The weak form factor for $B\to K_{1B}$ where $K_{1B}$ is the ${}^{1}P_1$ state is calculated in the light-cone sum rules (LCSR). Combining the quark model result for the form factor of $B\to K_{1A}$ with $K_{1A}$ being the ${}^{3}P_1$ state, we have larger values for $B\to K_1$ form factors than the previous LCSR results. The increased form factors reduce the discrepancy between theory and the experimental data for $B\to K_1\gamma$. Some phenomenological meanings are also discussed.

We investigate how to exploit the spin information imparted to the $Z$ boson in associated Higgs production at a future linear collider as an aid in distinguishing between $CP$-even and $CP$-odd Higgs bosons. We apply a generalized spin-basis analysis which allows us to study the possibilities offered by nontraditional choices of spin projection axis. In particular, we find that the $Z$ bosons produced in association with a $CP$-even Higgs via polarized collisions are in a single transverse spin state ($>90\%$ purity) when we use the $Zh$-transverse basis, provided that the $Z$ bosons are not ultrarelativistic (speed $<0.9c$). This same basis applied to the associated production of a $CP$-odd Higgs yields $Z$’s that are an approximately equal mixture of longitudinal and transverse polarizations. We present a decay angular distribution which could be used to distinguish between the $CP$-even and $CP$-odd cases. Finally, we make a few brief remarks about how this distribution would be affected if the Higgs boson turns out not to be a $CP$ eigenstate.

The staggered-fermion determinant is complex when the quark chemical potential $\mu$ is nonzero. Its fourth root, used in simulations with dynamical fermions, will have phase ambiguities that become acute when $\mathrm{Re}\mu$ is sufficiently large. We show how to resolve these ambiguities, but our prescription only works very close to the continuum limit. We argue that this regime is far from current capabilities. Other procedures require being even closer to the continuum limit, or fail altogether, because of unphysical discontinuities in the measure. At zero temperature the breakdown is expected when $\mathrm{Re}\mu \gtrsim m_{\pi }/2$, where $m_{\pi }$ is the pion mass. Estimates of the location of the breakdown at nonzero temperature are less certain.

In a study of $B^{+}\to J/\psi \gamma K^{+}$ decays, we find evidence for the radiative decay $X(3872)\to J/\psi \gamma$ with a statistical significance of $3.4\sigma$. We measure the product of branching fractions $\mathcal{B}(B^{+}\to X(3872)K^{+})·\mathcal{B}(X(3872)\to J/\psi \gamma )=(3.3\pm 1.0\pm 0.3)\times {10}^{-6}$, where the uncertainties are statistical and systematic, respectively. We also measure the branching fraction $\mathcal{B}(B^{+}\to {\chi }_{c1}{K}^{+})=(4.9\pm 0.2\pm 0.4)\times {10}^{-4}$. These results are obtained from $(287\pm 3)$ million $B\overline{B}$ decays collected at the ${\rm Y}(4S)$ resonance with the BABAR detector at the PEP-II $B$ Factory at SLAC.

We generalize the $(2+1)$-dimensional Yang-Mills theory to an anisotropic form with two gauge coupling constants $e$ and $e^{\prime }$. In axial gauge, a regularized version of the Hamiltonian of this gauge theory is $H_0+e^{\prime 2}{H}_1$, where $H_0$ is the Hamiltonian of a set of $(1+1)$-dimensional principal chiral sigma nonlinear models and $H_1$ couples charge densities of these sigma models. We treat $H_1$ as the interaction Hamiltonian. For gauge group SU(2), we use form factors of the currents of the principal chiral sigma models to compute the string tension for small $e^{\prime }$, after reviewing exact S-matrix and form-factor methods. In the anisotropic regime, the dependence of the string tension on the coupling constants is not in accord with generally-accepted dimensional arguments.

