{ 1{ Axions and Other Very Light Bosons Axions and Other Very Light Bosons, Part I (theory)

(by H. Murayama) In this section we list limits for very light neutral (pseudo) scalar bosons that couple weakly to stable matter. They arise if there is a global continuous symmetry in the theory that is spontaneously broken in the vacuum. If the symmetry is exact, it results in a massless Nambu{Goldstone (NG) boson. If there is a small explicit breaking of the symmetry, either already in the Lagrangian or due to quantum mechanical eeects such as anomalies, the would-be NG boson acquires a nite mass; then it is called a pseudo-NG boson. Typical examples are axions (A 0) 1], familons 2], and Majorons 3,4], associated, respectively, with spontaneously broken Peccei-Quinn 5], family , and lepton-number symmetries. This Review provides brief descriptions of each of them and their motivations. One common characteristic for all these particles is that their coupling to the Standard Model particles are suppressed by the energy scale of symmetry breaking, i.e. the decay constant f, where the interaction is described by the Lagrangian L = 1 f (@)J ; (1) where J is the Noether current of the spontaneously broken global symmetry. An axion gives a natural solution to the strong CP problem: why the eeective-parameter in the QCD Lagrangian L = CITATION: K. Hagiwara et al.