Using 3 fbˉ1 of proton-proton collisions, collected at centre-of-mass energies of √s = 7 and 8 TeV by the LHCb detector, several measurements of charmless B⁰s meson decays are made. A search is also performed for a highly suppressed B⁰ decay. First, the branching fraction of the B0⁰s→ ØØ decay is measured to be B(B⁰s→ ØØ) = (1.84 ± 0.05(stat) ± 0.07(syst) ± 0.11(fs=fd) ± 0.12(norm))X10ˉ⁵; where the third and fourth uncertainties arise from the fragmentation fraction fs/fd and the branching fraction of the normalisation mode. This represents a factor of five reduction in the statistical uncertainty compared to the previous best measurement. An upper limit on the branching fraction of the mode B⁰→ ØØ is set at B(B⁰→ ØØ) < 2.8 X 10ˉ⁸ (90% CL). This is a factor of seven improvement over the previous best measurement. An amplitude analysis of the B⁰s→ ØK+Kˉ decay is performed, wherein first observations of the decay modes B⁰s→ Øf´2(1525) and B⁰s→ ØØ(1680) are made. The branching fraction of the B⁰s→ Øf´2(1525) decay is measured to be B(B⁰s→ Øf´2 (1525)) = (1.63 ± 0.18(stat) ± 0.12(syst) ± 0.29(model) ± 0.17(norm)) X 10ˉ⁶; where the 'model' uncertainty arises from the choice of amplitude model. The longitudinal polarisation fraction of the decay B⁰s→ Øf´2(1525) is measured to be F0 = (86.6 ± 3.4 ± 0.8 ± 2.0 (model))%.

The Large Hadron Collider (LHC) has provided, and will continue to provide, data for collisions at the highest energies ever seen in a particle accelerator. A strong knowledge of the properties of amplitudes for Quantum Chromodynamics in the High Energy Limit is therefore important to interpret this data. We study this limit in the context of the High Energy Jets (HEJ) formalism. This formalism resums terms in the perturbative expansion of the cross-section that behave like αn/s log (s/-t)ⁿ¯¹, which are enhanced in this limit. Understanding this region is particularly important in certain key analyses at the LHC: for example, Higgs-boson- plus-dijet analyses where cuts are applied to pick out events with a large mjj and in many searches for new physics. In this thesis, we discuss two directions in which HEJ's accuracy has been improved. Firstly, we look at adding descriptions of partonic subprocesses which are formally sub-leading in the jet cross-section but Leading Logarithmic (LL) in the particular subprocess itself. This required the derivation of new effective vertices that describe the emission of a quark/anti-quark pair in a way that is consistent with the resummation procedure. The inclusion of such processes reduces HEJ's dependence on fixed-order calculations and marks an important step towards full Next-to-Leading Logarithmic (NLL) accuracy in the inclusive dijet cross-section. The second extension was to improve our description of events involving the emission of a Higgs boson along with jets. Specifically, we derive new effective vertices which keep the full dependence on the quark mass that appears in the loops that naturally arise in such amplitudes. The formalism is also simple enough to allow for any number of extra nal state jets in the process. Therefore, HEJ is unique in its ability to provide predictions for high-multiplicity Higgs-plus-jets processes with full nite quark mass e ects. Such a calculation is far beyond the reach of any xed order approach.

Pure Yang-Mills amplitudes with all external gluons carrying positive helicity, known as all-plus amplitudes, have an especially simple structure. The tree amplitudes vanish and, up to at least two loops, the loop-level amplitudes are related to those of N = 4 super-Yang-Mills (SYM) theory. This makes all-plus amplitudes a useful testing ground for new methods of simplifing more general classes of amplitudes. In this thesis we consider three new approaches, focusing on the structure before integration. We begin with the planar (leading-colour) sector. A D-dimensional local-integrand presentation, based on four-dimensional local integrands developed for N = 4 SYM, is developed. This allows us to compute the planar six-gluon, two-loop all-plus amplitude. Its soft structure is understood before integration, and we also perform checks on collinear limits. We then proceed to consider subleading-colour structures. A multi-peripheral colour decomposition is used to find colour factors based on underlying tree-level amplitudes via generalised unitarity cuts. This allows us to find the integrand of the full-colour, two-loop, five-gluon all-plus amplitude. Tree-level BCJ relations, satisfied by amplitudes appearing in the cuts, allow us to deduce all the necessary non-planar information for the full-colour amplitude from known planar data. Finally, we consider representations satisfying colour-kinematics duality. We discuss obstacles to finding such numerators in the context of the same five-gluon amplitude at two loops. The obstacles are overcome by adding loop momentum to our numerators to accommodate tension between the values of certain cuts and the symmetries of certain diagrams. Control over the size of our ansatz is maintained by identifying a highly constraining, but desirable, symmetry property of our master numerator.

