Dissertations / Theses on the topic 'Elementary Particles and Fields and String Theory'

The purpose of this thesis is to study a symmetry of string theory known as T-duality. We focus on a particular example establishing the equivalence between a quantized string moving in a circular space of radius R and a dual string moving in a similar space of radius 1/R . We will show that this duality implies that the momentum of the string in one picture becomes the number of times the string is wound around the circle in the dual picture. We present two proofs of T-duality. The first reflects the standard interpretation of T-duality as an isomorphism of quantum theories. The second approach is based on Hull's Double Field Theory.

The concept of faster than light travel in general relativity is examined, starting with a review of the Alcubierre metric. This spacetime, although incredible in its implications, has certain unavoidable problems which are discussed in detail. It is demonstrated that in order to describe faster than light travel without any ambiguities, a coordinate independent description is much more convenient. An alternative method of describing superluminal travel is then proposed, which has similarities to the Krasnikov tube.

This thesis is devoted to the study of two important applications of gauge-gravity duality: the cosmological singularity problem and conformal fluid dynamics. Gauge-gravity duality is a concrete dual relationship between a gauge theory (such as electromagnetism, the theories of weak and strong interactions), and a theory of strings which contains gravity. The most concrete application of this duality is the AdS/CFT correspondence, where the theory containing gravity lives in the bulk of an asymptotically anti-de-Sitter space-time, while the dual gauge theory is a deformation of a conformal field theory which lives on the boundary of anti-de-Sitter space-time(AdS). Our first application of gauge-gravity duality is to the cosmological singularity problem in string gravity. A cosmological singularity is defined as a spacelike region of space-time which is highly curved so that Einstein’s gravity theory can be no longer applied. In our setup the bulk space-time has low curvature in the far past and the physics is well described by supergravity (which is an extension of standard Einstein gravity). The cosmological singularity is driven by a time dependent string coupling in the bulk theory. The rate of change of the coupling is slow, but the net change of the coupling can be large. The dual description of this is a time dependent coupling of the boundary gauge theory. The coupling has a profile which is a constant in the far past and future and attains a small but finite value at intermediate times. We construct the supergravity solution, with the initial condition that the bulk space-time is pure AdS in the far past and show that the solution remains smooth in a derivative expansion without formation of black holes. However when the intermediate value of the string coupling becomes weak enough, space-time becomes highly curved and the supergravity approximation breaks down, mimicking a spacelike singularity. The resulting dynamics is analyzed in the dual gauge theory with a time dependent coupling constant which varies slowly. We develop an appropriate adiabatic expansion in the gauge theory in terms of coherent states and show that the time evolution continues to be smooth. We cannot, however, arrive at a definitive conclusion about the fate of the system at very late times when the coupling has again risen and supergravity again applies. One possibility is that the energy which has been supplied to the universe is simply extracted out and the space-time goes back to its initial state. This could provide a model for a bouncing cosmology. A second possibility is that dissipation leads to a thermal state at late time. If this possibility holds, we show that such a thermal state will be described either by a gas of strings or by a small black hole, but not by a big black hole. This means that in either case, the future space-time is close to AdS. We then apply gauge-gravity duality to conformal fluid dynamics. The long wavelength behavior of any strongly coupled system with a finite mean free path is described by an appropriate fluid dynamics. The bulk dual of a fluid flow in the boundary theory is a black hole with a slowly varying horizon. In this work we consider certain fluid flows which become supersonic in some regions. It is well known that such flows present acoustic analogs of ergoregions and horizons, where acoustic waves cannot propagate in certain directions. Such acoustic horizons are expected to exhibit thermal radiation of acoustic waves with temperature essentially given by the gradient of the velocity at the acoustic horizon. We find acoustic analogs of black holes in charged conformal fluids and use gauge-gravity duality to construct dual gravity solutions. A certain class of gravitational quasinormal wave modes around these gravitational backgrounds perceives a horizon. Upon quantization, this implies that these gravitational modes should have a thermal spectrum. The final issue that we study is fluid-gravity duality at zero temperature. The usual way of constructing gravity duals of fluid flows is by means of a small derivative expansion, in which the derivatives are much smaller than the temperature of the background black hole. Recently, it has been reported that for charged fluids, this procedure breaks down in the zero temperature limit. More precisely, corrections to the small derivative expansion in the dual gravity of charged fluid at zero temperature have singularities at the black hole horizon. In this case, fluid-gravity duality is not understood precisely. We explore this problem for a zero temperature charged fluid driven by a low frequency, small amplitude and spatially homogeneous external force. In the gravity dual, this force corresponds to a time dependent boundary value of the dilaton field. We calculate the bulk solution for the dilaton and the leading backreaction using a modified low frequency expansion. The resulting solutions are regular everywhere, establishing fluid-gravity duality to this order.

