We study the combined constraints on the compactification scale $1/R$ and the Higgs boson mass $m_H$ in the standard model with one or two universal extra dimensions. Focusing on precision measurements and employing the Peskin-Takeuchi S and T parameters, we analyze the allowed region in the ${(m}_H,1/R)$ parameter space consistent with current experiments. For this purpose, we calculate complete one-loop KK mode contributions to S, T, and U, and also estimate the contributions from physics above the cutoff of the higher-dimensional standard model. A compactification scale $1/R$ as low as 250 GeV and significantly extended regions of $m_H$ are found to be consistent with current precision data.

We discuss the Higgs boson mass sum rules in the minimal supersymmetric standard model (MSSM) in order to estimate the upper limits on the masses of top squarks as well as the lower bounds on the masses of the scalar Higgs boson state. The bounds on the scale of quark-lepton compositeness derived from the CDF Collaboration (Fermilab Tevatron) data and applied to the new extra gauge boson search are taken into account. These extra gauge bosons are considered in the framework of the extended $\mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_h\times \mathrm{SU}{\mathrm{}\left(2\right)\mathrm{}}_l$ model. In addition, we discuss the physics of rare decays of MSSM Higgs bosons in both CP-even and CP-odd sectors and also some extra gauge bosons.

The chiral properties of lattice fermions can be improved by altering either their fermion-gauge coupling or the pure gauge part of the action (or both). Using both perturbation theory and nonperturbative simulation, we compare a simple alteration of the gauge action [which encompasses the Wilson, Symanzik, Iwasaki, and doubly blocked from Wilson action in two coupling space (DWB2) actions], and hypercubic-blocked links in the fermion action. Perturbative tests include calculations of the potential, flavor-changing quark scattering amplitudes, and matching factors for currents. Nonperturbative tests include the potential, measurements of flavor symmetry breaking for staggered fermions, the behavior of topological objects, and properties of overlap actions. Our results display the bad properties of these actions as well as their good ones.

The H1 Collaboration recently reported a new analysis of data on the inelastic photoproduction of $J/\psi$ mesons at DESY’s HERA ep collider. We show that these new experimental results are well described by the color evaporation model for quarkonium production. Moreover, these new data require the introduction of resolved photon contributions in order to accommodate the results in the kinematic region where the fractional energy carried by the $J/\psi$ is small, demonstrating that colored perturbative $c\overline{c}$ states contribute to the production of a color singlet.

We study the role of a possible nonet of light scalar mesons in the still interesting $\overrightarrow{\eta }3\pi$ decay process, with the primary motivation of learning more about the scalars themselves. The framework is a conventional nonlinear chiral Lagrangian of pseudoscalars and vectors, extended to include the scalars. The parameters involving the scalars were previously obtained to fit the s-wave $\pi \pi$ and $\pi K$ scatterings in the region up to about 1 GeV as well as the strong decay ${\eta }^{\prime }\to \eta \pi \pi .$ At first, one might expect a large enhancement from diagrams including a light $\sigma \mathrm{}\left(560\right).$ However there is an amusing cancellation mechanism which prevents this from occurring. In the simplest model there is an enhancement of about 13 percent in the $\overrightarrow{\eta }3\pi$ decay rate due to the scalars. In a more complicated model which includes derivative type symmetry breakers, the cancellation is modified and the scalars contribute about 30 percent of the total decay rate (although the total is not significantly changed). The vectors do not contribute much. Our model produces a reasonable estimate for the related $a_0\mathrm{}\left(980\right)\mathrm{}-f_0\mathrm{}\left(980\right)\mathrm{}$ mixing strength, which has been a topic of current interest. Promising directions for future work along the present line are suggested.

We explore some important implications of the KamLAND measurment on the lepton flavor mixing matrix V and the neutrino mass matrix M. The model-independent constraints on nine matrix elements of V are obtained to a reasonable degree of accuracy. We find that nine two-zero textures of M are compatible with current experimental data, but two of them are only marginally allowed. Instructive predictions are given for the absolute neutrino masses, Majorana phases of $\mathrm{CP}$ violation, effective masses of the tritium beta decay, and neutrinoless double beta decay.

