We present a study of resonances in exclusive decays of $B$ mesons to ${\overline{D}}^{(*)}{D}^{(*)}K$. We report the observation of the decays $B\to {\overline{D}}^{(*)}{D}_{s1}^{+}(2536)$ where the $D_{s1}^{+}(2536)$ is reconstructed in the $D^{*0}{K}^{+}$ and $D^{*+}{K}_S^0$ decay channels. We report also the observation of the decays $B\to \psi (3770)K$ where the $\psi (3770)$ decays to ${\overline{D}}^0{D}^0$ and $D^{-}{D}^{+}$. In addition, we present the observation of an enhancement for the ${\overline{D}}^{*0}{D}^0$ invariant mass in the decays $B\to {\overline{D}}^{*0}{D}^{0}K$, at a mass of $({3875.1}_{-0.5}^{+0.7}\pm 0.5)\mathrm{MeV}/c^2$ with a width of $({3.0}_{-1.4}^{+1.9}\pm 0.9)\mathrm{MeV}$ (the first errors are statistical and the second are systematic). Branching fractions and spin studies are shown for the three resonances. The results are based on $347{\mathrm{fb}}^{-1}$ of data collected with the BABAR detector at the PEP-II $B$ factory.

We present a search for $B$ decays to a charged scalar meson $a_0^{+}$ and a ${\pi }^0$ where the $a_0^{+}$ decays to an $\eta$ meson and a ${\pi }^{+}$. The analysis was performed on a data sample consisting of $383\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric-energy $B$ Factory at SLAC. We find no significant signal and set an upper limit on the product branching fraction $\mathcal{B}(B^{+}\to a_0^{+}{\pi }^0)\times \mathcal{B}(a_0^{+}\to \eta {\pi }^{+})$ of $1.4\times {10}^{-6}$ at the 90% confidence level.

In an external constant magnetic field, so strong that the electron Larmour length is much shorter than its Compton length, we consider the modification of the Coulomb potential of a point charge owing to the vacuum polarization. We establish a short-range component of the static interaction in the Larmour scale, expressed as a Yukawa-like law, and reveal the corresponding “photon mass” parameter. The electrostatic force regains its long-range character in the Compton scale: the tail of the potential follows an anisotropic Coulomb law, decreasing away from the charge slower along the magnetic field and faster across. In the infinite-magnetic-field limit the potential is confined to an infinitely thin string passing though the charge parallel to the external field. This is the first evidence for dimensional reduction in the photon sector of quantum electrodynamics. The one-dimensional form of the potential on the string is derived that includes a $\delta$ function centered in the charge. The nonrelativistic ground-state energy of a hydrogenlike atom is found with its use and shown not to be infinite in the infinite-field limit, contrary to what was commonly accepted before, when the vacuum polarization had been ignored. These results may be useful for studying properties of matter at the surface of extremely magnetized neutron stars.

We study the evolution of linear perturbations in metric $f(R)$ models of gravity and identify a potentially observable characteristic scale-dependent pattern in the behavior of cosmological structures. While at the background level viable $f(R)$ models must closely mimic Lambda Cold Dark Matter, the differences in their prediction for the growth of large scale structures can be sufficiently large to be seen with future weak lensing surveys. While working in the Jordan frame, we perform an analytical study of the growth of structures in the Einstein frame, demonstrating the equivalence of the dynamics in the two frames. We also provide a physical interpretation of the results in terms of the dynamics of an effective dark energy fluid with a nonzero shear. We find that the growth of structure in $f(R)$ is enhanced, but that there are no small scale instabilities associated with the additional attractive “fifth force.” We then briefly consider some recently proposed observational tests of modified gravity and their utility for detecting the $f(R)$ pattern of structure growth.

The thermodynamical properties of dark energy are usually investigated with the equation of state $\omega ={\omega }_0+{\omega }_{1}z$. Recent observations show that our Universe is accelerating, and the apparent horizon and the event horizon vary with redshift $z$. Because definitions of the temperature and entropy of a black hole are used to describe the two horizons of the Universe, we examine the thermodynamical properties of the Universe, which is enveloped by the apparent horizon and the event horizon, respectively. We show that the first and the second laws of thermodynamics inside the apparent horizon in any redshift are satisfied, while they are broken down inside the event horizon in some redshifts. Therefore, the apparent horizon for the Universe may be the boundary of thermodynamical equilibrium for the Universe like the event horizon for a black hole.

