Search for Production of Heavy Particles Decaying to Top Quarks and Invisible Particles in $p\overline{p}$ Collisions at $\sqrt{s}=1.96\mathrm{TeV}$

We present a search for a new particle $T^{\prime }$ decaying to top quark via $T^{\prime }\to t+X$, where $X$ is an invisible particle. In a data sample with $4.8{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the CDF II detector at Fermilab in $p\overline{p}$ collisions with $\sqrt{s}=1.96\mathrm{TeV}$, we search for pair production of $T^{\prime }$ in the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ channel, $p\overline{p}\to t\overline{t}+X+X\to \ell \nu bq{q}^{\prime }b+X+X$. We interpret our results primarily in terms of a model where $T^{\prime }$ are exotic fourth generation quarks and $X$ are dark matter particles. Current direct and indirect bounds on such exotic quarks restrict their masses to be between 300 and $600\mathrm{GeV}/c^2$, the dark matter particle mass being anywhere below $m_{T^{\prime }}$. The data are consistent with standard model expectations, and we set 95% confidence level limits on the generic production of $T^{\prime }{\overline{T}}^{\prime }\to t\overline{t}+X+X$. For the dark matter model we exclude $T^{\prime }$ at 95% confidence level up to $m_{T^{\prime }}=360\mathrm{GeV}/c^2$ for $m_X\le 100\mathrm{GeV}/c^2$.

Figure 1

Reconstructed transverse mass of the $W$, $m_T^W$, for the standard model backgrounds, the observed data, and for three choices of ($m_{T^{\prime }}$, $m_X$).

Figure 2

Reconstructed transverse mass of the $W$, $m_T^W$, in signal-depleted control regions. Left, events with at least four jets and small missing transverse momentum ($<100\mathrm{GeV}/c$). Right, events with exactly three jets and large missing transverse momentum ($>100\mathrm{GeV}/c$).

Figure 3

Observed versus expected exclusion in ($m_{T^{\prime }}$, $m_X$) along with the cross-section upper limits.