We discuss the phenomenology of neutrino mixing with bulk fermions in the context of supernova physics. The constraints on the parameter space following from the usual energy loss argument can be relaxed by four orders of magnitude due to a feedback mechanism that takes place in a broad region of the parameter space. Such a mechanism also affects the protoneutron star evolution through a nontrivial interplay with neutrino diffusion. The consistency with the SN 1987A signal is discussed, as well as the implications for the deleptonization, cooling, composition of the neutrino flux and the delayed explosion scenario.

We present data on multiple production of jets with transverse energies near 20 GeV in $p\overline{p}$ collisions at $\sqrt{s}=1.8\mathrm{}\mathrm{TeV}.$ QCD calculations in the parton-shower approximation of PYTHIA and HERWIG and the next-to-leading order approximation of JETRAD are compared to the data for one, two, three, and four jet inclusive production. Transverse energy spectra and multiple jet angular and summed transverse-energy distributions are adequately described by the shower approximation while next-to-leading order calculations describe the data poorly.

We study the properties of the energy-momentum tensor in noncommutative gauge theories by coupling them to a weak external gravitational field. In particular, we show that the stress tensor of such a theory coincides exactly with that derived from a theory where a Seiberg-Witten map has been implemented (namely, the procedure is commutative). Various other interesting features are also discussed.

We show two analytical solutions of the tachyon inflation for which the spectrum of curvature (density) perturbations can be calculated exactly to linear order, ignoring both gravity and the self-interactions of the tachyon field. The main feature of these solutions is that the spectral indices are independent of scale.

We calculate the one loop gluon vacuum energy in the background of a color magnetic vortex for SU(2) and SU(3). We use zeta functional regularization to obtain analytic expressions suitable for numerical treatment. The momentum integration is turned to the imaginary axis and fast converging sums or integrals are obtained. We investigate numerically a number of profiles of the background. All of these vortex profiles allow for a lowering of the total vortex energy below the energy of the perturbative vacuum. In this problem bound states (tachyonic modes) are present for all profiles investigated, making them intrinsically unstable.

It is shown that a present acceleration with a past deceleration is a possible solution to the Friedmann equation by considering the Universe as a mixture of a scalar with a matter field and by including a nonequilibrium pressure term in the energy-momentum tensor. The dark energy density decays more slowly with respect to the time than the matter energy density does. The inclusion of the nonequilibrium pressure leads to a less pronounced decay of the matter field with a shorter period of past deceleration.

We analyze the transmission of supersymmetry breaking in brane-world models of pseudosupersymmetry. In these models two branes preserve different halves of the bulk supersymmetry. Thus supersymmetry is broken although each sector of the model is supersymmetric when considered separately. The world-volume theory on one brane feels the breakdown of supersymmetry only through two-loop interactions involving a coupling to fields from the other brane. In a 5D toy model with bulk vectors, we compute the diagrams that contribute to scalar masses on one brane and find that the masses are proportional to the compactification scale up to logarithmic corrections, $m^2\propto \left(2\mathrm{}\pi {R)}^{-2}\right[\ln \left(2\mathrm{}\pi {\mathrm{Rm}}_S\right)-1.1],$ where $m_S$ is an ultraviolet cutoff. Thus, for large compactification radii, where this result is valid, the brane scalars acquire a positive mass squared. We also compute the three-loop diagrams relevant to the Casimir energy between the two branes and find $E\propto \left(2\mathrm{}\pi {R)}^{-4}\{\right[\ln \left(2\mathrm{}\pi {\mathrm{Rm}}_S\right)-1.7{]}^2+0.2\mathrm{}\}.$ For large radii, this yields a repulsive Casimir force.

A simple method to canonically quantize noncommutative field theories is proposed. As a result, the elementary excitations of a $\mathrm{}(2n+1)\mathrm{}$-dimensional scalar field theory are shown to be bilocal objects living in an $\mathrm{}(n+1\mathrm{})\mathrm{}$-dimensional space-time. Feynman rules for their scattering are derived canonically. They agree, upon suitable redefinitions, with the rules obtained via star-product methods. The IR-UV connection is interpreted within this framework.

