We explore some of the consequences of dark-matter–photon interactions on structure formation, focusing on the evolution of cosmological perturbations and performing both an analytical and a numerical study. We compute the cosmic microwave background anisotropies and matter power spectrum in this class of models. We find, as the main result, that when dark matter and photons are coupled, dark matter perturbations can experience a new damping regime in addition to the usual collisional Silk damping effect. Such dark matter particles (having quite large photon interactions) behave like cold dark matter or warm dark matter as far as the cosmic microwave background anisotropies or matter power spectrum are concerned, respectively. These dark-matter–photon interactions leave specific imprints at sufficiently small scales on both of these two spectra, which may allow us to put new constraints on the acceptable photon–dark-matter interactions. Under the conservative assumption that the abundance of ${10}^{12}{M}_{\odot }$ galaxies is correctly given by the cold dark matter, and without any knowledge of the abundance of smaller objects, we obtain the limit on the ratio of the dark-matter–photon cross section to the dark matter mass ${\sigma }_{\gamma -\mathrm{DM}}{/m}_{\mathrm{DM}}\lesssim {10}^{-6}{\sigma }_{\mathrm{Th}}/\left(100\mathrm{}\hspace{0.5em}\mathrm{GeV}\right)\simeq 6\times {10}^{-33}\hspace{0.5em}{\mathrm{cm}}^2\hspace{0.5em}{\mathrm{GeV}}^{-1}.$

In the light cone frame, where the supermembrane theory and the matrix model are strikingly similar, the equations of motion admit an elegant complexification in even dimensional spaces. Although the explicit rotational symmetry of the target space is lost, the remaining unitary symmetries, apart from providing a simple and unifying description of all known solutions, suggest new ones for rotating spherical and toroidal membranes. In this framework the angular momentum is represented by U(1) charges which balance the nonlinear attractive forces of the membrane. We examine in detail a six-dimensional rotating toroidal membrane solution which lives in a three-torus $T^3$ and admits stable radial modes. In matrix theory it corresponds to a toroidal $\left(N-\mathrm{D}0\right)$-brane bound state. We demonstrate its existence and discuss its radial stability.

Given the path of a point particle, one can relate its acceleration and, in general, its kinematics to the curvature scalars of its trajectory. Using this, a general ansatz is made for the Yang-Mills connection corresponding to a non-Abelian point source. The Yang-Mills field equations are then solved outside the position of the point source under physically reasonable constraints such as finite total energy flux and finite total color charge. The solutions contain the Trautman solution; moreover two of them are exact whereas one of them is found using a series expansion in $1/R,$ where R is the retarded distance. These solutions are new and, in their most general form, are not gauge equivalent to the original Trautman solution.

The zero modes of the Dirac operator in the background of center vortex gauge field configurations in $R^2$ and $R^4$ are examined. If the net flux in $D=2\mathrm{}$ is larger than $1$ we obtain normalizable zero modes which are mainly localized at the vortices. In $D=4\mathrm{}$ quasinormalizable zero modes exist for intersecting flat vortex sheets with the Pontryagin index equal to $2.$ These zero modes are mainly localized at the vortex intersection points, which carry a topological charge of $\pm \frac{1}{2}.$ To circumvent the problem of normalizability the space-time manifold is chosen to be the (compact) torus $T^2$ and $T^4,$ respectively. According to the index theorem there are normalizable zero modes on $T^2$ if the net flux is nonzero. These zero modes are localized at the vortices. On $T^4$ zero modes exist for a nonvanishing Pontryagin index. As in $R^4$ these zero modes are localized at the vortex intersection points.

It is well known that the charged scalar perturbations of the Reissner-Nordström metric will decay slower at very late times than the neutral ones, thereby dominating in the late time signal. We show that, at the stage of quasinormal (QN) ringing, on the contrary, the neutral perturbations will decay slower for RN, RNAdS and dilaton black holes. The QN frequencies of the nearly extreme RN black hole have the same imaginary parts (damping times) for charged and neutral perturbations. An explanation of this fact is not clear but, possibly, is connected with the Choptuik scaling.

