In the case of the neutron, the combination of two negatively charged down quarks and one positively charged up quark results in a particle that is overall neutral in charge. The net charge of the neutron is zero, and therefore it is not equivalent to an antimatter particle.
The concept of matter-antimatter annihilation occurs when a particle and its corresponding antiparticle come into contact, resulting in the conversion of their mass into energy. However, in the case of the neutron, it is not composed of a particle and its corresponding antiparticle. The up and down quarks that make up the neutron are both matter particles, and their combination results in an overall neutral particle.
The up quark has a charge of +2/3, while the down quark has a charge of -1/3. When two down quarks combine with one up quark in the neutron, their charges add up to zero: (-1/3) + (-1/3) + (2/3) = 0. This cancellation of charges makes the neutron electrically neutral.
It is worth noting that the concept of matter-antimatter annihilation is applicable when there is a direct pairing of particles and their corresponding antiparticles, such as an electron and a positron (antielectron). In the case of quarks, they combine in various ways to form different particles, and the resulting combinations can have a net charge of zero, like the neutron.
So, while the neutron has a composition that involves positively and negatively charged quarks, it is not equivalent to an antimatter particle and does not undergo annihilation with itself.