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The new W mass from CDF is out, but… December 2, 2006

Posted by dorigo in news, physics, science.

… you will need to wait for a little while longer before you get the correct number.

That is, we do have a measurement: it is Mw = 80336+-48 MeV+- 100 MeV. The last number, however, is a blind offset that has been purposely left unknown until everything is approved and the analysis gets unblinded. This procedure has the aim  of avoiding the “experimenter’s bias”, which manifests as a tendency of favoring results that are in line with previous determinations of the same quantity.

This very precise result – the single best determination of the W mass so far – has been obtained with just 200 pb-1 of electron and muon data collected in Run II. If you thnk that we have more than 8 times that amount of data already on tape and waiting to be analyzed, you will concur that the future is bright.

CDF will probably reduce the total uncertainty of the W boson mass to less than 25 MeV by the time Run II ends. That is a very nice determination of a fundamental parameter of the standard model, and it carries quite some value by itself as a scientific result. However, many of us are interested in a precise number of the W and top quark masses because electroweak theory allows to determine indirectly (up to some theoretical uncertainty which is still small compared to experimental ones) the Higgs boson mass – which we would all love to know with precision!

I wrote about this in a former post, see my “physics made easy” page above (today I feel too lazy to find the link for you and place it here!)

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