We investigate the occurrence and nature of naked singularities in the gravitational collapse of a self-similar adiabatic perfect fluid in a five dimensional space-time. The naked singularities are found to be gravitationally strong in the sense of Tipler and thus violate the cosmic censorship conjecture.

The gravitational field of a cylindrical Nambu-Goto wall in vacuum spacetime is considered in order to clarify the interaction between Nambu-Goto membranes and gravitational waves. If one neglects the emission of gravitational waves by the wall motion, the spacetime becomes singular. It is also shown that the emission of gravitational waves does occur by the motion of the cylindrical wall if the initial data are singularity free. The energy loss rate due to the radiation of the gravitational waves agrees with that estimated from the test wall motion and the quadrupole formula for the gravitational wave emission. This is quite different from the oscillatory behavior of gravitating Nambu-Goto membranes: the presence of gravity induces the wall to lose its dynamical degree of freedom.

A formula for the initial mass spectrum of primordial black holes (PBHs), which can be used for the general case of a scale dependent spectral index, and for a wide class of models of gravitational collapse, is derived. The derivation is based on the Press-Schechter formalism. A comparative analysis of different types of initial mass spectra used in concrete calculations is carried out. It is shown that densities of background radiation $(\nu ,$ $\gamma )$ from PBH evaporation depend rather strongly on the type of gravitational collapse and on taking into account the spread of horizon masses at which PBHs can form. Constraints on the parameters of the primordial density perturbation amplitudes based on PBH evaporation processes and on atmospheric and solar neutrino data are obtained.

We study the quantum vacuum fluctuations around closed Friedmann-Robertson-Walker (FRW) radiation-filled universes with a nonvanishing cosmological constant. These vacuum fluctuations are represented by a conformally coupled massive scalar field and are treated in the lowest order of perturbation theory. In the semiclassical approximation, the perturbations are governed by differential equations which, properly linearized, become generalized Lamé equations. The wave function thus obtained must satisfy appropriate regularity conditions which ensure its finiteness for every field configuration. We apply these results to asymptotically anti–de Sitter Euclidean wormhole spacetimes and show that there is no catastrophic particle creation in the Euclidean region, which would lead to divergences of the wave function.

At low energy, the four-dimensional effective action of the ekpyrotic model of the universe is equivalent to a slightly modified version of the pre-big bang model. We discuss cosmological perturbations in these models. In particular we address the issue of matching the perturbations from a collapsing to an expanding phase. We show that, under certain physically motivated and quite generic assumptions on the high energy corrections, one obtains $n=0\mathrm{}$ for the spectrum of scalar perturbations in the original pre-big bang model (with a vanishing potential). With the same assumptions, when an exponential potential for the dilaton is included, a scale invariant spectrum $\mathrm{}(n=1\mathrm{})\mathrm{}$ of adiabatic scalar perturbations is produced under very generic matching conditions, both in a modified pre-big bang and ekpyrotic scenario. We also derive the resulting spectrum for arbitrary power law scale factors matched to a radiation-dominated era.

WIMP direct detection experiments are just reaching the sensitivity required to detect galactic dark matter in the form of neutralinos. Data from these experiments are usually analyzed under the simplifying assumption that the Milky Way halo is an isothermal sphere with Maxwellian velocity distribution. Observations and numerical simulations indicate that galaxy halos are in fact triaxial and anisotropic. Furthermore, in the cold dark matter paradigm galactic halos form via the merger of smaller subhalos, and at least some residual substructure survives. We examine the effect of halo modeling on WIMP exclusion limits, taking into account the detector response. Triaxial and anisotropic halo models, with parameters motivated by observations and numerical simulations, lead to significant changes which are different for different experiments, while if the local WIMP distribution is dominated by small scale clumps then the exclusion limits are changed dramatically.

