Particle physics theorists at TRIUMF, Brookhaven National Laboratory of the United States, and the University of British Columbia, and have proposed a new theory for the way that the anti-matter half of the universe could have disappeared at the Big Bang. The missing anti-matter is a violation of matter/anti-matter symmetry, and the source of a mystery that has puzzled physicists for decades. The proposal was published on November 19th, 2010, in Physical Review Letters, by David Morrissey and Sean Tulin from TRIUMF, Kris Sigurdson from UBC, and Hooman Davoudiasl, of Brookhaven.
The theory is unique in that it explains both 'dark matter' and 'anti-matter' all at once. Dark matter and anti-matter are often treated colloquially as being the same thing and interchangeable, but physicists understand them as being two different types of particles.
Anti-matter consists of particles that are nearly exactly the same as those that we know of in everyday life, with the charges reversed. An anti-proton is a regular proton with a negative charge; an anti-electron is a regular electron with a positive charge (also known as a positron). Anti-quarks (which make up anti-protons and anti-neutrons, when in different combinations) are the same as regular quarks with reversed charges. All anti-particles have the same mass, volume, spin, and may combine together the same way that regular particles form molecules. An entire anti-matter Earth could exist and few things would be different from our Earth except for the reversed charges, and the fact that if the anti-matter came into contact with our matter, both would annihilate and form energy.
Dark matter is made up of particles that have a mass, but do not give off or reflect any light; hence we call it 'dark'. We know dark matter must exist all around the universe, and that it must make up five times the total amount of regular matter we know of in the universe. This is because Albert Einstein's theory of General Relativity states that space-time curves in response to matter, and the higher the mass, the higher the curvature of space-time - but around the galaxies, the curvature of space-time is far higher than what it should be for our known matter - therefore there must be matter that we cannot detect. There are also many other gravitational effects that give confidence that dark matter must exist.
Theories have existed for years as to why there is an imbalance of matter and antimatter. The team from TRIUMF, UBC, and Brookhaven has devised a model that conserves the 'baryon number' of the universe, at zero. Baryon number is a quantum number that you get when you add the quarks in a particle together (protons have three quarks, each with a baryon number of 1/3, so when you add them, you get a baryon number of 1 in a proton, for instance). Particles of regular matter do not have negative baryon number, so this team of particle theorists hypothesize that dark matter are particles that have a negative baryon number, and exactly enough of it to make the total baryon of the universe equal zero.
Part of this model includes a mechanism of interactions that would bundle the concept of anti-matter with dark matter. The idea is that it was not matter and anti-matter that were created in equal amounts, but the total of anti-matter and matter balanced equally with the total of dark matter and a kind of 'anti-dark' matter - unequal amounts created by the decay of a newly hypothesized particle named "X" and its anti-matter counterpart, both which would have been created at the Big Bang. The X particle prefers to decay into ordinary matter, and anti-X prefers to decay into dark matter anti-particles, leaving extra matter and anti-dark matter that does not annihilate with each other. That way, there could be a little bit extra matter, and a little bit of this extra anti-dark matter. The anti-matter collides with most of the matter until there is none left, and the same for the dark matter side. What remains in the universe today is the little bit of extra matter particles and dark matter anti-particles.
One issue that makes the concept of dark matter difficult to grasp is that we do not experience its effect in everyday life. Experimentally however, scientists can isolate the observation of nuclei, and sometimes they see large, spontaneous movement for no obvious reason. This may be the result of a dark matter collision with a nucleus. Most experimentalists are looking for elastic scattering, where the particles bounce away from each other, but in this case, the hunt is for inelastic collisions where dark matter anti-particles transfer their energy to protons within nuclei and destroy them. It is estimated that universe currently is made up of approximately five times more dark matter than matter, by mass. This could also be explained by this new theory, if the dark matter particles simply each have a mass between 2 and 3 times that of a proton.
Morrissey believes the paper serves a practical purpose in suggesting a new way to use experimental data to look for new phenomena: physicists can search for certain types of dark matter by examining data from the Super Kamiokande experiment, initially studied to look for decaying protons. Further, the paper shows the interplay and connection between theory and experimentation. Theorists are helpful in identifying which experiments may be the most interesting and fruitful, and propose newer, better ways to examine the data collected. Experiments influence theory by providing data that may support one theory or another, or may be completely unexpected, and suggest the need for an entirely new theory.
This exchange between theorists and experimentalists goes on between people from TRIUMF, and those from universities, industries, and other laboratories around the world every day. It also supports innovation in science and greater understanding of the universe, which may lead to new technologies that impact everyday life in ways we cannot imagine.
- Jessica Coccimiglio, Communications Assistant
Image Credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research),The University of Tokyo.