Extensions of the standard model with universal extra dimensions are interesting both as phenomenological templates as well as model-building fertile ground. For instance, they are one the prototypes for theories exhibiting compressed spectra, leading to difficult searches at the LHC since the decay products of new states are soft and immersed in a large standard model background. Here we study the phenomenology at the LHC of theories with two universal extra dimensions. We obtain the current bound by using the production of second level excitations of electroweak gauge bosons decaying to a pair of leptons and study the reach of the LHC Run~II in this channel. We also introduce a new channel originating in higher dimensional operators and resulting in the single production of a second level quark excitation. Its subsequent decay into a hard jet and lepton pair resonance would allow the identification of a more model-specific process, unlike the more generic vector resonance signal. We show that the sensitivity of this channel to the compactification scale is very similar to the one obtained using the vector resonance.

We discuss two distinct aspects in supersymmetric quantum mechanics. First, we introduce a new class of operators A and A̅ in terms of anticommutators between the momentum operator and N+1 arbitrary superpotentials. We show that these operators reduce to the conventional ones which are the starting point in standard supersymmetric quantum mechanics. In this context, we argue furthermore that supersymmetry does not only connect Schrödinger-like operators, but also a more general class of differential operators. Second, we revisit the supersymmetric ε-system recently introduced in the literature by exploiting its intrinsic supersymmetry. Specifically, combining the Hamilton hierarchy method and the δ-expansion method, we determine an energy for the first excited state of the bosonic Hamiltonian close to that calculated in earlier works.

We propose an alternative formulation of the Standard Model which reduces the number of free parameters. In our framework, fermionic fields are assigned to fundamental representations of the Lorentz and the internal symmetry groups, whereas bosonic field variables transform as direct products of fundamental representations of all symmetry groups. This allows us to reduce the number of fundamental symmetries. We formulate the Standard Model by considering the SU(3) and SU(2) symmetry groups as the underlying symmetries of the fundamental interactions. This allows us to suggest a model, for the description of the interactions of the intermediate bosons among themselves and interactions of fermions, that makes use of just two parameters. One parameter characterizes the symmetric phase, whereas the other parameter (the asymmetry parameter) gives the breakdown strength of the symmetries. All coupling strengths of the Standard Model are then derived in terms of these two parameters. In particular, we show that all fermionic electric charges result from symmetry breakdown.

We propose a spinorial approach to the unified electroweak interactions, in which no use is made of spontaneous symmetry breakdown. No scalar particles are needed in order to break the symmetry. No reference is made to gauge symmetry. Our approach stresses the role of space–time and isospin symmetries in the build up of the electroweak model. Internal degrees of freedom, such as isospin, are incorporated in the theory by using spinors carrying isospin indices. All vector bosons are described by a rank 2 field in the spinorial and the isospinorial indices. Leptons are accomodated in a rank 1 spinor field and in a rank 2 isospin field as well. The dynamical variables of the theory are the chiral and isochiral components of these fields.

We study a noncommutative nonrelativistic fermionic field theory in $2+1$ dimensions coupled to the Chern-Simons field. We perform a perturbative analysis of the model and show that up to one loop the ultraviolet divergences are canceled and the infrared divergences are eliminated by the noncommutative Pauli term.

Received 25 October 2004DOI:https://doi.org/10.1103/PhysRevD.70.129905©2004 American Physical Society

Bose-Einstein condensation is studied as a phenomenon of spontaneous symmetry breakdown. The order parameter is the condensate wave function. We analyze the functional dependence of the free energy on the order parameter. An expression for the order parameter in terms of the Green's function of the theory is derived. A method for deriving the thermodynamics of a Bose-Einstein condensed system is provided. For the Bogoliubov condensate, we derive the London relation and an equation of state valid for very low temperatures. Finally, we discuss the loop expansion method, which provides a framework for computational calculations of the thermodynamics of Bose-Einstein condensed systems.

We study a noncommutative nonrelativistic theory in 2+1 dimensions of a scalar field coupled to the Chern-Simons field. In the commutative situation this model has been used to simulate the Aharonov-Bohm effect in the field theory context. We verified that, contrary to the commutative result, the inclusion of a quartic self-interaction of the scalar field is not necessary to secure the ultraviolet renormalizability of the model. However, to obtain a smooth commutative limit the presence of a quartic gauge invariant self-interaction is required. For small noncommutativity we fix the corrections to the Aharonov-Bohm scattering and prove that up to one loop the model is free from dangerous infrared/ultraviolet divergences.

In this paper we extend previous hydrodynamic equations, governing the motion of Bose-Einstein-condensed fluids, to include temperature effects. This allows us to analyze some differences between a normal fluid and a Bose-Einstein-condensed one. We show that, in close analogy with superfluid $^{4}\mathrm{He}$, a Bose-Einstein-condensed fluid exhibits the mechanocaloric and thermomechanical effects. In our approach we can explain both effects without using the hypothesis that the Bose-Einstein-condensed fluid has zero entropy. Such ideas could be investigated in existing experiments.

The asymmetry, between electric (E) and magnetic (H) fields of Maxwell's equation is here analyzed by using the concept of chirality. The chiral spinorial approach sets the stage for the construction of a more general theory of spin-1 particles than usual electrodynamics. Chiral components of a rank-2 spinor field are taken as the dynamic variables of the theory. A rank-2 spinor accommodates another particle (the magnetic photon). This new particle emerges naturally from chiral invariance arguments. The nonexistence, in nature, of such a particle is the reason for the nonexistence of monopoles and the asymmetry in Maxwell's equation. The existence of magnetic monopoles would restore the symmetry of Maxwell's equation. We establish, in this way, at a very formal level, the connection between magnetic monopoles and chiral asymmetry.

