In this work, the radiative $B_c\to D_s^{*}\gamma$ decay is investigated in the framework of QCD sum rules. The transition form factors responsible for the decay are calculated. The total branching ratio for this decay is estimated to be in the order of ${10}^{-5}$, so this decay can be measurable at LHC in the near future.

It is shown that it is possible to create successful models of flavor for both quarks and leptons using the discrete non-Abelian group $T^{\prime }$ by itself. Two simple realizations are presented that can be used as the starting point for more general scenarios. In addition to the minimal supersymmetric standard model particle content, the models include three generations of right-handed neutrinos and four scalar flavon fields. Three of the flavons are needed in the quark and charged lepton sector of the models, and the fourth flavon participates only in the neutrino sector.

We classify the unitary, renormalizable, Lorentz violating quantum field theories of interacting scalars and fermions, obtained improving the behavior of Feynman diagrams by means of higher space derivatives. Higher time derivatives are not generated by renormalization. Renormalizability is ensured by a “weighted power-counting” criterion. The theories contain a dimensionful parameter ${\Lambda }_L$, yet a set of models are classically invariant under a weighted scale transformation, which is anomalous at the quantum level. Formulas for the weighted trace anomaly are derived. The renormalization-group properties are studied.

The relationship between Hawking radiation emitted by nonextremal and extremal Reissner-Nordström black holes is critically analyzed. A careful study of a series of regular collapsing geometries reveals that the stress energy tensor stays regular in the extremal limit and is smoothly connected to that of nonextremal black holes. The unexpected feature is that the late time transients which played little role in the nonextremal case are necessary to preserve the well-defined character of the flux in the extremal case. The known singular behavior of the static energy density of extremal black holes is recovered from our series by neglecting these transients, when performing what turns out to be an illegitimate late time limit. Although our results are derived in two-dimensional settings, we explain why they should also apply to higher dimensional black holes.

We consider a thick $p$-brane embedded in an $n$-dimensional spacetime possessing radial symmetry in the directions orthogonal to the brane. We first consider a static brane, and find a general fine-tuning relationship between the brane and bulk parameters required for the brane to be flat. We then consider the cosmology of a time-dependent brane in a static bulk, and find the Friedmann equation for the brane scale factor $a(t)$. The singularities that would ordinarily arise when considering arbitrary codimensions are avoided by regularizing the brane, giving it a finite profile in the transverse dimensions. However, since we consider the brane to be a strictly local defect, we find that the transverse dimensions must have infinite volume, and hence gravity cannot be localized on the brane without resorting to some infrared cutoff.

We present an example of emergent spacetime as the hydrodynamic limit of a more fundamental microscopic theory. The low-energy, long-wavelength limit in our model is dominated by collective variables that generate an effective Lorentzian metric. This system naturally exhibits a microscopic mechanism allowing us to perform controlled signature change between Lorentzian and Riemannian geometries. We calculate the number of quasiparticles produced from a finite-duration Euclidean-signature event, where we take the position that to a good approximation the dynamics is dominated by the evolution of the linearized perturbations, as suggested by Calzetta and Hu [Phys. Rev. A 68, 043625 (2003)]. We adapt the ideas presented by Dray et al. [Gen. Relativ. Gravit. 23, 967 (1991)], such that the field and its canonical momentum are continuous at the signature-change event. We investigate the interplay between the underlying microscopic structure and the emergent gravitational field, focussing on its impact on quasiparticle production in the ultraviolet regime. In general, this can be thought of as the combination of trans-Planckian physics and signature-change physics. Further we investigate the possibility of using the proposed signature-change event as an amplifier for analogue “cosmological particle production” in condensed matter experiments.

We consider the effect of the ${\mathcal{R}}^4$ term in type IIA string theory on the supergravity background dual to $N_c$ D4-branes compactified on a circle with supersymmetry breaking boundary conditions. We study the dynamics of D8-branes in this perturbed geometry in the probe approximation. This leads to an analysis of higher derivative corrections in holographic QCD beyond the supergravity approximation. We make a rough estimate of the corrections to the masses of some of the lightest (axial) vector mesons. The corrections are suppressed by a factor of $(g_{\mathrm{YM}}^2{N}_c{)}^{-3}$ compared to their supergravity values. We find that the masses of these mesons increase from their supergravity values.

