support a zero momentum s-wave solution at E=m. Resonances resulting from scattering from a barrier can be explained in terms of the "crossing" theorem linking s-wave scattering from barriers to p-wave scattering from wells. A numerical procedure to extract phase shifts for general short range potentials is introduced and illustrated by considering relativistic scattering from a Gaussian potential well and barrier.