Laboratory for Particle Properties (Phi-lab)
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Neutron-antineutron oscillation search

<What is neutron-antineutron oscillation?>
Antiparticle is a particle which has the opposite charge of its corresponding particle. Neutrons have no charge, but up-quark and down-quark, which neutron is consisted of, have on-zero charge. Thus neutron and antineutron are different particles. For example, we can distinguish them by using pair-anihilation, which is a phenomenon that particle and corresponding antiparticle annihilate and emit a gamma-ray.

"Neutron-antineutron oscillation" is the phenomenon that a neutron(antineutron) change an antineutron(neutron) spontaneously. Neutron-antineutron oscillations have never been observed yet.

Neutrons and protons are one of the baryons, which is the particle group name. Baryons are assigned an integer +1 and antibaryons have -1. This integer is called baryon number. Almost all elementary particle processes do not change the sum of the baryon number, but neutron-antineutron oscillation does.

Short lifetime baryons' "baryon number non-conserved process" are supressed by its decay. Non-zero charge and stabele baryons, such as proton, cannot undergo proton-antiproton oscillation, due to electrical-charge conservation. Thus, neutron is the most convenient particle to search the baryon nunber non-conserved process.

<Baryon asymmetry problem in cosmology>
It is considered that there are alot of particles and a few of antiparticles in the universe, which is proved by both the CMB observations and observing the abundance of helium. This particle-antiparticle assymmetry implies unknown baryon-number violating processes.

<European Spallation Source>
ESS is now under construction in Sweden. ESS is new neutron-generating facility and it will become the most powerful neutron source in the world. Our neutron-antineutron oscillation experiment will be done at the 200 meter beamline in ESS. If flying neutron in the beamline change to antineutron, this antineutron would undergo pair annihilation in the carbon target situated at the end of beamline. We can prove the existence of new baryon number non-conserved process by detecting gamma-ray derived from the pair annihilation.
ESS(from ESS web site)

<Neutron-antineutron search at Phi-lab>
Phi-lab takes part in the neutron-antineutron oscillation searching collaboration at ESS. Our simulation proposed the neutron optical system which may improve the figure of merit by about 200 times of the ILL(the Institut Laue-Langevin) experiment during 1989 and 1991.

The scheme of the ESS beamline

Neutron-Antineutron Oscillations: Theoretical Status and Experimental Prospects