The production of single top quarks through the weak interaction is of great interest in particle physics, as it provides the only means to directly measure the CKM matrix element |Vtb|, one of the few remaining Standard Model parameters which has yet to be precisely constrained by direct experimental measurements. This thesis describes a search for Wt-channel single top quark production using proton-proton collision data collected at a center-of-mass energy of 7 TeV by the ATLAS detector at the Large Hadron Collider, corresponding to a total integrated luminosity of 35 pb-1. Collision events having exactly two oppositely-charged electrons or muons, a single high-transverse-momentum jet, and large missing or total transverse energy are selected, resulting in a sample with enhanced single top purity. 38 data events are found to survive the selection, in good agreement with the expected sum from all signal and background predictions of 38.6 ± 6.0. The Wt content of the selected sample is predicted to amount to 4.3 ± 0.3 events. The resulting cross-section measurement is 10 +20-10 (stat) +20-10 (syst) pb. As this level of precision is insufficient to claim evidence or discovery of this process, an upper limit is also derived for the parameter of interest, resulting in σ(pp→Wt) < 70 pb at 95% confidence level.

The production of single top quarks through the weak interaction is of great interest in particle physics, as it provides the only means to directly measure the CKM matrix element |Vtb|, one of the few remaining Standard Model parameters which has yet to be precisely constrained by direct experimental measurements. This thesis describes a search for Wt-channel single top quark production using proton-proton collision data collected at a center-of-mass energy of 7 TeV by the ATLAS detector at the Large Hadron Collider, corresponding to a total integrated luminosity of 35 pb-1. Collision events having exactly two oppositely-charged electrons or muons, a single high-transverse-momentum jet, and large missing or total transverse energy are selected, resulting in a sample with enhanced single top purity. 38 data events are found to survive the selection, in good agreement with the expected sum from all signal and background predictions of 38.6 ± 6.0. The Wt content of the selected sample is predicted to amount to 4.3 ± 0.3 events. The resulting cross-section measurement is 10 +20-10 (stat) +20-10 (syst) pb. As this level of precision is insufficient to claim evidence or discovery of this process, an upper limit is also derived for the parameter of interest, resulting in σ(pp→Wt) < 70 pb at 95% confidence level.

In this thesis we use effective field theories of the strong interactions to improve our understanding of several quantities in the Standard Model of particle physics (SM). We also examine constraints on an extension of the SM scalar sector and study the implications for the Higgs mass. We first examine an approach to extracting the Cabibbo-Kobayashi-Maskawa matrix element |Vub| via the relationship between the B meson decays B -> Xu l nu and B -> Xs gamma, where Xi is any final state hadron containing a quark of flavour i. Model dependence is reduced in this approach since the non-perturbative shape function at leading order is universal and drops out; however the perturbative expansion at next-to-leading order is found to be poorly behaved. We carry out a renormalon analysis of the relationship between these spectra to examine higher order perturbative corrections and compare the fixed-order and log expansions. Our analysis can be used to estimate the perturbative uncertainty in the extraction of |Vub|, which we show to be relatively small. Next we take a step towards the broader goal of summing large phase space logarithms from a variety of jet algorithms using Soft Collinear Effective Theory (SCET). We develop a consistent approach to implementing arbitrary phase space constraints in SCET and demonstrate the connection between cutoffs in SCET and phase space limits. By considering several jet algorithms at next-to-leading order, we gain some insight into factorization of final state jets. In particular, we point out the connection between the ultraviolet regulator and factorization. Finally we consider a scalar sector that contains a colour-octet electroweak-doublet scalar, in addition to the SM Higgs. This extension contains the only scalar representations that Yukawa-couple to quarks and are consistent with minimal flavour violation. We examine constraints from electroweak precision data, direct production from LEPII and the Tevatron, and from flavour physics. We find both the Higgs and new scalars can be simultaneously light, with masses of O(100 GeV). The data also allows all the scalars to be heavy, with masses of O(1 TeV). The presence of the additional scalars removes the preference for a light Higgs, which normally emerges from fits to electroweak precision data.