Axigluons are colored heavy neutral gauge bosons that couple to quarks through an axial vector current and the same strong coupling as gluons. The most important model-independent manifestation of axigluons is the generation of a forward–backward asymmetry in top-antitop quark production at $p\overline{p}$ collisions, which originates from the charge asymmetry. We update our previous analysis for the inclusive QCD induced forward–backward asymmetry and define a new observable that is more sensitive to the effect than the forward–backward asymmetry. Furthermore, we find a lower limit of 1.2 TeV at 90% C.L. on the axigluon mass from recent measurements of the asymmetry at Tevatron. Also at LHC, the charge asymmetry is sizable in suitably selected samples. We evaluate this asymmetry in the central region for different selection cuts and show that, like at Tevatron, the charge asymmetry can probe larger values of the axigluon mass than the dijet mass distribution.

We suggest searching for excited charmonium states in $e^{+}{e}^{-}$ annihilation via double charmonium production at $\sqrt{s}=10.6\mathrm{GeV}$ with $B$ factories, based on a more complete leading order calculation including both QCD and QED contributions for various processes. In particular, for the $C=+$ states, the ${\chi }_{c0}(nP)(n=2,3)$ and ${\eta }_c(mS)(m=3,4)$ may have appreciable potentials to be observed, while for the $C=-$ states, the ${\eta }_c{h}_c$ production and especially the ${\chi }_{c1}{h}_c$ production might provide opportunities for observing the $h_c$ with higher statistics in the future. A brief discussion for the X(3940) observed in the double charmonium production is included.

Exact vacuum solutions with a nontrivial torsion for the Einstein-Gauss-Bonnet theory in five dimensions are constructed. We consider a class of static metrics whose spacelike section is a warped product of the real line with a nontrivial base manifold endowed with a fully antisymmetric torsion. It is shown that requiring solutions of this sort to exist, fixes the Gauss-Bonnet coupling such that the Lagrangian can be written as a Chern-Simons form. The metric describes black holes with an arbitrary, but fixed, base manifold. It is shown that requiring its ground state to possess unbroken supersymmetries fixes the base manifold to be locally a parallelized three-sphere. The ground state turns out to be half-BPS, which could not be achieved in the absence of torsion in vacuum. The Killing spinors are explicitly found.

In the gravitational Higgs mechanism graviton components acquire mass in the process of spontaneous breaking of diffeomorphisms (via scalar vacuum expectation values). Recently, the gravitational Higgs mechanism was discussed in the context of obtaining massive gravity directly in four dimensions. We discuss a setup, with all diffeomorphisms spontaneously broken by scalars, where the gravitational Higgs mechanism results in massive gravity in Minkowski space with no nonunitary propagating modes. This is achieved by including higher derivative terms in the scalar sector. Decoupling the nonunitary mode (trace of the graviton) then requires tuning a single parameter, namely, the cosmological constant. We also discuss a setup where only spatial diffeomorphisms are broken. In this case we obtain massive gravity in Minkowski space with only transverse modes propagating, so the mode count is the same as for a massless graviton.

We calculate the magnetic field generated during bubble collisions in a first-order electroweak phase transition that may occur for some choices of parameters in the minimal supersymmetric standard model. We derive equations of motion from the electroweak Lagrangian that couple the Higgs field and the gauge fields of the standard model sector. We show that for sufficiently gentle collisions, where the Higgs field is relatively unperturbed in the bubble overlap region, the equations of motion can be linearized so that in the absence of fermions the charged $W^{\pm }$ fields are the source of the electromagnetic current for generating the seed fields. Solutions of the equations of motion for the charged gauge fields and Maxwell’s equations for the magnetic field in $O(1,2)$ space-time symmetry are expressed in closed form by applying boundary conditions at the time of collision. Our results indicate that the magnetic fields generated by charged $W^{\pm }$ fields in the collision are comparable to those found in previous work. The magnetic fields so produced could seed galactic and extra-galactic magnetic fields observed today.

We study the azimuthal angle decorrelation of forward jets in deep inelastic scattering. We make predictions for this observable at HERA describing the high energy limit of the relevant scattering amplitudes with quasi-multi-Regge kinematics together with a collinearly-improved evolution kernel for multiparton emissions.