From Birkoff's theorem, the geometry in four spacetime dimensions outside a spherically symmetric and static, gravitating source must be given by the Schwarzschild metric. This metric therefore satisfies the Einstein vacuum equations. If the mass which gives rise to the Schwarzschild spacetime geometry is concentrated within a radius of r=2M, a black hole will form. Non-accelerating particles (freely falling) traveling through this geometry will do so along parametrized curves called geodesics, which are curved space generalizations of straight paths. These geodesics can be found by solving the geodesic equation. In this thesis, the geodesic structure in the Schwarzschild geometry is investigated with an attempt to generalize the solution to higher dimensions.

We study constraints imposed on a general unitary two-dimensional conformal field theory by modular invariance. We begin with a review of previous bounds on the conformal dimension Delta1 of the lowest primary operator assuming unitarity, a discrete spectrum, modular invariance, cL, cR > 1, and no extended chiral algebra. We then obtain bounds on the conformal dimensions Delta2, Delta3 using no additional assumptions. We also show that in order to find a bound for Delta4 or higher Deltan, we need to assume a larger minimum value for ctot that grows logarithmically with n. We next extend the previous results to remove the requirement that our two-dimensional conformal field theories have no extended chiral algebra. We then show that modular invariance also implies an upper bound on the total number of states of positive energy less than ctot=24 (or equivalently, states of conformal dimension between ctot=24 and ctot=12), in terms of the number of negative energy states. Finally, we consider the case where the CFT has a gravitational dual and investigate the gravitational interpretation of our results. Using the AdS3/CFT2 correspondence, we obtain an upper bound on the lightest few massive excitations (both with and without the constraint of no chiral primary operators) in a theory of 3D matter and gravity with Lambda < 0. We show our results are consistent with facts and expectations about the spectrum of BTZ black holes in 2+1 gravity. We then discuss the upper and lower bounds on number of states and primary operators in the dual gravitational theory, focusing on the case of AdS3 pure gravity.

CP violation is an important condition to explain the preponderance of baryons in our universe, yet the available CP violation in the Standard Model (SM) via the so-called Cabibbo-Kobayashi-Maskawa mechanism seems to not provide enough CP violation. Thus searching for new sources of CP violation is one of the central tasks of modern physics. In this thesis, we focus on a new possible source of CP violation which generates triple-product correlations in momenta which can appear in neutron and nuclear radiative β decay. We show that at low energies such a CP violating correlation may arise from the exotic coupling of nucleon, photon and neutrino that was proposed by Harvey, Hill, and Hill (HHH). One specialty of such an exotic HHH coupling is that it does not generate the well-known CP-violating terms such as ``D-term'', ``R-term'', and neutron electric dipole moment, in which particle's spins play critical role. We show that such a new HHH-induced CP violating effect is proportional to the imaginary part of c5gv, where gv is the vector coupling constant in neutron and nuclear β decay, and c5 is the phenomenological coupling constant that appears in chiral perturbation theory at O(M-2) with M referring to the nucleon or nuclear mass. We consider a possible non-Abelian hidden sector model, which is beyond the SM and may yield a nontrivial Im(c5). The available bounds on both Im(c5) and Im(gv) are considered, and a better limit on Im(c5) can come from a direct measurement in radiative beta decay. We calculate the competitive effect that arises from the general parameterization of the weak interaction that was proposed by Lee and Yang in 1956. We also show that in the proposed measurements, the CP-violating effect can be mimicked by the SM via final-state interactions (FSI). For a better determination of the bound of Im(c5), we consider the FSI-induced mimicking effect in full detail in O(α) as well as in leading recoil order. To face ongoing precision measurements of neutron radiative β decay of up to 1% relative error, we sharpen our calculations of the CP conserving pieces of neutron radiative β decay by considering the largest contributions in O(α2): the final-state Coulomb corrections as well as the contributions from two-photon radiation.

