UBC and TRIUMF physicists have proposed a unified explanation for dark matter and the so-called baryon asymmetry–the apparent imbalance of matter with positive baryon charge and antimatter with negative baryon charge in the Universe.
The visible Universe appears to be made of atoms, and each of these atoms carries a positive baryon charge equal to total number of protons and neutrons in its nucleus.
However, since the discovery of antimatter in 1932, researchers have wondered why the Universe doesn’t hold a neutral baryon charge–requiring as much negatively charged antimatter as positively charged matter. This net asymmetry of particles over antiparticles remains one of the biggest unsolved mysteries in physics.
We have presented a novel mechanism to generate dark matter and baryon densities simultaneously. Decays of a massive X1 state split baryon number between SM quarks and antibaryons in a hidden sector. These hidden antibaryons constitute the dark matter. An important signature of this mechanism is the destruction of baryons by the scattering of hidden dark matter.
The SM (Standard Model) is extended to include a new hidden sector of states with masses near a GeV and very weak couplings to the SM. Such sectors arise in many well-motivated theories of physics beyond the SM, and have received much attention within the contexts dark matter models, and high luminosity, low-energy precision measurements.
The main idea underlying our mechanism is that some of the particles in the hidden sector are charged under a generalization of the global baryon number (B) symmetry of the SM (Standard Model). This symmetry is not violated by any of the relevant interactions in our model. Instead, equal and opposite baryon asymmetries are created in the visible and hidden sectors, and the Universe has zero total B.
These asymmetries are generated when
(i) the TeV-scale states X1 and its antiparticle ¯X1 (carrying equal and opposite B charge) are generated non-thermally in the early Universe (e.g., during reheating), and
(ii) X1 decays into visible and hidden baryonic states. The X1 decays violate quark baryon number and CP, and occur away from equilibrium. Both the visible and hidden baryons are stable due to a combination of kinematics and symmetries.
We present a novel mechanism for generating both the baryon and dark matter densities of the Universe. A new Dirac fermion X carrying a conserved baryon number charge couples to the standard model quarks as well as a GeV-scale hidden sector. CP-violating decays of X, produced nonthermally in low-temperature reheating, sequester antibaryon number in the hidden sector, thereby leaving a baryon excess in the visible sector. The antibaryonic hidden states are stable dark matter. A spectacular signature of this mechanism is the baryon-destroying inelastic scattering of dark matter that can annihilate baryons at appreciable rates relevant for nucleon decay searches.
In our model, the hidden sector consists of two massive Dirac fermions Xa (a = 1, 2, with masses mX2 > mX1 & TeV), a Dirac fermion Y , and a complex scalar (with masses mY ∼ GeV). These fields couple through the “neutron portal” (XUcDcDc) and a Yukawa interaction. Many variations on these operators exist, corresponding to different combinations of quark flavors and spinor contractions. With this set of interactions one can define a generalized global baryon number symmetry that is conserved
Observations of the the big bang’s afterglow, the cosmic microwave background, by the WMAP satellite now show about 4.6 per cent of the Universe (by density) is comprised of atoms, with about five times more dark matter (23 per cent).
The cosmic balancing act proposed by the researchers may explain why the measured densities of dark matter and atoms differ only by a factor of five.
The researchers also predict an entirely new method to detect dark matter.
“Occasionally a dark-matter antiparticle may collide with and annihilate an ordinary atomic particle, releasing a burst of energy,” says Sigurdson. “While extremely rare, this means dark matter might be observed in nucleon decay experiments on Earth that look for the spontaneous decay of protons.
A potentially spectacular signature of our model is that rare processes can transfer baryon number from the hidden to the visible sector. Effectively, antibaryonic dark matter states can annihilate baryons in the visible sector through inelastic scattering. These events mimic nucleon decay into a meson and a neutrino, but are distinguishable from standard nucleon decay by the kinematics of