Top Quark Pair plus Large Missing Energy at the LHC

TL;DR

This work addresses the collider challenge of detecting new physics in $pp\to T\bar T\to t\bar t A^0 A^0$ events, i.e., $t\bar t$ plus large missing energy, at the LHC by examining both fermionic top partners ($T'$) and scalar stops ($\tilde t$) that decay via $T\to t A^0$. It develops a robust kinematic framework, including a transverse-mass based discrimination and a novel complex-valued reconstruction of the leptonic top mass $m_t^r$, to suppress dominant SM backgrounds and map the discovery reach in the $(m_T,m_A)$ plane for 100 fb$^{-1}$. The study finds substantial observability for moderate mass splittings $\Delta M_{TA}$ (up to about $\sim 250$ GeV at $m_T=1$ TeV), with similar reach for scalar and fermionic partners, while very small splittings challenge detection. Distinguishing scalar from fermionic top partners in this minimal channel proves difficult, indicating the potential need for extended decay structures or alternative observables to achieve a clear spin determination, thereby guiding future LHC search strategies in top-partner scenarios.

Abstract

We study methods of extracting new physics signals in final states with a top-quark pair plus large missing energy at the LHC. We consider two typical examples of such new physics: pair production of a fermionic top partner (a $T'$ in Little Higgs models for example) and of a scalar top partner (a $\tilde{t}$ in SUSY). With a commonly-adopted discrete symmetry under which non Standard Model particles are odd, the top partner is assumed to decay predominantly to a top quark plus a massive neutral stable particle $A^0$. We focus on the case in which one of the top quarks decays leptonically and the other decays hadronically, $pp \to {\tt} A^0A^0 X \to bj_1j_2 \bar b\ell^- \barνA^0A^0 X + c.c.$, where the $A^0$s escape detection. We identify a key parameter for the signal observation: the mass splitting between the top partner and the missing particle. We reconstruct a transverse mass for the lepton-missing transverse energy system to separate the real $W$ background from the signal and propose a definition for the reconstructed top quark mass that allows it to take unphysical values as an indication of new physics. We perform a scan over the two masses to map out the discovery reach at the LHC in this channel. We also comment on the possibility of distinguishing between scalar and fermionic top partners using collider signatures.

Figures (11)

• Figure 1: Leading order QCD cross section for top partner pair production at the LHC, as a function of its mass. The solid line corresponds to a spin-$1\over 2$ particle, the dashed line to a spin-$0$ state. The two dashed horizontal lines indicate the cross sections for the SM background processes $t\bar{t}$ and $t\bar{t} Z$ with tree-level matrix elements. The left panel shows the results before $T$ decay, and the right panel includes the decay branching fractions to the semi-leptonic final state $bj_1j_2\ \bar{b}\ell^- \bar{\nu} + E\!\!\!\!\slash_{T}$, before any kinematical acceptance.
• Figure 2: Transverse momentum distributions for the top quark from QCD $\tt$ production (the top curve), $\tt Z$ production (dot-dashed), and from $T$ decays for $m_T=1$ TeV and $m_A=800,\ 200$ GeV, respectively. For comparison, we include the $p_T$ of the fermionic $T$ (long-dashed).
• Figure 3: Differential distributions for (a) the transverse momentum of the charged lepton $p_T(\ell)$, (b) the missing transverse momentum $p_T^{miss}=E\!\!\!\!\slash_{T}$, and (c) the effective transverse mass of the final state system, respectively.
• Figure 4: Kinematical correlation of events between $E\!\!\!\!\slash_{T}$ and $M^{\rm eff}_T$ for (a) the QCD $t\bar{t}$ background, (b) and (c) the $T$ signal with $m_A=200,\ 800$ GeV respectively. The color codes indicate the size of the cross sections.
• Figure 5: Normalized opening angle distributions in the transverse plane $\phi_{t-bl}$ for the signal $m_A=200,\ 800$ GeV (solid), and the $t\bar{t}$ background (solid), $Wb\bar{b} jj$ (dashed), and $t\bar{t} Z$ (dot-dashed).