Institute for Research in Fundamental Sciences
Tehran, Iran
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Naseh A.,Institute for Research in Fundamental Sciences
Physical Review D | Year: 2016

For a generic conformal field theory (CFT) in four dimensions, the scale anomaly dictates that on conformally flat backgrounds, the universal part of entanglement entropy across a sphere [Cuniv(S2)] is positive. Based on this fact, we explore the consequences of assuming a positive sign for Cuniv(S2) on such backgrounds in a four-dimensional scale-invariant theory (SFT). In the absence of a dimension two scalar operator O2 in the spectrum of a SFT, we show that this assumption suggests that SFT is a CFT. In the presence of O2, we show that this assumption can fix the coefficient of the nonlinear coupling term ∫d4xgRO2 to a conformal value. © 2016 American Physical Society.

Chen X.,University of Texas at Dallas | Namjoo M.H.,Institute for Research in Fundamental Sciences
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

Standard Clocks in the primordial epoch leave a special type of features in the primordial perturbations, which can be used to directly measure the scale factor of the primordial universe as a function of time a(t), thus discriminating between inflation and alternatives. We have started to search for such signals in the Planck 2013 data using the key predictions of the Standard Clock. In this Letter, we summarize the key predictions of the Standard Clock and present an interesting candidate example in Planck 2013 data. Motivated by this candidate, we construct and compute full Standard Clock models and use the more complete prediction to make more extensive comparison with data. Although this candidate is not yet statistically significant, we use it to illustrate how Standard Clocks appear in Cosmic Microwave Background (CMB) and how they can be further tested by future data. We also use it to motivate more detailed theoretical model building. © 2014 The Authors.

Farzan Y.,Institute for Research in Fundamental Sciences
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2015

It is well-known that in addition to the standard LMA solution to solar anomaly, there is another solution called LMA-Dark which requires Non-Standard Interactions (NSI) with effective couplings as large as the Fermi coupling. Although this solution satisfies all the bounds from various neutrino oscillation observations and even provides a better fit to low energy solar neutrino spectrum, it is not as popular as the LMA solution mainly because no model compatible with the existing bounds has been so far constructed to give rise to this solution. We introduce a model that provides a foundation for such large NSI with strength and flavor structure required for the LMA-Dark solution. This model is based on a new U(1)' gauge interaction with a gauge boson of mass 10 MeV under which quarks as well as the second and third generations of leptons are charged. We show that observable effects can appear in the spectrum of supernova and high energy cosmic neutrinos. Our model predicts a new contribution to the muon magnetic dipole moment and new rare meson decay modes. © 2015 The Author.

Fareghbal R.,Institute for Research in Fundamental Sciences
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2010

We show that for a certain low frequency limit, the wave equation of a generic massive scalar field in the background of the spacelike warped AdS3 black hole can be written as the Casimir of an SL(2,R) symmetry. Two sets of SL(2,R) generators are found which uncover the hidden SL(2,R)×SL(2,R) symmetry of the solution. This symmetry is only defined locally and is spontaneously broken to U(1)×U(1) by a periodic identification of the Φ coordinate. By using the generator of the identification we read the left and right temperatures (TL,TR) of the proposed dual conformal field theory which are in complete agreement with the WAdS/CFT conjecture. Moreover, under the above condition of the scalar wave frequency, the absorption cross section of the scalar field is consistent with the two-point function of the dual CFT. © 2010 Elsevier B.V.

Alishahiha M.,Institute for Research in Fundamental Sciences
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

For a field theory with a gravitational dual, following Susskind's proposal we define holographic complexity for a subsystem. The holographic complexity is proportional to the volume of a codimension one time slice in the bulk geometry enclosed by the extremal codimension two hypersurface appearing in the computation of the holographic entanglement entropy. The proportionally constant, up to a numerical order of 1 factor is GR where G is the Newton constant and R is the curvature of the space-time. We study this quantity in certain holographic models. We also explore a possible relation between the defined quantity and fidelity appearing in quantum information literature. © 2015 American Physical Society.

Goldouzian R.,Institute for Research in Fundamental Sciences
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

The presence of the anomalous top-quark flavor-changing neutral-current (FCNC) interactions leads to the production of same-sign top quarks in proton-proton collisions. The results of a search for events with same-sign dileptons and b jets conducted by the CMS Collaboration with 10.5fb-1 of data collected in pp collisions at s=8TeV are used to obtain the constraints on the strength of top-quark FCNC interactions. The 95% confidence level upper limits on the branching ratios of top-quark decays to a light quark q=u,c and a gauge or a Higgs boson are set to be BR(t→uγ)<1.27%, BR(t→uZ)<0.8%, BR(t→ug)<1.02%, and BR(t→uH)<4.21%. The sensitivity of future searches in the same-sign top-quark channel is also presented. © 2015 American Physical Society.

