#### Date of Award

8-24-2012

#### Embargo Period

10-16-2012

#### Degree Type

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics

#### Advisor(s)

Lincoln Wolfenstein

#### Abstract

In this thesis, two of the most exciting areas in high energy physics nowadays are studied: neutrino physics and collider physics, which play important roles in the intensity and energy frontiers, respectively.

For neutrino physics, assuming that the neutrino mass matrix is dominated by a term with the permutation symmetry S_{2}, it is possible to explain neutrino data only if the masses are quasi-degenerate. A sub-dominant term with an approximate μ−τ symmetry leads to an approximate tri-bimaximal form. Experimental consequences are discussed.

In this thesis several channels that contain missing-energy signal in the final states will be investigated with minimal model assumptions. Channels of this type are very challenging to analyze at the Large Hadron Collider (LHC), since this approach offers only a few kinematical handles. I start with a channel that has a clean signal of two leptons and missing energy. This signature generally arises from pair production of heavy charged particles which each decay into a lepton and a weakly interacting stable particle. This class of processes is analyzed with minimal model assumptions by considering all possible combinations of spin 0, ½ or 1, and of weak iso-singlets, -doublets, or -triplets for the new particles. Adding to existing work on mass and spin measurements, two new variables for spin determination and an asymmetry for the determination of the couplings of the new particles are introduced. It is shown that these observables allow one to independently determine the spin and the couplings of the new particles, except for a few cases that turn out to be indistinguishable at the LHC. These findings are corroborated by results of an alternative analysis strategy based on an automated likelihood test.

I then study decays of the form C → ℓ+ℓ−A (ℓ = e, μ), including the possibility that this three-body decay is preceded by an additional decay step D → jC. Here A, C and D are heavy new-physics particles and j stands for a quark jet. It is assumed that A escapes direct detection in a collider experiment, so that one cannot kinematically reconstruct the momenta of the new particles. Instead, information about their properties can be obtained from invariant-mass distributions of the visible decay products, i. e. the di-lepton (ℓℓ) and jet-lepton (jℓ) invariant-mass distributions. All possible spin configurations and renormalizable couplings of the new particles are considered, and explicit expressions for the invariant-mass distributions are derived, in a formulation that separates the coupling parameters from the spin and kinematic information. In a numerical analysis, it is shown how these properties can be determined independently from a fit to the m_{ℓℓ} and mjℓ distributions.

Finally, I will take a model-independent approach to searching for new physics involving the top quark. The experimental signatures for new physics involving top quarks at the LHC may be characteristic, yet challenging to disentangle. I systematically parameterize generic interactions of a new particle that couples to the top quark and optimize the search strategy for the new particles at the LHC and propose the study for their properties. Several variables are proposed and a detailed numerical study is performed for the determination of spins and couplings of new heavy particles.

#### Recommended Citation

Chen, Chien-Yi, "New Physics Beyond the Standard Model with Weakly Interacting Particles" (2012). *Dissertations*. 118.

http://repository.cmu.edu/dissertations/118