Negative pions, or π- mesons, are subatomic particles that belong to the family of mesons. They are composed of a quark and an antiquark, specifically an anti-up quark and a down quark. Negative pions are unstable and have a mass about 273 times that of an electron. They play a significant role in mediating the strong nuclear force, which holds atomic nuclei together. These particles are produced in high-energy processes, such as cosmic ray interactions and particle collisions in accelerators. Negative pions can decay into other particles, including muons and neutrinos, with a mean lifetime of about 26 nanoseconds. Their study helps physicists understand fundamental forces and the structure of matter.