Axions

In Particle Physics
* Motivation: Their existence is required by the Peccei-Quinn mechanism for enforcing strong PT symmetry; > s.a. neutron.
* Properties: A pseudoscalar, a negative-parity, spin-zero particle, a very light cousin of the neutral pion; Its mass and couplings are related to a parameter of the Peccei-Quinn theory; It is currently thought that its mass is likely to be less than a meV (based on SN1987A neutrino production), and more than a micro-eV (based on non-overproduction in early universe).
* Searches: The axion interacts directly with quarks and leptons, and indirectly with nucleons and photons; 2006, All of the current searches are based on the axion-photon interaction (the reason is related to the Primakoff effect); The PVLAS experiment reports seeing birefringence for laser photons in a magnetic field (best explained if a photon from the PVLAS laser combines with a photon in the vacuum to produce an axion); 2007, The PVLAS team failed to reproduce their results, says tiny rotation in the polarization of laser light that they reported last year does not support the existence of axions, but rather is an artefact related to how the experiment had been performed; New experiments question usefulness of other planned experiments.
@ General references: Sikivie PT(96)dec; Csáki et al PRL(02) [photon-axion oscillation and supernova dimming]; Raffelt JPA(07) [rev].
@ Experimental searches: Collar et CAST he/03-in; news pw(04)nov [CAST]; Sikivie hp/06-in [and bounds]; news pw(06)jul [PVLAS collaboration experiment]; van Bibber & Rosenberg PT(06)aug; Bignami & Dupays pw(06)nov [PVLAS signal]; Mohapatra & Nasri PRL(07) + pw(07)feb [reconciling PVLAS and CAST's findings]; news pw(07)jul [PVLAS follow-up]; news pw(07)nov [new results question merit of experiments].

Theory > s.a. dilaton [axion-dilaton gravity].
* Idea: A theory of a 2-form B (Kalb-Ramond field), with field strength the 3-form H = dB (axion field), and equations of motion d*H = 0 and the Bianchi identity dH = 0; 2004, They are a viable dark matter candidate.
* Action: In 4 dimensions, it is

S = (1/16) H *H = (1/16) H_abc H^abc dv .

* Symmetry: B B + dA, with A a 1-form; The axion is massless if this symmetry is exact.
* Degrees of freedom: In D-dimensional spacetime, B has (D–3)(D–2)/2 physical degrees of freedom; Thus, in D = 4, it is equivalent to a (free massless) scalar field , with *H = d.
* Axion charge: Inside a closed 2-surface S,

q = _S B = _V H = _V *d .

* Axionic black holes: They can have axionic charge if non-static; Use the same metric as without axions, H = = 0, but B belongs to H²(M; R) (can tell only if something couples to B); > s.a. black hole hair.
@ References: Turner PRP(90); Srednicki ht/02-in [rev]; Svrcek & Witten JHEP(06) [in string theory].

And Astrophysics / Cosmology > s.a. dark matter; quintessence.
* 1996: Considered as dark matter candidates, but the window appears to be very small.
* 2002: Considered as possible explanation for dimming of supernovas, alternative to universal acceleration.
* 2006: Combined constraints suggest that its mass is bounded by 3 10^–3 eV > m_a > 10^–6 eV.
* 2007: Absence of X-rays from axion decays in galaxy clusters leads to estimate > 10²³ s.
@ General references: Hagmann et al PRL(98) [search]; Massó & Redondo JCAP(05) [evading bounds]; Raffelt JPA(07) [rev].
@ Cosmology, theory: Kim ap/98-in, ap/98-in, ap/00; Sikivie ap/06-in [rev].
@ Axion stars: Iwazaki PLB(00) [and X-rays], PLB(00) [and uhe cosmic rays]; > s.a. types of black holes.
@ Searches: Fairbairn et al PRL(07) + pn(07)may [indirect, from transparency of Sun to photons]; Grin et al PRD(07) [from 2-photon decay]; Riemer-Sørensen et al PRL(07)ap/07 [bound on lifetime from galaxy cluster X-ray emission].

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