Baryons, along with mesons, are hadrons, particles composed of quarks. Examples of bosons are photons, gluons, phonons, helium-4 nuclei and all mesons. Collisions between particles were too energetic to allow quarks to combine into mesons or baryons. The nuclear force acts between hadrons, known as mesons and baryons. The D mesons are the lightest particle containing charm quarks. The charm quarks were observed bound with charm antiquarks in mesons. Particles are sorted into groups as mesons or baryons. D mesons (), charmed Sigma baryons (), and other charmed particles. The force mediators for these are other hadrons called mesons. Examples of mesons commonly produced in particle physics experiments include pions and kaons. Pions are the lightest mesons and, more generally, the lightest hadrons. As in the case of mesons and baryons, glueballs would be QCD color charge neutral. Pions, which are mesons with zero spin, are composed of first-generation quarks. Also, bosons can be either elementary, like photons, or a combination, like mesons. The neutral B mesons, and , spontaneously transform into their own antiparticles and back.