First Measurement of the $W$-Boson Mass in Run II of the Tevatron

We present a measurement of the $W$-boson mass using $200{\mathrm{pb}}^{-1}$ of data collected in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ by the CDF II detector at run II of the Fermilab Tevatron. With a sample of 63 964 $W\to e\nu$ candidates and 51 128 $W\to \mu \nu$ candidates, we measure $M_W=80413\pm {34}_{\mathrm{stat}}\pm {34}_{\mathrm{syst}}=80413\pm 48\mathrm{MeV}/c^2$. This is the most precise single measurement of the $W$-boson mass to date.

Figure 1

The $Z\to ee$ (top panel) and $Z\to \mu \mu$ (bottom panel) mass fits, showing the data (points) and the simulation (histogram). The arrows indicate the fitting range.

Figure 2

The distribution of $E/pc$ for the $W\to e\nu$ data (points) and the best-fit simulation (histogram) including the small jet background (shaded). The arrows indicate the fitting range used for the electron energy calibration. The jet background, which is barely visible on this scale, contributes a negligible uncertainty in the calibrations of the electron energy scale and the amount of radiative material.

Figure 3

Left panel: The $u_{\parallel }$ distribution for the electron channel data (points) and simulation (histogram). Right panel: The $|\overrightarrow{u}|$ distribution for the muon channel. The mean and rms of the histograms agree between data and simulation, within the statistical precisions of $\approx 1\%$.

Figure 4

The $m_T$ distribution of the data (points) and the best-fit simulation template (histogram) including backgrounds (shaded), for muons (top panel) and electrons (bottom panel). The arrows indicate the fitting range. The ${\chi }^2/(\mathrm{dof})$ for the electron channel distribution receives large contributions from a few bins near $65\mathrm{GeV}/c^2$, which do not bias the mass fit.