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 the 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]; Carosi et al CP(08);
Melissinos PRL(09)
[proposed optical-cavity search].
@ In solids: Wilczek PRL(87)
[action and effects]; Qi et al PRB(08)
+ Franz Phy(08) [in topological insulators].
Theory > s.a. dilaton [axion-dilaton
gravity]; supersymmetric theories [axionic extension of standard model].
* 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
)
Habc Habc
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. quintessence; types
of dark matter.
* 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 > ma > 10–6 eV.
* 2007: Absence of X-rays
from axion decays in galaxy clusters leads to estimate
>
1023 s.
@ General references: Massó & Redondo JCAP(05)
[evading bounds]; Raffelt JPA(07)
[rev]; Wantz & Shellard a0910.
@ Cosmology, theory: Kim ap/98-in, ap/98-in, ap/00;
Sikivie LNP(08)ap/06 [rev];
Mack a0911/PRD [inflation].
@ As
cold-dark-matter candidate: Hwang & Noh PLB(09)-a0902;
Duffy & van Bibber a0904/NJP.
@ Axion stars: Iwazaki PLB(00)
[and X-rays], PLB(00)
[and uhe cosmic rays]; > s.a. types
of black holes.
@ Searches: Hagmann et al PRL(98);
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]; Burrage et al PRL(09)
[AGNs]; Hannestad et al a0911-in [cosmological bounds].
@ Evidence, gamma rays: Roncadelli et al AIP(08)-a0902 [propagation];
Sánchez-Conde et al PRD(09)-a0905.
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send feedback and suggestions to bombelli at olemiss.edu – modified 3
nov
2009