Number 731, May 12, 2005
by Phil Schewe and Ben Stein
Most Precise Mass Calculation For Lattice QCD
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 Davieset 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
lattice QCD.
Andreas Kronfeld (ask@fnal.gov, 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.
(Allisonet al., Physical Review Letters,6 May 2005; Lattice QCD website
at http://lqcd.fnal.gov/)
Neutrino Pulsar
A new hypothesis suggests that we should be able
to see beams of TeV (trillion electron volt) neutrinos coming from
certain pulsars in the sky. A pulsar is a rotating neutron star
possessing high magnetic fields and spewing energy in a searchlight
pattern, usually observed at radio wavelengths.
According to
Bennett Link of Montana State University, the potent nature of a
young, rapidly spinning neutron star---emitting the energy of our
sun but from a surface 5 billion times smaller, and in the form of x
rays---creates electric fields of fantastic strength, some 1015
volts. These fields will whip protons in the vicinity up to PeV
(1015 eV) energies. When such protons collide with the x rays
emanating from the star, delta particles (essentially heavy protons)
can be created. When these subsequently decay energetic neutrinos
are formed.
This whole production mechanism---proton acceleration,
delta creation, daughter neutrino cascades---sweeps around like the
radio waves normally seen from a pulsar. With the right detector,
the pulsar would reveal itself through neutrinos. If such a neutron
star were as far away as our sun, the Earth would receive about a
million 50-TeV neutrinos per square cm per second. Actual pulsars
are, of course, much further away from us.
Nevertheless, Link
(link@physics.montana.edu) estimates that there are about 10
neutrino pulsars within a distance of 15,000 light years from
Earth. He believes that these energetic sources might result in
about 10 neutrino detections per year in a square-kilometer
detector, which is about the effective size of the so-called IceCube
facility being built now.
Neutrino pulsars could be the brightest continuous high-energy neutrino
sources in the universe and their detection would help to bolster the
idea of neutrino astronomy. (Link
and Burgio, Physical Review Letters, 13 May 2005)
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