Why do we exist?

More precisely, why is there so much more matter than antimatter in our universe? When the universe cooled down after the big bang, radiation converted to particle-antiparticle pairs. If these pairs combine again, they annihilate into energy. How come, we today have more matter left to form our planets, stars and even galaxies? Unfortunately up to now theory cannot answer this question, yet there are some criteria that would cause an imbalance in the universe. One of those is violation of parity and time reversal invariance. Thus it matters in which spacial and temporal direction a reaction occurs. A possible candidate for a so-called PT violence is the existence of a permanent neutron electric dipole moment.

Neutrons consist of two down quarks (charge -1/3 each) and one up quark (charge 2/3). On top they are extended particles, so it could be assumed that we find a charge distribution inside. An asymmetric charge distribution would therefore cause an electric dipole moment. The n2edm experiment, which is run at Paul Scherrer Institute in Swiss aims at improving the precision which excludes the existence of a permanent electric dipole moment.

Today the lower limit of the neutron electric dipole moment is 2.9·10-26 ecm.