Before the Large Hadron Collider (LHC) was up and running, the top quark was discovered at Tevatron, a facility with two experiments, CDF and D0.
The world average value of the top quark mass comes from a combination of both LHC and Tevatron measurements. Currently, the top quark mass estimated from direct measurements such as those in the chart below, according to the Particle Data Group is 172.9 ± 0.4 GeV, and this result would pull it down modestly from that value by a few tenths of a GeV or so to more than 172.5 GeV, but less than the current global average value.
But, a recent reanalysis of the data from D0 found a subtle systemic error in some of the D0 measurements that inflated their measurements of the top quark mass. The impact of correcting this error is shown below. It makes the range of mass measurements much tighter than before and lowers the world average (to the extent that this correction is accepted as correct).
Implications
This lower top quark mass measurement, which drives down the global average measurement of the top quark pole mass, also puts pressure on a variety of theoretical cues that favor a higher top quark mass such as: (1) global electroweak observable based fits with the W boson and Higgs boson masses which favor a more massive top quark; (2) an extended Koide's rule estimate of the top quark mass using only the electron and muon masses as inputs, predicted a top quark mass of 173.263 947 ± 0.000 006 GeV; and (3) the top quark mass necessary to make the sum of the square of the fundamental fermion masses equal to half of the square of the Higgs vev is 174.045 477 GeV. The top quark mass necessary to make the sum of the square of the fundamental Standard Model particle masses equal to the square of the Higgs vev is a bit lower, but still more than 173.5 GeV given current Higgs boson mass estimates.
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