Strong CP problem is one of the biggest mysteries in the standard model of particle
physics. R. Peccei and H. Quinn proposed an elegant solution to solve the strong CP
problem by introducing the Peccei-Quinn (PQ) mechanism, and S. Weinberg and F.
Wilczek showed that the PQ mechanism results in a new particle, axion. Though the
original Peccei-Quinn-Weinberg-Wilczek (PQWW) axion was ruled out by experimental
results, the invisible axion models such as Kim-Shifman-Vainshtein-Zakharov (KSVZ)
and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) models still remain as candidates. Since
the axion is expected extremely light and stable, it is also being considered as a good
candidate of dark matter which is another missing piece of the puzzle in the universe.
Since P. Sikivie proposed an experimental technique for axion search using a mi-
crowave resonant cavity under an external magnetic field, many experiments attempted
to search the axion, however, it has not discovered to date. The experiments excluded
a number of mass ranges, however, there are vast ranges remaining unexplored.
The Center for Axion and Precision Physics Research (CAPP) of Institute for Basic
Science (IBS) is dedicated for the mission to search the axion. In the Center, several
parallel experiments have been built and are trying to discover the axion. In this pre-
sentation, one of the axion search experiment, the CAPP-8TB experiment, dedicated to
a mass around 6.7 μeV is presented. The experiment has excluded a mass range with a
certain level of sensitivity, and searching in broader range is in preparation. Activities on
research and development of improvements of experiments at CAPP are also discussed.