Dark Matter-Motivated Searches for Exotic 4th Generation Quarks in Tevatron and Early LHC Data

TL;DR

This paper analyzes collider prospects for dark matter in models with exotic 4th-generation quarks $T'$ decaying as $T'\to tX$, yielding the signature $t\bar t + \slash{E}_T$ with $m_{T'}$ in the 300–600 GeV range. Using LO simulations with MadGraph/MadEvent, Pythia, and PGS4, it estimates signals and dominant backgrounds for Tevatron and early LHC data, finding that the fully hadronic channel provides the strongest reach. At the Tevatron, 20 fb$^{-1}$ can exclude $m_{T'}$ up to about 455 GeV (hadronic) and probe discovery up to around 400 GeV for favorable $m_X$; at the 10 TeV LHC with 0.3 fb$^{-1}$, exclusions reach up to about 600 GeV and discoveries up to ~490 GeV, with the hadronic channel again being most powerful. The results imply that DM scenarios such as WIMPless models—including potential explanations of DAMA/CoGeNT—can be tested with existing Tevatron data and early LHC runs, highlighting a strong complementarity between collider searches and direct/indirect detection efforts.

Abstract

We determine the prospects for finding dark matter at the Tevatron and LHC through the production of exotic 4th generation quarks T' that decay through T' \to t X, where X is dark matter. The resulting signal of t \bar{t} + \met has not previously been considered in searches for 4th generation quarks, but there are both general and specific dark matter motivations for this signal, and with slight modifications, this analysis applies to any scenario where invisible particles are produced in association with top quarks. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and 600 GeV, and the dark matter's mass may be anywhere below m_T'. We simulate the signal and main backgrounds with MadGraph/MadEvent-Pythia-PGS4. For the Tevatron, we find that an integrated luminosity of 20 fb^-1 will allow 3σdiscovery up to m_T' = 400 GeV and 95% exclusion up to m_T' = 455 GeV. For the 10 TeV LHC with 300 pb^-1, the discovery and exclusion sensitivities rise to 490 GeV and 600 GeV. These scenarios are therefore among the most promising for dark matter at colliders. Perhaps most interestingly, we find that dark matter models that can explain results from the DAMA, CDMS and CoGeNT Collaborations can be tested with high statistical significance using data already collected at the Tevatron and have extraordinarily promising implications for early runs of the LHC.

Figures (6)

• Figure 1: Distributions of missing transverse energy $\,{\slash E_T}$, transverse mass $m_T^W$, number of jets $N(\text{jets})$, and jet pair invariant mass $m_{jj}$ for signal and backgrounds for the 10 TeV LHC in the semi-leptonic channel. Each of the observables has been plotted after the precuts coming before it in the list, and the chosen precut has been marked by a vertical line. For signal, the masses $(m_{T'}, m_X) = (300~\text{GeV}, 1~\text{GeV})$, $(400~\text{GeV}, 1~\text{GeV})$, and $(500~\text{GeV}, 1~\text{GeV})$ have been chosen for illustration. The $W$ and $Z$ samples were simulated with a cut on $\,{\slash E_T}>80~\text{GeV}$ and at least 3 jets in the parton-level generation. See text for details.
• Figure 2: ${\slash E_T}$, $N(\text{jets})$, and $H_T$ distributions for signal and backgrounds for the 10 TeV LHC in the hadronic channel. The top two panels show distributions of $\,{\slash E_T}$ and $N(\text{jets})$ after the previous cuts in the precut table, with the position of the precut marked with a vertical dashed line. The lower two panels show distributions of $\,{\slash E_T}$ and $H_T$ after all precuts. The hadronic top contribution is negligible after the $\,{\slash E_T}>100~\text{GeV}$ cut and has therefore been omitted in the remaining plots. For signal, the masses $(m_{T'}, m_X) = (300~\text{GeV}, 1~\text{GeV})$, $(400~\text{GeV}, 1~\text{GeV})$, and $(500~\text{GeV}, 1~\text{GeV})$ have been chosen for illustration. The $W$ and $Z$ samples were simulated with a cut on $\,{\slash E_T} > 80~\text{GeV}$ and at least 3 jets in the parton-level generation. See text for details.
• Figure 3: 95% CL Tevatron exclusion contours for the semi-leptonic channel (left) and the hadronic channel (right) for integrated luminosities 2, 5, 10, and $20~\text{fb}^{-1}$. For each point in parameter space, the cut with the best significance has been chosen.
• Figure 4: 3$\sigma$ (Gaussian equivalent) Tevatron discovery contours for the semi-leptonic channel (left) and the hadronic channel (right) for integrated luminosities 2, 5, 10, and $20~\text{fb}^{-1}$. For each point in parameter space, the cut with the best significance has been chosen.
• Figure 5: 95% CL exclusion contours for a 10 TeV LHC run in the semi-leptonic channel (left) and the hadronic mode (right), for integrated luminosities 100, 200, and $300~\text{pb}^{-1}$. For each point in parameter space, the cut with the best significance has been chosen.