In this thesis, we mainly study the structure of $D_9$-brane Chan-Paton factors in Type II orientifold $O_p^{\pm}$ theories. An explicit structure is found through a thorough and systematic derivation. For this purpose, we give a complete analysis of the worldsheet parity square action for all p-q open strings (i.e, the strings starting from $D_p$-branes and ending on $D_q$-branes), which is an extension of Gimon-Polchinski's work on the parity square action for p-q strings with $p-q$ even. We also formulate the rules for computing the scattering amplitudes of open strings ending on D-branes. These are important to build phenomenological models in string theory. As an application to the mathematical aspects of string theory, we confirm the proposal that the D-brane charges in Type II orientifolds are classified by the KR-theory groups. All these results will appear in our paper\footnote{Dongfeng Gao and Kentaro Hori, Clifford Algebras and Chan-Paton Factors, to appear.}. We also show some preliminary results on the B-type D-branes in the orientifolds of linear sigma models. A typical linear sigma model has both a geometric phase and a Landau-Ginzberg phase over its K\"ahler moduli space. The B-type D-branes are described by suitable categories in a linear sigma model and its phases. The orientifold projection induces parity mappings on the various categories. By looking at the transportation of invariant branes in the moduli space, we find the relations among the parity mappings of different phases in the quintic model.

In this thesis, we mainly study the structure of $D_9$-brane Chan-Paton factors in Type II orientifold $O_p^{\pm}$ theories. An explicit structure is found through a thorough and systematic derivation. For this purpose, we give a complete analysis of the worldsheet parity square action for all p-q open strings (i.e, the strings starting from $D_p$-branes and ending on $D_q$-branes), which is an extension of Gimon-Polchinski's work on the parity square action for p-q strings with $p-q$ even. We also formulate the rules for computing the scattering amplitudes of open strings ending on D-branes. These are important to build phenomenological models in string theory. As an application to the mathematical aspects of string theory, we confirm the proposal that the D-brane charges in Type II orientifolds are classified by the KR-theory groups. All these results will appear in our paper\footnote{Dongfeng Gao and Kentaro Hori, Clifford Algebras and Chan-Paton Factors, to appear.}. We also show some preliminary results on the B-type D-branes in the orientifolds of linear sigma models. A typical linear sigma model has both a geometric phase and a Landau-Ginzberg phase over its K\"ahler moduli space. The B-type D-branes are described by suitable categories in a linear sigma model and its phases. The orientifold projection induces parity mappings on the various categories. By looking at the transportation of invariant branes in the moduli space, we find the relations among the parity mappings of different phases in the quintic model.

A search for Supersymmetry is presented. The search is done with final states containing jets, missing transverse energy and no leptons, using 4.7 fb-1 of proton-proton collisions at sqrt(s)=7 TeV recorded by the ATLAS detector at the Large Hadron Collider. The analysis is carried out using new kinematic variables in two distinct signal regions with and without b-jet tagging. No significant excess above the Standard Model background expectation is observed, and limits are set on the Supersymmetry models. The results are interpreted as limits on the minimal supergravity framework, as well as a simplified Supersymmetry model of Gluino pair-production

A search for Supersymmetry is presented. The search is done with final states containing jets, missing transverse energy and no leptons, using 4.7 fb-1 of proton-proton collisions at sqrt(s)=7 TeV recorded by the ATLAS detector at the Large Hadron Collider. The analysis is carried out using new kinematic variables in two distinct signal regions with and without b-jet tagging. No significant excess above the Standard Model background expectation is observed, and limits are set on the Supersymmetry models. The results are interpreted as limits on the minimal supergravity framework, as well as a simplified Supersymmetry model of Gluino pair-production