Using $13.3{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ data recorded with the CLEO II and CLEO II.V detector configurations at CESR, we have searched for $f_J\mathrm{}\left(2220\right)\mathrm{}$ decays to $K_S^0{K}_S^0$ in untagged two-photon interactions. We report an upper limit on the product of the two-photon partial width and the branching fraction, ${\Gamma }_{\gamma \gamma }\mathcal{B}\left(f_J\mathrm{}\right(2220)\mathrm{}\to K_S^0{K}_S^0),$ of less than 1.1 eV at the 95% confidence level; systematic uncertainties are included. This data set is four times larger than that used in the previous CLEO publication.

We systematically study proton decay in the minimal supersymmetric SU(5) grand unified theory. We find that although the available parameter space of soft masses and mixings is quite constrained, the theory is still in accord with experiment.

A fully realistic unified theory is constructed, with $\mathrm{SU}\left(5\right)\mathrm{}$ gauge symmetry and supersymmetry both broken by boundary conditions in a fifth dimension. Despite the resulting explicit breaking of $\mathrm{SU}\left(5\right)\mathrm{}$ locally at a boundary of the dimension, when the size of the extra dimension is taken to be large precise predictions emerge for gauge coupling unification, ${\alpha }_s{(M}_Z)=0.118\mathrm{}\pm 0.003,$ and for Yukawa coupling unification, $m_b{(M}_Z)=3.3\mathrm{}\pm 0.2\mathrm{GeV}.$ The 5D theory is then valid over a large energy interval from the compactification scale, $M_c\simeq 1\times {10}^{15}\mathrm{GeV},$ to the scale of strong coupling, $M_s\simeq 1\times {10}^{17}\mathrm{GeV}.$ A complete understanding of the Higgs sector of the minimal supersymmetric standard model is given, with explanations for why the Higgs triplets are heavy, why the Higgs doublets are protected from a large tree-level mass, and why the $\mu$ and B parameters are naturally generated to be of order the supersymmetry breaking scale. All sources of proton decay from operators of dimension four and five are forbidden, while a new origin for baryon number violating dimension six operators is found to be important. The exchange of the superheavy gauge boson, with a brane-localized kinetic energy interaction, leads to ${\tau }_p\approx {10}^{34}\mathrm{yr},$ with several branching ratios determined in terms of a single mixing parameter. The theory is only realistic for an essentially unique choice of matter location in the fifth dimension: the ten-plets of the first two generations must lie in the bulk, with all other matter located on the $\mathrm{SU}\left(5\right)\mathrm{}$ preserving boundary. Several aspects of flavor follow from this geometry: only the third generation possesses an $\mathrm{SU}\left(5\right)\mathrm{}$ mass relation, and the lighter two generations have only small mixings with the heaviest generation except for neutrinos. The entire superpartner spectrum is predicted in terms of only two free parameters. The squark and slepton masses have sizes determined by their location in the fifth dimension, allowing a significant experimental test of the detailed structure of the extra dimension. Lepton flavor violation is found to be generically large in higher dimensional unified theories with nontrivial matter geometries, providing soft supersymmetry breaking operators are local up to the compactification scale. In our theory this forces a common location for all three neutrinos, predicting large neutrino mixing angles. Rates for $\overrightarrow{\mu }e\gamma ,$ $\overrightarrow{\mu }eee,$ $\overrightarrow{\mu }e$ conversion and $\overrightarrow{\tau }\mu \gamma$ are larger in our theory than in conventional 4D supersymmetric grand unified theories, and, once superpartner masses are measured, these rates are completely determined in terms of two leptonic mixing angles. Proposed experiments probing $\overrightarrow{\mu }e$ transitions will probe the entire interesting parameter space of our theory.

