A Pati–Salam model from branes

We explore the possibility of embedding the Pati–Salam model in the context of Type I brane models. We study a generic model with U(4)_C×U(2)_L×U(2)_R gauge symmetry and matter fields compatible with a Type I brane configuration. Examining the anomaly cancellation conditions of the surplus abelian symmetries we find an alternative hypercharge embedding that is compatible with a low string/brane scale of the order of 5–7 TeV, when the U(4)_C and U(2)_R brane stack couplings are equal. Proton stability is assured as baryon number is associated to a global symmetry remnant of the broken abelian factors. It is also shown that this scenario can accommodate an extra low energy abelian symmetry that can be associated to lepton number. The issue of fermion and especially neutrino masses is also discussed.     

Inflation in warped geometries

We argue that brane anti-brane inflation in string theory de-Sitter vacua of Kachru–Kallosh–Linde–Trivedi (KKLT) is captured by the dynamics of a D3-brane probe in the local KKLT model constructed in hep-th/0203041. This provides a framework to study in a controllable way corrections to the inflationary slow roll parameter η due to conformal symmetry breaking in a warped geometry throat. We compute the leading correction to η for the inflation in the Klebanov–Tseytlin throat geometry. We find that in certain regime this correction tends to decrease η. Computations in a different regime suggest however that it is unlikely that η1 can be achieved with the D3-brane throat inflation.     

Gauge unification in type I/I′ models

We discuss whether the (MSSM) unification of gauge couplings can be accommodated in string theories with a low (TeV) string scale. This requires either power law running of the couplings or logarithmic running extremely far above the string scale. In both cases it is difficult to arrange for the multiplet structure to give the MSSM result. For the case of power law running there is also enhanced sensitivity to the spectrum at the unification scale. For the case of logarithmic running there is a fine tuning problem associated with the light closed string Kaluza Klein spectrum which requires gauge mediated supersymmetry breaking on the “visible” brane with a dangerously low scale of supersymmetry breaking. Evading these problems in low string scale models requires a departure from the MSSM structure, which would imply that the success of gauge unification in the MSSM is just an accident.     

Strong CP problem and mirror world: the Weinberg–Wilczek axion revisited

A new possibility for solving the strong CP problem is suggested. It is based on the concept of a mirror world of particles, with the gauge symmetry and Lagrangian completely identical to that of the observable particles. We assume that the ordinary and mirror sectors share the same Peccei–Quinn symmetry realized à la Weinberg–Wilczek, so that the θ-terms are simultaneously canceled by the axion VEV in both worlds. This property remains valid even if the symmetry between two sectors is spontaneously broken and the weak scale of the mirror world is larger than the ordinary weak scale, in which case also the mirror QCD scale becomes larger than the ordinary one. In this situation our axion essentially represents a Weinberg–Wilczek axion of the mirror world with quite a large mass, while it couples the ordinary particles like an invisible axion. The experimental and astrophysical limits are discussed and an allowed parameter window is found with the Peccei–Quinn scale f_a10⁴–10⁵ GeV and the axion mass m_a1 MeV, which can be accessible for future experiments. We also show that our solution to the strong CP problem is stable against the Planck scale induced effects.     

Inflation and late-time acceleration in braneworld cosmological models with varying brane tension

Braneworld models with variable brane tension λ introduce a new degree of freedom that allows for evolving gravitational and cosmological constants, the latter being a natural candidate for dark energy. We consider a thermodynamic interpretation of the varying brane tension models, by showing that the field equations with variable λ can be interpreted as describing matter creation in a cosmological framework. The particle creation rate is determined by the variation rate of the brane tension, as well as by the brane–bulk energy-matter transfer rate. We investigate the effect of a variable brane tension on the cosmological evolution of the Universe, in the framework of a particular model in which the brane tension is an exponentially dependent function of the scale factor. The resulting cosmology shows the presence of an initial inflationary expansion, followed by a decelerating phase, and by a smooth transition towards a late accelerated de Sitter type expansion. The varying brane tension is also responsible for the generation of the matter in the Universe (reheating period). The physical constraints on the model parameters, resulting from the observational cosmological data, are also investigated.     

4D gravity on a BPS brane in 5D AdS-Minkowski space

We calculate small correction terms to gravitational potential near an asymmetric BPS brane embedded in a 5D AdS-Minkowski space in the context of supergravity. The normalizable wave functions of gravity fluctuations around the brane describe only massive modes. We compute such wave functions analytically in the thin wall limit. We estimate the correction to gravitational potential for small and long distances, and show that there is an intermediate range of distances in which we can identify 4D gravity on the brane below a crossover scale. The 4D gravity is metastable and for distances much larger than the crossover scale the 5D gravity is recovered.     

Effective Gravity in Randall–Sundrum Infinite Brane World

The gravity induced on the brane in the Randall–Sundrum (RS) infinite brane world is briefly reviewed. We also discuss the possibility of the absence of black hole configuration in this model based on the argument of the AdS/CFT correspondence.     