In this thesis, the general framework of supersymmetric quantum mechanics and the path integral approach will be presented (as well as the worked out example of the supersymmetric harmonic oscillator). Then the theory will be specialized to the case of supersymmetric quantum mechanics on Riemannian manifolds, which will start from a supersymmetric Lagrangian for the general case and the special case for S2. Afterwards, there will be a discussion on the superfield formalism. Concluding this thesis will be the Hamiltonian formalism followed by the inclusion of deforma- tions by potentials.

We present comprehensive analysis of the light and strange disconnected-sea quarks contribution to the nucleon electric and magnetic form factors. The lattice QCD estimates of strange quark magnetic moment GsM (0) = −0.064(14)(09) μN and the mean squared charge radius ⟨r2s⟩E = −0.0043(16)(14) fm2 are more precise than any existing experimental measurements and other lattice calculations. The lattice QCD calculation includes ensembles across several lattice volumes and lattice spacings with one of the ensembles at the physical pion mass. We have performed a simultaneous chiral, infinite volume, and continuum extrapolation in a global fit to calculate results in the continuum limit. We find that the combined light-sea and strange quarks contribution to the nucleon magnetic moment is−0.022(11)(09) μN and to the nucleon mean square charge radius is −0.019(05)(05) fm2. The most important outcome of this lattice QCD calculation is that while the combined light-sea and strange quarks contribution to the nucleon magnetic moment is small at about 1%, a negative 2.5(9)% contribution to the proton charge radius and a relatively larger positive 16.3(6.1)% contribution to the neutron charge radius come from the sea quarks in the nucleon. For the first time, by performing global fits, we also give predictions of the light-sea and strange quarks contributions to the nucleon electric and magnetic form factors at the physical point and in the continuum and infinite volume limits in the momentum transfer range of 0 ≤ Q2 ≤ 0.5 GeV2.

With the use of chiral theory of mesons [1], [2] we evaluate the decay rate of η′ → π+π−π+π−. Our theoretical study of this problem is different from the previous theo- retical study [3] and our predicted result is in a good agreement with the experiment. In this chiral theory we evaluate Feynman diagrams up to one loop and the decay rate is calculated with the use of triangle and box diagrams. The ρ0 meson includes in both type of diagrams as a resonance state. Divergent integrals in the loop calculations are regularized with the use of n-dimensional ’t Hooft-Veltman regularization technique. At the last step to obtain the decay rate, the phase space integral has been calculated.

In this thesis, the clean angular observables in the $\bar{B} \to D^{*+} \ell^- \bar{\nu}_{\ell}$ angular distribution is studied. Similar angular observables are widely studied in $B \to K^* \mu^+ \mu^-$ decays. We believed that these angular observables may have different sensitivities to different new physics structures.

