15. WIMPS AND MACHOS
The neutralino is expected to interact very weakly with the particles of the Standard Model. Neutralinos are excellent candidates to form the dark matter of the universe so most investigations of WIMP dark matter have concentrated on the neutralino.
15.1 ACCELERATOR SEARCHES
We have seen that the energy released in proton-proton collisions in the LHC manifests itself as particles that fly away in all directions (see Figure 11-4). While most collisions produce known particles, on rare occasions new, exotic ones may be produced, including the neutralino, according to some supersymmetric models.
Unsuccessful searches for supersymmetric particles have been reported so far at particle accelerators throughout the world, thus substantial regions of prime neutralino-dark matter parameter space have already been eliminated. Does this imply a death knell for low-energy supersymmetry? Not necessarily, since only a small portion of the allowed mass range under 1 TeV has been explored so far.
LHC is now ready for action following a two-year shut down. The collision energy available now will be 13 TeV, a significant increase over the initial three-year LHC run, which began with a collision energy of 7 TeV, rising to 8 TeV. These enormous energies may be sufficient to create neutralinos, although the detectors at LHC will not be able to “see” them directly. Instead, the system might find a group of particles moving in one direction but nothing in the other, according to Malcolm Fairbairn of Kings College London in the UK. The only way that could happen is if there was something else on the move that the detectors could not pick up and that might then be a dark matter particle.
15.2 OTHER SEARCHES
In 1981 a team led by Marc Davis of Harvard University performed one of the first galactic surveys. It was realized that galaxies are not arranged in a uniform pattern but congregate into big clusters, each containing hundreds of thousands of galaxies. These make intricate patterns known as the “cosmic web” where dark matter is the skeleton on which ordinary matter is suspended.
There is one thing that dark matter does to reveal itself. It bends the light that passes through it. Using a technique called gravitational lensing, scientists are able to create maps of the Universe’s dark matter. The team that is responsible for this work hopes to map one-eighth of our Universe, amounting to millions of galaxies.
One way to detect dark matter is by monitoring how it behaves using the existing dark matter “maps” as reference. Dark matter particles usually pass through normal matter, unimpeded. Occasionally some will collide with the nucleus of an atom causing it to recoil like a pool ball. This collision is expected to create gamma rays: extremely high-energy light.
In 2014, using data from NASA’s powerful Fermi telescope, researchers found an area in our Milky Way galaxy that seems to be glowing with gamma rays, possibly from dark matter. The patterns fit theoretical models although it could also have originated from energetic stars called pulsars, or from collapsing stars.
A team of researchers at Durham University in the UK lead by Richard Massey devised an ingenious way to detect dark matter. We know dark matter particles can occasionally collide with normal matter, but dark matter might also bump into itself. Massey’s team has recently monitored colliding galaxies. They expected all the dark matter in the galaxies to pass straight through, but instead some of it slowed down, lagging behind the galaxy it belonged to, indicating it had interacted with other dark matter.
Search for dark matter particles are also being conducted in underground laboratories, deep inside old mines and inside mountains. If dark matter particles pass through the earth all the time, one should be able to spot the flashes of gamma rays from these collisions. The challenge is to isolate the signal from that of cosmic ray interactions with matter. Cosmic rays are primarily high-energy protons and atomic nuclei mainly originating outside our Solar System. The impact of cosmic rays with atmospheric nuclei may produce showers of secondary particles that penetrate the Earth’s atmosphere and sometimes even reach the earth surface. Deep underground experiments are shielded from cosmic rays by the earth’s crust.
The XENON100 experiment located deep underground inside the Gran Sasso mountain in central Italy, is a new dark matter search experiment, that uses liquid xenon (LXe) in a time projection chamber (TPC) to search for Xe nuclear recoils resulting from the scattering of dark matter WIMPs. The XENON100 experiment reported a null result in a recent paper published in August 2015. Interestingly, another team (the DAMA experiment) from the same laboratory, using a different detector, had claimed for years that their experiment had detected dark matter, a claim that later proved to be false.