Number 526, February 13, 2001 by Phil Schewe, James
Riordon, and Ben Stein
All-metal Superconductivity at 40K
In the early days
of superconductivity research most of the samples were metals or combinations
of metals, and the mechanism for producing the super-current state was
described by the BCS theory, named for John Bardeen, Leon Cooper, and
Robert Schrieffer. In this theory electrons pair up and eventually fall
into a single quantum state in which the moving electron pairs are immune
from electrical resistance---the hallmark of superconductivity---courtesy
of a wavelike flexing of the crystal of atoms. An equivalent way of
expressing this idea is to say that electrons pair up by exchanging
phonons.
And then the Woodstock
of Physics happened in 1987; this was the physics meeting at which a
series of ceramic compounds (e.g., yttrium-barium-copper oxide) were
revealed to superconduct up to 90 K and higher. Suddenly oxide superconductors
were the rage, the BCS theory was felt to be inadequate to describing
the new "high-temperature" materials, and study of the intermetallics
(i.e., using only metallic elements) languished by comparison. Now this
might change.
At a meeting on
January 10, in Sendai Japan, Jun Akimitsu of the Aoyama-Gakuin University
reported that they had observed superconductivity in a magnesium-boron
compound at 39 K, a transition temperature almost twice the size for
any previous intermetallic material. More recently a group at the Ames
Lab at Iowa State (Paul Canfield, 515-294-6270, canfield@ameslab.gov)
has taken a MgB2 sample and scrutinized it using different isotopes
of B. Not only are the samples superconducting, at 40.2 K when using
boron with an atomic mass of 10 and at 39.2 K for a boron mass of 11,
but the dependence on isotope shows that even at this temperature, the
BCS mechanism (which stipulates how the transition temperature should
change with the mass of the isotope) is at work. (Bud'ko et al., Physical
Review Letters, 26 February 2001; text at Physics
News Select)
The Higgs Search at Fermilab
The standard model
of particle physics says that in the early moments of the universe many
of the particles we know, such as the electron and the quark, were endowed
with mass in a process called the Higgs mechanism (named for physicist
Peter Higgs). Indeed the process is implicitly still at work, behind
the scenes, and an associated particle for this purpose, the Higgs boson,
should be lurking in the vacuum. By adding a lot of energy to a small
volume of space, one should be able to make the Higgs show itself, and
this effort is given by the highest priority by particle physicists.
Electron-positron
collisions at CERN in Geneva might have accomplished this, but the CERN
machine had to be shut down before enough evidence could definitely
settle the issue (Update
502). The rudimentary data from CERN indicated a possible Higgs
mass at around 115 GeV. Now, across the Atlantic, Fermilab's Tevatron
machine will resume its operations in March 2001 and will run for five
years, and it too will search for the Higgs. Fermilab can, in principle,
search for Higgs's as massive as 180 GeV, but CERN's efforts will have
helped Fermilab, at least at first, in sifting the incoming returns
for signs of the Higgs. With its new higher luminosity (essentially
the intensity of the beams), the Tevatron should be able to produce
about 1015 proton-antiproton collisions.
In the small fraction
of these in which a quark and antiquark meet nearly head on, a Higgs
can be assembled mainly in three ways: (1) via the fusion of two gluons;
(2) in the company of a W boson; and (3) accompanied by a top and an
anti-top quark. According to current theoretical estimates these three
modes should respectively produce about this many 115-GeV Higgs particles:
15,000, 4500, and 120.
Ironically it is
mode 3 which is of greatest interest to a group of Fermilab scientists.
Stephen Parke (630-840-4517, parke@fnal.gov) says
that mode 1 is much better suited to producing heavier Higgs. Mode 3,
although rarer than mode 2, has a more distinctive signature. In mode
3 each of the top quarks decays into a bottom (b) quark plus a W boson,
while the Higgs itself decays quickly into a pair of b quarks (actually
a b and anti-b). In turn the W's decay into either a lepton (such as
electron or muon) plus neutrino or into two quark jets. The final ensemble
of daughter particles would therefore be four b quarks, a lepton, two
other quark jets, and a neutrino which, although it can't be detected,
would leave a telltale shortfall of momentum in a particular direction.
Because of this striking signature, and because the Tevatron detectors
have greatly improved their sensitivity to b quarks, Parke feels that
mode 3 will be "the Cinderella discovery mode for Higgs production,
long overlooked but eventually the queen." The expected number
of Higgs produced in this way, 120, is not a large number, and Fermilab
will be under pressure to produce a higher-than-planned number of collisions.
(Goldstein et al., Physical Review Letters, 26 February 2001;
text at Physics News Select)
The First Drafts of the Human Genome
The first drafts
of the human genome are reported this week in the journals Nature
and Science respectively by an academic consortium coordinated
by the National Institutes of Health and the Department of Energy, and
by the corporation Celera Genomics. Both groups estimate that humankind?s
DNA instruction set has only between 30,000-40,000 genes, only 50% more
than the roundworm and much lower than the 100,000 genes once estimated
for the human species. But human complexity may arise in other ways,
both teams speculate. For example, a mechanism known as alternative
splicing enables different proteins to be made from a single gene. And
compared to invertebrates, humans and other vertebrates apparently make
more complex proteins from similar protein substructures. Physicists
have been and will continue to play a key part in elucidating the genome,
through developing powerful data analysis methods (Update
491), finding patterns in so-called junk DNA (non-protein-coding
regions of DNA; Update
202), and introducing faster techniques for sequencing DNA (Update
171), to name a few of many contributions.
The Near-Shoemaker Spacecraft
The near-Shoemaker
spacecraft has made a semi-soft landing on the asteroid Eros, the first
time an asteroid has been visited by an object sent from Earth. NEAR-Shoemaker,
which had been in orbit around the Manhattan-sized rock for the past
year, settled down on the surface and radio signals from the craft showed
that it still had life. See the web
site.
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