Searcharxiv⌕ Search

arXiv subjects

T. N. Tomaras

Publications and source records attributed to T. N. Tomaras.

At least 19 recordsLinked to original sources
arXiv2014

Is the Standard Model saved asymptotically by conformal symmetry?

It is pointed out that the top-quark and Higgs masses and the Higgs VEV satisfy with great accuracy the relations 4m_H^2=2m_T^2=v^2, which are very special and reminiscent of analogous ones at Argyres - Douglas points with enhanced conformal symmetry. Furthermore, the RG evolution of the corresponding Higgs self-interaction and Yukawa couplings λ(0)=1/8 and y(0)=1 leads to the free-field stable point λ(M_Pl)= \dot λ(M_Pl)=0 in the pure scalar sector at the Planck scale, also suggesting enhanced conformal symmetry. Thus, it is conceivable that the Standard Model is the low-energy limit of a distinct special theory with (super?) conformal symmetry at the Planck scale. In the context of such a "scenario" one may further speculate that the Higgs particle is the Goldstone boson of (partly) spontaneously broken conformal symmetry. This would simultaneously resolve the hierarchy and Landau pole problems in the scalar sector and would provide a nearly flat potential with two almost degenerate minima at the electroweak and Planck scales.

hep-ph↗
arXiv2014

Constraining Dark Energy through the Stability of Cosmic Structures

For a general dark-energy equation of state, we estimate the maximum possible radius of massive structures that are not destabilized by the acceleration of the cosmological expansion. A comparison with known stable structures constrains the equation of state. The robustness of the constraint can be enhanced through the accumulation of additional astrophysical data and a better understanding of the dynamics of bound cosmic structures.

astro-ph.CO↗
arXiv2011

Geodesic deviation and particle creation in curved spacetimes

A quantum mechanical picture, relating accelerated geodesic deviation to creation of massive particles via quantum tunneling in curved background spacetimes, is presented. The effect is analogous to pair production by an electric field and leads naturally to production of massive particles in de Sitter and superluminal FRW spacetimes. The probability of particle production in de Sitter space per unit volume and time is computed in a leading semiclassical approximation and shown to coincide with the previously obtained expression.

gr-qc↗
arXiv2010

Classical ultrarelativistic bremsstrahlung in extra dimensions

The emitted energy and the cross-section of classical scalar bremsstrahlung in massive particle collisions in D=4+d dimensional Minkowski space M_D as well as in the brane world M_4 \times T^d is computed to leading ultra-relativistic order. The particles are taken to interact in the first case via the exchange of a bulk massless scalar field Φand in the second with an additional massless scalar ϕconfined together with the particles on the brane. Energy is emitted as Φradiation in the bulk and/or ϕradiation on the brane. In contrast to the quantum Born approximation, the classical result is unambiguous and valid in a kinematical region which is also specified. For D=4 the results are in agreement with corresponding expressions in classical electrodynamics.

hep-th↗
arXiv2007

If LHC is a Mini-Time-Machines Factory, Can We Notice?

Assuming the hypothesis of TeV-scale multi-dimensional gravity, one can imagine that at LHC not only mini-black-holes (MBH) will be intensively created, but also other exotic gravitational configurations, including hypothetical mini-time-machines (MTM). Like MBH, they should quickly evaporate, but one can wonder if their temporal existence at the moment of high-energy collision can leave any traces in the observable data. We briefly discuss five thinkable effects: (i) change of the energy spectrum due to the frequency-filtration property of MTM, (ii) possible production of anomalously energetic particles, accelerated by passing many times through gravitational field inside the MTM, (iii) acceleration of particle decays, since the proper time of a particle moving inside MTM can strongly exceed the laboratory time, (iv) CPT and naive unitarity violation (thermalization) due to effective non-local interactions caused by MTM and to possible ambiguity in the population of closed world-lines inside MTM, (v) collective effects due to conversion of a single particle into a bunch of its co-existing copies within the MTM. Despite possible particle-antiparticle conversion inside MTM, they do not seem to produce any specific CP-violation effects.

