A measurement of the jet multiplicity in di-leptonically decaying ttbar events (i.e. ttbar → (ee, μμ, eμ) + missing transverse momentum + jets) is presented, using 4.66 fb -1 of data collected from √ s = 7 TeV pp collisions at the CERN Large Hadron Collider. In order to mitigate the effects of systematic uncertainties associated with jet energy measurements, a measurement of the ratio of the number of di-lepton ttbar events with N + 2 jets to Z → ℓ ℓ + N jet events is also determined. The results are unfolded for detector effects and compared to the particle-level predictions of several Monte Carlo generators, showing generally good agreement.

This dissertation describes a search for the Standard Model Higgs boson in Z+photon channel with the ATLAS detector at the LHC using 4.6 fb⁻¹ of proton-proton collisions at center of mass energy 7 TeV in 2011 and 20.7 fb⁻¹ of proton-proton collisions at center of mass energy 8 TeV in 2012. The distribution of the mass difference between M_Zphoton and M_Z is compared to the Standard Model (SM) background expectations. No significant deviation from the SM prediction is observed and the upper limits on the signal strength μ of a Higgs boson with a mass between 120 and 150 GeV are derived. The expected exclusion limits at 95% confidence level range between 7.3 and 22.3 times the predicted Standard Model cross section. The observed exclusion limits range between 5.3 and 38.7 times the Standard Model cross section. For a Higgs boson with mass of 125.5 GeV, the expected and observed limits are 13.2 and 17.0 times the Standard Model respectively.

A search for photons originating in the decay of a neutral long-lived particle produced in proton-proton collisions at sqrt(s) = 7 TeV is presented. The search was performed in the diphoton plus missing transverse energy final state, using the full data sample of 4.8 fb-1 of 7 TeV proton-proton collisions collected in 2011 with the ATLAS detector at the CERN Large Hadron Collider. The analysis exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction of photons, and utilizes the excellent time resolution of the calorimeter as an independent cross-check of the results. The search was conducted in the context of Gauge Mediated Supersymmetry Breaking models, where the lightest neutralino is the next-to-lightest supersymmetric particle and has a finite lifetime. In the family of models investigated, supersymmetric particles are produced in pairs due to R-parity conservation, eventually decaying to a pair of neutralinos, each subsequently decaying to a photon and a gravitino. The gravitinos escape the detector, giving rise to missing energy, while the photons can appear not to originate from the primary vertex of the event, and are measured with a delay with respect to the collision time. No excess was observed above the background expected from Standard Model processes. The results were used to set exclusion limits at 95% CL in the two-dimensional parameter space defined by the supersymmetry breaking scale and the lifetime of the lightest neutralino.

We present a search for electron antineutrino disappearance at the Chooz nuclear power plant in Chooz, France. Using the Double Chooz far detector and 101.5 days of detector run time, we measure sin^2(2θ(subscript 13)) = 0.086 ± 0.041 (stat.) ± 0.030 (syst.) from a rate and shape fit. A combined analysis of T2K and Double Chooz data finds that sin^2(2θ(subscript 13)) = 0 is excluded at the 3σ level.

A vacuum, a classically stable state, can decay to another vacuum by virtue of quantum tunneling. Although vacuum decay is an interesting topic itself in field theory, when it combines with gravity, it has wider applications and plays an essential role to understand the very early universe. The semi-classical solution of vacuum decay is well-described by the WKB approximation both in flat space and in curved space. Meanwhile, if we consider the configuration space of fields, we encounter many kinds of problems related with gravitational fields since the mode spectrum in curved space turns out to have an infinite number of negative modes in a de Sitter background. Despite of infinitely many negative modes, the regime in the weak gravity limit can be smoothly connected to flat space. To understand this, I discuss the nature of vacuum decay and various topics about negative modes in this thesis.

