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News from ICTP 93 - What's New
Dark matter may account for more than 90 percent of the
matter in the universe. Yet its presence is so flimsy and whimsical
that no one has ever been able to see it-not yet that is.
The WIMP Factor
Recent advances in detection equipment may bring the 70-year-old mystery of dark matter to a successful conclusion. A group of scientists participating in the Dark Matter experiment (DAMA) at Gran Sasso National Laboratories in Italy, with help from colleagues at the Chinese Academy of Sciences' Institute of High Energy Physics, claims that their experiments over the past 4 years have revealed the presence of 'weakly interacting massive particles' or WIMPs, which may contribute the largest share of energy to the universe. The research group is led by Rita Bernabei and Pierluigi Belli, who are affiliated with the University of Rome Tor Vergata and the National Institute of Nuclear Physics (INFN) in Italy.
No one should underestimate the importance of WIMPs' shadowy presence.
Scientists believe that the gravitational pull they supply keeps
the Milky Way from spinning out of control and flying apart.
Physicists have long suspected the presence of dark matter and
there's plenty of proof-both theoretical and experimental--to
back them up. Particles with verifiable mass--protons or neutrinos,
for example--are simply not 'bulky' enough to create a force of
gravity of sufficient strength to hold stars and other heavenly
objects in place as they whirl around our universe at warp speed.
For most physicists, the issue is not whether dark matter exists,
but what is the nature of these elusive particles.
At times, researchers have pointed to such newly discovered particles
as neutrinos or to massive stars labelled brown dwarfs. But subsequent
theories and experiments have shown that neither these particles
nor these stars have what it takes to fill the void: that is,
they do not possess the physical features or act in ways that
can account for the galaxies and clusters of celestial bodies
that we observe in the sky. So the world's particle hunters have
turned to WIMPs. These 'dark' particles, which have defied detection
thus far, are now close to being cornered.
The conclusions of the DAMA researchers were first announced in
Rome in January 1999 and subsequently presented to a group of
physicists at the Fourth International Symposium on Sources and
Detection of Dark Matter in the Universe held in Marina del Rey,
California, in the United States, in February 2000.
Meanwhile, particle hunters from the Cold Dark Matter Search (CDMS)
group at Stanford University in the United States, relying on
another detection technique, suggested that their Italian counterparts
may have detected not WIMPs but neutrons--a more ordinary particle
that also emits flashes of light when subject to highly sophisticated
laboratory conditions. Neutrons have already been ruled out as
the constituent ingredient in dark matter; so, in the U.S. researchers'
view, the hunt must continue.
The debate over dark matter, regardless of the outcome, holds
the potential to both confirm and expand the research efforts
of theoretical physicists at ICTP, the University of Trieste,
the International School for Advanced Studies (SISSA) and elsewhere.
After all, theoretical discussions during the past half century
have provided the insights and inspiration for examining the deepest
reaches of our universe and enhancing our understanding of the
elementary constituents and forces that gave rise to the universe's
birth and its current state of physical being.
As advances in technology place us on the threshold of discovering
the makeup of dark matter, this historic finding is likely to
raise as many questions as it solves. As one researcher recently
noted, "the Copernican revolution told us we're not the center
of the universe. The (imminent) discovery of dark matter will
tell us we're not the main matter of the universe." All of
which makes the 'WIMP factor' that much more intriguing.
Antonio Masiero
Astrophysics Sector
International School for Advanced Studies (SISSA)
For more detailed information about the scientific debate on
WIMPs, see G. Jungman, M. Kamionkowski and K. Griest, "Supersymmetric
Dark Matter," Physics Reports 267 (1996), pp. 195-373,
and A. Masiero, "Dark Matter in the Universe,"
in S. Bonometto, J.R. Primack and A. Provenzale, eds., Proceedings
of the International School of Physics "E. Fermi" Course
CXXXII (Varenna, Italy: IOS Press, 1996).