The hierarchy problem of the electroweak scale is an intriguing puzzle which can conceivably be solved during upcoming runs of the Large Hadron Collider (LHC). Supersymmetry (SUSY) is an attractive potential solution to this problem, though the fact that no supersymmetric particles have been discovered thus far raises fine tuning of most models to &order;(1&percnt;). Extradimensional SUSY is especially interesting in light of this fact since certain models are easier to reconcile with LHC data. This thesis discusses two different extradimensional SUSY scenarios: auto-concealment and Maximally Natural SUSY (MNSUSY). The auto-concealment mechanism applies when the lightest ordinary superymmetric particle (LOSP), a brane localized state, promptly decays to the Kaluza-Klein (KK) tower of a bulk lightest supersymmetric particle (LSP). This dynamically realizes the compression mechanism for hiding SUSY as decays into the more numerous heavier KK LSP states are favored. LHC limits on LOSP squarks weaken to ≲ 450 GeV while limits on LOSP right-handed sleptons evaporate. Slepton searches perform poorly in this case because LHC analyses are blind to compressed slepton spectra. To help fill this gap, this thesis presents a monojet-like search sensitive to the very compressed range 3 GeV < m _&itilde; − m^&Tilde;<sub style='position: relative; left: -.7em;'>x</sub>⁰<sub style='position: relative; left: -.6em;'>1</sub> < 24 GeV. The analysis should allow LHC14 with 100 fb^−1 to search for degenerate left-handed selectrons and smuons in the compressed region up to m _{&itilde;_L} ≲ 150 GeV. In addition, it should be sensitive to m _{&itilde;_L} ≲ 110 GeV for auto-concealed SUSY. To expand the class of extradimensional SUSY models the LHC is able to test this thesis also describes the Lagrangian and mass matrices of MNSUSY, which are needed to implement this model in the Feynman rules generator program FeynRules for use in Monte Carlo programs for collider simulations. It also describes benchmark scenarios useful for the first collider phenomenology studies.

Parton distribution functions (PDFs) provide a description of the quark and gluon content of the proton. They are important input into theoretical calculations of hadronic observables, and are obtained by fitting to a wide range of experimental data. The NNPDF approach to fitting PDFs provides a robust and reliable determination of their central values and uncertainties. The PDFs are modelled using neural networks, while the uncertainties are generated through the use of Monte Carlo replica datasets. In this thesis I provide an in depth description of development of the latest NNPDF determination: NNPDF3.0. A number of novel adaptations to the genetic algorithm and network structure are outlined and the results of tests as to their effectiveness are shown. Centrally, the use of closure tests, where artificial data is generated according to a known theory and used to perform a fit, has been instrumental in both the development and validation of the NNPDF3.0 approach. The results of these tests, which demonstrate the ability of our methodology to reproduce a known underlying law, are investigated in detail. Finally, results from the NNPDF3.0 PDF sets are presented. The parton distributions obtained are compared with results from other PDF collaborations, and PDFs fit to limited datasets are also discussed. Physical observables relevant for future collider runs are presented and compared to other determinations.

Since the discovery of the Higgs boson in 2012, the search for new physics beyond the Standard Model has been greatly intensified. At the CERN Large Hadron Collider (LHC), ATLAS searches for new physics entail looking for new particles by colliding protons together. Presented here is a search for a new form of quark matter called Vector-like Quarks (VLQ), which are hypothetical particles that are expected to have mass around a few TeV. VLQ can come in a variety of forms and can couple to their Standard Model (SM) quark counterparts, particularly to the third generation. They are necessary in several beyond the SM theories in order to solve the hierarchy problem. This search uses 36.1 fb−1of proton-proton collision data collected with the ATLAS detector at the LHC from August 2015 to October 2016. Only events with two leptons of the same charge, or three leptons, plus b-jets and high missing transverse energy are considered in the main analysis. This signature is rarely produced in the SM, which means the backgrounds in this analysis are relatively low. This analysis is sensitive to specific predicted decay modes from pair production of an up-type VLQ with a charge of +2/3, T, an up-type VLQ with a charge of +5/3, T5/3, and a down-type quark with a charge of −1/3, B, as well as single production of T5/3. There is another theorized VLQ that this analysis is not sensitive to: B−4/3, due to its primary decay mode, which is unable to produce the final-state signature of interest. The results from this analysis suggest only a slight deviation of data from SM backgrounds reaching as high as 1.89σ, which does not indicate evidence for VLQ. A mostly frequentist statistical technique, called the CLS Method, is used to interpret the data and set limits on the T, B, and T5/3 signal models. Using this method, exclusion limits are set at the 95% confidence level, effectively excluding T mass below 0.98 TeV, T5/3 mass below 1.2 TeV, and B mass below 1.0 TeV, assuming singlet branching ratios. Also, branching ratio independent limits are set on the T and B VLQ.

