Measurement of the Mass Difference between $t$ and $\overline{t}$ Quarks

We present a direct measurement of the mass difference between $t$ and $\overline{t}$ quarks using $t\overline{t}$ candidate events in the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ channel, collected with the CDF II detector at Fermilab’s 1.96 TeV Tevatron $p\overline{p}$ Collider. We make an event by event estimate of the mass difference to construct templates for top quark pair signal events and background events. The resulting mass difference distribution of data is compared to templates of signals and background using a maximum likelihood fit. From a sample corresponding to an integrated luminosity of $5.6{\mathrm{fb}}^{-1}$, we measure a mass difference, $\Delta M_{\mathrm{top}}=M_t-M_{\overline{t}}=-3.3\pm 1.4(\mathrm{stat})\pm 1.0(\mathrm{syst})\mathrm{GeV}/c^2$, approximately 2 standard deviations away from the $CPT$ hypothesis of zero mass difference.

Figure 1

Distributions of $\Delta m_{\mathrm{reco}}$ and $\Delta m_{\mathrm{reco}}^{(2)}$ used to extract $\Delta M_{\mathrm{top}}$ for zero $b$-tagged (nontagged) events and one or more $b$-tagged (tagged) events. The data are overlaid with the predictions from the KDE probability distributions assuming $\Delta M_{\mathrm{top}}=-4\mathrm{GeV}/c^2$ (solid red line) and $\Delta M_{\mathrm{top}}=0\mathrm{GeV}/c^2$ (dashed blue line).