Scientific Series

I will review an old (Greenberg and Schweber, 1958) and undeservedly forgotten idea in quantum field theory. This idea allows one to reformulate QFT as a Hamiltonian theory of physical (rather than bare) particles and their direct interactions. The dressed particle approach is scattering-equivalent to the traditional one, however it doesn\'t require renormalization and may provide a valuable tool for calculations of wave functions of bound states and time evolution.

Explorations of the possibility that the quark masses, and more generally the particle mass spectra, could be dynamically generated in the context of massless QCD will be presented. The basic idea is that the large degeneracy of the free massless QCD could lead to a large quark condensate and its corresponding mass. Under the presence of this very massive quark, the other five ones could acquire smaller masses as argued by Fritzsch in his Democratic Symmetry Breaking scheme. Further, the lepton and neutrinos could get their masses through their interaction with quarks mediated by radiative corrections. In this case the stronger electromagnetic corrections could imply the larger lepton masses with respect to the neutrino ones, since these are only weakly interacting with the quarks.

The Hamiltonian of traditionally adopted detector models features out of diagonal elements between the vacuum and the one particle states of the field to be detected. We argue that reasonably good detectors, when written in terms of fundamental fields, have a more trivial response on the vacuum. In particular, the model configuration ``detector in its ground state + vacuum of the field\' generally corresponds to a stable bound state of the underlying theory (e.g. the hydrogen atom in a suitable QED with electrons and protons) and therefore should be also an eigenstate of the model Hamiltonian. As a concrete example, we study a consistent ``fundamental\' toy field theory where a stable particle can capture a light quantum and form a quasi-stable state. To such stable particle correspond eigenstates of the full theory, as is shown explicitly by using a dressed particle formalism at first order in perturbation theory. We then write the corresponding Hamiltonian for a model detector (at rest) where the stable particle and the quasi-stable configurations correspond to the two internal levels, ``ground\' and ``excited\', of the detector. The accelerated version of this Hamiltonian is inevitably model dependent emph{i.e.} it will generally depend on how the stable particle/detector is forced along the accelerated trajectory. However, in its most basic version, the accelerated detector doesn\'t see Unruh radiation.

Consider the quantum predictions for EPR-type measurements on two systems with Hilbert space of dimension at least 3 in any maximally entangled state. I show that the only possible hidden variables model of these probabilities that satisfies both Shimony\'s and Jarrett\'s condition of parameter independence (or `locality\') and Jones and Clifton\'s condition of conditional parameter independence (or `constrained locality\') is trivial, i.e. given by the quantum probabilities themselves. I shall attempt to discuss also the meaning of the conditions and of this result.

Many string theorists and cosmologists have recently turned their attention to building and testing string theory models of inflation. One of the main goals is to find novel features that could distinguish stringy models from their field theoretic counterparts. This is difficult because, in most examples, string theory is used to derived an effective theory operating at energies well below the string scale. However, since string theory provides a complete description of dynamics also at higher energies, it may be interesting to construct inflationary models which take advantage of this distinctive feature. I will discuss recent progress in this direction using p-adic string theory - a toy model of the bosonic string for which the full series of higher dimensional operators is known explicitly - as a playground for studying string cosmology to all order in $alpha\'$. The p-adic string is a nonlocal theory containing derivatives of all orders and this structure is also ubiquitous in string field theory. After discussing the difficulties (such as ghosts and classical instabilities) that arise in working with higher derivative theories I will show how to construct generic inflationary models with infinitely many derivatives. Novel features include the possibility of realizing slow roll inflation with a steep potential and large nongaussian signatures in the CMB.

We analyze the trans-Planckian problem and its formulation in the context of cosmology, black-hole physics, and analogue models of gravity. In particular, we discuss the phenomenological approach to the trans-Planckian problem based on modified, locally Lorentz-breaking, dispersion relations (MDR). The main question is whether MDR leave an detectable imprint on macroscopic physics. In the framework of the semi-classical theory of gravity, this question can be unambiguously answered only through a rigorous formulation of quantum field theory on curved space with MDR. In this context, we propose a momentum-space analysis of the Green\'s function, which will hopefully lead to the correct renormalization of the stress tensor.

I will describe antiferromagnets and superconductors near quantum phase transitions. There is a remarkable analogy between their dynamics and the holographic description of Hawking radiation from black holes. I will show how insights from this analogy have shed light on experiments on the cuprate high temperature superconductors.

The dynamics of particles moving in a medium defined by its relativistically invariant stochastic properties is investigated. For this aim, the force exerted on the particles by the medium is defined by a stationary random variable as a function of the proper time of the particles. The equations of motion for a single one-dimensional particle are obtained and numerically solved. A conservation law for the drift momentum of the particle during its random motion is shown. Moreover, the conservation of the mean value of the total linear momentum for two particles repelling each other according to the Coulomb interaction also follows. Therefore, the results indicate the realization of a kind of stochastic Noether theorem in the system under study.

We introduce a formalism allowing us to localize a certain class of theories with an infinite number of derivatives (nonlocal), which include effective actions of string field theory. The number of degrees of freedom is finite and the Cauchy problem, Hamiltonian and quantization are all well-defined. As applications, the rolling tachyon of cubic string field theory and some cosmological toy models are considered.

Geometries produced by brane intersections preserving eight supercharges are constructed. Typical examples of such configurations are given by fundamental strings ending on D branes and by brane webs. Consistency conditions of supergravity are shown to impose certain requirements on the locations of the sources, and these restrictions are found to be in a perfect agreement with results of the probe analysis. This agreement serves as a nontrivial test of the duality between open and closed strings. Some applications to AdS/CFT correspondence are also discussed.