Neutralinos in dark matter

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
Funding: Université de Montréal



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