[Paper Review] Four-top production and t tbar + missing energy events at multi TeV e+e- colliders

M. Battaglia, Géraldine Servant|arXiv (Cornell University)|May 25, 2010

Particle physics theoretical and experimental studies10 references3 citations

TL;DR

This paper investigates $Z^\prime$-mediated four-top and $t\bar{t}+E_{\text{miss}}$ final states at 3 TeV $e^+e^-$ colliders like CLIC, using a $U(1)^\prime$ model with suppressed couplings to light SM states. It shows that these signatures—especially $t\bar{t}$ invariant mass peaks and $E_{\text{miss}}$ distributions—enable clean $Z^\prime$ mass reconstruction with ~3.5 GeV accuracy, offering a powerful probe of composite top and dark matter models.

ABSTRACT

Four-top production and top pair production in association with missing energy at e+e- colliders are sensitive probes of beyond-the-Standard-Model physics. We consider Standard Model (SM) extensions containing a new U(1)' which couples preferably to the most massive states of the SM such as the top quark or Dark Matter but has suppressed couplings to all the light states of the SM, as inspired by Randall-Sundrum-like setups or theories of partial fermion compositeness. These simple models are poorly constrained by experimental data but lead to striking new signatures at colliders. In this note we consider Z' production in association with a top quark pair in 3 TeV e+e- collisions at CLIC, leading to interesting four-top final states and ttbar+missing energy events.

Motivation & Objective

• To explore the discovery potential of $Z^\prime$ resonances in $e^+e^-$ collisions via four-top and $t\bar{t}+E_{\text{miss}}$ final states.
• To assess the sensitivity of CLIC to $Z^\prime$ couplings to top quarks and WIMPs in models with partial compositeness or Randall-Sundrum-like structures.
• To demonstrate that $t\bar{t}$ invariant mass reconstruction and $E_{\text{miss}}$ signatures provide clean, background-suppressed probes of new physics.
• To quantify the $Z^\prime$ mass resolution achievable in $t\bar{t}t\bar{t}$ final states using kinematic fitting and detector simulation.

Proposed method

• The study employs an effective $U(1)^\prime$ model with a $Z^\prime$ gauge boson coupling preferentially to right-handed top quarks and WIMPs, with suppressed couplings to light SM fermions via kinetic mixing.
• Signal events are generated using CalcHep at 3 TeV $e^+e^-$ center-of-mass energy, with $g_t^{Z^\prime} = g_N^{Z^\prime} = 3$, followed by parton showering with Pythia 6.125.
• Full detector simulation is performed using Mokka and ILD-based reconstruction tools, with event reconstruction via Marlin processors.
• For $t\bar{t}+E_{\text{miss}}$, selection requires $E_{\text{miss}} < 1900$ GeV, jet clustering, and $M_{\text{jet}}$ in 125–225 GeV range to identify top quarks.
• For $t\bar{t}t\bar{t}$, event selection includes high particle multiplicity (>50), energy and thrust cuts, and a kinematic fit using Pufitc to improve resolution.
• The $Z^\prime$ mass is reconstructed from the upper endpoint of the $t\bar{t}$ invariant mass distribution after reconstruction and kinematic fitting.

Experimental results

Research questions

• RQ1 Can $e^+e^-$ colliders like CLIC observe $Z^\prime$ resonances in $t\bar{t}t\bar{t}$ and $t\bar{t}+E_{\text{miss}}$ final states at 3 TeV?
• RQ2 How does the $t\bar{t}$ invariant mass distribution enable $Z^\prime$ mass reconstruction in the presence of detector effects and beamstrahlung?
• RQ3 What is the significance of $E_{\text{miss}}$-based signatures for probing WIMP dark matter in $Z^\prime$-mediated $t\bar{t}+E_{\text{miss}}$ events?
• RQ4 How do kinematic fitting and event selection reduce SM backgrounds in multi-top final states?
• RQ5 What is the achievable mass resolution for $Z^\prime$ in $t\bar{t}t\bar{t}$ events at 1 ab$^{-1}$ of integrated luminosity?

Key findings

• The $t\bar{t}$ invariant mass distribution in $t\bar{t}Z^\prime \to t\bar{t}t\bar{t}$ events preserves the upper endpoint well after reconstruction, enabling precise $Z^\prime$ mass determination.
• For $M_{Z^\prime} = 360$ GeV, the $Z^\prime$ mass can be reconstructed with a statistical accuracy of approximately 3.5 GeV using 1 ab$^{-1}$ of integrated luminosity.
• After selection, 240 signal events are observed in the $t\bar{t}$ mass peak for $M_{Z^\prime} = 360$ GeV, with negligible SM background.
• The $t\bar{t}+E_{\text{miss}}$ channel yields 648 signal events and 22 SM background events for 200 fb$^{-1}$, indicating a clean signature with low background.
• The signal cross section for $t\bar{t}t\bar{t}$ at $M_{Z^\prime} = 360$ GeV is 4.2 fb, significantly exceeding the SM rate of 0.03 fb at 3 TeV.
• The dominant SM background for $t\bar{t}t\bar{t}$ comes from $t\bar{t}W+jets$ (0.4 fb) and $t\bar{t}WW+Njets$ (0.6 fb), both much smaller than the signal.