The K_L-K_S mass difference is a promising quantity to reveal new phenomena which lie outside the standard model. A state-of-art perturbation theory calculation has be en performed at next-to-next-to-leading order (NNLO) and a 40% error is quoted in the final result. We develop and demonstrate non-perturbative techniques needed to calculate the K_L-K_S mass difference, ΔM_K, in lattice QCD and carry out exploratory calculations. The calculations are performed on a 2+1 flavor, domain wall fermion, 16³ x 32 ensemble with a 421 Mev pion and a 24³ x 64 lattice ensemble with a 329 MeV pion. In the $16^3$ lattice calculation, we drop the double penguin diagrams and the disconnected diagrams. The short distance part of the mass difference in a 2+1 flavor calculation contains a quadratic divergence cut off by the lattice spacing. Here, this quadratic divergence is eliminated through the Glashow-Iliopoulos-Maiani (GIM) mechanism by introducing a quenched charm quark. We obtain a mass difference ΔM_K which ranges from 6.58(30) x 10⁻¹² MeV to 11.89(81) x 10⁻¹² MeV for kaon masses varying from 563 MeV to 839 MeV. On the 24³ lattice, we include all the diagrams and perform a full calculation. Our result is for a case of unphysical kinematics with pion, kaon and charmed quark masses of 330, 575 and 949 MeV respectively. We obtain ΔM_K=3.19(41)(96) x 10⁻¹² MeV, quite similar to the experimental value. Here the first error is statistical and the second is an estimate of the systematic discretization error. An interesting aspect of this calculation is the importance of the disconnected diagrams, a dramatic failure of the OZI rule.

A search for undiscovered particles decaying into top-antitop quark pairs produced in proton-proton collisions with the ATLAS experiment at the Large Hadron Collider utilizing 20.3 fb$^-1$ of data collected at $\sqrt{s}=8$ \tev\ center-of-mass energy during the 2012 data taking period is presented. The invariant mass spectrum of events containing multiple jets, exactly one lepton, and missing transverse energy and which are consistent with the decay of a top-antitop quark pair is studied and found to be consistent with that predicted by the Standard Model. Upper limits on the production cross section times branching ratio of several benchmark signal models are set at a 95\% confidence level.

This thesis presents complementary measurements of the neutrino oscillation parameter sin²2θ₁₃ made by observing the electron antineutrino flux from two nuclear reactors in Chooz, France. Antineutrinos are identified through both products of the inverse beta decay interaction, bar νₑ + p → e⁺ + n, in a high-precision liquid scintillator detector located approximately 1 km from the reactors. The most sensitive signal channel involves neutron captures by a gadolinium dopant, while a search for neutron captures on hydrogen provides a high-statistics validation. In both channels, the value of sin²2θ₁₃ is revealed by an energy- and reactor power-dependent deficit of antineutrino-like events, compared to a reactor simulation. All analyses produce results consistent with one another and with findings of other experiments. These datasets also expose features of the antineutrino spectrum not predicted in reactor flux models. Data from the newly inaugurated near detector, anticipated in the final part of this thesis, holds unique potential to clarify these features, pursue signals from sterile neutrinos, and contribute to global knowledge of three-neutrino mixing.