hep-th↗
arXiv2007

On n-point Amplitudes in N=4 SYM

The computation of n-point planar amplitudes in N=4 SYM at strong coupling is known to be reduced to the search for solutions of the integrable 2d SO(4,2) sigma-model with growing asymptotics on the world-sheet and to the study of their Whitham deformations induced by an epsilon-regularization, which breaks both integrability and SO(4,2) symmetry. A multi-parameter (moduli) family of such solutions is constructed for n=4. They all correspond to the same s and t and some are related by SO(4,2) transformations. Nevertheless, they lead to different regularized areas, whose minimum is the Alday-Maldacena solution. A brief review of results on n-point amplitudes is also provided, with special emphasis on the underlying equivalence of the above regularized minimal area in AdS and a double contour integral along the same boundary, two purely geometric quantities.

hep-th↗
arXiv2006

Brane-bulk energy exchange : a model with the present universe as a global attractor

The role of brane-bulk energy exchange and of an induced gravity term on a single braneworld of negative tension and vanishing effective cosmological constant is studied. It is shown that for the physically interesting cases of dust and radiation a unique global attractor which can realize our present universe (accelerating and 0<Omega_{m0}<1) exists for a wide range of the parameters of the model. For Omega_{m0}=0.3, independently of the other parameters, the model predicts that the equation of state for the dark energy today is w_{DE,0}=-1.4, while Omega_{m0}=0.03 leads to w_{DE,0}=-1.03. In addition, during its evolution, w_{DE} crosses the w_{DE}=-1 line to smaller values.

hep-th↗
arXiv2005

On the Need for Phenomenological Theory of P-Vortices or Does Spaghetti Confinement Pattern Admit Condensed-Matter Analogies?

Usually the intuition from condensed-matter physics is used to provide ideas for possible confinement mechanisms in gauge theories. Today, with a clear but puzzling ``spaghetti'' confinement pattern, arising after a decade of lattice computer experiments, which implies formation of a fluctuating net of peculiar magnetic vortices rather than condensation of the homogeneously distributed magnetic monopoles, the time is coming to reverse the logic and search for similar patterns in condensed matter systems. The main thing to look for in a condensed matter setup is the simultaneous existence of narrow tubes ($P$-vortices or 1-branes) of direction-changing electric field and broader tubes (Abrikosov lines) of magnetic field, a pattern dual to the one, presumably underlying confinement in gluodynamics. As a possible place for this search we suggest systems with coexisting charge-density waves and superconductivity.

hep-th↗
arXiv2004

Searching for Extra Dimensions in High Energy Cosmic Rays

We present a study of the decay of an intermediate mini black hole at the first impact of a cosmic ray particle with the atmosphere, in the context of D-brane world scenarios with TeV scale gravity and large extra dimensions. We model the decay of the black hole using the semiclassical approximation and include the corrections coming from energy loss into the bulk. Extensive simulations show that mini black hole events are characterized by essentially different multiplicities and wider lateral distributions of the air showers as a function of the energy of the incoming primary, as compared to standard events. Implications for their detection and some open issues on their possible discovery are also briefly addressed.

hep-ph↗
arXiv2004

Cosmic Ray Signals from Mini Black Holes in Models with Extra Dimensions: An Analytical/Monte Carlo Study

We present a study of the multiplicities, of the lateral distributions and of the ratio of the electromagnetic to the hadronic components in the air showers, generated by the collision in the atmosphere of an incoming high energy cosmic ray and mediated by the formation of a mini black hole, predicted in TeV scale gravity models with large extra dimensions. The analysis is performed via a large scale simulation of the resulting cascades over the entire range ($10^{15}-10^{19}$eV) of ultra high initial energies, for several values of the number of large extra dimensions, for a variety of altitudes of the initial interaction and with the energy losses in the bulk taken into account. The results are compared with a representative of the standard events, namely the shower due to the collision of a primary proton with a nucleon in the atmosphere. Both the multiplicities and the lateral distribution of the showers show important differences between the two cases and, consequently, may be useful for the observational characterization of the events. The electromagnetic/hadronic ratio is strongly fluctuating and, thus, less decisive for the altitudes considered.