One of the most important results emerging from string theory is the gauge gravity duality (AdS/CFT correspondence) which tells us that certain problems in particular gravitational backgrounds can be exactly mapped to a particular dual gauge theory a quantum theory very similar to the one explaining the interactions between fundamental subatomic particles. The chief merit of the duality is that a difficult problem in one theory can be mapped to a simpler and solvable problem in the other theory. The duality can be used both ways. Most of the current theoretical framework is suited to study equilibrium systems, or systems where time dependence is at most adiabatic. However in the real world, systems are almost always out of equilibrium. Generically these scenarios are described by quenches, where a parameter of the theory is made time dependent. In this dissertation I describe some of the work done in the context of studying quantum quench using the AdS/CFT correspondence. We recover certain universal scaling type of behavior as the quenching is done through a quantum critical point. Another question that has been explored in the dissertation is time dependence of the gravity theory. Present cosmological observations indicate that our universe is accelerating and is described by a spacetime called de-Sitter(dS). In 2011 there had been a speculation over a possible duality between de-Sitter gravity and a particular field theory (Euclidean SP(N) CFT). However a concrete realization of this proposition was still lacking. Here we explicitly derive the dS/CFT duality using well known methods in field theory. We discovered that the time dimension emerges naturally in the derivation. We also describe further applications and extensions of dS/CFT.

Polarized deep inelastic scattering experiments play a vital role in the exploration of the spin structure of the proton. The polarized proton-proton collider at RHIC provides direct access to the gluon spin distribution through longitudinal double spin asymmetry measurements of inclusive jets, pions, and dijets. This thesis presents the measurement of the dijet double differential cross-section in proton-proton collisions at center of mass energies of √s = 500 GeV. The data represent an integrated luminosity of 8.7 pb-1 recorded by the STAR detector during the 2009 RHIC run. A comprehensive jet analysis was performed to determine the ideal jet algorithm and jet parameters used in √s = 500 GeV collisions at the STAR detector. The cross-section is measured as a function of the dijet invariant mass (30 ≤ Mij ≤ 152 GeV) in the mid rapidity region with a maximum rapidity range of |ymax| ≤ 0.8. This result shows agreement with theoretical next-to-leading order pQCD calculations, motivating the use of dijet asymmetries at STAR to further constrain the shape of Δg(x).

In the nanoscale metrology industry, there is a need for low-cost instruments, which have the ability to probe the structrure and elemental composition of thin films. This dissertation, describes the research performed to design and simulate a miniature Cylindrical Mirror Analyzer, (CMA), and Auger Electron Spectrometer, (AES). The CMA includes an integrated coaxial thermionic electron source. Electron optics simulations were performed using the Finite Element Method, (FEM), software COMSOL. To address the large Secondary Electron, (SE), noise, inherent in AES spectra, this research also included experiments to create structures in materials, which were intended to suppress SE backgound noise in the CMA. Laser Beam Machining, (LBM), of copper substrates was used to create copper pillars with very high surface areas, which were designed to supress SE’s. The LBM was performed with a Lumera SUPER RAPID‐HE model Neodymium Vanadate laser. The laser has a peak output power of 30 megawatts, has a 5x lens and a spot size of 16 μm. The laser wavelength is in the infrared at 1064 nm, a pulse width of 15 picoseconds, and pulse repetition rate up to 100 kHz. The spectrometer used in this research is intended for use when performing chemical analysis of the surface of bulk materials and thin films. It is applicable for metrology of thin films, as low as 0.4 nm in thickness, without the need to perform destructive sample thinning, which is required in Scanning Tranmission Electron Microscopy, (STEM). The spectrometer design is based on the well known and widely used coaxial cylinder capacitor design known as the Cylindrical Mirror Analyzer, (CMA). The coaxial tube arrangement of the CMA allows for placing an electron source,which is mounted in the center of the inner cylinder of the spectrometer. Simulation of the electron source with an Einzel Lens was also performed. In addtion, experiments with thin film coatings and Laser Beam Machining to supress Secondary Electron emission noise within the Auger electron spectrum were completed. Design geometry for the miniature CMA were modeled using Computer Aided Design, (CAD). Fixed Boundary Conditions, (BC), were applied and the geometry was then meshed for FEM. The electrostatic potential was then solved using the Poisson equation at each point. Having found the solution to the electrostatic potentials, electron flight simulations were performed and compared with the analytical solution. From several commercially available FEM modeling packages, COMSOL Multiphysics was chosen as the research platform for modeling of the spectrometer design. The CMA in this design was reduced in size by a factor of 4 to 5. This enabled mounting the CMA on a 2 ¾ in flange compared to the commercial PHI model 660 CMA which mounts onto a 10 in flange. Results from the Scanning Electron Microscopy measurements of the Secondary Electron emission characteristics of the LBM electron suppressor will also be presented.