In this paper, we present a new scenario of the early universe that contains a pre-big bang ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy condition without developing any ghostlike instabilities. Thus the contracting universe goes through a nonsingular bounce and evolves smoothly into the expanding post-big bang phase. The curvature perturbation acquires a scale-invariant spectrum well before the bounce in this scenario. It is sourced by the scale-invariant entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism recently proposed by Lehners et al. Since the background geometry is nonsingular at all times, the curvature perturbation remains nearly constant on superhorizon scales. It emerges from the bounce unscathed and imprints a scale-invariant spectrum of density fluctuations in the matter-radiation fluid at the onset of the hot big bang phase. The ekpyrotic potential can be chosen so that the spectrum has a red tilt, in accordance with the recent data from WMAP. As in the original ekpyrotic scenario, the model predicts a negligible gravity wave signal on all observable scales. As such “new ekpyrotic cosmology” provides a consistent and distinguishable alternative to inflation to account for the origin of the seeds of large-scale structure.

A near-Milne universe produces a very red spectrum of vacuum quantum fluctuations but has the potential to produce near-scale-invariant thermal fluctuations. This happens if the energy and entropy are mildly subextensive, for example, if there is a Casimir contribution. Therefore, one does not need to invoke corrections to Einstein gravity (as in loop quantum cosmology) for a thermal scenario to be viable. Neither do we need the energy to scale like the area, as in scenarios where the thermal fluctuations are subject to a phase transition in the early universe. Some odd features of this model are pointed out: whether they are fatal or merely unusual will need to be investigated further.

We elaborate on a model of conformal dark energy (dynamical dark energy measured by the conformal age of the universe) recently proposed in [H. Wei and R. G. Cai, arXiv:0708.0884] where the presentday dark energy density was taken to be ${\rho }_q\equiv 3{\alpha }^2{m}_P^2/{\eta }^2$, where $\eta$ is the conformal time and $\alpha$ is a numerical constant. In the absence of an interaction between the ordinary matter and dark energy field $q$, the model may be adjusted to the present values of the dark energy density fraction ${\Omega }_q\simeq 0.73$ and the equation of state parameter $w_q<-0.78$, if the numerical constant $\alpha$ takes a reasonably large value, $\alpha \gtrsim 2.6$. However, in the presence of a nontrivial gravitational coupling of $q$-field to matter, say $\tilde{Q}$, the model may be adjusted to the values ${\Omega }_q\simeq 0.73$ and $w_q\simeq -1$, even if $\alpha \sim \mathcal{O}(1)$, given that the present value of $\tilde{Q}$ is large. Unlike for the model in [R. G. Cai, arXiv:0707.4049], the bound ${\Omega }_q<0.1$ during big bang nucleosynthesis (BBN) may be satisfied for almost any value of $\alpha$. Here we discuss some other limitations of this proposal as a viable dark energy model. The model draws some parallels with the holographic dark energy; we also briefly comment on the latter model.

The scalar sector of the simplest version of the 3-3-1 electroweak model is constructed with three Higgs triplets only. We show that a relation involving two of the constants of the model, two vacuum expectation values of the neutral scalars, and the mass of the doubly charged Higgs boson leads to important information concerning the signals of this scalar particle.

We report on a study of exclusive radiative decays ${\rm Y}(nS)\to \gamma +\mathcal{R}$ ($n=1$, 2, 3), with $\mathcal{R}$ a narrow resonant hadronic state decaying into four or more charged particles (plus possible neutrals). Using data collected from the CLEO III detector at the Cornell Electron Storage Ring, we present upper limits of order ${10}^{-4}$ for such bottomonium two-body decays as a function of the mass $M_{\mathcal{R}}$ recoiling opposite the photon.

We use current-current interaction to calculate the fragmentation functions to describe the production of spin-$1/2$, spin-$1/2^{\prime }$, and spin-$3/2$ baryons with massive constituents in a relativistic quark-diquark model. Our results are in their analytic forms and are applicable for singly, doubly, and triply heavy baryons. We discuss the production of ${\Omega }_{bbc}$, ${\Omega }_{bcc}$, and ${\Omega }_{ccc}$ baryons in some detail. The results are satisfactorily compared with those obtained for triply heavy baryons calculated in a perturbative regime within reasonable values of the parameters involved.

