Elementary Particles

14. WIMPS AND MACHOS

There is another reason why physicists are in the hot pursuit of supersymmetric particles. That has to do with the enigmatic dark matter that dominates the mass of the universe and holds galaxies together.

Neutralino, the lightest supersymmetric particle is expected to be stable and electrically neutral. They fall under a general category of particles called WIMPs (weakly interacting massive particle). WIMPs and MACHOs (massive (astrophysical) compact halo object) are two of the most plausible, although fundamentally different, dark matter candidates along with the illusive neutrino.

The hypothetical WIMP is a hitherto undiscovered weakly interacting particle thought to have been created in the big bang. It is conjectured that the abundance of WIMPS is large enough today to make up for the dark matter in its entirety. The masses typically required for WIMPs are in the range 1 GeV–1 TeV. In fact, numerous elementary particles could fall under the WIMP class, including supersymmetric particles such as neutralinos, photinos, higgsinos or sneutrinos, and new (undiscovered) heavy neutrinos.

On the other hand, MACHOs are a class of astronomical objects that include primordial black holes created during the big bang, neutron stars, or white dwarf stars that are invisible. They could even be brown dwarf stars with masses less than 0.08 the solar mass (M_o), or Jupiter like objects (10^{-3}M_o) that are plentiful in the galactic halo and incapable of emitting or absorbing light.

There is one important difference between WIMPs and MACHOs. WIMPs are non-baryonic while MACHOS are typically (but not always) formed from baryonic material. Baryonic MACHOs were eliminated as candidates for dark matter because there are not enough of them around in the Universe to make up dark matter.

Neutrinos were once hypothesized to be dark matter candidates. This idea was introduced in 1980 by a team of Russian scientists lead by VA Lyubimov. These illusive particles were created in great abundance during the big bang. They are capable of moving unimpeded through the galactic core and even through the Earth, yet they are very hard to detect because they interact weakly with matter. Neutrinos have small, nonzero mass, but due to their sheer number, even their small mass seemed enough to make up for the bulk of the dark matter. Yet, when Carlos Frenk of the University of Durham in UK tried to create a computer simulation of a universe full of hot dark matter (neutrinos are good candidates for hot dark matter because they are light and are able to move fast), his simulation resulted in a cosmos containing an enormous supercluster of galaxies that does not exist in reality. Thus it seems unlikely now that Standard Model neutrinos are dark matter candidates.

Instead, dark matter must be cold and slow-moving. Scientists are now focusing at the non-baryonic WIMPs to account for the universe’s missing mass—these extremely hard to detect particles could fill space in colossal numbers and form invisible halos around galaxies, adding vastly to their masses. WIMPs themselves would have to be fairly massive to account for all the missing mass of the universe but they could only interact weakly with normal matter.

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