The incompressible Navier–Stokes equations from black hole membrane dynamics

We consider the dynamics of a d+1 space–time dimensional membrane defined by the event horizon of a black brane in (d+2)-dimensional asymptotically Anti-de Sitter space–time and show that it is described by the d-dimensional incompressible Navier–Stokes equations of non-relativistic fluids. The fluid velocity corresponds to the normal to the horizon while the rate of change in the fluid energy is equal to minus the rate of change in the horizon cross-sectional area. The analysis is performed in the Membrane Paradigm approach to black holes and it holds for a general non-singular null hypersurface, provided a large scale hydrodynamic limit exists. Thus we find, for instance, that the dynamics of the Rindler acceleration horizon is also described by the incompressible Navier–Stokes equations. The result resembles the relation between the Burgers and KPZ equations and we discuss its implications.     

Stabilizing the axion by discrete gauge symmetries

The axion solution to the strong CP problem makes use of a global Peccei–Quinn U(1) symmetry which is susceptible to violations from quantum gravitational effects. We show how discrete gauge symmetries can protect the axion from such violations. PQ symmetry emerges as an approximate global symmetry from discrete gauge symmetries. Simple models based on Z_N symmetries with N=11,12, etc., are presented realizing the DFSZ axion and the KSVZ axion. The discrete gauge anomalies are canceled by a discrete version of the Green–Schwarz mechanism. In the supersymmetric extension our models provide a natural link between the SUSY breaking scale, the axion scale, and the SUSY-preserving μ term.     

High resolution cw cars spectroscopy of the q-branch of the ν2 band in C2H2

The fully resolved spectrum of the Q-branch of the ν₂ band in acetylene has been obtained by cw CARS spectroscopy with a resolution of 40 MHz. The dispersion of χ⁽³⁾ and the linewidth of this Raman mode were investigated over the pressure range 0.05–5 atm. The constant of rotational-vibrational coupling was measured to be £ = (5.91 ± 0.05) × 10^-3 cm^-1.     

The Cosmological Slingshot Scenario in details

We generalize the Cosmological Slingshot Scenario for a Slingshot brane moving in a Klebanov–Strassler throat. We show that the horizon and isotropy problems of standard cosmology are avoided, while the flatness problem is acceptably alleviated. Regarding the primordial perturbations, we identify their vacuum state and elucidate the evolution from the quantum to the classical regimes. Also, we calculate their exact power spectrum showing its compatibility with current data. We discuss the bouncing solution from a four dimensional point of view. In this framework the radial and angular motion of the Slingshot brane are described by two scalar fields. We show that the bouncing solution for the scale factor in String frame is mapped into a monotonically increasing (in conformal time) solution in the Einstein frame. We finally discuss about the regularity of the geometry in Einstein frame.     

Sub-Doppler spectroscopy by sub-micron thin Cs vapour layer

A unique extremely thin cell (ETC) with the thickness of Cs atomic vapour layer in the range of 150–300 nm has been developed for the first time. All six hyperfine transitions of D₂ line (852 nm) of Cs, i.e. 6S_1/2(F=3)→6P_3/2(F=2–4) and 6S_1/2(F=4)→6P_3/2(F=3–5) are very well resolved in the transmission and fluorescence spectra, whereas in a cell of usual length they are not resolved due to Doppler broadening. It is demonstrated that a robust and widely used simple laser-diode technique is enough to observe separately atomic hyperfine transitions at the intensity up to 100 mW/cm² thanks to the ETC usage. We report also the first results obtained with the new ETC of 300 nm thickness filled with natural Rb. The usage of ETC allowed us to completely resolve each particular hyperfine transition of both ⁸⁵Rb and ⁸⁷Rb isotopes in D₁ line fluorescence spectrum.     

Brane Universes Tested by Supernovae Ia

We discuss observational constraints coming from supernovae Ia imposed on the behaviour of the Randall-Sundrum models. In the case of dust matter on the brane, the difference between the best-fit Perlmutter model with a -term and the best-fit brane models becomes detectable for redshifts z > 1.2. It is interesting that brane models predict brighter galaxies for such redshifts which is in agreement with the measurement of the z = 1.7 supernova. We also demonstrate that the fit to supernovae data can also be obtained, if we admit the super-negative dark energy (phantom matter) p = – (4/3) on the brane, where the dark energy in a way mimics the influence of the cosmological constant. It also appears that the dark energy enlarges the age of the universe which is demanded in cosmology. Finally, we propose to check for dark radiation and brane tension by the application of the angular diameter of galaxies minimum value test. We point out the existence of coincidence problem for the brane tension parameter.     

Reheating a multi-throat universe by brane motion

We propose a mechanism of reheating after inflation in multi-throat scenarios of warped extra dimensions. Validity of an effective field theory on the standard model (SM) brane requires that the position of the SM brane during inflation be different from the position after inflation. The latter is supposed to be near the tip of the SM throat but the former is not. After inflation, when the Hubble expansion rate becomes sufficiently low, the SM brane starts moving towards the tip and eventually oscillates. The SM fields are excited by the brane motion and the universe is reheated. Since interaction between the brane position modulus and the SM fields is suppressed only by the local string scale, the modulus effectively decays into the SM fields.     

