We determine the first correction in inverse powers of heavy-quark masses to the ratio of the differential decay rates of B-pev and D-pev at the kinematical point of zero hadronic recoil (maximum q? within the quark model. In this particular case of heavy-to-light meson transitions, the heavy-quark limit gives a definite prediction, and theoretical uncertainties should be minimal. This result is potentially important for the extraction of the Kobayashi-Maskawa element v,, from future measurements.PACS number(s): 13.20. Jf, 11.30.Hv, 12.15.Ff, 13.20.F~

Conservation of helicity is applied to the scattering of relativistic electrons by an infinitely long solenoid of arbitrarily small radius (Aharonov-Bohm scattering). It is also shown that the Born approximation, which fails when applied to Aharonov-Bohm scattering in the Schrodinger equation formalism, gives correct results in the case of the Dirac equation.

Physics teachers have long employed video clips to study moving objects in their classrooms and instructional labs. A number of approaches exist, both free and commercial, for tracking the coordinates of a point using video. The main characteristics of the method described in this paper are: it is simple to use; coordinates can be tracked using any standard Internet navigator; it can be used with examples available on our webserver or with a modified example in the user's own computer; the web pages with new sequences of images are easy to upload to a web server; it is possible to track and analyze any complex one- or two-dimensional motion of objects; and the code is open source. The setup is very simple, intuitive, and can easily be scaled up.

We studied theoretically the excitonic energy levels and the optical absorption spectra for double quantum wells,both symmetric and asymmetric, in the presence of an homogeneous magnetic field. Within the effective mass approach, we expanded the excitonic wave-function, in an orthogonal basis formed by products of electron and hole wave-functions in the growth direction z, and one particle solutions of the magnetic Hamiltonian in the x − y plane. We applied our method to the case of AlxGa1−xAs, for which we showed how the exciton wave-function vary, and how the basis functions are mixed in a non trivial way by the effect of the Coulomb potential. By taking into account all the mixing between the elements in our base, we get anti-crossings between excited excitonic states not reported previously. 73.20.Dx,71.35.Ji

We studied, theoretically, the optical absorption spectra for a slightly asymmetric double quantum well (DQW), in the presence of electric and magnetic fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al .33 Ga .67 As DQW show clearly the different behavior in the luminescence peaks for the indirect exciton IX and left direct exciton DX as a function of the external electric field. We show that the presence of a peak near the DX peak, attributed to an impurity bound left DX in the experimental results, could be a consequence of the non-trivial mixing between excitonic states.