An improved Hamiltonian constraint operator is introduced in loop quantum cosmology. Quantum dynamics of the spatially flat, isotropic model with a massless scalar field is then studied in detail using analytical and numerical methods. The scalar field continues to serve as “emergent time”, the big bang is again replaced by a quantum bounce, and quantum evolution remains deterministic across the deep Planck regime. However, while with the Hamiltonian constraint used so far in loop quantum cosmology the quantum bounce can occur even at low matter densities, with the new Hamiltonian constraint it occurs only at a Planck-scale density. Thus, the new quantum dynamics retains the attractive features of current evolutions in loop quantum cosmology but, at the same time, cures their main weakness.

Since the work of Mac-Dowell-Mansouri it is well known that gravity can be written as a gauge theory for the de Sitter group. In this paper we consider the coupling of this theory to the simplest gauge invariant observables that is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce exactly the dynamics of relativistic particles coupled to gravity, the gauge charges carried by Wilson lines being the mass and spin of the particles. Insertion of Wilson lines breaks in a controlled manner the diffeomorphism symmetry of the theory and the gauge degree of freedom are transmuted to particles degree of freedom.

Recently the class of purely magnetic nonrotating dust spacetimes has been shown to be empty [L. Wylleman, Classical Quantum Gravity 23, 2727 (2006).]. It turns out that purely magnetic rotating dust models are subject to severe integrability conditions as well. One of the consequences of the present paper is that also rotating dust cannot be purely magnetic when it is of Petrov type $D$ or when it has a vanishing spatial gradient of the energy density. For purely magnetic and nonrotating perfect fluids on the other hand, which have been fully classified earlier for Petrov type $D$ [C. Lozanovski, Classical Quantum Gravity 19, 6377 (2002).], the fluid is shown to be nonaccelerating if and only if the spatial density gradient vanishes. Under these conditions, a new and algebraically general solution is found, which is unique up to a constant rescaling, which is spatially homogeneous of Bianchi type ${\mathrm{VI}}_0$, has degenerate shear, and is of Petrov type $\mathrm{I}(M^{\infty })$ in the extended Arianrhod-McIntosh classification. The metric and the equation of state are explicitly constructed and properties of the model are briefly discussed. We finally situate it within the class of normal geodesic flows with degenerate shear tensor.

I investigate the possibility that the observed curvature perturbation is due to a massive vector field. To avoid generating a large scale anisotropy the vector field is not taken to be driving inflation. Instead it is assumed to become important after inflation when it may dominate the Universe and imprint its perturbation spectrum before its decay, as in the curvaton scenario. It is found that, to generate a scale invariant spectrum of perturbations, the mass-squared of the vector field has to be negative and comparable to the Hubble scale during inflation. After inflation the mass-squared must become positive so that the vector field engages into oscillations. It is shown that such an oscillating vector field behaves as pressureless matter and does not lead to large scale anisotropy when it dominates the Universe. The possibility of realizing this scenario in supergravity is also outlined.

We examine phantom dark energy models derived from a scalar field with a negative kinetic term for which $V(\varphi )\to \infty$ asymptotically. All such models can be divided into three classes, corresponding to an equation of state parameter $w_{\varphi }$ with asymptotic behavior $w_{\varphi }\to -1$, $w_{\varphi }\to w_0<-1$, and $w_{\varphi }\to -\infty$. We derive the conditions on the potential $V(\varphi )$ which lead to each of these three types of behavior. For models with $w_{\varphi }\to -1$, we derive the conditions on $V(\varphi )$ which determine whether or not such models produce a future big rip. Observational constraints are derived on two classes of these models: power-law potentials with $V(\varphi )=\lambda {\varphi }^{\alpha }$ (with $\alpha$ positive or negative) and exponential potentials of the form $V(\varphi )=\beta e^{\lambda {\varphi }^{\alpha }}$. It is shown that these models spend more time in a state with ${\Omega }_m\sim {\Omega }_{\varphi }$ than do corresponding models with a constant value of $w_{\varphi }$, thus providing a more satisfactory solution to the coincidence problem.

We obtain the leading order interaction between the graviton and the neutral scalar boson in the context of noncommutative field theory. Our approach makes use of the Ward identity associated with the invariance under a subgroup of symplectic diffeomorphisms.

