Pair production of heavy Q=2/3 singlets at LHC

TL;DR

The study assesses LHC discovery potential for heavy $Q=2/3$ quark singlets ($T$) produced in pairs via QCD, with decays $T\to Wb$, $Zt$, $Ht$, focusing on the final state where one $W$ decays leptonically and the other hadronically. Using a particle-level simulation chain (HELAS/ALPGEN, PYTHIA, ATLFAST) and realistic $b$-tagging, the authors reconstruct heavy-quark candidates from $m_T^{\text{had}}$ and $m_T^{\text{lep}}$ and demonstrate observable mass peaks after optimized cuts. For $m_T=500$ GeV, a $5\sigma$ discovery is achievable with as little as $\mathcal{L} \approx 2.7$ fb$^{-1}$, and with $\mathcal{L}=100$ fb$^{-1}$ the reach extends to $m_T \approx 1$ TeV; for $m_T=1$ TeV, a $5\sigma$ discovery requires $\mathcal{L} \approx 85$ fb$^{-1}$ and can probe up to about $1.1$ TeV. The results highlight the complementarity between $T\bar T$ and $Tj$ production and the role of precision electroweak constraints in constraining the allowed parameter space, offering a path to either discovering a new quark or tightening bounds on its mass and mixing.

Abstract

We examine the LHC discovery potential for new Q=2/3 quark singlets T in the process gg,qq -> T Tbar -> W+ b W- bbar, with one W boson decaying hadronically and the other one leptonically. A particle-level simulation of this signal and its main backgrounds is performed, showing that heavy quarks with masses of 500 GeV or lighter can be discovered at the 5 sigma level after a few months of running, when an integrated luminosity of 3 fb^-1 is collected. With a luminosity of 100 fb^-1, this process can signal the presence of heavy quarks with masses up to approximately 1 TeV. Finally, we discuss the complementarity among T Tbar, Tj production and indirect constraints from precise electroweak data in order to discover a new quark or set bounds on its mass.

Figures (6)

• Figure 1: Transverse momentum of: (a) the fastest jet; (b) the fastest $b$ jet; (c) the charged lepton. Missing transverse momentum (d); total transverse energy (e). The histograms are normalised to a total number of 2000 events.
• Figure 2: Reconstructed masses of the heavy quarks decaying hadronically (a) and semileptonically (b). The histograms are normalised to a total number of 2000 events.
• Figure 3: Reconstructed masses of the heavy quarks decaying hadronically (a) and semileptonically (b), after the selection cuts in Eq. (\ref{['ec:cut500']}). The dashed lines correspond to the SM predictions, while the full lines represent the SM plus a new 500 GeV quark.
• Figure 4: Reconstructed masses of the heavy quarks decaying hadronically (a) and semileptonically (b), after the selection cuts in Eq. (\ref{['ec:cut1000']}). The dashed lines correspond to the SM predictions, while the full lines represent the SM plus a new 1 TeV quark.
• Figure 5: (a) Cross sections for $T \bar{T}$ production (full line) and $Tj$ production, in the latter case for $V_{Tb}=0.1$ (dotted line), and for $V_{Tb}$ derived from the $\mathrm{T}$ parameter (dashed line). (b) Upper bounds on $|V_{Tb}|$ and values suggested by the $\mathrm{T}$ parameter (black line) and the Little Higgs relation $V_{Tb} = m_t/m_T$ (grey line).