We consider the supersymmetric seesaw mechanism induced by the exchange of heavy ${\mathrm{SU}\left(2\right)\mathrm{}}_W$ triplet states, rather than “right-handed” neutrino singlets, to generate neutrino masses. We show that in this scenario the neutrino flavor structure tested at low-energy in the atmospheric and solar neutrino experiments is directly inherited from the neutrino Yukawa couplings to the triplets. This allows us to predict the ratio of the $\overrightarrow{\tau }\mu \gamma$ (or $\overrightarrow{\tau }e\gamma )$ and $\overrightarrow{\mu }e\gamma$ decay rates in terms of the low-energy neutrino parameters. Moreover, once the model is embedded in a grand unified model, quark-flavor violation can be linked to lepton-flavor violation.

In the decoupling limit $M_{A^0}^2\gg M_Z^2,$ the heavy $\mathrm{CP}$-even, $\mathrm{CP}$-odd, and charged Higgs boson masses are nearly degenerate, $\sin \left(\beta -\alpha \right)$ approaches $1,$ and the lightest $\mathrm{CP}$-even Higgs boson almost displays the same properties as the standard model Higgs boson. But the top and bottom squark sectors can change this pattern through radiative corrections. We find that there are parameter regions at small and moderate $\tan \beta$ in the minimal supersymmetry standard model under experimental constraints of $(g-{2)}_{\mu },$ $\overrightarrow{b}s\gamma ,$ and lower bounds of Higgs boson and sparticle masses, where ${\sin }^2\left(\beta -\alpha \right)$ is damped (say below 0.8), which has a significant effect on Higgs couplings $g_{h^0\mathrm{VV}}{(V=W}^{\pm }{,Z}^0)$ and $g_{h^0\gamma \gamma }.$ We discuss its impact on the lightest $\mathrm{CP}$-even Higgs boson production at $\gamma \gamma$ colliders.

We analyze the recent NA48 data for the reaction $K_L\to {\pi }^0\gamma \gamma$ with and without the assumption of vector meson dominance (VMD). We find that the data are well described by a three-parameter expression inspired by $\mathcal{O}{(p}^6)$ chiral perturbation theory. We also find that it is impossible to fit the shape of the decay distribution and the overall rate simultaneously if one imposes the VMD constraints on the three parameters. We comment on the different fits and their implications for the $\mathrm{CP}$-conserving component of the decay $K_L\to {\pi }^0{e}^{+}{e}^{-}.$

We introduce a Wilson line distribution function $W_{\tau }\mathrm{}\left(v\right)\mathrm{}$ to study gluon saturation at small Feynman $x_F,$ or large $\tau =\ln {(1/x}_F).\mathrm{}$ This new distribution can be obtained from the distribution $W_{\tau }\left(\alpha \right)$ of the color glass condensate model and the Jalilian-Marian–Iancu–McLerran–Weigert–Leonidov–Kovner (JIMWLK) renormalization group equation. $W_{\tau }\mathrm{}\left(v\right)\mathrm{}$ is physically more relevant, and mathematically simpler to deal with because of unitarity of the Wilson line $v.\mathrm{}$ A JIMWLK equation is derived for $W_{\tau }\mathrm{}\left(v\right);$ its properties are studied. These properties are used to complete Mueller’s derivation of the JIMWLK equation, though for $W_{\tau }\mathrm{}\left(v\right)\mathrm{}$ and not $W_{\tau }\left(\alpha \right).\mathrm{}$ They are used to derive a generalized Balitsky-Kovchegov equation for higher multipole amplitudes. They are also used to compute the unintegrated gluon distribution at $x_F=0,$ yielding a completely flat spectrum in transverse momentum squared ${\mathbf{k}}^2,$ with a known height. This is similar but not identical to the mean field result at small ${\mathbf{k}}^2.$

We analyze an extension of the minimal supersymmetric standard model including the dominant trilinear and bilinear R-parity violating contributions. We take the trilinear terms from the superpotential and the bilinear terms from the superpotential as well as the scalar potential. We compute the neutrino masses induced by those couplings and determine the allowed ranges of the R-parity violating parameters that are consistent with the latest SNO results, atmospheric data and the CHOOZ constraint. We also estimate the effective mass for neutrinoless double beta decay in such scenarios.