The Lorentzian anti-de Sitter/conformal field theory correspondence implies a map between local operators in supergravity and nonlocal operators in the CFT. By explicit computation we construct CFT operators which are dual to local bulk fields in the semiclassical limit. The computation is done for general dimension in global, Poincaré and Rindler coordinates. We find that the CFT operators can be taken to have compact support in a region of the complexified boundary whose size is set by the bulk radial position. We show that at finite $N$ the number of independent commuting operators localized within a bulk volume saturates the holographic bound.

We calculate the entropy of 4-charge extremal black holes in Type IIA supersting theory by using Sen’s entropy function method. Using the low-energy effective actions in both 10D and 4D, we find precise agreements with the Bekenstein-Hawking entropy of the black hole. We also calculate the higher-order corrections to the entropy and find that they depend on the exact form of the higher-order corrections to the effective action.

We study cosmological self-reproduction in models of inflation driven by a scalar field $\varphi$ with a noncanonical kinetic term ($k$-inflation). We develop a general criterion for the existence of attractors and establish conditions selecting a class of $k$-inflation models that admit a unique attractor solution. We then consider quantum fluctuations on the attractor background. We show that the correlation length of the fluctuations is of order $c_s{H}^{-1}$, where $c_s$ is the speed of sound. By computing the magnitude of field fluctuations, we determine the coefficients of Fokker-Planck equations describing the probability distribution of the spatially averaged field $\varphi$. The field fluctuations are generally large in the inflationary attractor regime; hence, eternal self-reproduction is a generic feature of $k$-inflation. This is established more formally by demonstrating the existence of stationary solutions of the relevant Fokker-Planck equations. We also show that there exists a (model-dependent) range ${\varphi }_R<\varphi <{\varphi }_{\max }$ within which large fluctuations are likely to drive the field towards the upper boundary $\varphi ={\varphi }_{\max }$, where the semiclassical consideration breaks down. An exit from inflation into reheating without reaching ${\varphi }_{\max }$ will occur almost surely (with probability 1) only if the initial value of $\varphi$ is below ${\varphi }_R$. In this way, strong self-reproduction effects constrain models of $k$-inflation.

Using quenched lattice QCD simulations we investigate the continuum limit of baryon-strangeness correlation and other related conserved charge-flavor correlations for temperatures $T_c<T\le 2T_c$. By working with lattices having large temporal extents $(N_{\tau }=12,10,8,4)$ we find that these quantities are almost independent of the lattice spacing, i.e., robust. We also find that these quantities have very mild dependence on the sea quark mass and acquire values which are very close to their respective ideal gas limits. Our results also confirm robustness of the Wroblewski parameter.

In this paper causal geodesic completeness of Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological models is analyzed in terms of generalized power expansions of the scale factor in coordinate time. The strength of the found singularities is discussed following the usual definitions due to Tipler and Królak. It is shown that while classical cosmological models are both timelike and lightlike geodesically incomplete, certain observationally allowed models which have been proposed recently are lightlike geodesically complete.

The modes of vibration of hanging and partially supported strings provide useful analogies to scalar fields travelling through spacetimes that admit conformally flat spatial sections. This wide class of spacetimes include static, spherically symmetric spacetimes. The modes of a spacetime where the scale factor depends as a power law on one of the coordinates provide a useful starting point and yield a new classification of these spacetimes on the basis of the shape of the string analogue. The family of corresponding strings follow a family of curves related to the cycloid, denoted here as hypercycloids (for reasons that will become apparent). Like the spacetimes that they emulate, these strings exhibit horizons, typically at their bottommost points where the string tension vanishes; therefore, hanging strings may provide a new avenue for the exploration of the quantum mechanics of horizons.

We consider a modified gravity theory, $f(R)=R-a/R^n+b{R}^m$, in the metric formulation, which has been suggested to produce late-time acceleration in the Universe, while satisfying local fifth-force constraints. We investigate the parameter range for this theory, considering the regimes of early and late-time acceleration, big bang nucleosynthesis, and fifth-force constraints. We conclude that it is difficult to find a unique range of parameters for consistency of this theory.