What is Antiparticle?
{{Antimatter}} There is an '''antiparticle''' corresponding to most kinds of [[particle physics|particle]]. It has the same [[mass]] and opposite [[electric charge]].<ref> The exceptions are massless [[boson]]s such as the [[photon]] and the [[graviton]].</ref> Even electrically neutral particles, such as the [[neutron]], are not identical to their antiparticle. In the example of the neutron, the 'ordinary' particle is made out of [[quarks]], and the antiparticle out of antiquarks.<ref>The laws of nature were thought to be symmetric between particles and antiparticles until [[CP violation]] experiments found that [[time-reversal symmetry]] is violated in nature. This small asymmetry is involved in [[baryogenesis]], how our universe came to consist almost entirely of matter, with almost no free antimatter.</ref> Particle-antiparticle pairs can [[Antimatter#Annihilation|annihilate]] each other if they are in appropriate [[quantum state]]s. They can also be produced in various processes. These processes are used in [[particle accelerator]]s to create new particles and to test theories of [[particle physics]]. High energy processes in nature can create antiparticles. These are visible in [[cosmic ray]]s and in certain [[nuclear reaction]]s. The word [[antimatter]] properly refers to (elementary) antiparticles, composite antiparticles made with them (such as [[antihydrogen]]), and larger assemblies of either. == History == In 1932, soon after the prediction of [[positron]]s by [[Paul Dirac]], [[Carl Anderson]] found that cosmic-ray collisions produced these particles in a [[cloud chamber]] {{ndash}} a particle detector in which moving [[electron]]s (or positrons) leave behind trails as they move through the gas. The antiproton and antineutron were found by [[Emilio Segrè]] and [[Owen Chamberlain]] in 1955 at the [[University of California, Berkeley]]. Since then the antiparticles of many other subatomic particles have been created in particle accelerators. In recent years, complete atoms of [[antimatter]] have be