The enhancon mechanism is a specific phenomenon in string theory which resolves a certain naked spacetime singularity arising in the supergravity description related to N = 2 supersymmetric pure gauge theory. After reviewing the problem of singularities in general relativity as well as in string theory, and discussing the prototypical enhancon example constructed by wrapping D6-branes on a K3 surface, the thesis presents three generalisations to this static spherically symmetric case pertaining to large N SU{N) gauge theory. First we will use orientifolds to show how the enhancon mechanism also works in similar situations related to SO(2N + 1), USp(2N) and SO{2N) gauge theories. Second we will wrap D-brane distributions on K3 to obtain the enhancon in oblate, toroidal and prolate shapes. Third we will study a rotating enhancon configuration and consider its implications for the black hole entropy and the second law of thermodynamics.

String theory is to date the best candidate for the "theory of everything" which would describe all four fundamental interactions, including gravity, on the same footing. The general description of black holes and other spacetimes with horizons remains a key aspect of quantum gravity that must be addressed by a consistent theory, such as string theory. Also, the resolution of singularities is a fundamental problem in a quantum theory of gravity. At the same time, the recent revolution in observational cosmology creates a pressing need to accommodate fundamental issues such as cosmic acceleration and cosmological horizons within the framework of string theory. Therefore, the focus of this thesis is on new aspects of gravitation from a point of view of string theory. In the first part of this thesis, after an introduction in the subject, we deal with the resolution of singularities in string theory. In particular, we describe one of the intrinsically stringy mechanisms to resolve singularities, namely "Enhancon mechanism". We generalize the basic enhancon solution by constructing solutions without spherical symmetry.
The second part of the thesis is devoted to investigating different aspects of holography. The AdS/CFT correspondence is a concrete realization of the holographic principle. Such correspondence is referred to as duality in the sense that the supergravity (closed string) description of D-branes and the field theory (open string) description are different formulations of the same physics. This way, the infrared (IR) divergences of quantum gravity in bulk are equivalent to ultraviolet (UV) divergences of dual field theory living on the boundary. A novel method to renormalize the stress-energy of gravity and provide a measure of gravitational mass was proposed. We used this method to study locally asymptotically anti-de Sitter spacetimes with nonzero NUT charge and charged black holes configurations in de Sitter spacetime, respectively.

Starting from the 10-dimensional low-energy string effective action for the graviton and dilaton, we study cosmological implications of string theory. We find some solutions for the string equations of motion both in vacuum and with the presence of matter.
The scale factors r and R, which can be interpreted as the radii of the universe, tend to evolve in opposite directions: one radius expands and the other shrinks.
We also study the flatness problem and propose an alternative solution to this problem.
The behaviour of the radii r and R rear the Planck length $({ sim}10 sp{-33}$cm) is studied in detail.
The significance of our results lies in the fact that in the context of string theory, we may have a good chance of observing several large spatial dimensions, with other internal spatial dimensions remaining small and unobserved from a macroscopic point of view.

In string theory, some techniques have been developed to calculate scattering amplitudes. Because of the $SL(2, doubc)$ invariance for tree-level amplitudes, a physical inconsistency arises; the amplitudes, instead of being finite, diverge. Two techniques can be used to remedy this problem. To avoid the overcounting configurations, we can (a) divide the amplitude by the $SL(2, doubc)$ volume or (b) insert three ghosts in the amplitude integral. Independently of which one of the two ways is followed, a number of two-dimensional integrations over the complex plane remain to be performed. This thesis explores some ways to replace some of these two-dimensional integrals by two unidimensional contour integrals. Consequently, this transformation reveals a new way to think about the ghosts and their role in amplitude calculations. In fact, we can consider the ghosts as perpendicular vectors to the contour along which the integration is performed.

