Sharif Digital Repository

After a decade of gravitational microlensing experiments, a dozen microlensing candidates in the direction of the stars of the Large Magellanic Cloud (LMC) have been detected by the EROS and MACHO groups. Recently it was shown that the distribution of the duration of the observed LMC microlensing events is significantly narrower than what is expected from the standard halo model. In this article we make the same comparison, using non-standard halo models and considering the contribution of non-halo components of the Milky Way such as the disc, spheroid and the LMC itself in the microlensing events. Comparing the theoretical and experimental widths of the distribution of the duration of... 

Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected when the light of remote stars crosses a Galactic - disk or halo - refractive medium such as a molecular cloud. We present the promising results of a test performed with the ESO-NTT, and consider its potential  

The dynamics of the Magellanic Stream (MS) as a series of clouds extending from the Magellanic Clouds (MCs) to the south Galactic pole is affected by the distribution and the amount of matter in the Milky Way. We calculate the gravitational effect of the Galactic disk on the MS in the framework of modified Newtonian dynamics (MOND) and compare with observations of the Stream's radial velocity. We consider the tidal force of the Galaxy, which strips material from the MCs to form the MS, and, using a no-halo model of the Galaxy, we ignore the effect of the drag of the Galactic halo on the MS. We also compare the MONDian dynamics with that in logarithmic and power-law dark halo models and show... 

We construct a model based on a new U(1) X gauge symmetry and a discrete Z 2 symmetry under which the new gauge boson is odd. The model contains new complex scalars which carry U(1) X charge but are singlets of the Standard Model. The U(1) X symmetry is spontaneously broken but the Z 2 symmetry is maintained, making the new gauge boson a dark matter candidate. In the minimal version there is only one complex scalar field but by extending the number of scalars to two, the model will enjoy rich phenomenology which comes in various phases. In one phase, CP is spontaneously broken. In the other phase, an accidental Z 2 symmetry appears which makes one of the scalars stable and therefore a dark... 

A perturbative description of Large Scale Structure is a cornerstone of our understanding of the observed distribution of matter in the universe. Renormalization is an essential and defining step to make this description physical and predictive. Here we introduce a systematic renormalization procedure, which neatly associates counterterms to the UV-sensitive diagrams order by order, as it is commonly done in quantum field theory. As a concrete example, we renormalize the one-loop power spectrum and bispectrum of both density and velocity. In addition, we present a series of results that are valid to all orders in perturbation theory. First, we show that while systematic renormalization... 

Shell galaxies are understood to form through the collision of a dwarf galaxy with an elliptical galaxy. Shell structures and kinematics have been noted to be independent tools to measure the gravitational potential of the shell galaxies. We compare theoretically the formation of shells in Type I shell galaxies in different gravity theories in this work because this is so far missing in the literature. We include Newtonian plus dark halo gravity, and two non-Newtonian gravity models, MOG and MOND, in identical initial systems. We investigate the effect of dynamical friction, which by slowing down the dwarf galaxy in the dark halo models limits the range of shell radii to low values. Under... 

Recent observations of gravitational waves motivate investigations for the existence of primordial black holes (PBHs). We propose the observation of gravitational microlensing of distant quasars for the range of infrared to the submillimeter wavelengths by sublunar PBHs as lenses. The advantage of observations in the longer wavelengths, comparable to the Schwarzschild radius of the lens (i.e., Rsch≃λ) is the detection of the wave optics features of the gravitational microlensing. The observation of diffraction pattern in the microlensing light curve of a quasar can break the degeneracy between the lens parameters and determine directly the lens mass as well as the distance of the lens from... 

The galactic scale challenges of dark matter such as 'missing satellite' problem and 'too big to fail' problem are the main caveats of standard model of cosmology. These challenges could be solved either by implementing the complicated baryonic physics or it could be considered as an indication to a new physics beyond the standard model of cosmology. The modification of collisionless dark matter models or the standard initial conditions are two promising venues for study. In this work, we investigate the effects of the deviations from scale invariant initial curvature power spectrum on number density of dark matter haloes. We develop the non-Markov extension of the excursion set theory to... 

We present a Vector Dark Matter (VDM) model that explains the 3.5 keV line recently observed in the XMM-Newton observatory data from galaxy clusters. In this model, dark matter is composed of two vector bosons, V and V', which couple to the photon through an effective generalized Chern-Simons coupling, gV. V' is slightly heavier than V with a mass splitting mV' - mV 3.5 keV. The decay of V' to V and a photon gives rise to the 3.5 keV line. The production of V and V' takes place in the early universe within the freeze-in framework through the effective gV coupling when mV' < T < Λ, Λ being the cut-off above which the effective gV coupling is not valid. We introduce a high energy model that... 

