Measuring the W mass with ATLAS

11 February 2008

The evolution of the measured W mass and its precision since its discovery in 1983 (from PDG).

25 years after its first measurement, measuring the W mass with high precision can still reveal a lot about the Higgs boson and more.

The W boson and its mass has played a central role in particle physics, and it will continue to do so at the LHC, adding to its colourful history.

Discovered at CERN's SPS in 1983, the W boson was a major corner stone in the experimental verification of the Standard Model (SM). Back then UA1 and UA2 only had 10 W events in total, but were still able to estimate the W mass to "about 80 GeV", beautifully verifiying the prediction of the SM.

Once the W mass was accurately measured, it allowed for a precise prediction of the top mass, before the top quark was discovered in 1995. Repeating history, it is now the centerpiece for predicting the Higgs mass. However, the Higgs mass prediction requires a very precise W mass measurement. We currently know the W mass to be 80.403 +/- 0.029 GeV [PDG2007], and this W mass uncertainty is the main limitation to the accuracy of the Higgs mass prediction. Therefore, a more precise measurement is highly desirable.

At the LHC, we expect to have about 60 million reconstructed W events in 10 fb-1, which means after a year of running at nominal luminosity (which probably translates into about 3 years from now). Only decays into electrons and muons are used, as the hadronic decay modes completely disappear in (irreducible) background. Such large statistics complicate the analysis, but at the same time it is what allows us to improve the precision. We have used fast analysis routines and other "tricks", and it will certainly be interesting to see how the ATLAS software will deal with data samples of this size.

Using the Z for calibration, we - the ATLAS W mass group - believe that we can improve the precision significantly beyond the present uncertainty of 29 MeV.

By how much? Well, the short answer is an uncertainty somewhere around 5-10 MeV, depending on a few issues. For more information, please read our note (ATL-COM-PHYS-2007-047) and the coming W mass CSC note.

The current constraint on the Higgs mass as a function of the W boson and top quark masses. The uncertainty (blue oval) projected onto the Higgs mass direction (perpendicular to the green band) is largest in the W mass direction, signifying that the W mass uncertainty is the most limiting parameter (from LEP EWWG)

I have enjoyed very much working on this analysis, which has not only taught me about precision calibration but also given me the chance to work with such excellent people as Nathalie Besson, Maarten Boonekamp, Esben Klinkby, and Sascha Mehlhase.

And who knows - Nature might very well have more discoveries in store for us, which once again could be seen indirectly by an ever more precise W mass measurement.

Colin Barras


Troels Petersen