rpp2010-rev-axions - 1 AXIONS AND OTHER SIMILAR PARTICLES...

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– 1– AXIONS AND OTHER SIMILAR PARTICLES Revised January 2010 by C. Hagmann (LLNL), H. Murayama (UC Berkeley), G.G. Raffelt (MPI Physics), L.J. Rosenberg (U. of Washington), and K. van Bibber (LLNL). Introduction In this section, we list coupling-strength and mass limits for 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 effects such as anomalies, the boson acquires a mass and is called a pseudo-NG boson. Typical examples are axions ( A 0 ) [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 μ μ φ , (1) where J μ is the Noether current of the spontaneously broken global symmetry. If f is very large, these new particles interact very weakly. Detecting them would provide a window to physics far beyond what can be probed at accelerators. Axions remain of particular interest because the Peccei- Quinn (PQ) mechanism remains perhaps the most credible scheme to preserve CP in QCD. Moreover, the cold dark matter of the universe may well consist of axions and they are searched for in dedicated experiments with a realistic chance of discovery. Originally it was assumed that the PQ scale f A was re- lated to the electroweak symmetry-breaking scale v weak = ( 2 G F ) 1 / 2 = 247 GeV. However, the associated “standard” and “variant” axions were quickly excluded—we refer to the Listings for detailed limits. Here we focus on “invisible axions” with f A v weak as the main possibility. CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34
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– 2– Axions have a characteristic two-photon vertex, inherited from their mixing with π 0 and η . It allows for the main search strategy based on axion-photon conversion in external magnetic fields [5], an effect that also can be of astrophysical interest. While for axions the product “ Aγγ interaction strength × mass” is essentially fixed by the corresponding π 0 properties, one may consider more general axion-like particles (ALPs) where the two parameters are independent. Several experiments have recently explored this more general parameter space. I. THEORY I.1 Peccei-Quinn mechanism and axions The QCD Lagrangian includes a CP-violating term L Θ = ¯ Θ ( α s / 8 π ) G μνa ˜ G a μν , where π ¯ Θ + π is the effective Θ parameter after diagonalizing quark masses, G is the color field strength tensor, and ˜ G its dual. Limits on the neutron electric dipole moment [6] imply | ¯ Θ | < 10 10 even though ¯ Θ = O (1) is otherwise completely satisfactory. The spontaneously broken
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