We present a class of constraints on products and combinations of Yukawa couplings for $R$ parity violating (${\mathrm{R̸}}_p$) and lepton flavor conserving as well as violating semileptonic decays of $K$ mesons into light pseudoscalar mesons along with two charged leptons at $1\sigma$ and $2\sigma$ levels. We compare the constraints obtained by semileptonic rare decays with pure leptonic rare decays and find that most of these bounds are now improved over the existing ones. We also study the forward-backward asymmetry in the decays of $K^{+}\to {\pi }^{+}{l}^{+}{l}^{-}$ ($l=e$ and $\mu$) in the absence of tensor terms. The asymmetry is found to be up to $O({10}^{-3})$ ($O({10}^{-1})$) for the electron and muon modes, respectively. The asymmetry is found to be as large as $O({10}^{-1})$ in the case of $K^{+}\to {\pi }^{+}{\mu }^{+}{\mu }^{-}$.

We solve the Dirac equation in the presence of a constant magnetic field in $(3+1)$ and $(2+1)$ dimensions. Quantizing the fermion field, we calculate the $\overline{\psi }\psi$ condensate from first principles for parity conserving and violating Lagrangians for arbitrary field strength. We make a comparison with the results already known in the literature for some particular cases and point out the relevance of our work for possible physical applications.

We derive generalized Lüscher formulas for finite size corrections in a theory with a general dispersion relation. For the ${\mathrm{AdS}}_5\times S^5$ superstring these formulas encode leading wrapping interaction effects. We apply the generalized $\mu$-term formula to calculate finite size corrections to the dispersion relation of the giant magnon at strong coupling. The result exactly agrees with the classical string computation of Arutyunov, Frolov, and Zamaklar. The agreement involved a Borel resummation of all even loop orders of the BES/BHL dressing factor thus providing a strong consistency check for the choice of the dressing factor.

For the most general supersymmetric solutions of type IIB supergravity consisting of a warped product of $Ad{S}_5$ with a five-dimensional manifold $M_5$, we construct an explicit consistent Kaluza-Klein reduction on $M_5$ to minimal $D=5$ gauged supergravity. Thus, any solution of the gauged supergravity can be uplifted on $M_5$ to obtain an exact solution of type IIB supergravity. We also show that for general $Ad{S}_4\times S{E}_7$ solutions, where $S{E}_7$ is a seven-dimensional Sasaki-Einstein manifold, and for a general class of supersymmetric solutions that are a warped product of $Ad{S}_4$ with a seven-dimensional manifold $N_7$, there is an analogous consistent reduction to minimal $D=4$ gauged supergravity.

We study solutions for rigidly rotating strings on a two-sphere. Among them we find two limiting cases that have a particular interest, one is the already known giant magnon and the other we call the single spike solution. The limiting behavior of this last solution is a string infinitely wrapped around the equator. It differs from that solution by the existence of a single spike of height $\overline{\theta }$ that points toward the north pole. We study its properties and compute its energy $E$ and angular momentum $J$ as a function of $\overline{\theta }$. We further generalize the solution by adding one angular momentum to obtain a solution on $S^3$. We find a spin chain interpretation of these results in terms of free fermions and the Hubbard model but the exact relation with the same models derived from the field theory is not clear.

We explore the cosmological content of Salam-Sezgin six-dimensional supergravity, and find a solution to the field equations in qualitative agreement with observation of distant supernovae, primordial nucleosynthesis abundances, and recent measurements of the cosmic microwave background. The carrier of the acceleration in the present de Sitter epoch is a quintessence field slowly rolling down its exponential potential. Intrinsic to this model is a second modulus which is automatically stabilized and acts as a source of cold dark matter, with a mass proportional to an exponential function of the quintessence field (hence realizing varying mass particle models within a string context). However, any attempt to saturate the present cold dark matter component in this manner leads to unacceptable deviations from cosmological data—a numerical study reveals that this source can account for up to about 7% of the total cold dark matter budget. We also show that (1) the model will support a de Sitter energy in agreement with observation at the expense of a miniscule breaking of supersymmetry in the compact space; (2) variations in the fine structure constant are controlled by the stabilized modulus and are negligible; (3) “fifth” forces are carried by the stabilized modulus and are short range; (4) the long time behavior of the model in four dimensions is that of a Robertson-Walker universe with a constant expansion rate $(w=-1/3)$. Finally, we present a string theory background by lifting our six-dimensional cosmological solution to ten dimensions.