Just a few days ago the CDF collaboration announced their new measurement of the W boson mass, with a considerable improvement over the precision of the current world average for that quantity. Now DZERO, the competitor experiment, has also published their own new measurement, which is based on a statistics of 4.3 inverse femtobarns - twice as many data as the ones used by CDF.

The new DZERO measurement is quite compatible with the CDF result: DZERO finds M(W)=80367+-26 MeV. The uncertainty is larger, mainly because of their systematic uncertainty on the electron energy calibration. Note that this measurement only exploits the W -> eν final state, while CDF uses both electron and muon final states.

The figure below shows the fits to two of the three kinematical quantities sensitive to the W boson mass: the transverse mass of the electron-neutrino system (left) and the electron transverse momentum distribution (center). The red points are experimental data, the blue curve is the fit result, and the black histogram represent the estimated background component. At the bottom there are the residuals of the fit, showing that the modeling of the observed distributions is generally very good.

DZERO combines the new result with its own earlier determinations of the W mass, obtaining the result

M(W)= 80375 +- 23 MeV           (DZERO combined)

Combined with the CDF result and the other past measurements, this should end up allowing us to know the W mass to a precision of 13 MeV or so, i.e. less than two parts in ten thousand. I am eager to see the new world average and the resulting global Standard Model fits... Stay tuned, they can't take long!