Showing papers in "Physical Review in 2005"

Device model for the operation of polymer/fullerene bulk heterojunction solar cells

Abstract: We have developed a numerical device model that consistently describes the current-voltage characteristics of polymer:fullerene bulk heterojunction solar cells. Bimolecular recombination and a temperature- and field-dependent generation mechanism of free charges are incorporated. It is demonstrated that in poly[2-methoxy-5-(3('),7(')-dimethyloctyloxy)-p-phenylene vinylene]- (OC1C10-PPV-) and [6,6]-phenyl C-61-butyric acid methyl ester- (PCBM-) (1:4 wt. %) based solar cells space-charge effects only play a minor role, leading to a relatively constant electric field in the device. Furthermore, at short-circuit conditions only 7% of all free carriers are lost due to bimolecular recombination. The model predicts that an increased hole mobility together with a reduction of the acceptor strength of 0.5 eV will lead to a maximum attainable efficiency of 5.5% in the PPV/PCBM-based solar cells.

Final state interactions in hadronic B decays

Abstract: There exist many experimental indications that final-state interactions (FSIs) may play a prominent role not only in charmful $B$ decays but also in charmless $B$ ones. We examine the final-state rescattering effects on the hadronic $B$ decay rates and their impact on direct CP violation. The color-suppressed neutral modes such as ${B}^{0}\ensuremath{\rightarrow}{D}^{0}{\ensuremath{\pi}}^{0},{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0},{\ensuremath{\rho}}^{0}{\ensuremath{\pi}}^{0},{K}^{0}{\ensuremath{\pi}}^{0}$ can be substantially enhanced by long-distance rescattering effects. The direct CP-violating partial rate asymmetries in charmless $B$ decays to $\ensuremath{\pi}\ensuremath{\pi}/\ensuremath{\pi}K$ and $\ensuremath{\rho}\ensuremath{\pi}$ are significantly affected by final-state rescattering, and their signs are generally different from that predicted by the short-distance (SD) approach. For example, direct CP asymmetry in ${B}^{0}\ensuremath{\rightarrow}{\ensuremath{\rho}}^{0}{\ensuremath{\pi}}^{0}$ is increased to around 60% due to final-state rescattering effects whereas the short-distance picture gives about 1%. Evidence of direct CP violation in the decay ${\overline{B}}^{0}\ensuremath{\rightarrow}{K}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}$ is now established, while the combined BABAR and Belle measurements of ${\overline{B}}^{0}\ensuremath{\rightarrow}{\ensuremath{\rho}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$ imply a $3.6\ensuremath{\sigma}$ direct CP asymmetry in the ${\ensuremath{\rho}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ mode. Our predictions for CP violation agree with experiment in both magnitude and sign, whereas the QCD factorization predictions (especially for ${\ensuremath{\rho}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$) seem to have some difficulty with the data. Direct CP violation in the decay ${B}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$ is very small ($\ensuremath{\lesssim}1%$) in the standard model even after the inclusion of FSIs. Its measurement will provide a nice way to search for new physics as in the standard model QCD penguins cannot contribute (except by isospin violation). Current data on $\ensuremath{\pi}K$ modes seem to violate the isospin sum-rule relation, suggesting the presence of electroweak penguin contributions. We have also investigated whether a large transverse polarization in $B\ensuremath{\rightarrow}\ensuremath{\phi}{K}^{*}$ can arise from the final-state rescattering of ${D}^{(*)}{\overline{D}}_{s}^{(*)}$ into $\ensuremath{\phi}{K}^{*}$. While the longitudinal polarization fraction can be reduced significantly from short-distance predictions due to such FSI effects, no sizable perpendicular polarization is found owing mainly to the large cancellations occurring in the processes $\overline{B}\ensuremath{\rightarrow}{D}_{s}^{*}\overline{D}\ensuremath{\rightarrow}\ensuremath{\phi}{\overline{K}}^{*}$ and $\overline{B}\ensuremath{\rightarrow}{D}_{s}{\overline{D}}^{*}\ensuremath{\rightarrow}\ensuremath{\phi}{\overline{K}}^{*}$, and this can be understood as a consequence of CP and SU(3) [CPS] symmetry. To fully account for the polarization anomaly (especially the perpendicular polarization) observed in $B\ensuremath{\rightarrow}\ensuremath{\phi}{K}^{*}$, FSI from other states or other mechanism, e.g., the penguin-induced annihilation, may have to be invoked. Our conclusion is that the small value of the longitudinal polarization in $VV$ modes cannot be regarded as a clean signal for new physics.

Search for Extraterrestrial Point Sources of High Energy Neutrinos with AMANDA-II Using Data Collected in 2000-2002

Abstract: Search for Extraterrestrial Point Sources of High Energy Neutrinos with AMANDA-II Using Data Collected in 2000-2002

Resonant exit time in stochastic and deterministic systems

Abstract: The motion of a particle in the field of a time-dependent potential is studied here both at absolute zero and in the presence of thermal agitation. The potential executes either random fluctuations or deterministic harmonic oscillations. Assuming absorbing boundaries it is always possible to find an exit time ${\ensuremath{\tau}}_{\mathrm{ex}}(\ensuremath{\kappa})$ which has a local minimum as a function of the potential flip rate $\ensuremath{\kappa}$. Thus resonant activation, usually associated with diffussive systems, exists in purely deterministic systems as well. Thermal agitation merely extends the range of admissible initial conditions and renders all exit times finite.