Direct Measurement of the Mass Difference between Top and Antitop Quarks

We present a measurement of the mass difference between $t$ and $\overline{t}$ quarks in $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ final states of $t\overline{t}$ events in $1{\mathrm{fb}}^{-1}$ of data collected with the D0 detector from Fermilab Tevatron Collider $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$. The measured mass difference of $3.8\pm 3.7\mathrm{GeV}$ is consistent with the equality of $t$ and $\overline{t}$ masses. This is the first direct measurement of a mass difference between a quark and its antiquark partner.

Figure 1

Values of the measured mean $\Delta$ from MC pseudo experiments as a function of ${\Delta }^{\mathrm{in}}$, parameterized by straight lines for (a) $e+\mathrm{\text{jets}}$ and (b) $\mu +\mathrm{\text{jets}}$ MC events. Dotted lines represent complete equality between measured and input values. Results from pseudo experiments with same ${\Delta }^{\mathrm{in}}$ but different $m_{\mathrm{sum}}$ correspond to the extra points for fixed ${\Delta }^{\mathrm{in}}$ (see text).

Figure 2

Fitted contours of equal probability for the two-dimensional likelihoods as a function of $m_t$ and $m_{\overline{t}}$ for (a) $e+\mathrm{\text{jets}}$ and (b) $\mu +\mathrm{\text{jets}}$ data. The boxes, representing the bins in the two-dimensional histograms of the likelihoods, have areas proportional to the bin contents, set equal to the value of the likelihood evaluated at the bin center.

Figure 3

Projections of the likelihoods onto the $\Delta$ axis for (a) $e+\mathrm{\text{jets}}$ and (b) $\mu +\mathrm{\text{jets}}$ data.