A photo of the “birth of the universe” published
in February by NASA has enabled scientists to estimate
the age of the universe at 13.7 billion years. It has also
kicked off a race to detect dark matter, which presumably
makes up more than 95% of the universe. “This photo
shows that visible matter forms 4% of the universe, and
that invisible dark matter accounts for 23%, the rest being
dark energy, about which we know nothing,” explains
Claude Leroy, director of the Particle Physics Group in
the Department of Physics at Université de Montréal.
Professor Leroy believes that this dark matter is composed
of neutralinos. Together with other researchers in the
Project to Identify Sypersymmetric Candidates for Dark
Matter (Picasso), he has been trying to expose these phantom
characters. Neutralinos are “exotic particles” of
neutral electric charge; according to the NASA analyses
they have a mass of a few hundred giga-electron volts (between
100 and 500 times the mass of a proton). The galaxies may
be surrounded by an immense cloud of neutralinos, which
make up most of the matter in the universe. These particles
travel through us all the time (luckily we don’t
even notice), and pass through the Earth from one side
to the other without changing their trajectory.
Neutralinos are assumed to exist by particle physics, but
researchers have not managed to observe them yet because
present colliders are not powerful enough to produce them.
The members of the Picasso group thus decided to search
for them in cosmic rays, by building neutralino traps.
A prototype sensor has already been assembled by professors
Louis Lessard and Claude Leroy, and it has been tested
successfully. The trap consists of polymerized gel in which
droplets of superheated freon are held in suspension. “Freon
contains fluorine, the element most likely to react through
one of the neutralino interaction modes,” explain
Louis Lessard. “If a neutralino hits a fluorine nucleus,
the droplet of freon will turn into a gas bubble. Supersensitive
microphones can detect the sound wave from this mini-explosion,
and computers trace the wave profile.”
The present phase of work is aimed at solving the problems
of building large volume detectors, as researchers hope
to produce 400 detectors containing 70 litres of gel each
in the next two years. In three years, these detectors
will be installed more than two kilometres underground
in the Sudbury neutrino observatory as part of a broad
international project launched by a consortium of Canadian
universities. The detectors must be placed in underground
galleries so that no other type of radiation, cosmic or
terrestrial, will disturb the experiment, which should
extend over several years.
Researcher: Claude Leroy
Telephone: (514) 343-6722
Email: claude.leroy@umontreal.ca
Funding: Université de Montréal
