Results from the Tevatron experiments CDF and Dě are combined by the Tevatron Electroweak Working Group. In particular, combinations of the mass of the top quark and mass and width of the W boson are used in the Standard-Model analyses here.
Combinations and analyses are performed with published and preliminary measurements usually twice a year: in winter (around February/March) and in summer (around July). The results, figures and detailed write-ups are posted below.
For questions and comments contact: Martin Grünewald (E-Mail No-SPAM: Martin DOT Grunewald AT cern DOT ch)
A translation of (older versions of) this page into Haitian Creole by Susan Basen is available here, and a translation into Ukrainian by Mario Pozner is available here. No guarantee whatsoever on the validity of these translations is implied.
Status of March 2012:
The combinations from the LEPEWWG are used to perform stringent tests
the Standard Model of particle physics by comparing the precise
results with theory predictions. The conclusion is that the Minimal
Standard Model is able to describe nearly all the LEP measurements
rather well; there is no compelling need for introducing new phenomena
beyond those foreseen by the Standard Model. Furthermore, exploiting
theory relationships, the experimental results allow us, among other
things, to predict the masses of heavy fundamental particles, such as
the top quark and the W boson, which are then compared to the direct
measurements. This checks the correctness of the prediction and thus
of the theory in this area. The bar chart on the left displays this
comparison for the mass of the W boson: The top part shows the direct
measurements, the bottom part shows the indirect constraints valid
within the Minimal Standard Model.
Separately shown is the measurement from the NuTeV collaboration, which has recently published its final result on the ratio of neutral current to charged current reactions in neutrino-nucleon scattering. This measurement, when interpreted as a measurement of the mass of the W boson, shows an interesting deviation, at the level of 2.6 to 2.8 standard deviations, from the other indirect constraints.
Of particular interest is the constraint on the mass of the Higgs
boson, because this fundamental ingredient of the Standard Model has
not been observed yet. The figure on the left shows the Delta-chi2
curve derived from high-Q2 precision electroweak measurements,
performed at LEP and by SLD, CDF, and D0, as a function of the
Higgs-boson mass, assuming the Standard Model to be the correct theory
of nature. The preferred value for its mass, corresponding to the
minimum of the curve, is at 94 GeV, with an experimental uncertainty
of +29 and -24 GeV (at 68 percent confidence level derived from Delta
chi2 = 1 for the black line, thus not taking the theoretical
uncertainty shown as the blue band into account). This result is only
little affected by the low-Q2 results such as the NuTeV measurement
discussed above. |
While this is not a proof that the Standard-Model Higgs boson actually exists, it does serve as a guideline in what mass range to look for it. The precision electroweak measurements tell us that the mass of the Standard-Model Higgs boson is lower than about 152 GeV (one-sided 95 percent confidence level upper limit derived from Delta chi2 = 2.7 for the blue band, thus including both the experimental and the theoretical uncertainty). This limit increases to 171 GeV when including the LEP-2 direct search limit of 114 GeV shown in yellow (see below).
The Tevatron experiments CDF and Dě also search for the Standard-Model Higgs boson; the most recent combined result (July 2011) excluding the mass range of 156 GeV to 177 GeV at 95%CL. The LHC experiments exclude a range of 127 GeV to 600 GeV (December 2011 LHC presentations of ATLAS and CMS).
More information can be found in the LEP Electroweak Working Group Work Page, although access is restricted for most items.