The exclusive rare decays B -> K l(bar) l and B_c -> D(D*) l(bar) l in a relativistic quark model

We study the exclusive rare decay B -> K l(bar) l. We calculate the relevant form factors within a relativistic constituent quark model, for the first time without employing the impulse approximation. The calculated form factors are used to evaluate differential decay rates and polarization observables. We present results on the q2-dependence of a set of observables with and without long-distance contributions. A similar analysis is done for the exclusive rare decays B_c -> D (D*) l(bar) l with special emphasis on the cascade decay B_c -> D* (-> D pi) l(bar) l. We derive a four-fold angular decay distribution for this process in terms of helicity amplitudes including lepton mass effects. The four-fold angular decay distribution allows to define a number of physical observables which are amenable to measurement. We compare our results with the results of other studies.Comment: 28 pages, 12 figure

Exciton dynamics in WSe2 bilayers

We investigate exciton dynamics in 2H-WSe2 bilayers in time-resolved photoluminescence (PL) spectroscopy. Fast PL emission times are recorded for both the direct exciton with $\tau_{D}$ ~ 3 ps and the indirect optical transition with $\tau_{i}$ ~ 25 ps. For temperatures between 4 to 150 K $\tau_{i}$ remains constant. Following polarized laser excitation, we observe for the direct exciton transition at the K point of the Brillouin zone efficient optical orientation and alignment during the short emission time $\tau_{D}$. The evolution of the direct exciton polarization and intensity as a function of excitation laser energy is monitored in PL excitation (PLE) experiments.Comment: 4 pages, 3 figure

Exciton states in monolayer MoSe2: impact on interband transitions

We combine linear and non-linear optical spectroscopy at 4K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In 1-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A- and B-exciton emission of 220 meV. In 2-photon PLE we detect for the A- and B-exciton the 2p state 180meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than 2 orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A- and B-exciton. Our post-Density Functional Theory calculations show in the conduction band along the $K-\Gamma$ direction a local minimum that is energetically and in k-space close to the global minimum at the K-point. This has a potentially strong impact on the polarization and energy of the excitonic states that govern the interband transitions and marks an important difference to MoS2 and WSe2 monolayers.Comment: 8 pages, 3 figure

Nucleon QCD sum rules in nuclear matter including four-quark condensates

We calculate the nucleon parameters in nuclear matter using the QCD sum rules approach in Fermi gas approximation. Terms up to 1/q^2 in the operator product expansion (OPE) are taken into account. The higher moments of the nucleon structure functions are included. The complete set of the nucleon expectation values of the four-quark operators is employed. Earlier the lack of information on these values has been the main obstacle for the further development of the approach. We show that the four-quark condensates provide the corrections of the order 20% to the results obtained in the leading orders of the OPE. This is consistent with the assumption about the convergence of the OPE. The nucleon vector self-energy \Sigma_v and the nucleon effective mass m^* are expressed in terms of the in-medium values of QCD condensates. The numerical results for these parameters at the saturation value of the density agree with those obtained by the methods of nuclear physics.Comment: 38 pages, 5 figure

Anisotropic flow of strange particles at RHIC

Space-time picture of the anisotropic flow evolution in Au+Au collisions at BNL RHIC is studied for strange hadrons within the microscopic quark-gluon string model. The directed flow of both mesons and hyperons demonstrates wiggle structure with the universal antiflow slope at |y| < 2 for minimum bias events. This effect increases as the reaction becomes more peripheral. The development of both components of the anisotropic flow is closely related to particle freeze-out. Hadrons are emitted continuously, and different hadronic species are decoupled from the system at different times. These hadrons contribute differently to the formation and evolution of the elliptic flow, which can be decomposed onto three components: (i) flow created by hadrons emitted from the surface at the onset of the collision; (ii) flow produced by jets; (iii) hydrodynamic flow. Due to these features, the general trend in elliptic flow formation is that the earlier mesons are frozen, the weaker their elliptic flow. In contrast, baryons frozen at the end of the system evolution have stronger v2.Comment: proceedings of the conference SQM2004 (September 2004, Cape Town, South Africa

Robust optical emission polarization in MoS2 monolayers through selective valley excitation

We report polarization resolved photoluminescence from monolayer MoS2, a two-dimensional, non-centrosymmetric crystal with direct energy gaps at two different valleys in momentum space. The inherent chiral optical selectivity allows exciting one of these valleys and close to 90% polarized emission at 4K is observed with 40% polarization remaining at 300K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9T indicating robust, selective valley excitation.Comment: 5 pages, 3 figure