hep-ph↗
arXiv2003

Cosmological evolution with brane-bulk energy exchange

The consequences for the brane cosmological evolution of energy exchange between the brane and the bulk are analysed in detail, in the context of a non-factorizable background geometry with vanishing effective cosmological constant on the brane. A rich variety of brane cosmologies is obtained, depending on the precise mechanism of energy transfer, the equation of state of brane-matter and the spatial topology. An accelerating era is generically a feature of our solutions. In the case of low-density flat universe more dark matter than in the conventional FRW picture is predicted. Spatially compact solutions are found to delay their recollapse.

hep-th↗
arXiv2003

D-branes and the Standard Model

We perform a systematic study of the Standard Model embedding in a D-brane configuration of type I string theory at the TeV scale. We end up with an attractive model and we study several phenomenological questions, such as gauge coupling unification, proton stability, fermion masses and neutrino oscillations. At the string scale, the gauge group is U(3)_color x U(2)_weak x U(1)_1 x U(1)_bulk. The corresponding gauge bosons are localized on three collections of branes; two of them describe the strong and weak interactions, while the last abelian factor lives on a brane which is extended in two large extra dimensions with a size of afew microns. The hypercharge is a linear combination of the first three U(1)s. All remaining U(1)s get masses at the TeV scale due to anomalies, leaving the baryon and lepton numbers as (perturbatively) unbroken global symmetries at low energies. The conservation of baryon number assures proton stability, while lepton number symmetry guarantees light neutrino masses that involve a right-handed neutrino in the bulk. The model predicts the value of the weak angle which is compatible with the experiment when the string scale is in the TeV region. It also contains two Higgs doublets that provide tree-level masses to all fermions of the heaviest generation, with calculable Yukawa couplings; one obtains a naturally heavy top and the correct ratio m_b/m_tau. We also study neutrino masses and mixings in relation to recent solar and atmospheric neutrino data.

hep-th↗
arXiv2002

Brane-induced decay of the Kaluza-Klein vacuum

The enhancement in the decay rate of the Kaluza-Klein vacuum due to the presence of a brane is studied, both in the test brane approximation and beyond it. Spontaneous brane materialization in the Kaluza-Klein vacuum is also described.

hep-th↗
arXiv2002

Induced Gravity on RS Branes

It is shown that a localized four-dimensional Einstein term, induced by quantum corrections, modifies significantly the law of gravity in a Randall-Sundrum brane world. In particular, the short-distance behavior of gravity changes from five- to four-dimensional, while, depending on the values of parameters, there can be an intermediate range where gravity behaves as in five dimensions. The spectrum of graviton fluctuations around the brane, their relative importance for the gravitational force, and the relevance of their emission in the bulk for the brane cosmology are analysed. Finally, constraints on parameters are derived from energy loss in astrophysical and particle physics processes.

hep-th↗
arXiv2001

Pair creation and axial anomaly in light-cone $QED_2$

The 1+1 dimensional massive Dirac equation is solved exactly in light-cone coordinates for $x^+ > 0$ and $x^- > -L$, in the presence of an arbitrary $x^+$ dependent electric field. Our solution resolves the ambiguity at $p^+ = 0$. We also obtain the one loop rate for pair production for a positive electric field, compute the expectation values of the vector and axial vector currents, and recover the well known anomaly $e^2 E/π$ in the divergence of the latter. A final intriguing result is that the theory seems to exhibit a phase transition in the limit of infinite $L$.

hep-th↗
arXiv2000

Back-Reaction In Lightcone QED

We consider the back-reaction of quantum electrodynamics upon an electric field E(x_+) = - A'_-(x_+) which is parallel to x^3 and depends only on the lightcone coordinate x_+ = (x^0 + x^3)/\sqrt{2}. Novel features are that the mode functions have simple expressions for arbitrary A_-(x_+), and that one cannot ignore the usual lightcone ambiguity at zero + momentum. Each mode of definite canonical momenta k_+ experiences pair creation at the instant when its kinetic momentum p_+=k_+ - e A_-(x_+) vanishes, at which point operators from the surface at x_- =-\infty play a crucial role. Our formalism permits a more explicit and complete derivation of the rate of particle production than is usually given. We show that the system can be understood as the infinite boost limit of the analogous problem of an electric field which is homogeneous on surfaces of constant x^0.

hep-ph↗