Number 552,
August 20, 2001 by Phil Schewe, James Riordon, and Ben Stein
Doubly Strange Nuclei
"Doubly strange" nuclei, each containing two strange quarks,
have been produced by an international team at Brookhaven National Laboratory
(Bob Chrien, Brookhaven, 631-344-3903, chrien@bnl.gov). Doubly strange
nuclei are presumed to play an important part in the role of strange
matter in neutron stars and in the formation of the early universe.
They also promise to deliver new information on the forces that hold
a nucleus together.
Previously, various groups have reported single candidates for a nucleus
containing two strange quarks. However, these candidates were inconsistent
with one another and never independently confirmed, according to the
Brookhaven team, which has now observed hundreds of these novel particles.
A "doubly strange" nucleus, or lambda-lambda hypernucleus
as it's more formally known, consists of the usual protons and neutrons
plus two lambda particles, each made of an up, a down, and a strange
quark. In effect, a lambda is a neutron with one of its quarks, a down
quark, replaced by a strange quark. The doubly strange nuclei produced
at BNL can be thought of as a sort of heavy hydrogen nucleus consisting
of a deuteron (proton and neutron) and two lambdas.
To create the doubly strange nuclei, the researchers aimed a beam of
K- mesons at a beryllium target surrounded by appropriate
detection equipment. (The K mesons, each containing an up quark and
an anti-strange quark, were created by colliding protons at a tungsten
target in BNL's Alternating Gradient Synchrotron.) The absorption of
a K- meson by a beryllium nucleus resulted in a reshuffling
of quarks in which two nucleons were converted into two lambda particles,
each of which contains a strange quark.The doubly strange nuclei subsequently
decayed into lighter particles by emitting pi mesons (the lightest kind
of quark-antiquark pair) which were detected by the experimenters, and
identified by their characteristic energies.
Having created many of these nuclei, the Brookhaven researchers believe
that they have a reliable production technique. They hope to learn more
about the force that binds lambdas together and thereby round out knowledge
of nuclear forces.
In addition, the production of doubly strange nuclei constitutes a
strong argument against the existence of the "H particle,"
a hypothesized bag of six quarks instead of the usual two or three.
That's because bringing two lambdas together in a nucleus would make
it energetically favorable for them to decay into an H, as long as the
lambdas are not too tightly bound; however, the researchers did not
observe any evidence of such a decay. (Ahn et al., upcoming article
in Physical Review Letters; also see Brookhaven
press release.)
The Antiproton's Mass and Charge
The antiproton's mass and charge have been measured to within 60 parts
per billion, affording new tests of quantum mechanics. Paul Dirac's
1930 prediction of a whole shadow family of particles, antiparticle
counterparts of the known particles, was quickly borne out. In 1932
the anti-electron, the positron, was discovered and in 1955 antiprotons
(p-bar) were made artificially in an accelerator for the first time.
Since that time physicists have sought to determine that antimatter
plays by the same rules as ordinary matter.
An excellent place for these studies is at the CERN Antiproton Decelerator
in Geneva, where antiprotons are created in high energy collisions,
then collected, cooled, decelerated, and directed toward a number of
experimental setups. One such experiment, staffed by a Japanese-European
collaboration, sends the antiprotons into a bottle of cold helium.
About a million of the p-bars at a time ingratiate themselves into helium
atoms, essentially taking the place of an electron and, at least in
principle, obeying all known laws of atomic physics, including the ability
to make quantum jumps between energy states of this exotic "antiprotonic"
helium atom.
In fact the p-bar intruder begins in a somewhat circular orbit but after
about one microsecond undergoes a transition to a closer orbit. It does
this again and again until the antiproton eventually annihilates with
a proton or neutron in the helium nucleus.
Before this happens, however, the CERN scientists have more than enough
time to perform some crucial atomic physics, including the first-ever
measurement of ultraviolet transitions in this kind of exotic atom.
Not waiting for the transitions to occur, the researchers actually induce
them with a beam of laser light.
Knowing the laser frequency at which the transitions occur allows one
to calculate a number proportional to the antiproton charge squared
times the antiproton mass. When this number is combined with a separate
measurement of the antiproton's motion in an atom trap (see Update
426), which supplies a value for the ratio of the antiproton's charge
to its mass (a ratio measured with uncertainties of only 90 parts per
trillion), then a separate value for the mass and charge of the antiproton
can be determined. In this case the values agree with those of the proton
(allowing for the opposite charge) to within 60 parts per billion. (Hori
et al., Physical Review Letters, 27 August 2001; contact
Masaki Hori, masaki.hori@cern.ch, 41-22-762-8306, or John Eades at CERN,
john.eades@cern.ch; also see CERN
website.)
Is Alpha, like Pi, a Fundamental Constant?
Is alpha, like pi, a fundamental constant, or does it change over time?
Pi, the ratio of a circle's circumference to its diameter (pi can be
defined in other ways too) doesn't seem to be changing, but alpha, the
symbol for the fine structure constant, might be.
Alpha is a measure of the intrinsic strength of the electromagnetic
force and thus determines how strong an atom is bound and what kind
of light is absorbed or emitted by the atom when an electron inside
the atom moves from one internal quantum state to another.
In 1999 a group of scientists at the University of New South Wales
in Australia reported some positive evidence that alpha was not staying
the same (See Update
410). The evidence for a changing alpha--at the level of a part
in 100,000, according to a new report being issued by the same group--consists
of the spacings of pairs of absorption lines of metal atoms in gas clouds
in front of quasars at various redshifts. The spacings are proportional
to alpha squared. The new observations suggest that alpha is growing
bigger.
This, if confirmed by further tests, runs counter to the law which
prescribes that elasticized objects lose their holding power with the
years. Swimsuits might droop with age, but atoms would get stronger
as time goes by. (Webb
et al., Physical Review Letters, 27 August.)
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