PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 731 May 12, 2005
by Phillip F. Schewe, 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 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 lattice QCD.  Andreas Kronfeld
(ask{at}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. 
(Allison et 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 10^15
volts.  These fields will whip protons in the vicinity up to PeV
(10^15 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{at}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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