Nonlinear realization of <Emphasis Type="Italic">N</Emphasis>=2 superconformal symmetry and brane effective actions

Since there is an incompatibility of simultaneously nonlinear breaking the superconformal symmetry and the dilatation symmetry with the dilaton taken as the compensator field, in the present paper is shown an alternative mechanism of spontaneous breaking the N=2 superconformal symmetry to the N=0 case. By using the approach of nonlinear transformations one finds that it leads to a space-filling brane theory with Weyl scale W(1,3) symmetry. The dynamics of the resulting Weyl scale invariant brane, along with that of other Nambu–Goldstone fields, is derived in terms of the building blocks of the vierbein and the covariant derivative from the Maurer–Cartan one-forms. A general coupling of the matter fields localized on the brane world volume to these NG fields is also constructed.     

Phase diagram of SrSi2 to 40 kbar and 1200°C

The pressure-temperature phase diagram of dimorphic SrSi₂ was investigated in the pressure range 10–40 kbar and temperature range 600–1200°C in a belt-type apparatus. The phase boundary between cubic SrSi₂I (SrSi₂-type) and tetragonal SrSi₂II (-ThSi₂-type) was studied by X-ray techniques in quenched samples. The curvature of the equilibrium line was not resolved. Molar volumes of both polymorphs have been measured in the temperature range 20–400°C at ambient pressure. From the slope of the equilibrium line in the pressure-temperature diagram and the change in molar volume, approximate values of the entropy and heat of transition SrSi₂(I–II) have been calculated.

In the silicon sublattice of both polymorphs of SrSi₂ the topologically simplest three-dimensional three-connected nets are found to be slightly distorted from their ideal configurations. The ideal SrSi₂-type net represents the three-connected analogy to the four-connected diamond net.     

A braneworld universe from colliding bubbles

Much work has been devoted to the phenomenology and cosmology of the so-called braneworld universe, where the (3+1)-dimensional universe familiar to us lies on a brane surrounded by a (4+1)-dimensional bulk spacetime that is essentially empty except for a negative cosmological constant and the various modes associated with gravity. For such a braneworld cosmology, the difficulty of justifying a set of preferred initial conditions inevitably arises. The various proposals for inflation restricted to the brane only partially explain the homogeneity and isotropy of the resulting braneworld universe because the three-dimensional homogeneity and isotropy of the bulk must be assumed a priori. In this Letter we propose a mechanism by which a brane surrounded by AdS space arises naturally in such a way that the homogeneity and isotropy of both the brane and the bulk are guaranteed. We postulate an initial false vacuum phase of (4+1)-dimensional de Sitter, or possibly Minkowski, space subsequently decaying to a true vacuum of anti-de Sitter space, assumed discretely degenerate. This decay takes place through bubble nucleation. When two bubbles of the true AdS vacuum eventually collide, because of the degeneracy of the true AdS vacuum, a brane (or domain wall) inevitably forms separating the two AdS phases. It is on this brane that we live. The SO(3,1) symmetry of the collision geometry ensures the three-dimensional spatial homogeneity and isotropy of the universe on the brane as well as of the bulk. In the semi-classical (→0) limit, this SO(3,1) symmetry is exact. We sketch how the leading quantum corrections translate into cosmological perturbations.     

A method for identification of interfering signals in EPR dating of tooth enamel

In this paper, a method for identifying interfering EPR signals in fossil tooth enamel is presented. This method consists of optimizing EPR parameters to enhance spectral resolution of the fossil signals followed by a g-factor and intensity normalized subtraction of a high-dosed, modern tooth enamel sample spectrum. By scanning the modern tooth enamel sample with the same parameters as the fossil sample, the difference spectrum of the fossil and modern irradiated tooth enamel samples can resolve numerous hyperfine and other interfering signals. This method was successfully applied to a fossil sample studied elsewhere (Grün, R., 1998a. Ancient TL 16, 51–55; Grün, R., Clapp, R., 1996. Ancient TL 14, 1–5; Martin Jonas, 1997. Ph.D. Thesis, Cambridge University; Jonas, M., Grün, R., 1997. Radiat. Meas. 27, 49–58; Vanhaelewyn et al., 2000. Appl. Radiat. Isot. 52, 1317–1326). This sample has shown discrepancies in dose estimations obtained from the power absorption curve versus those obtained from the first derivative spectra (Grün, R., 1998b. Radiat. Meas. 29, 177–193). The reason for this, and other discrepancies, are accounted for by the signals resolved using the method presented here.     

Brane world scenarios

We review proposals of brane world models which attempt to combine gauge theories with gravity at TeV scale by confining the gauge theory to a three-brane embedded in higher dimensional bulk. Gravity, however, propagates in the directions transverse to the brane as well.