UC Berkeley

Many of the proposed solutions to the hierarchy and naturalness problems postulate new ``partner'' fields to the Standard Model (SM) particles. Determining the spins of these new particles will be critical in distinguishing among the various possible SM extensions, yet proposed methods rely on the underlying models. We propose a new model-independent method for spin measurements which takes advantage of quantum interference among helicity states. By looking at the azimuthal angular dependence of the differential cross section in the production followed by decay of a new particle species one can determine its spin by looking at the various cosine modes. We demonstrate that this method will be able to discriminate scalar particles from higher spin states at the ILC, and discuss application to higher spins and possible uses at the LHC. Supersymmetry and Universal Extra Dimensions models prove problematic at the LHC because missing energy signatures result in too many unknowns while reconstructing events. However, warped extra dimension models in certain setups allow for events whose kinematics can be fully reconstructed. In such scenarios, the heavy spin-2 Kaluza-Klein (KK) graviton provides a unique signature with a $cos{(4 phi)}$ mode.We study the feasibility of this approach to measuring the spin of the KK graviton in the warped Randall-Sundrum Model at the LHC.

In chapter 5 of this thesis, taking a phenomenological approach, we study a color sextet scalar at the LHC. We focus on the QCD production of a color sextet pair $Phi_6bar{Phi}_{6}$ through $gg$ fusion and $qbar{q}$ annihilation. Its unique coupling to $bar{psi^c}psi$ allows the color sextet scalar to decay into same-sign diquark states, such as $Phi_6to tt/tt^*$. We propose a new reconstruction in the multijet plus same sign dilepton with missing transverse energy samples ($bb+ell^pmell^pm+cancel{E}_T+Nj$, $Ngeq 6$) to search for on-shell $ttbar{t}bar{t}$ final states from sextet scalar pair production. Thanks to the large QCD production, the search covers the sextet mass range up to 1 TeV for 100 fb$^{-1}$ integrated luminosity.