International Journal of Modern Physics A

It has recently been argued that codimension-two braneworlds offer a promising line of attack on the cosmological constant problem, since in such models the Hubble rate is not directly related to the brane tension. We point out challenges to building more general models where the brane content is not restricted to pure tension. In order to address these challenges, we construct a thick brane model which we linearize around a well known static solution. We show that the model's cosmology does reduce to standard FRW behaviour, but find no hint of a self-tuning mechanism which might help solve the cosmological constant problem whithin the context of non-supersymmetric Einstein gravity.

The potential between branes in a compact space obeys a modified Poisson equation. Although the Green's functions for simple spaces such as the torus are known, we illustrate the use of a well-established method from solid state physics that may be used to find the potential on a general compact space. This potential is relevant for inflationary scenarios, and in particular helps determine whether a given model satisfies the slow-roll conditions. We show that models with branes at small angles generally give enough inflation.

Brane Gas Cosmology is an M-theory motivated attempt to reconcile aspects of the standard cosmology based on Einstein's theory of general relativity. Dilaton gravity, when incorporating winding p-brane states, has verified the Brandenberger–Vafa mechanism —a string-motivated conjecture which explains why only three of the nine spatial dimensions predicted by string theory grow large. Further investigation of this mechanism has argued for a hierarchy of subspaces, and has shown the internal directions to be stable to initial perturbations. These results, however, are dependent on a rolling dilaton, or varying strength of Newton's gravitational constant G_N. In these proceedings we show that it is not possible to stabilize the dilaton and maintain the stability of the internal directions within the standard Brane Gas Cosmology setup.

For a large region of parameter space involving the cosmological constant and mass parameters, we discuss spacetime solutions that are effectively Minkowskian on large time and distance scales. A negative energy fluid is involved, resulting in a scale factor oscillating about a constant, with a frequency determined by the size of a negative cosmological constant. Gravity itself induces a coupling between the ghost-like and normal fields, and we find that this results in stochastic rather than unstable behavior. Ghosts could also allow for the existence of Lorentz invariant fluctuating solutions of finite energy density.

A field theory is proposed where the regular fermionic matter and the dark fermionic matter are different states of the same "primordial" fermion fields. In regime of the fermion densities typical for normal particle physics, each of the primordial fermions splits into three generations identified with regular fermions. In a simple model, this fermion families birth effect is accompanied with the right lepton numbers conservation laws. It is possible to fit the muon to electron mass ratio without fine tuning of the Yukawa coupling constants. When fermion energy density becomes comparable with dark energy density, the theory allows new type of states - Cosmo-Low Energy Physics (CLEP) states. Neutrinos in CLEP state can be both a good candidate for dark matter and responsible for a new type of dark energy. In the latter case the total energy density of the universe is less than it would be in the universe free of fermionic matter at all. The (quintessence) scalar field is coupled to dark matter but its coupling to regular fermionic matter appears to be extremely suppressed.

We show that the origin of the dark matter and dark energy problems originates in the assumption of standard Einstein gravity that Newton's constant is fundamental. We discuss an alternate, conformal invariant, metric theory of gravity in which Newton's constant is induced dynamically, with the global induced one which is effective for cosmology being altogether weaker than the local induced one needed for the solar system. We find that in the theory dark matter is no longer needed, and that the accelerating universe data can be fitted without fine-tuning using a cosmological constant as large as particle physics suggests. In the conformal theory then it is not the cosmological constant which is quenched but rather the amount of gravity that it produces.

Observations to date cannot distinguish neutron stars from self-bound bare quark stars on the basis of their gross physical properties such as their masses and radii alone. However, their surface luminosity and spectral characteristics can be significantly different. Unlike a normal neutron star, a bare quark star can emit photons from its surface at super-Eddington luminosities for an extended period of time. We present a calculation of the photon bremsstrahlung rate from the bare quark star's surface, and indicate improvements that are required for a complete characterization of the spectrum. The observation of this distinctive photon spectrum would constitute an unmistakable signature of a strange quark star and shed light on color superconductivity at stellar densities.