We present four-dimensional gauge theories that describe physics on five-dimensional curved (warped) backgrounds, which includes bulk fields with various spins (vectors, spinors, and scalars). Field theory on the ${\mathrm{AdS}}_5$ geometry is examined as a simple example of our formulation. Various properties of bulk fields on this background, e.g., the mass spectrum and field localization behavior, can be achieved within a fully four-dimensional framework. Moreover, that gives a localization mechanism for massless vector fields. We also consider supersymmetric cases, and show in particular that the conditions on bulk masses imposed by supersymmetry on warped backgrounds are derived from a four-dimensional supersymmetric theory on the flat background. As a phenomenological application, models are shown to generate hierarchical Yukawa couplings. Finally, we discuss possible underlying mechanisms which dynamically realize the required couplings to generate curved geometries.

The structure of the commutator algebra for conformal quantum mechanics is considered. Specifically, it is shown that the emergence of a dimensional scale by renormalization implies the existence of an anomaly or quantum-mechanical symmetry breaking, which is explicitly displayed at the level of the generators of the SO(2,1) conformal group. Correspondingly, the associated breakdown of the conservation of the dilation and special conformal charges is derived.

We consider the matrix model associated with a pp-wave background and construct supersymmetric branes. In addition to the spherical membrane preserving 16 supersymmetries, one may construct rotating elliptic membranes preserving 8 supersymmetries. The other branch describes rotating 1/8 BPS hyperbolic branes in general. When the angular momentum vanishes, the hyperbolic brane becomes 1/4 Bogomol’nyi-Prasad-Sommerfield (BPS) preserving 8 real supersymmetries. It may have the shape of a hyperboloid of one or two sheets embedded in the flat three space. We study the spectrum of the worldvolume fields on the hyperbolic branes and show that there are no massless degrees. We also compute the spectrum of the 0-2 strings.

The Einstein initial-value equations in the extrinsic curvature (Hamiltonian) representation and conformal thin sandwich (Lagrangian) representation are brought into complete conformity by the use of a decomposition of symmetric tensors which involves a weight function. In stationary spacetimes, there is a natural choice of the weight function such that the transverse traceless part of the extrinsic curvature (or canonical momentum) vanishes.

Analytical approximations for $〈{\phi }^2〉$ and $〈T_{\nu }^{\mu }〉$ of a quantized scalar field in static spherically symmetric spacetimes are obtained. The field is assumed to be both massive and massless, with an arbitrary coupling $\xi$ to the scalar curvature, and in a zero temperature vacuum state. The expressions for $〈{\phi }^2〉$ and $〈T_{\nu }^{\mu }〉$ are divided into low- and high-frequency parts. The contributions of the high-frequency modes to these quantities are calculated for an arbitrary quantum state. As an example, the low-frequency contributions to $〈{\phi }^2〉$ and $〈T_{\nu }^{\mu }〉$ are calculated in asymptotically flat spacetimes in a quantum state corresponding to the Minkowski vacuum (Boulware quantum state). The limits of the applicability of these approximations are discussed.

We argue that the cosmological constant problem can be solved in a braneworld model with infinite-volume extra dimensions, avoiding no-go arguments applicable to theories that are four dimensional in the infrared. Gravity on the brane becomes higher dimensional at super-Hubble distances, which entails that the relation between the acceleration rate and vacuum energy density flips upside down compared to the conventional one. The acceleration rate decreases with increasing energy density. The experimentally acceptable rate is obtained for an energy density larger than $(1\mathrm{}\mathrm{TeV}{)}^4.$ The results are stable under quantum corrections because supersymmetry is broken only on the brane and stays exact in the bulk of infinite-volume extra space. The consistency of 4D gravity and cosmology on the brane requires the quantum gravity scale to be around ${10}^{-3}\mathrm{eV}.$ Testable predictions emerging within this approach are (i) simultaneous modifications of gravity at submillimeter and the Hubble scales and (ii) Hagedorn-type saturation in TeV energy collisions due to the Regge spectrum with a spacing equal to ${10}^{-3}\mathrm{eV}.$