Sharif Digital Repository

In this thesis, we first explain how we can construct three-point amplitudes for both massive and massless particles by solely knowing the Lorentz invariance and spin or helicity of particles. Then, using BCFW recursion relation, we determine the tree-level amplitudes. To calculate the loop amplitude, we apply the unitarity of Smatrix.The unitarity relates the tree-level amplitudes to loop level. Finally, I discuss some exciting topics in this field, including color-kinematics duality, some relevant applications of contemporary S-matrix theory in effective field theories  

During years, supersymmetry has been a candidate for resolution of Higgs mass fine-tuning problem that is quadratic dependence of scalar mass radiative correction to UV-physics. So, supersymmetry phenomenology is of great importance. As accelerators have reached energy of 13 TeV in recent years and sparticles have not yet been observed, it is important to have a theory in hand, describing SUSY breaking dynamics with scale of SUSY breaking above the energies currently accessible to accelerators. As a result of mass-sum-rule constraint which holds in theories with spontaneous SUSY breaking at tree level and leads to light sparticles and even tachyonic directions, there are currently no... 

One of the open questions in particle physics and cosmology is matter-antimatter asymmetry in the Universe. Observations at different scales in the early universe indicates that the number of baryons and leptons are greater than the number of anti-baryons and anti-leptons. Various mechanisms in the last 50 years, have been proposed to generate this asymmetry from a symmetric Universe. They can be classified into two groups: models which add new particles and interactions and models which consider gravity and time dependent background fields. The basic difference is that if they respect CPT symmetry or not. In this thesis we take the second approach. We study the neutrino sector of the... 

The dRGT theory of massive gravity is a consistent theory of interacting massive spin 2 particles. As an effective field theory compared to observational data, its UV cutoff is rather low. Namely, unitarity is violated at low energies in scattering of longitudinal modes. In this dissertation, we give a brief review of the dRGT theory and its challenges. Of particular interest is the question of its UV completion in a more consistent model. Then, we show that this theory can be obtained from a topological gauge theory in one higher dimension. After dimensional reduction and symmetry breaking via boundary conditions, we obtained four dimensional effective field theories including the dRGT plus... 

Effective field theory is a practical method for model building and explaining experimental/observational data. These theories are constructed based on knowing low energy degrees of freedom and symmetries (accidental or fundamental). By construction, these theories are applicable up to a cut-off scale and their predictions beyond that are not reliable. For applications to higher energies a UV completion is needed through which modifications are applied or new heavy degrees of freedom are introduced. In order to complete a theory in a Wilsonian approach, scattering amplitudes must satisfy some constraints. These constraints are derived from basic assumptions like Lorentz invariance,... 

In this manuscript we used the method of coset construction to describe theories of spontaneous symmetry breaking. The mathematical details are included in chapter 2. The utility of the method is that without any knowledge of the UV part of a theory we can construct IR part by just knowing the symmetry breaking pattern. We used the method for some symmetry breaking patterns to construct general relativity, massive gravity and also some symmetry breaking patterns corresponding to conformal gravity  

Exponential expansion of the universe is an important issue in early-time and late-time cosmology . In the widely accepted standard model of cosmology, there are two assumed exponentially expanding phases , early-time inflationary phase and late-time dark energy phase . On the other hand, there are ideas from string theory that limit the possibility of constructing the de Sitter space, which are categorized in the Swampland program . In this work, we first examine no-scale models of de Sitter space in supergravity from the point of view of de Sitter conjectures . In this regard, we observe that satisfying these conjectures limits the parameter and field space and the trans-Planckian... 

Swampland conjectures are set of conditions that must be satisfied in a quantum field theory in order to be compatible with a theory of quantum gravity. These conditions have been deduced from the features of field theories, which are derived from the compactification of the additional dimensions of the string theory. However, it is thought that the mpatibility of these conditions with the theory of quantum gravity is independent of string theory. One of the important reasons is that the swampland conjectures can be obtained by examining the physics of black holes. Black hole is one of the quantum states of any quantum gravity theory. In this thesis, first we will have an overview of the... 

Today with respect to the particle physics experiments and astrophysical observations we know that Neutrinos are massive. Free streaming of light Neutrinos supresses the density field of matter and opposes gravitational growth, so that clusters formed at fixed mass are fewer and hence more biased than for a pure CDM density field. One way to study the effects of Neutrinos on the formation of large-scale structures is to examine the matter power-spectrum and corellation function in the presence of massive Neutrinos.In the presence of massive Neutrinos, the matter power-spectrum is suppressed in small scales and also correlation function and clustering bias are changed in comparison to the CDM... 

In this thesis, we study different aspects of tilted Dirac materials from hydrodynamics and holography approach. Dirac materials are among the most important structures which have been introduced in recent years in condensed matter physics where graphene is the simplest of them. Classification of space groups allows nonsymmorphic structures where the low energy electronic degrees of freedom can be described by tilted Dirac cones. In the long wavelength and time limit, these materials can be effectively described by a metric which is called tilted metric. Dirac materials are usually strongly coupled and cannot be studied using the common perturbative techniques; therefore hydrodynamics and... 

The top quark is the heaviest of elementary particles. It gives the dominant contribution to the radiative correction to the Higgs self coupling constant which itself determines the structure of the electroweak vacuum. Therefore, it is extremely important to have a precise measurement of the top quark mass. On the other hand, whereas the lifetime of top quark is short and rapidly decays to lighter particles, it is not easy to compute and measure its mass with a desired precision. In this work, we propose a measurement method to determine the top quark mass in the t▁t→leptons +jets channel by analyzing the decay of the b-hadron in the top quark decay chain. Through the reconstruction of the... 

Supersymmetry breaking mechanism is an unanswered question. Which is not allowed to break at tree-level in MSSM due to supertrace restrictions. Most theories suppose it’s broken in a dark section, much higher energies than electro-weak and its impacts as soft SUSY breaking terms emerged in Lagrangian. Here, in a different approach, an extension of MSSM with a U(1) and Stueckelberg superfield along with nonlinear gauge transformation, provides an additional condition to present a tree-level SUSY breaking mechanism. Mass mixing term in Stueckelberg’s Lagrangian or kinetic mixing are two methods of such an extension. The particles mass terms, solving the Higgsino mass parameter problem µ,... 

In the framework of quantum field theory, countless consistent theories can be constructed. However, it is understood that only a small subset of these theories can be consistently embedded in a theory of quantum gravity (QG). In string theory, as the best candidate for quantum gravity, there is a landscape of effective field theories (EFTs) which are descended from some compactification of extra dimensions. EFTs which are not derived from a compactification is said to belong to swampland. From a bottom-up point of view, the aim of the swampland program is to find a list of criteria which an EFT must respect so that it survives the swampland. The trans-Planckian censorship conjecture (TCC)...