Closing in on a new W mass measurement by CDF November 17, 2006Posted by dorigo in news, physics, science.
The W mass measurement is a very complex business in hadron colliders. W bosons can be identified with relative ease when they decay to a lepton-neutrino pair, because our detector is very well suited for measuring the charged track produced by a muon in the central tracker and in the outer muon detectors, or the energy deposit left by the electron in the calorimeter. However, the neutrino escapes undetected, and the measurement of its momentum transverse to the beam axis must be extrapolated from the energy imbalance in the transverse plane measured by the calorimeter system.
A precise measurement of the W mass entails a detailed understanding of very subtle effects, such as imperfections of the magnetic field in the tracker, imprecise knowledge of the amount of dead material in the detector (which causes gamma radiation through bremsstrahlung, modifying the response to electrons), a detailed modeling of the calorimeter response to soft hadrons, and the like.
It is not surprising, therefore, that a W mass measurement takes a long time to produce. We have started collecting data in Run II in 2002, and we have not produced one yet. But we are getting close. And the total uncertainty, once electron and muon channel results get combined in a single measurement, is going to be very small!
I cannot divulgate the uncertainty no more than I can write what the final result is. Well, I do not even know the final result yet, because the analysis is still blinded – not even the authors know it until they get the information on the random offset of all Monte Carlo templates used for the fits. But I do know that the uncertainty is smaller than that of any previous measurements.
That is, better than each of the Opal, Aleph, Delphi, L3 measurements (not better than their combinations yet, but close!). And of course better than previous determinations at the Tevatron.
One thing to note: the measurement is based on just 200 pb-1 of data, but we already have collected eight times more! So it’s getting better… But it will take a long time to analyze all the data with this level of precision.
Below you can find the summary of past measurements of the W boson mass. Stay tuned, in a couple of weeks I should be able to give you the real numbers from CDF…
Just a note: the last three determinations are indirect results, and are somewhat more model dependent than the direct determinations from the Tevatron and Lep2 (two rows on top).