We analyze some aspects of the fluidity of Bose-Einstein (BE) condensed systems. We show that a two-fluid picture of a condensed system holds true. The condensate is a quantum fluid component of the system. This component of the fluid satisfies the continuity equation and a generalized Bernoulli equation. Under certain conditions, the condensate component becomes a classical superfluid. We suggest that condensed systems might exhibit mechanochalorical and thermomechanical effects, in close analogy with superfluid helium-4. Quantized vortices are also expected to appear in a BE condensed system. We discuss the structure of quantized vortices.

In this paper we apply the chiral spinorial approach to the description of spin 32 particles. Chiral components of rank 3 spinor fields are considered to be the dynamical variables of the theory. The free Lagrangian is built from the same principles as in the case of spin 1 and spin 2 particles: chiral symmetry at the free field level and chiral symmetry breakdown at the interaction level. We show how the chiral spinorial approach provides an unambiguous Lagrangian approach for massless spin 32 particles. This approach provides a fairly rich set of effective Lagrangians for the interaction of spin 32 particles.

In this letter we address the question on whether gauge invariance can be derived from general basic principles. We show that, within the chiral spinorial approach developed by us, gauge invariance can be seen as a result of the left–right asymmetry of nature. This is valid for Abelian and non-Abelian gauge theories. In order to select the right couplings of the chiral components, we just have to require masslessness of the photon (or gluon) and renormalizability of the theory.

In this letter we apply an alternative approach, recently developed, to the description of massless particles of arbitrary spin to the case of spin-two particles. This provides a non-geometrical approach to the theory of linearized gravitation. Within this method the chiral components of a spinor field are treated as independent field variables. The free field Lagrangian is built up from the requirement of chiral invariance. This formulation is parallel to the neutrino theory and leads to a formulation that generalizes, to particles of spin-two, the two-component neutrino theory. At the free field level the analog of curvature tensor, spin connection tensor, and metric tensor are independent quantities. By introducing left–right asymmetric linear interactions of these chiral components we get the linearized gravitation theory.

Using 2-body trees on a flat space background, it is shown that the actions A[g,phi]=integral d(4)x root-g [(R/2 kappa)+(1/2)(g(mu nu)partial derivative(mu)phi partial derivative(mu)phi+lambda R phi(2))] and (A) over bar[(g) over bar,<(phi)over bar>]=integral d(4)x root-(g) over bar[((R) over bar/2 kappa)+(1/2)(g) over bar(mu nu)partial derivative(mu)<(phi)over bar>partial derivative(nu)phi] describe the same theory at the tree-level in this case. We also demonstrate the quantum equivalence (at one-loop) of the barred and unbarred systems for lambda=-1/6 (conformal coupling).

In this paper we deal with an alternative approach to the description of massless particles of arbitrary spin. Within this scheme chiral components of a spinor field are regarded as fundamental quantities and treated as independent field variables. The free field Lagrangian is built up from the requirement of chiral invariance. This formulation is parallel to the neutrino theory and allows for a formulation that generalizes, to particles of arbitrary spin, the two-component neutrino theory. We achieve a spinor formulation of electrodynamics. In the case of the photon, the nonzero helicity components satisfy Weyl’s equations and are associated to observables (electromagnetic fields) whereas the zero helicity components are related to nonobservables (electromagnetic potentials). Within the spinor formulation of electrodynamics the minimal coupling substitution follows as a consequence of the linearity of the interaction and the preference of nature for chiral components, that is, of the left–right asymmetry of nature.

The lowest order invariant amplitudes for the 2\ensuremath{\rightarrow}2 processes concerning the action S[g,\ensuremath{\varphi}]=F${\mathit{d}}^{4}$x \ensuremath{\surd}-g [2R/${\mathrm{\ensuremath{\kappa}}}^{2}$+1/2(${\mathit{g}}^{\mathrm{\ensuremath{\mu}}\ensuremath{\nu}}$${\mathrm{\ensuremath{\partial}}}_{\mathrm{\ensuremath{\mu}}}$\ensuremath{\varphi}${\mathrm{\ensuremath{\partial}}}_{\ensuremath{\nu}}$\ensuremath{\varphi}+\ensuremath{\lambda}R${\mathrm{\ensuremath{\varphi}}}^{2}$)] are computed. It is found that these results do not depend on the value of \ensuremath{\lambda}; therefore, it is impossible to measure the R${\mathrm{\ensuremath{\varphi}}}^{2}$ coupling parameter at tree-level scattering in this case. It is also shown that the theory described by the above action is equivalent at the classical level to Einstein's theory with a massless minimally coupled scalar field provided that 1+\ensuremath{\lambda}${\mathrm{\ensuremath{\kappa}}}^{2}$${\mathrm{\ensuremath{\varphi}}}^{2}$/4>0. Some consequences for cosmology and black holes are discussed.

We discuss the astrophysical consequences of the process \ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}, when this reaction is mediated by either Majorons or composite neutral leptons, and present the constraints on their coupling coming from stellar energy loss arguments. We also discuss the effect of nuclear absorption when this reaction is mediated by pions, and show that no significant output of energy is provided in this case. Finally, we comment on the importance of these processes in cosmology.