|Uncertainties in units of MeV/c2 on the latest result for the mass of the W boson, which was determined to be 80,387 ± 19 MeV/c2. The total uncertainty amounts to 0.02 percent.|
On Feb. 29, 2012, CDF submitted to the journal Physical Review Letters the world’s most precise measurement of the W boson mass. More recently, on Nov. 4, 2013, we submitted a long paper to Physical Review D documenting this measurement in detail. This has long been a practice at CDF: making important results available quickly and then proceeding to document them carefully so that physicists can really understand what was done.
Discovered at CERN in 1983, the W boson is one of the particles responsible for mediating electroweak interactions. Among other decay paths, it can decay into an electron or muon and a neutrino.
In Tevatron Run I (1990), using 1,722 W bosons, we determined the W boson mass to be 79,910 ± 390 MeV/c2. Between Run I and Run II, we completely rebuilt all of the tracking chambers, which would help in making a more accurate measurement. Thanks to the Accelerator Division, the Tevatron’s beam luminosity increased, and thus in 2007 we announced a new measurement based on more than 60 times the number of W boson events from 1990, helping significantly diminish the uncertainty of 390 MeV/c2. We found the mass to be 80,413 ± 48 MeV/c2 based on 115,092 events. At the time, this was the world’s single most precise measurement of the W mass.
For the 2012 measurement, our goal was to reduce the 48 MeV/c2 uncertainty to below that of the previous world average of 23 MeV/c2.
The largest single systematic uncertainty in this measurement was ± 23 MeV/c, which was the uncertainty on the calibration of the momentum of the decay electron and muon. The collaboration improved the calibration of the electron energy scale using, in decays of both the W and the Z boson, the ratio of its energy to its momentum. We verified this calibration technique by applying it to the measurement of the mass of the Z boson decaying into two electrons. We established the momentum calibration using J/ψ– and Υ-to-muon pair decays and again verified it using the Z-to-muon pair mass measurement. In the current measurement, the uncertainty of the electron and muon energy scale is reduced to ± 7 MeV/c2 (see top figure).
The February 2012 measurement of the W boson mass from CDF is 80,387 ± 19 MeV/c2, which was obtained using 470,126 candidates in which a W decays into an electron and a neutrino and 624,708 candidates in which a W decays into a muon and a neutrino. The combined world average now yields a W boson mass of 80,385 ± 15 MeV/c2.
What has changed since February 2012? The Higgs boson has been discovered, and its mass has been determined to high accuracy, allowing a prediction of the W boson mass of 80,359 ± 11 MeV/c2. The comparison of this prediction with the combined world average places bounds on non-Standard Model physics.
What next? CDF is analyzing four times more data to try to achieve a precision of about 10 MeV/c2. It is a challenging target. Stay tuned!
—edited by Andy Beretvas