聚（N-異丙基丙烯酸醯胺）（PNIPAM）微凝膠是一種内部有化学交联的三維網絡結構的軟膠體。這種微凝膠的物理性質是介於硬球膠體和超軟軟的星形聚合物或者膠束之間的。根據微粒的柔軟程度，微凝膠可以發生網絡互穿或者形變。因此塞滿微凝膠的體系的有效體積分數可以遠遠超過硬球系統裏的緊密堆積體積分數。這樣的系統會出現比硬球系统更加豐富的相行為。然而現在仍然缺乏對軟膠體系統在超過緊密堆積體積分數時變現的機構和性質的研究。特別是有些理論預測出的相行為仍然還沒被實際的實驗觀測的到。 / 本論文採用了一種新穎的方式去研究微凝膠體系的微流變，這種方法結合了粒子追蹤微流變和磁鑷系統。這種方法本質上是通過一種消逝波（產生於全內反射顯微鏡（TIRM）中的固液介面）作為入射光來探測靠近平直表面上事先植入的探針微粒（直徑為幾個微米）的位移。這個儀器通過記錄來自探針微粒的散射光強度來追蹤植入的探針微粒在垂直於固體水平面上的熱運動。對於位移的記錄可以達到十個納米的精度，使得它成為很靈敏的空間位置探測器。再者，通過添加了磁鑷系統，使得我們能夠有效地通過震蕩的力來在三維空間裏操控植入的微粒。通過控制探針微粒的運動，可以測量微凝膠懸浮液裏局部原位的粘彈性質。我們研究了濃度依賴和溫度依賴的PNIPAM微凝膠懸浮液的結構變化和相行為。 通過微流變的研究，讓我們第一次通過分析微凝膠懸浮液的粘彈性，確認觀測到了由有效體積分數導致的可逆轉的流體態-玻璃態-劉體態的相轉變過程。 / Soft colloids such as poly(N-isopropylamide) (PNIPAM)-based microgels are colloidal particles that consist of chemically cross-linked three-dimensional polymer networks. The physical nature of these microgel particles thus lies in between that of hard-sphere colloids, and ultrasoft star polymers as well as micelles. Due to the softness of the particles, microgels can interpenetrate or compress. As a result, microgels can be packed to effective volume fractions far above solid particles close packing, leading to the existence of much richer phase behavior when compared to simple hard colloidal particles. However, there is still a lack of knowledge on the structure and properties of soft colloid suspensions at and above close packing, and in particular some theoretically predicted phase behavior has not yet been reproduced by the experimental studies. / This thesis presents a novel approach to study the rheological properties of soft microgel suspensions using a combination of particle-tracking microrheology and magnetic tweezers. We essentially employ an evanescent wave (generated by a solid/liquid interface in the total internal reflection microscopy (TIRM)) as the incident light source to probe the displacement of an embedded probe particle (of a few micrometers diameter) near a flat surface. By measuring the scattered intensity, this technique allows tracking of the thermal motion of the embedded particle perpendicular to the solid surface to a precision of tens of nanometers, making it a highly sensitive spatial detector. Moreover, the integration of a magnetic driving force into the TIRM enables us to effectively manipulate the embedded particle in three dimensions by an oscillatory force so that the local viscoelastic properties of the microgel suspensions can be measured by resolving the particle motion. We investigated the concentration- and temperature-dependent on the structural ordering and phase behavior of PNIPAM microgel suspensions. Microrheology allows us, at a first time, to identify an effective volume fraction driven re-entrant liquid-glass-liquid phase transition by looking at the viscoelastic properties of the suspensions. We show that this result is related to the softness of the microgel particles under a confined condition, and discuss our findings in view of the existing theoretical predictions for soft particles. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Hua, Li. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstracts also in Chinese. / 摘要 --- p.i / Abstract --- p.ii / Acknowledgement --- p.iv / Chapter 1 --- p.1 / Chapter 1.1 --- Introduction to colloids --- p.1 / Chapter 1.1.1 --- Phase transition in hard colloidal system --- p.2 / Chapter 1.1.2 --- Phase transition in soft colloidal system --- p.3 / Chapter 1.2 --- Overview of phase transition in microgels --- p.4 / Chapter 1.3 --- Simulation of soft colloids’ phase diagram --- p.8 / Chapter 1.4 --- Reference and Notes --- p.12 / Chapter 2 --- p.15 / Chapter 2.1 --- Overview of the microrheology methods --- p.15 / Chapter 2.1.1 --- Passive techniques of microrheology --- p.16 / Chapter 2.1.2 --- Active techniques for microrheology --- p.23 / Chapter 2.2 --- Microrheometer based on incorporating Magnetic Tweezers to Total Internal Reflection Microscopy --- p.29 / Chapter 2.2.1 --- Particle tracking system - TIRM --- p.30 / Chapter 2.2.2 --- Magnetic Tweezers as driven force --- p.32 / Chapter 2.2.3 --- Calibration of the magnetic force --- p.35 / Chapter 2.3 --- Reference and Notes --- p.37 / Chapter 3 --- p.44 / Chapter 3.1 --- Overview of the series of experiments --- p.44 / Chapter 3.2 --- PNIPAM microgel synthesis and characterization --- p.44 / Chapter 3.3 --- Microrheology of PNIPAM microgels suspension --- p.47 / Chapter 3.3.1 --- Volume Fraction dependence measurements --- p.49 / Chapter 3.3.2 --- Temperature depended measurements --- p.53 / Chapter 3.4 --- References and Notes --- p.55 / Chapter 4 --- p.56 / Chapter 4.1 --- Discussion and Conclusion --- p.56 / Chapter 4.2 --- Future Perspectives --- p.59 / Chapter 4.3 --- References and Notes --- p.61

A search for squarks and gluinos in all hadronic final states in \sqrt(s) = 13 TeV proton-proton collisions using an integrated luminosity of 13.3 fb^-1 collected by the ATLAS detector at the LHC is presented. The search is the first to use Recursive Jigsaw Reconstruction, a technique to impose a particular decay tree interpretation on events. The decay tree is resolved using jigsaw rules, which define boosts between the relevant reference frames to define an uncorrelated basis of variables to describe the decay. The Recursive Jigsaw Reconstruction variables are used to define a set of selections with sensitivity to pair produced squarks and gluinos. No excess is observed over the Standard Model background. Results are interpreted in simplified models where squarks and gluinos are pair produced and decay to jets and the lightest supersymmetric particle. These limits substantially extend the region of supersymmetric phase space excluded by previous searches.