Modified gravitational theories can provide alternatives to dark energy as an explaination for the observed late-time cosmic acceleration. Several examples of low-curvature corrections to the Einstein-Hilbert action are studied. These models generically contain unstable de Sitter solutions and, depending on the parameters of the theory, can exhibit late time accelerating attractor solutions.

Chiral Lagrangians are used to compute the production rate of photons from the hadronic phase of relativistic nuclear collisions. Special attention to the role of the a₁ pseudovector is paid. Calculations that include strange meson reactions, form factors, the use of consistent vector spectral densities, the emission from a quark-gluon plasma, and primordial nucleon-nucleon collisions reproduce the photon spectra measured at the Super Proton Synchrotron (SPS). Some predictions for the Relativistic Heavy Ion Collider (RHIC) are made.

We present the ideas leading to the construction of non-linear sigma model on the fuzzy supersphere . In a recent article¹ Bott projectors have been used to obtain the fuzzy model. Here we obtain the supersymmetric extensions of these projectors and their fuzzy versions. The latter are then used to construct the non-linear sigma model on . We discuss the interpretation of the resulting model as a finite-dimensional matrix model.

The "complementary" Ansatz, Tr(M_ν)=0, where M_ν is the prediagonal neutrino mass matrix, seems a plausible approximation for capturing in a self contained way some of the content of Grand Unification. We study its consequences in the form of relations between the neutrino masses and CP violation phases.

We analyze the effects of the current neutrino data on thermal leptogenesis and lepton flavor violating and conserving processes in a fully left-right extension of the minimal supersymmetric model. The model has several phases from both the heavy and light neutrino sectors: two CKM-type phases, and four Majorana phases, which give new contributions to CP-violating parameters and leptogenesis. We study observable effects of these phases in most general neutrino mixing scenarios, with either hierarchical, inverse hierarchical or quasi-degenerate light and heavy neutrinos. The CP violating phases in both the heavy and light neutrino sectors of the left-right supersymmetric model have unique features which may distinguish the model from other supersymmetric scenarios.

We review the results on neutrino propagation in neutral and charged media under strong magnetic fields . It is shown that the neutrino energy density gets a magnetic contribution in both charged and neutral media, which is linear in the magnetic field, of first order in G_F, and independent of the charge density. This new term enters as a correction to the neutrino kinetic energy and produces an anisotropic contribution to the neutrino index of refraction. As a consequence, in a neutral medium a highly anisotropic resonant level-crossing condition takes place for the oscillation between electron-neutrinos and the other neutrino species. Possible cosmological applications are presented.

In this talk, I discuss possible bounds on the Lorentz and CPT violating parameters in the Higgs sector of the so called minimal Standard-Model Extension (SME). The main motivation to this study is coming from the fact that unlike the parameters in the fermion and gauge sector, there are no published bounds on the parameters in the Higgs sector. From the one-loop contributions to the photon propagator the bounds on the CPT-even asymmetric coefficients are obtained and the c_μν coefficients in the fermion sector determine the bound on the CPT-even symmetric coefficients. The CPT-odd coefficient is bounded from the non-zero vacuum expectation value of the Z-boson.

Hadron spectroscopy still follows basic 1966 physics of Sakharov, Zeldovich and Nambu. Constituent quark quasiparticles with effective masses and non-abelian color gauge electric and magnetic interactions fit all masses and magnetic moments of ground state mesons and baryons containing no more than one heavy or strange or heavy quark. Flavor antisymmetry explains absence of low-lying exotics and suggests diquark-triquark model for Θ⁺ pentaquark.

A nonlinear chiral Lagrangian framework for scalar mesons below 2 GeV [the I =l/2 states: κ(900) and ; the I=1 states: a₀(980) and a₀(1450); and the I=0 states: α(550), f₀(980), f₀(1370), f₀(1500) and f₀(1710)] is presented. This framework is formulated in terms of two meson nonets with a mixing mechanism dictated by SU(3) symmetry and its breakdown.

We compare the predictions of two distinct dipole models for inclusive and exclusive diffractive processes. While only one of these dipole models contains perturbative saturation dynamics, we show that the predictions of both models are fully consistent with the available HERA data, indicating no compelling evidence for saturation at present HERA energies.

