The LEP Electroweak Working Group

The LEP Electroweak Working Group (LEP EWWG) combines the measurements of the four LEP experiments ALEPH, DELPHI, L3 and OPAL on electroweak observables, such as cross sections, masses and various couplings of the heavy electroweak gauge bosons, properly taking into account the common systematic uncertainties. These combined precision electroweak results are then publicised as the ``best'' LEP averages, confronting theories such as the Standard Model of particle physics. We also compare or combine LEP results with electroweak results from other experiments, notably NuTeV, CDF, DØ and SLD.

Results from the Tevatron experiments CDF and DØ 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. 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 DØ 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).



Of course, the Higgs boson is also searched for directly. Only the direct observation of the Higgs boson constitutes a proof of its existence. The LEP Higgs Working Group (LEP HWG) combines the results on the direct searches for the Higgs boson at LEP. These direct searches lead to the conclusion that the Standard-Model Higgs boson must be heavier than 114.4 GeV (95 percent confidence level limit), as indicated by the excluded area drawn in yellow in the figure above, which is well compatible with the above prediction. Thus, the mass of the Standard-Model Higgs boson is now restricted to a small range of values by the data. In fact, towards the end of the LEP running in the year 2000, tantalising hints for the direct observation of the Higgs signal, corresponding to a Higgs-boson mass around 116 GeV, have been detected; a mass value well compatible with the constraints derived from the precision electroweak measurements. Please consult the LEP Higgs Working Group (LEP HWG) and their final publication on searches for the Higgs boson of the Minimal Standard Model, preprint: CERN-EP/2003-011 (25-April-2003), eprint: hep-ex/0306033, and published in: Phys. Lett. B 565 (2003) 61 for the details.

Resources (including preliminary results):

Resources (including only final results):

Workshops:

LEP-2: above the Z pole

Complete LEP-2 article with all chapters and figures

The Z pole (LEP-1 and SLC)

Complete Z-pole article with all chapters and figures

More information can be found in the LEP Electroweak Working Group Work Page, although access is restricted for most items.

Modified: Mon Aug 4 15:54:02 METDST 2003 by Martin Grünewald