Precise Measurement of the $W$-Boson Mass with the CDF II Detector

We have measured the $W$-boson mass $M_W$ using data corresponding to $2.2{\mathrm{fb}}^{-1}$ of integrated luminosity collected in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ with the CDF II detector at the Fermilab Tevatron collider. Samples consisting of 470 126 $W\to e\nu$ candidates and 624 708 $W\to \mu \nu$ candidates yield the measurement $M_W=80387\pm {12}_{\mathrm{stat}.}\pm {15}_{\mathrm{syst}.}=80387\pm 19\mathrm{MeV}/c^2$. This is the most precise measurement of the $W$-boson mass to date and significantly exceeds the precision of all previous measurements combined.

Figure 1

The $Z\to \mu \mu$ (top) and $Z\to ee$ (bottom) mass fits, showing the data (points), the best-fit simulation template (histogram) and the photon-pole contribution (shaded). The arrows indicate the fitting range.

Figure 2

The distribution of $E/p$ for the $W\to e\nu$ data (points) and the best-fit simulation (histogram) including the small jet background. The arrows indicate the fitting range used for the electron energy calibration.

Figure 3

The $m_T$ distribution for muons (top) and the $p_T^{\ell }$ distribution for electrons (bottom). The data (points) and the best-fit simulation template (histogram) including backgrounds (shaded) are shown. The arrows indicate the fitting range.

Figure 4

The 68% C.L. ellipses of $M_W$ and the top quark mass $m_{\mathrm{top}}$, using the Tevatron-average value [22] of $m_{\mathrm{top}}$. Also shown as bands are the allowed ranges for the SM Higgs boson mass, based on direct searches conducted at LEP [23], the Tevatron [24], and the LHC [25, 26].