The figure shown below represents the best measurement of the top quark mass ever obtained by a single experiment, and it is a determination with a less than 1% total uncertainty. It has been approved last week by the CDF experiment at Fermilab.

The CDF experiments collects proton-antiproton collisions delivered by the Tevatron collider, which imparts the projectiles with 1 TeV of energy each, for a center-of-mass energy of 2 TeV. This is still the highest energy ever achieved by a collider, although the record is going to be soon stripped off Fermilab by the Large Hadron Collider, which is due to start colliding protons with other protons at 7 TeV of energy this coming fall.

CDF collected 630 top pair candidates from 4.3 inverse femtobarns of collisions (about 300 trillions of them) in the "single lepton" decay topology: events must have a leptonic W boson decay ($W \to e \nu$or $W \to \mu \nu$, id est with an energetic electron or muon decay), plus four hadronic jets. The reconstruction fit for the top quark mass uses a matrix element technique which determines both the most likely top quark mass and the offset from a perfect calibration of the jet energy, the so-called "jet energy scale" factor.

Jet energies are the least well determined quantities in the top decay, and an imprecise calibration is the largest source of systematic uncertainty: by fitting for both quantities together the uncertainty in the top quark mass is minimized, through the use of a two-dimensional likelihood function.

The figure shows the two quantities determined together as contours of equal probability. The top mass is measured as $M_t = 172.6 \pm 0.9 \pm 0.7 \pm 1.1 GeV$ (the first uncertainty is statistical, the second is the residual one due to the jet energy scale systematics, and the third one is due to other systematical sources of error), or 172.6 +- 1.6 GeV.

This is a less than percent uncertainty, and the best result in the World.

Kudos to my colleagues in CDF for this new stellar result! And kudos to old members of CDF like me, who using merely 7 candidates measured the top mass for the first time back in 1994 as 174 GeV, a measurement which is amazingly still less than one standard deviation away from the most credited numbers!