A team of theoretical physicists have produced the best prediction
of a particle’s mass. And within days of their paper being submitted
to Physical Review Letters, that very particle’s mass was accurately
measured at Fermilab, providing striking confirmation of the predicted
value. How do the known particles acquire the mass they have? The answer
might come from lattice QCD, the name for a computational approach to
understanding how quarks interact.
Imagine quarks placed at the interstices of a crystal-like structure.
Then let the quarks interact with each other via the exchange of gluons
along the links between the quarks. The gluons are the designated carriers
of the strong nuclear force under the general auspices of the theory
called quantum chromodynamics (QCD). From this sort of framework the
mass of the known hadrons (quark-containing composite particles such
as mesons and baryons) can be calculated.
Until recently, however, the calculations were marred by a crude approximation.
A big improvement came only in 2003, when uncertainties in mass predictions
went from the 10% level to the 2% level (see Davies
et al., Physical Review Letters, 16 January 2004). The mass of
the proton, for example, could be calculated within a few percent of
the actual value. Progress has come from a better treatment of the light
quarks and from greater computer power.
Together the improvements provide the researchers with a realistic treatment
of the "sea quarks," the virtual quarks whose ephemeral presence has
a noticeable influence over the "valence" quarks that are considered
the nominal constituents of a hadron. A proton, for example, is said
to consist of three valence quarks---two up quarks and one down quark---plus
a myriad of sea quarks that momentarily pop into existence in pairs.
Now, for the first time, the mass of a hadron has been predicted with
Andreas Kronfeld (firstname.lastname@example.org, 630-840-3753) and his colleagues at
Fermilab, Glasgow University, and Ohio State report a mass calculation
for the charmed B meson (Bc, for short, consisting of an anti-bottom
quark and a charmed quark). The value they predict is 6304 +/- 20 MeV---the
remarkable precision stems not only from the improvements discussed
above, but also from the researchers' methods for treating heavy quarks.
A few days after they submitted their Letter for publication, the first
good experimental measurement of the same particle was announced 6287
+/- 5 MeV.
This successful confirmation is exciting, because it bolsters confidence
that lattice QCD can be used to calculate many other properties of hadrons.
et al., Physical Review Letters,6 May 2005; Lattice QCD website