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Houri Ziaeepour

Publications and source records attributed to Houri Ziaeepour.

49 records · Page 3Linked to original sources
arXiv2006

Color Glass Condensate in Brane Models or Don't Ultra High Energy Cosmic Rays Probe $10^{15}eV$ Scale ?

In a previous work hep-ph/0203165 we have studied propagation of relativistic particles in the bulk for some of most popular brane models. Constraints have been put on the parameter space of these models by calculating the time delay due to propagation in the bulk of particles created during the interaction of Ultra High Energy Cosmic Rays with protons in the terrestrial atmosphere. The question was however raised that probability of hard processes in which bulk modes can be produced is small and consequently, the tiny flux of UHECRs can not constrain brane models. Here we use Color Glass Condensate (CGC) model to show that effects of extra dimensions are visible not only in hard processes when the incoming particle hits a massive Kaluza-Klein mode but also through the modification of soft/semi-hard parton distribution. At classical level, for an observer in the CM frame of UHECR and atmospheric hadrons, color charge sources are contracted to a thin sheet with a width inversely proportional to the energy of the ultra energetic cosmic ray hadron and consequently they can see an extra dimension with comparable size. Due to QCD interaction a short life swarm of partons is produced in front of the sheet and its partons can penetrate to the extra-dimension bulk. This reduces the effective density of partons on the brane or in a classical view creates a delay in the arrival of the most energetic particles if they are reflected back due to the warping of the bulk. In CGC approximation the density of swarm at different distance from the classical sheet can be related and therefore it is possible (at least formally) to determine the relative fraction of partons in the bulk and on the brane at different scales. Results of this work are also relevant to the test of brane models in hadron colliders like LHC.

hep-ph↗
arXiv2004

Quest for Fats: Roles for a Fat Dark Matter (WIMPZILLA)

Since 1990's the detection of extremely energetic air showers and precise astronomical measurements have proved that our knowledge about fundamental laws of Nature is far from being complete. These observations have found convincing evidences against two popular believes: The spectrum of Cosmic Rays would have a steep cutoff at energies around $10^{19}eV$ (GZK cutoff) and the contravortial quantity called Cosmological Constant (dark energy) should be strictly zero. They have been important additions to the yet unsolved mystery of the nature of dark matter.For both phenomena many models have been suggested. The top-down model decay of a Superheavy Dark Matter(SDM), also called WIMPZILLA as the origin of the Ultra High Energy Cosmic Rays (UHECRs) - is one of the most favorite candidates. Here we show that a meaningful constraints on the mass, lifetime and cosmological contribution of SDM is possible only if the energy dissipation of the remnants is precisely taken into account. We discuss the simulation of relevant processes and their evolution in the cosmological environment. We show that such a dark matter can be the dominant component of Cold Dark Matter (CDM) with a relatively short lifetime. Moreover, the equation of State of the Universe in this model fits the Supernova type Ia data better than a stable dark matter. If a small fraction of the mass of the SDM decays to an axion-like scalar field, its condensation can also explain the dark energy without need for extreme fine tuning of the parameters. Presumably, a meta-stable dark matter can explain 3 mysteries of Physics and Cosmology. Finally we review some of the particle physics and cosmological issues related to SDM and its associated quintessence field.

astro-ph↗
arXiv2003

Solving Two Puzzles in One Go: Quintessence from a Decaying Dark Matter

The mystery of the Dark energy is not just its smallness. The extreme fine tuning of its density in the Early Universe such that it permits galaxy formation before creating a new inflationary epoch is its other unexplained aspect. Here we propose a model based on the very slow decay of a Super Heavy Dark Matter (SDM), presumed candidate for the origin of the Ultra High Energy Cosmic Rays (UHECRs). A small part of the remnants energy is in the form of a scalar field which decoheres and behaves like a quintessence field. This model does not need a special form for the potential and a simple $ϕ^4$ theory or an axion like scalar is enough. The equation of state of the Dark Energy becomes very close to a cosmological constant and is very well consistent with observations.

astro-ph↗
arXiv2003

Quintessence From The Decay of a Superheavy Dark Matter

We investigate the possibility of replacing the cosmological constant with gradual condensation of a scalar field produced during the decay of a superheavy dark matter. The advantage of this class of models to the ordinary quintessence is that the evolution of the dark energy and the dark energy are correlated and cosmological coincidence problem is solved. This model does not need a special form for the quitessence potential and even a simple $ϕ^4$ theory or an axion like scalar is enough to explain the existence of the Dark Energy. We show that the model has an intrinsic feedback between energy density of the dark matter and the scalar field such that for a large volume of the parameter space the equation of state of the scalar field from very early in the history of the Universe is very close to a cosmological constant. Other aspects of this model are consistent with recent CMB and LSS observations.

astro-ph↗
arXiv2003

Quintessence From a Decaying Dark Matter

It is a known fact that a quintessence model with $w_q < -1$ fits the publicly available Super Nova (SN) type Ia data better than a model with cosmological constant or $w_q > -1$. Two types of models have this property: Scalar fields with unconventional kinetic term and models with cosmological constant and a slowly decaying Cold Dark Matter (CDM). In this work we investigate the possibility of replacing the cosmological constant in the latter models with gradual condensation of a scalar field produced during the decay of the CDM and present some preliminary results. The advantage of this class of models to the ordinary quintessence is that the evolution of the dark energy and CDM are correlated and cosmological coincidence problem is solved or at least reduced to the fine tuning of the coupling between decaying CDM and quintessence field i.e the Hierarchy problem. Here we show that for part of the parameter space these models are consistent with present estimation of cosmological parameters.