As an alternative to dark matter models, Modified Gravity (MOG) theory is a covariant modification of Einstein gravity. The theory introduces two additional scalar fields and one vector field. The aim is to explain the dynamics of astronomical systems based only on their baryonic matter. The effect of the vector field in the theory resembles a Lorentz force where each particle has a charge proportional to its inertial mass. The weak field approximation of MOG is derived by perturbing the metric and the fields around Minkowski space-time. We obtain an effective gravitational potential which yields the Newtonian attractive force plus a repulsive Yukawa force. This potential, in addition to the... 

We present a dark matter model for explaining the observed 130 GeV photon line from the galaxy center. The dark matter candidate is a vector boson of mass mV with a dimensionless coupling to the photon and Z boson. The model predicts a double line photon spectrum at energies equal to mV and mV(1-mZ2/4mV2) originating from the dark matter annihilation. The same coupling leads to a mono-photon plus missing energy signal at the LHC. The entire perturbative parameter space can be probed by the 14 TeV LHC run. The model has also a good prospect of being probed by direct dark matter searches as well as the measurement of the rates of h→γγ and h→Zγ at the LHC  

In this paper, we propose a new strategy for the pixel-lensing observations of M87 in the Virgo cluster using theHubble Space Telescope(HST). In contrast to a previous observational strategy by Baltz et al., we show that a few days of intensive observations with the duration of ~90 min in eachHSTorbit will increase the observational efficiency of high-magnification events by more than one order of magnitude. We perform a Monte Carlo simulation for this strategy and we show that the number of high-magnification microlensing events will increase at the rate of 4.2 events per day with a typical transit time-scale of 16 h. We also examine the possibility of observing mini-halo dark matter... 

The excursion set approach is a framework for estimating how the number density of nonlinear structures in the cosmic web depends on the expansion history of the universe and the nature of gravity. A key part of the approach is the estimation of the first-crossing distribution of a suitably chosen barrier by random walks having correlated steps: The shape of the barrier is determined by the physics of non-linear collapse, and the correlations between steps by the nature of the initial density fluctuation field. We describe analytic and numerical methods for calculating such first up-crossing distributions. While the exact solution can be written formally as an infinite series, we show how to... 

We study the possibility of magnetic mass detection using the gravitational microlensing technique. Recently, the theoretical effect of magnetic mass in NUT space on the microlensing light curve has been studied. It has been shown that in the low photometric signal-to-noise ratio and sampling rate of MACHO experiment light curves, no signature of the NUT factor has been found. In order to increase the sensitivity of magnetic mass detection, we propose a systematic search for microlensing events, using the currently running alert systems and complementary telescopes for monitoring Large Magellanic Clouds stars. This observation strategy provides the lowest observable limit of the NUT factor,... 

We study the theoretical signature of magnetic masses on the light curve of the gravitational microlensing effect in NUT space. The light curves for microlensing events in NUT space are presented and contrasted with those caused by lensing produced by normal matter. In the next step, associating the magnetic mass to the massive astrophysical compact halo objects (MACHOs), we try to see the effect on the light curves of microlensing candidates observed by the MACHO group. The presence or absence of this feature in the observed microlensing events can shed light on the question of the existence of magnetic masses in the Universe  

A search for long-lived particles decaying into muon pairs is performed using proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment at the LHC in 2017 and 2018, corresponding to an integrated luminosity of 101 fb−1. The data sets used in this search were collected with a dedicated dimuon trigger stream with low transverse momentum thresholds, recorded at high rate by retaining a reduced amount of information, in order to explore otherwise inaccessible phase space at low dimuon mass and nonzero displacement from the primary interaction vertex. No significant excess of events beyond the standard model expectation is found. Upper limits on branching... 

The first collider search for dark matter arising from a strongly coupled hidden sector is presented and uses a data sample corresponding to 138 fb−1, collected with the CMS detector at the CERN LHC, at s = 13 TeV. The hidden sector is hypothesized to couple to the standard model (SM) via a heavy leptophobic Z′ mediator produced as a resonance in proton-proton collisions. The mediator decay results in two “semivisible” jets, containing both visible matter and invisible dark matter. The final state therefore includes moderate missing energy aligned with one of the jets, a signature ignored by most dark matter searches. No structure in the dijet transverse mass spectra compatible with the...