Hydrodynamics is an effective theory that is extremely successful in describing a wide range of physical phenomena in liquids, gases and plasmas. However, our understanding of the structure of the theory, its microscopic origins and its behaviour at strong coupling is far from complete. To understand how an effective theory of dissipative hydrodynamics could emerge from a closed microscopic system, we analyse the structure of effective Schwinger-Keldysh Closed-Time-Path theories. We use this structure and the action principle for open systems to derive the energy-momentum balance equation for a dissipative fluid from an effective CTP Goldstone action. Near hydrodynamical equilibrium, we construct the first-order dissipative stress-energy tensor and derive the Navier-Stokes equations. Shear viscosity is shown to vanish, while bulk viscosity and thermodynamical quantities are determined by the form of the effective action. The exploration of strongly interacting states of matter, particularly in the hydrodynamic regime, has been a major recent application of gauge/string duality. The strongly coupled theories involved are typically deformations of large-$N$ SUSY gauge theories with exotic matter that are unusual from a low-energy point of view. In order to better interpret holographic results, an understanding of the weak-coupling behaviour of such gauge theories is essential. We study the exact and SUSY-broken N=1 and N=2 super-QED with finite densities of electron number and R-charge, respectively. Despite the fact that fermionic fields couple to the chemical potentials, the strength of scalar-fermion interactions, fixed by SUSY, prevents a Fermi surface from forming. This is important for hydrodynamical excitations such as zero sound. Intriguingly, in the absence of a Fermi surface, the total charge need not be stored in the scalar condensates alone and fermions may contribute. Gauss-Bonnet gravity is a useful laboratory for non-perturbative studies of the higher derivative curvature effects on transport coefficients of conformal fluids with holographic duals. It was previously known that shear viscosity can be tuned to zero by adjusting the Gauss-Bonnet coupling, λ_GB, to its maximal critical value. To understand the behaviour of the fluid in this limit, we compute the second-order transport coefficients non-perturbatively in λ_GB and show that the fluid still produces entropy, while diffusion and sound attenuation are suppressed at all order in the hydrodynamic expansion. We also show that the theory violates a previously proposed universal relation between three of the second order transport coefficients. We further compute the only second-order coefficient thus far unknown, λ₂, in the N=4 super Yang-Mills theory with the leading-order 't Hooft coupling correction. Intriguingly, the universal relation is not violated by these leading-order perturbative corrections. Finally, by adding higher-derivative photon field terms to the action, we study charge diffusion and non-perturbative parameter regimes in which the charge diffusion constant vanishes.

The prescription of R. M. Wald for determining a thermodynamic first law for stationary, axisymmetric, asymptotically flat black holes in general theories of gravity is applied to the effective Lagrangian for the bosonic sector of low-energy heterotic super-string theory. It is found that the presence of the gauge fields necessitates an alteration of the prescription. Specifically, the Lie derivatives with respect to the Killing vector of the gauge fields are non-vanishing. This introduces new terms which ensure gauge-invariance of the final result in a natural way.

In this thesis we review the gauge/gravity duality and how it can be used to compute the thermodynamic properties and low-energy excitations of holographic quantum liquids - strongly-interacting field theories with a non-zero density of matter. We then study in detail the charge density excitations of two such liquids, the D3/D7 theory and the RN-AdS₄ theory, by computing the poles of their charge density Green's functions, and their charge density spectral functions. Although it is not a Landau Fermi liquid, the charge density excitations of the D3/D7 theory display many of the same properties as one, including a collisionless/hydrodynamic crossover as the temperature is increased. In contrast to this, the charge density (and energy density) excitations of the RN-AdS₄ theory do not share these properties but behave in a way that cannot be explained by Landau's theory of interacting fermionic quasiparticles. This is consistent with other results which indicate that this is not a Landau Fermi liquid.