astro-ph↗
arXiv2002

Testing Brane World Models with Ultra High Energy Cosmic Rays

The arrival time coherence of particles in the Ultra High Energy Air Showers where the center of mass energy of the interaction is of the order of $10^{15} eV$, puts strict constraint on the propagation of particles in a hypothetical extra-dimension. We first argue that at such high energies bulk modes and massive KK-modes can be produced abundantly and in many models their phase space volume is larger than confined modes. Then, we study the minimum propagation time in one and two-brane models and show that a large part of the parameter space of these models are ruled out unless the confinement of fields is proteced by symmetries up to energies not accessible even to the high energy tail of Ultar High Energy Cosmic Rays (UHECRs). As a by-product we confirm the result obtained in some previous works about the close relation between a small Cosmological Constant and the hierarchy problem.

hep-ph↗
arXiv2002

Statistical Identification of XMM-Newton Sources Using XID Database

The XMM identification programme (XID) is accumulating Optical/IR images and spectra to identify thousands of XMM sources. The XID Database is the main repository of this data and permits easy selection and correlation of XMM X-ray and UV (from Optical Monitor) data with Optical/IR observations. Here we discuss the methodology and some preliminary results of statistical classification and identification of X-ray sources using the data from XID Database. We show that in the medium range of X-ray fluxes and optical/UV magnitudes most of XMM sources are AGNs in agreement with previous observations. Moreover, AGNs are mostly concentrated in a small area of (U/g-r)-HR2/HR3 plane. At present the confirmed identifications are too scarce to permit statistical identification of individual sources without taking their spectra. Nonetheless, these types of investigation help to better understand general characteristics of classes of astronomical objects in X-ray and its relation with optical/IR ones.

astro-ph↗
arXiv2001

Two-Brane Models and BBN

We obtain a class of solutions for the AdS$_5$ two-brane models by imposing the observed value of cosmological constant and Newton coupling constant on the visible brane. When all terms up to the first order of matter density are included, the cosmological evolution on the observable brane depends on the equation of state of the matter and consequently when the pressure exists, the cosmology of these models deviates from FLRW cosmology. We show that it is possible to choose the matter equation of state on the hidden brane to neutralize its contribution on the cosmological evolution of the visible brane. We compare the prediction of these models for primordial {\it $^4$He} yield with observations. In standard BBN with $n_ν^{light} = 3$ this brane model is ruled out. If in addition to 3 SM neutrinos there is one light sterile neutrino, this model reconciles the observed {\it $^4$He} yield with a high $Ω_b \sim 0.033 h^{-2}$ suggested by BOOMERANG and MAXIMA experiments.

hep-ph↗
arXiv2000

Searching the Footprint of WIMPZILLAs

We constrain mass, lifetime and contribution of a very slowly decaying Ultra Heavy Dark Matter (UHDM) by simulating the cosmological evolution of its remnants. Most of interactions which participate in energy dissipation are included in the numerical solution of the Boltzmann equation. Cross-sections are calculated either analytically or by using PYTHIA Monte Carlo program. This paper describes in detail our simulation. To show the importance of the distribution of matter in constraining WIMPZILLA characteristics, we consider two extreme cases: a homogeneous universe, and a local halo with uniform distribution. We show that in a homogeneous universe, the decay of UHDM with a mass \sim 10^15 GeV and a lifetime \sim a few times τ_0 the age of the Universe, can not explain the flux of observed Ultra High Energy Cosmic Rays (UHECRs). This shows the importance of nearby sources, notably galactic halo. In a uniform clump with an over-density of \sim 200 extended to 100 kpc or more, the lifetime must be \sim 10 - 100 τ_0 or the contribution in the DM must be proportionally smaller. We also compare our calculation with observed gamma-rays at E \sim 10^11 eV by EGRET and CASA-MIA limit at E \sim 10^15 eV. They are compatible with a UHDM with relatively short lifetime.

astro-ph↗
arXiv2000

Cosmic Equation of State, Quintessence and Decaying Dark Matter

If CDM particles decay and their lifetime is comparable to the age of the Universe, they can modify its equation of state. By comparing the results of numerical simulations with high redshift SN-Ia observations we show that this hypothesis is consistent with present data. Fitting the simplest quintessence models with constant $w_q$ to data leads to $w_q \lesssim -1$. We show that a universe with a cosmological constant or quintessence matter with $w_q \sim -1$ and a decaying Dark Matter has an effective $w_q < -1$ and fits SN data better than stable CDM or quintessence models with $w_q > -1$.

astro-ph↗
arXiv1999

Initial Data Set For Cosmology: Application to Matching Condition

In Einstein theory of gravity the initial configuration of metric field and its time derivative are related to matter configuration by four equations called constraints. We use the method of conformal metrics (York Method) to solve constraints and find an analytic set of consistent initial data for linearized Einstein field equations in a perturbed constant curvature space-time. They are explicitly covariant and more compact than decomposition of quantities to scalar, vector and tensor. This method is independent of type and physics of matter fields and is extendable to higher-order perturbative calculations. As an application example, we apply this method to two commonly used matching conditions during a phase transition and compare and interpret the results.

astro-ph↗
arXiv1998

Can Dark Matter be Ultra Heavy Particles ?

The detection of High Energy Cosmic Rays (HECR) with energies around and beyond GZK expected cutoff has introduced the idea of existence of a decaying Ultra Heavy Dark Matter (UHDM). If this type of particles make a substantial part of the CDM, their decay can have important implications for evolution of the large structures and high energy backgrounds. Here we present preliminary results of numerical solution of Boltzmann equations in presence of a decaying CDM. The evolution of baryons and photons energy are calculated. We find that in a homogeneous universe, UHDM can not have a large contribution to the CDM. However, this hypothesis can probably survive if one takes into account the clumsiness of the matter in the Universe.

astro-ph↗