A search for new physics in events with an energetic jet and large missing transverse momentum is performed with the ATLAS detector, using 2011 dataset corresponding to 4.7 fb−1 integrated luminosity. A model–independent approach is adopted, making predictions in various kinematic regions sensitive to potentially new physics scenarios. Data–driven background determination methods are developed to obtain robust predictions of the Standard Model expectations of the number of events in each probed kinematic region. No deviation from the Standard Model expectation is observed, and the results are hence interpreted in the context of the ADD scenario of Large Extra Dimensions, and pair production of WIMP dark matter candidates. This results in the world’s tightest constraints on the size of the D-dimensional Planck scale as the fundamental parameter of the ADD theory. The constraints obtained on Dark Matter suppression scale are stronger than those obtained from dedicated direct and indirect dark matter experiments for a large range of WIMP masses. Furthermore, data-driven estimates of various Standard Model processes contributing to the mono-jet final state allows a precise direct measurement of the invisible decay width of the Z boson. This results in a measurement at 5% precision level, comparable to the results of the L3 experiment, and better than all other LEP direct measurements.

In this thesis we use effective field theories of the strong interactions to improve our understanding of several quantities in the Standard Model of particle physics (SM). We also examine constraints on an extension of the SM scalar sector and study the implications for the Higgs mass. We first examine an approach to extracting the Cabibbo-Kobayashi-Maskawa matrix element |Vub| via the relationship between the B meson decays B -> Xu l nu and B -> Xs gamma, where Xi is any final state hadron containing a quark of flavour i. Model dependence is reduced in this approach since the non-perturbative shape function at leading order is universal and drops out; however the perturbative expansion at next-to-leading order is found to be poorly behaved. We carry out a renormalon analysis of the relationship between these spectra to examine higher order perturbative corrections and compare the fixed-order and log expansions. Our analysis can be used to estimate the perturbative uncertainty in the extraction of |Vub|, which we show to be relatively small. Next we take a step towards the broader goal of summing large phase space logarithms from a variety of jet algorithms using Soft Collinear Effective Theory (SCET). We develop a consistent approach to implementing arbitrary phase space constraints in SCET and demonstrate the connection between cutoffs in SCET and phase space limits. By considering several jet algorithms at next-to-leading order, we gain some insight into factorization of final state jets. In particular, we point out the connection between the ultraviolet regulator and factorization. Finally we consider a scalar sector that contains a colour-octet electroweak-doublet scalar, in addition to the SM Higgs. This extension contains the only scalar representations that Yukawa-couple to quarks and are consistent with minimal flavour violation. We examine constraints from electroweak precision data, direct production from LEPII and the Tevatron, and from flavour physics. We find both the Higgs and new scalars can be simultaneously light, with masses of O(100 GeV). The data also allows all the scalars to be heavy, with masses of O(1 TeV). The presence of the additional scalars removes the preference for a light Higgs, which normally emerges from fits to electroweak precision data.

A search for new physics in events with an energetic jet and large missing transverse momentum is performed with the ATLAS detector, using 2011 dataset corresponding to 4.7 fb−1 integrated luminosity. A model–independent approach is adopted, making predictions in various kinematic regions sensitive to potentially new physics scenarios. Data–driven background determination methods are developed to obtain robust predictions of the Standard Model expectations of the number of events in each probed kinematic region. No deviation from the Standard Model expectation is observed, and the results are hence interpreted in the context of the ADD scenario of Large Extra Dimensions, and pair production of WIMP dark matter candidates. This results in the world’s tightest constraints on the size of the D-dimensional Planck scale as the fundamental parameter of the ADD theory. The constraints obtained on Dark Matter suppression scale are stronger than those obtained from dedicated direct and indirect dark matter experiments for a large range of WIMP masses. Furthermore, data-driven estimates of various Standard Model processes contributing to the mono-jet final state allows a precise direct measurement of the invisible decay width of the Z boson. This results in a measurement at 5% precision level, comparable to the results of the L3 experiment, and better than all other LEP direct measurements.