We study Berry’s phase in the D0-D4-brane system. When a D0-brane moves in the background of D4-branes, the first excited states undergo a holonomy described by a non-Abelian Berry connection. At weak coupling this is an $SU(2)$ connection over ${\mathbf{R}}^5$, known as the Yang monopole. At strong coupling, the holonomy is recast as the classical gravitational precession of a spinning particle. The Berry connection is the spin connection of the near-horizon limit of the D4-branes, which is a continuous deformation of the Yang and anti-Yang monopole.

We revisit the calculation of gravitational radiation through the use of Weyl scalars. We point out several possible problems arising from gauge and tetrad ambiguities and ways to address them. Our analysis indicates how relatively simple corrections can be introduced to remove these ambiguities.

We explore the classical stability of topological black holes in $d$-dimensional anti-de Sitter spacetime, where the horizon is an Einstein manifold of negative curvature. According to the gauge invariant formalism of Ishibashi and Kodama, gravitational perturbations are classified as being of scalar, vector, or tensor type, depending on their transformation properties with respect to the horizon manifold. For the massless black hole, we show that the perturbation equations for all modes can be reduced to a simple scalar field equation. This equation is exactly solvable in terms of hypergeometric functions, thus allowing an exact analytic determination of potential gravitational instabilities. We establish a necessary and sufficient condition for stability, in terms of the eigenvalues $\lambda$ of the Lichnerowicz operator on the horizon manifold, namely $\lambda \ge -4(d-2)$. For the case of negative mass black holes, we show that a sufficient condition for stability is given by $\lambda \ge -2(d-3)$.

The detailed formulation for loop quantum cosmology (LQC) in the Bianchi I model with a scalar massless field has been constructed. In this paper, its effective dynamics is studied in two improved strategies for implementing the LQC discreteness corrections. Both schemes show that the big bang is replaced by the big bounces, which take place up to 3 times, once in each diagonal direction, when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. These two strategies give different evolutions: In one scheme, the effective dynamics is independent of the choice of the finite sized cell prescribed to make Hamiltonian finite; in the other, the effective dynamics reacts to the macroscopic scales introduced by the boundary conditions. Both schemes reveal interesting symmetries of scaling, which are reminiscent of the relational interpretation of quantum mechanics and also suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale.

Preheating after inflation involves large, time-dependent field inhomogeneities, which act as a classical source of gravitational radiation. The resulting spectrum might be probed by direct detection experiments if inflation occurs at a low enough energy scale. In this paper, we develop a theory and algorithm to calculate, analytically and numerically, the spectrum of energy density in gravitational waves produced from an inhomogeneous background of stochastic scalar fields in an expanding universe. We derive some generic analytical results for the emission of gravity waves by stochastic media of random fields, which can test the validity/accuracy of numerical calculations. We contrast our method with other numerical methods in the literature, and then we apply it to preheating after chaotic inflation. In this case, we are able to check analytically our numerical results, which differ significantly from previous works. We discuss how the gravity-wave spectrum builds up with time and find that the amplitude and the frequency of its peak depend in a relatively simple way on the characteristic spatial scale amplified during preheating. We then estimate the peak frequency and amplitude of the spectrum produced in two models of preheating after hybrid inflation, which for some parameters may be relevant for gravity-wave interferometric experiments.

We show that the use of higher dimensional wrapped branes can significantly extend the inflaton field range compared to brane inflation models which use $D3$-branes. We construct a simple inflationary model in terms of 5-branes wrapping a 2-cycle and traveling towards the tip of the Klebanov-Strassler throat. Inflation ends when the branes reach the tip of the cone and self-annihilate. Assuming a quadratic potential for the brane it is possible to match the cosmic microwave background data in the Dirac-Born-Infeld regime, but we argue that the backreaction of the brane is important and cannot be neglected. This scenario predicts a strong non-Gaussian signal and possibly detectable gravitational waves.

We show that the existence of cosmic strings can be strongly constrained by the next generation of gravitational lensing surveys at radio frequencies. We focus on cosmic string loops, which simulations suggest would be far more numerous than long (horizon-sized) strings. Using simple models of the loop population and minimal assumptions about the lensing cross section per loop, we estimate the optical depth to lensing and show that extant radio surveys such as CLASS have already ruled out a portion of the cosmic string model parameter space. Future radio interferometers, such as LOFAR and especially SKA, may constrain $G\mu /c^2<{10}^{-9}$ in some regions of parameter space, outperforming current constraints from pulsar timing and the cosmic microwave backgound by up to two orders of magnitude. This method relies on direct detections of cosmic strings, and so is less sensitive to the theoretical uncertainties in string network evolution that weaken other constraints.