The Alan Sondheim Mail Archive


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Date: Thu, 12 May 2005 15:07:26 -0400
From: physnews@aip.org
To: sondheim@PANIX.COM
Subject: Physics News Update 731

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@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@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)

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
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