The experiments performed at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab have discovered a state of matter called the strongly coupled quark-gluon plasma (sQGP). The strong coupling has limited the ability of the standard theory to describe such matter, namely Quantum Chromodynamics (QCD). However, string theory's anti-de Sitter/conformal field theory (AdS/CFT) correspondence has provided a new way to study the situation and in an analytical manner. So far, hydrodynamic properties of RHIC's plasma, such as elliptic flow and longitudinal expansion, have been seen to follow from classical supergravity calculations. In this dissertation I discuss some of the field's development as well as the research done by the author and collaborators.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.
Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1071. Adviser: Sheldon Katz. Includes bibliographical references (leaves 117-123) Available on microfilm from Pro Quest Information and Learning.

The main purpose of Relativistic Heavy Ion Collider (RHIC) program is to study the Quark-Gluon Plasma (QGP), a deconfined state of matter believed to be created in ultra-relativistic heavy ion collisions. Heavy quarks, expected to be produced during the earlier stages of heavy ion collisions, serve as an important probe of the QGP.‎ ‎The following dissertation presents measurements of single muons resulting from the semileptonic decay of heavy flavor quarks in the rapidity range of $1.4 < vertetavert < 1.9$ for Cu+Cu nuclei collisions at $sqrt{s_{NN}}=200$ GeV measured by the PHENIX experiment. Single muon spectra were measured for three different centrality classes (0 - 20 \% , 20 - 40 \%, 40 - 94 \%) within the $p_{T}$ range of 1.0 - 4.0 GeV/c.‎ ‎To calculate single muon spectra, a full background estimate was statistically subtracted from inclusive spectra of muon candidate tracks reconstructed in the PHENIX muon arms. The background was predicted and estimated with a ``Hadron Cocktail", a full-scale data-driven Monte Carlo simulation. The hadron cocktail approach was originally developed and implemented to measure single muon production for Run-5 p+p collisions. First, the relevant light hadrons are generated with a ``realistic'' input (ratios of different particle species and $p_{T}$ spectra). The generated tracks are then propagated through the PHENIX detector geometry using GEANT. At the last step, introduced and implemented specifically for this analysis, the simulated tracks were embedded into real events and finally reconstructed with the PHENIX muon arms reconstruction software. This was done to realistically reproduce detector performance due to effects caused by colliding heavy ions. The hadron cocktail method provides a much better alternative to the previously attempted purely data-driven peacemeal approaches which suffer from very large systematic uncertainties.‎ ‎Finally, using baseline single muon measurements for p+p collisions, nuclear modification factors for all of the above specified centralities have been measured. These are the first measurements of single muon spectra and nuclear modification factors at forward angles for any two heavy colliding ion systems.

With the start of the age of the Large Hadron Collider (LHC) two challenges face theoreticians and computational physicists. The first is about understanding theories beyond the Standard Model and producing verifiable predictions that can be tested against what the LHC and subsequent machines would produce. The second is to improve computational methods so that the new experimental precision is matched by a theoretical one. But this improvement is also crucial for the detection of potential deviations from Standard Model predictions and possibly also finding the elusive Higgs. This work tries to address problems in both areas. In the first part we study the effects of adding tension in considering a black-hole on a brane. Such black-holes are predicted by some models as potential phenomena at the LHC. We calculate the effects of adding tension on observable quantities of black-holes, namely, quasinormal mode frequencies and Hawking radiation, and we show how this improves predictions. In the second part we investigate the computational problem of extending the Britto-Cachazo-Feng-Witten (BCFW) method to 1-loop level. The BCFW has been successfully used in recent years to compute scattering amplitudes at tree-level by suitably complex-shifting external momenta and reducing diagrams to simpler ones. In our investigation we establish that the BCFW can be extended to 1-loop, which means that 1-loop integrands can be treated as trees and can be broken down further into even simpler trees using the BCFW. We explicitly look at the effects of the shift for the lowest three n-point cases, but also demonstrate how the result extends to arbitrary n.