Maleknejad A.,Institute for Research in Fundamental Sciences | Sheikh-Jabbari M.M.,Institute for Research in Fundamental Sciences | Soda J.,Kyoto University
Physics Reports | Year: 2013

The isotropy and homogeneity of the cosmic microwave background (CMB) favors "scalar driven" early Universe inflationary models. However, gauge fields and other non-scalar fields are far more common at all energy scales, in particular at high energies seemingly relevant to inflation models. Hence, in this review we consider the role and consequences, theoretical and observational, that gauge fields can have during the inflationary era. Gauge fields may be turned on in the background during inflation, or may become relevant at the level of cosmic perturbations. There have been two main classes of models with gauge fields in the background, models which show violation of the cosmic no-hair theorem and those which lead to isotropic FLRW cosmology, respecting the cosmic no-hair theorem. Models in which gauge fields are only turned on at the cosmic perturbation level, may source primordial magnetic fields. We also review specific observational features of these models on the CMB and/or the primordial cosmic magnetic fields. Our discussions will be mainly focused on the inflation period, with only a brief discussion on the post inflationary (p)reheating era. © 2013 Elsevier B.V.

Maleknejad A.,Institute for Research in Fundamental Sciences
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

We present a leptogenesis scenario associated with inflationary models involving non-Abelian gauge fields within the standard model (SM) of particle physics. We show that this class of inflationary models generates intrinsic birefringent gravitational waves that following Alexander, Peskin, and Sheikh-Jabbari [Phys. Rev. Lett. 96, 081301 (2006)], through the gravitational chiral anomaly in the SM, can naturally create a net lepton number density. The CP-violating interaction is produced by tensor fluctuations of the gauge field, while the efficiency of this process is determined by the effective background value of the gauge field. We demonstrate that this mechanism can create the observed value of baryon to photon number density in a natural range of parameters of these models. © 2014 American Physical Society.

Sheikh-Jabbari M.M.,Institute for Research in Fundamental Sciences
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012

Gauge-flation, non-Abelian gauge field inflation, which was introduced in Maleknejad and Sheikh-Jabbari (2011) [4] and analyzed more thoroughly in Maleknejad and Sheikh-Jabbari (2011) [5], is a model of inflation driven by non-Abelian gauge fields minimally coupled to Einstein gravity. In this model a certain rotationally invariant combination of gauge fields plays the role of the inflaton. Recently, the chromo-natural inflation model was proposed (Adshead and Wyman, 2012 [8]) which besides the non-Abelian gauge fields also involves an axion field. In this short Letter we show that the model involving axions, indeed allows for various slow-roll trajectories for different values of its parameters: A specific trajectory discussed in Adshead and Wyman (2012) [8] starts from a "small axion" region, while the trajectory considered in Maleknejad and Sheikh-Jabbari (2011) [4,5] corresponds to a "large axion" region. © 2012 Elsevier B.V.

Saberi A.A.,University of Tehran | Saberi A.A.,Institute for Research in Fundamental Sciences
Physics Reports | Year: 2015

Percolation is the simplest fundamental model in statistical mechanics that exhibits phase transitions signaled by the emergence of a giant connected component. Despite its very simple rules, percolation theory has successfully been applied to describe a large variety of natural, technological and social systems. Percolation models serve as important universality classes in critical phenomena characterized by a set of critical exponents which correspond to a rich fractal and scaling structure of their geometric features. We will first outline the basic features of the ordinary model. Over the years a variety of percolation models has been introduced some of which with completely different scaling and universal properties from the original model with either continuous or discontinuous transitions depending on the control parameter, dimensionality and the type of the underlying rules and networks. We will try to take a glimpse at a number of selective variations including Achlioptas process, half-restricted process and spanning cluster-avoiding process as examples of the so-called explosive percolation. We will also introduce non-self-averaging percolation and discuss correlated percolation and bootstrap percolation with special emphasis on their recent progress. Directed percolation process will be also discussed as a prototype of systems displaying a nonequilibrium phase transition into an absorbing state. In the past decade, after the invention of stochastic Löwner evolution (SLE) by Oded Schramm, two-dimensional (2D) percolation has become a central problem in probability theory leading to the two recent Fields medals. After a short review on SLE, we will provide an overview on existence of the scaling limit and conformal invariance of the critical percolation. We will also establish a connection with the magnetic models based on the percolation properties of the Fortuin-Kasteleyn and geometric spin clusters. As an application we will discuss how percolation theory leads to the reduction of the 3D criticality in a 3D Ising model to a 2D critical behavior. Another recent application is to apply percolation theory to study the properties of natural and artificial landscapes. We will review the statistical properties of the coastlines and watersheds and their relations with percolation. Their fractal structure and compatibility with the theory of SLE will also be discussed. The present mean sea level on Earth will be shown to coincide with the critical threshold in a percolation description of the global topography. © 2015 Elsevier B.V.

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