Text 435, 72 rader
Skriven 2005-05-13 21:59:15 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 731
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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)

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