Top partners at the CERN LHC: Spin and mass measurement

If one takes naturalness seriously and also assumes a weakly coupled extension of the standard model (SM) then there are expectations for phenomenology that can be inferred in a model-independent framework. The first such expectation is that there is likely to be some colored particle with mass $\mathcal{O}(\mathrm{TeV})$ that cancels the top loop contribution to the quadratic divergence of the Higgs mass. In this paper we begin a model-independent analysis of the phenomenology of this “top partner,” $t^{\prime }$. We make one additional assumption that it is odd under a parity which is responsible for the stability of a WIMP dark matter candidate, $N$. We focus on three questions to be explored at the LHC: discovery opportunities, mass determination, and spin determination of this top partner. We find that within a certain region of masses for the $t^{\prime }$ and $N$, $t^{\prime }{\overline{t}}^{\prime }$ is easily discovered in the $t\overline{t}+2N$ decay with the tops decaying fully hadronically. We show that without having to rely on other channels for new physics that for a given $t^{\prime }$ spin the masses of $t^{\prime }$ and $N$ can be measured using kinematic information (e.g. average ${\mathrm{E̸}}_{\mathrm{T}}$ or $H_T$) and total cross section. A degeneracy due to the spin remains, but with several hundred ${\mathrm{fb}}^{-1}$ of luminosity we demonstrate potentially useful new methods for determining the $t^{\prime }$ spin over a wide range of masses. Our methods when could be useful for distinguishing supersymmetric and nonsupersymmetric models.