The energy-momentum relations for massive and massless particles are E=p²/2m and E=pc respectively. According to Einstein, these two different expressions come from the same formula . Quarks and partons are believed to be the same particles, but they have quite different properties. Are they two different manifestations of the same covariant entity as in the case of Einstein's energy-momentum relation? The answer to this question is YES. It is possible to construct harmonic oscillator wave functions which can be Lorentz-boosted. They describe quarks bound together inside hadrons. When they are boosted to an infinite-momentum frame, these wave functions exhibit all the peculiar properties of Feynman's parton picture. This formalism leads to a parton distribution corresponding to the valence quarks, with a good agreement with the experimentally observed distribution.

We investigate moduli field dynamics in supergravity/M-theory like set ups where we turn on fluxes along some or all of the extra dimensions. As has been argued in the context of string theory, we observe that the fluxes tend to stabilize the squashing (or shape) modes. Generically we find that at late times the shape is frozen while the radion evolves as a quintessence field. At earlier times we have a phase of radiation domination where both the volume and the shape moduli are slowly evolving. However, depending on the initial conditions and the parameters of the theory, like the value of the fluxes, curvature of the internal manifold and so on, the dynamics of the internal manifold can be richer with interesting cosmological consequences, including inflation.

We study analytically the dynamical formation of lower dimensional branes at the endpoint of brane-antibrane inflation through the condensation of topological defects of the tachyon field which describes the instability of the initial state. We then use this information to quantify the efficiency of the reheating which is due to the coupling of time dependent tachyon background to massless gauge fields which will be localized on the final state branes. We improve upon previous estimates indicating that this can be an efficient reheating mehcanism for observers on the brane.

I review CP violation in the standard model (SM). I also describe the predictions for CP violation in the B system, along with signals for physics beyond the SM. I stress the numerous contributions of Pat O'Donnell to this subject. Finally, I discuss a new method for measuring new-physics parameters in B decays. This knowledge will allow us to partially identify any new physics which is found, before its direct production at high-energy colliders.

We describe here a method for detecting physics beyond the standard model via CP violation in B→ρπ decays. Using a Dalitz-plot analysis to obtain α, along with an analytical extraction of the various tree (T) and penguin (P) amplitudes, we obtain a criterion for the absence of new physics (NP). This criterion involves the comparison of the measured |P/T| ratio with its value as predicted by QCD factorization. We show that the detection of NP via this method has a good efficiency when compared with the corresponding technique using B→ππ decays.

I briefly review CP violation in the B system, concentrating on B→πK decays. I discuss how to deal with electroweak-penguin contributions to these decays using flavour SU(3). With these, I show that the entire unitarity triangle can be extracted from measurements of B→πK decays. Finally, I examine the signals for new physics in these decays and the possibilities for measuring them.

Recent advances in improving extractions of |V_cb| and m_b from spectra of semileptonic inclusive B decay are reported. Results of a general moment analysis of the lepton energy spectrum and the hadronic invariant mass spectrum are summarized. The calculation of the general structure functions for semileptonic B decay is reported, which has allowed the calculation of the terms for the hadronic invariant mass moments to be carried out. Recent theoretical advances and improvements in experimental data has allowed extractions of the CKM matrix element |V_cb| to improve to the 2% level.

The three body decays , may be used to measure both sin2β and cos2β. Crucial to the cos2β measurement is the resonant contribution to the three body decay from p-wave excited D_s, states. If these p-wave states are the newly discovered D_s(2317) and D_s(2460) then they are below the D^(*)K threshold and hence do not contribute to . The three body decays can then be used to measure sin2β without resonant dilution and to look for new physics in transition.

Inclusive weak radiative and semileptonic B decays play a very important role. They are used to determine the weak mixing angles V_cb, and V_ub. Furthermore the weak radiative decays provide an important constraint on extensions of the standard model, in particular low energy supersymmetry. I review the theoretical understanding of these processes with particular emphasis on the seminal work of Patrick O'Donnell.