This thesis presents an analysis of the central exclusive production (CEP) of &chi;_c mesons in proton-proton collisions at a centre-of-mass energy of 8 TeV, using data collected with the LHCb detector corresponding to an effective integrated luminosity of 126.3 ± 4.3 pb⁻¹. Candidate &chi;_c decays are reconstructed in the K⁺K⁻ and π⁺π⁻ final states, where the final state particles have pseudorapidities between 2.5 and 4.5. The products of the cross sections and branching fractions are measured as &sigma; x B(&chi;_c0 &rarr; K⁺K⁻) = 28.0 ± 2.8 ± 13.9 pb, &sigma; x B(&chi;_c0 &rarr; π⁺π⁻) = 20.3 ± 2.4 ± 8.1 pb, &sigma; x B(&chi;_c2 &rarr; K⁺K⁻) &LT; 12.1 pb, &sigma; x B(&chi;_c2 &rarr; π⁺π⁻) &LT; 10.5 pb, where the first uncertainties are statistical and the second systematic, and limits are set at the 95&percnt; confidence level. A major source of systematic uncertainty is the determination of the fraction of the selected data which is CEP rather than inelastic backgrounds. Also described is HERSCHEL, a new subdetector installed at LHCb during the first long shutdown of the LHC. This is a system of forward shower counters which will provide CEP analyses with event-by-event rejection of inelastic backgrounds. The pseudorapidity coverage and detection efficiency of HERSCHEL are determined using Monte Carlo simulations. The pseudorapidity coverage extends to |&eta;&| &ge; 10 due to the showering of collision products at the aperture limit. With a photoelectron threshold yield of 0.2 times the yield in the mean minimum bias event, the detection efficiency for single diffractive events is predicted to be 89&percnt; in the forward direction and 91&percnt; in the backward direction.

This dissertation describes a search for the invisible decays of dark matter particles produced in association with a Z boson, where the latter decays to a charged lepton pair. The dataset for this search includes 13.3 1/fb of collisions recorded in 2015 and 2016 at a centre-of-mass energy of 13 TeV in the ATLAS detector at the Large Hadron Collider in Geneva, Switzerland. The invisible particles manifest themselves as missing transverse momentum, or MET, in the detector, while the charged leptons of interest are electron (e+e-) or muon (mu+mu-) pairs. The models simulated for this study are vector mediated simplified models with Dirac fermionic dark matter particles with couplings g_q = 0.25, g_X = 1 and g_l = 0 . The main background to this analysis, ZZ->llvv, is irreducible, as it shares the same signature as the signal. It is estimated with Monte Carlo simulations including contributions from both qq->ZZ and gg->ZZ production modes. Where possible, other backgrounds are estimated using data-driven techniques and reduced through various selection criteria. The final search is performed by looking for a deviation from the Standard Model background expectation in the MET distribution using two signal regions, e+e- and mu+mu-. This is done using statistical tools to make a likelihood fit and set a 95% confidence level limit as no deviations are found. Limits are placed on the presented model of dark matter for mediator masses up to 400 GeV and for a range of dark matter masses from 1 to ~200 GeV. / Graduate

A search for Higgs boson pairs produced in the decay of high mass exotic resonances is presented. The search uses the bb̄bb̄ final state, analyzing 139 fb⁻¹ of proton–proton collision data at √s = 13 TeV collected with the ATLAS detector. Spin-0 and spin-2 benchmark signal models are considered and no significant deviation from the Standard Model prediction is observed. The search is combined with a complementary analysis for lower mass resonances to set upper limits on the production cross-section times HH branching ratio of new resonances in the mass range of 251 GeV to 5 TeV. In addition, the methodology for the in-situ calibration of a novel double-b-tagging algorithm (Xbb2020) using gluon to bb̄ decays is presented. Preliminary scale factors for Monte-Carlo simulation are computed using 139 fb⁻¹ of √s = 13 TeV pp collision data collected with the ATLAS detector. The completed calibration will allow the Xbb2020 algorithm to be used in future ATLAS searches for H → bb̄ decays.

This work presents the search for Dark Matter particles associated with the Higgs Boson decaying into a b b-bar quark pair. The dark matter search result is based on proton-proton collision data collected at a center-of-mass energy of 13 TeV by the ATLAS detector during Run II. The results are interpreted in the context of a simplified model (Z’-2HDM) which describes the interaction of dark matter and standard model particles via new heavy mediator particles. The new powerful Higgs tagging techniques, which exploit the jet substructure and heavy flavor information to a large extent, are developed to improve the search sensitivity of the search. The target physics signals are signature with an optimized search region and interpreted with background estimation result statistically.