Top Partners at the LHC: Spin and Mass Measurement

TL;DR

This work investigates a naturalness-motivated, model-independent scenario with a top partner $t'$ and a parity-odd neutral particle $N$ (LPOP), focusing on the LHC signature $t'ar t' o tar t+2N$ in the all-hadronic channel. The authors develop a framework where $t'$ can be either a fermion or a scalar, with $N$’s spin constrained by the decay vertex; they propose mass and spin measurement strategies that combine kinematic variables, production cross sections, and novel rapidity-based observables. They show promising discovery prospects in large regions of parameter space, and outline methods to determine $m_{t'}$ and $m_N$ while addressing a degeneracy between SUSY and non-SUSY realizations. Spin discrimination relies on beam-line and directional asymmetries together with pseudorapidity correlations, potentially enabling distinction between SUSY-like and non-SUSY top partners with sufficient luminosity. Overall, the paper provides a practical, model-independent toolkit for probing naturalness-inspired top partners at the LHC and offers pathways to extend the analysis to more complex spectra.

Abstract

If one takes naturalness seriously and also assumes a weakly coupled extension of the Standard Model (SM) then there are predictions for phenomenology that can be inferred in a model independent framework. The first such prediction is that there must be some colored particle with mass O(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'. 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' and N, t'\bar{t'} is easily discovered in the t\bar{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 a given t' spin the masses of t' and N can be measured using kinematic information (e.g. average MET or H_T) and total cross section. A degeneracy due to the spin remains, but with several hundred inverse fb of luminosity we demonstrate potentially useful new methods for determining the t' spin over a wide range of masses. Our methods could be useful for distinguishing supersymmetric and non-supersymmetric models.

Figures (5)

• Figure 1: Significance of signal for $t'$ fermion $N$ scalar for $10\;\mathrm{fb}^{-1}$. The contours (from left to right) represent significance of $>15\sigma$,$>10\sigma$, $>5\sigma$, $>3\sigma$, and $<3\sigma$. The region $m_{t'}-m_N<200\,\mathrm{GeV}$ is not investigated.
• Figure 2: Significance of signal for $t'$ scalar $N$ fermion for $10\;\mathrm{fb}^{-1}$ on the left and and $100\,\mathrm{fb}^{-1}$ on the right. The contours (from left to right) represent significance of $>15\sigma$,$>10\sigma$, $>5\sigma$, $>3\sigma$, and $<3\sigma$. The region $m_{t'}-m_N<200\,\mathrm{GeV}$ is not investigated.
• Figure 3: Signal to background plots. At left, the case $t'$ fermion, $N$ scalar. Contours (left to right) are $S/B = 40, 20, 10, 5, 1$. At right, the case $t'$ scalar, $N$ fermion. Contours are $S/B = 10, 5, 1, 0.1$.
• Figure 4: Top (a): contour plots of kinematic variables, demonstrating that they all measure the same function of $(m_{t'},m_N)$. On the left is the case of $t'$ fermion $N$ scalar; at right, $t'$ scalar $N$ fermion. Bottom (b): the same plots, with contours of constant cross section superimposed. At left, $t'$ fermion $N$ scalar; at right, $t'$ scalar $N$ fermion. Approximately, the kinematic variables are all sensitive only to the mass difference, while the cross section is sensitive to the $t'$ mass. ($\left<H_t\right>$ is in red, $\left<|\hbox{${\rm \not\! E}_{\rm T}$}|\right>$ is in blue, $\left<M_{eff}\right>$ is in purple, $M_{T2}^{max}$ is in gold, and in (b) $\sigma$ is in black.)
• Figure 5: Distribution of events in the $(\eta_+,\eta_-)$ plane for two points with similar cross-section and kinematics but different spins, with one and two sigma contours. At left: $t'$ fermion, mass 700 GeV, $N$ scalar, mass 400 GeV, $(\sigma_+, \sigma_-) = (1.31,1.01)$; at right, $t'$ scalar, mass 500 GeV, $N$ fermion, mass 150 GeV, $(\sigma_+, \sigma_-) = (1.52,0.90)$. In the lighter ($t'$ scalar) case, there is on average more boost, so the ellipse is stretched more along the $\eta_+$ axis.