PDF《AXIONS》

C 1C AXIONS Written in August

2007 by C.

Hagmann (LLNL), H. Murayama (UC Berkeley), G.G. Ra?elt (MPI Physics), L.J. Rosenberg (U. of Washington), and K. van Bibber (LLNL). Introduction: In this section, we list mass and coupling- strength limits for very light neutral scalar or pseudoscalar bosons that couple weakly to normal matter and radiation. Such bosons may arise from a global spontaneously broken U(1) symmetry, resulting in a massless Nambu-Goldstone (NG) boson. If there is a small explicit symmetry breaking, either already in the Lagrangian or due to quantum-mechanical e?ects such as anomalies, the boson acquires a mass and is called a pseudo-NG boson. Typical examples are axions (A0) [1,2], familons [3] and Majorons [4], associated, respectively, with a spontaneously broken Peccei-Quinn, family and lepton-number symmetry. A common characteristic among these light bosons φ is that their coupling to Standard-Model particles is suppressed by the energy scale that characterizes the symmetry breaking, i.e., the decay constant f. The interaction Lagrangian is L = f?1 J? ?? φ , (1) where J? is the Noether current of the spontaneously broken global symmetry. If f is very large, these new particles interact very weakly. Conversely, detecting them would provide a win- dow to physics far beyond what can be probed at accelerators. The interest in global symmetries and the associated NG bosons has somewhat waned except for the case of axions where it has held steady since they were proposed

30 years ago. This is because the Peccei-Quinn (PQ) mechanism remains perhaps the most credible scheme to preserve CP in QCD;

axions are a plausible candidate for the cold dark matter of the universe and they are searched for in experiments with a realistic chance of discovery. Originally it was assumed that the PQ scale fA was related to the electroweak symmetry-breaking scale vweak = ( √ 2GF)?1/2 =

247 GeV. However, the associated standard and variant axions were quickly excluded, leaving CITATION: C. Amsler et al. (Particle Data Group), PL B667,

1 (2008) (URL: http://pdg.lbl.gov) July 16,

2008 10:42 C 2C invisible axions with fA vweak as the main possibility. We refer to the Listings for limits on standard and variant axions, whereas here we focus primarily on very low-mass, very weakly-interacting axions and axion-like particles. I. THEORY I.1 Peccei-Quinn mechanism and axions: QCD includes a CP-violating Lagrangian LΘ = ? Θ (αs/8π) G?νa ? Ga ?ν, where ?π ≤ ? Θ ≤ +π is the e?ective Θ parameter after diagonalization of the quark masses, G is the color ?eld strength tensor, and ? G its dual. Experimental limits on the neutron electric dipole moment [5] imply | ? Θ| <

? 10?10 even though ? Θ = O(1) is otherwise completely satisfactory. The spontaneously broken global Peccei-Quinn symmetry U(1)PQ was introduced to solve this strong CP problem [1], and an axion is the pseudo-NG boson of U(1)PQ [2]. This symmetry is exact on the classical level, but is broken quantum mechanically due to the axion'

s anomalous triangle coupling to gluons, L = ? Θ ? φA fA αs 8π G?νa ? Ga ?ν , (2) where φA is the axion ?eld and fA the axion decay constant. Color anomaly factors have been absorbed in the normalization of fA which is de?ned by this Lagrangian. Thus normalized, fA is the quantity that enters all low-energy phenomena [6]. Non- perturbative e?ects induce a potential for φA whose minimum is at φA = ? Θ fA, thereby canceling the ? Θ term in the QCD Lagrangian, and thus restoring the CP symmetry. The resulting axion mass is given by mAfA ≈ mπfπ where mπ =