PQ axion, with the higher excitations having masses spaced at ~1/R, where R is the radius of the compactified extra dimensions. Decay lifetimes for these KK axions are much shorter due to their higher mass, e.g. for ga??=10-11GeV-1 the lifetime of the PQ

1
Searches For Kaluza-Klein Axions
B. Morgan1, N.J.C. Spooner1, S.M. Paling1, T.B.
Lawson1, J.C. Davies1, K. Zioutas2, D.H.H.
Hoffmann3, J. Jacoby4.
Introduction Axions in Extra Dimensions The
axion arises as a solution to the strong CP
problem. The Lagrangian for QCD includes a term
of the form
Searching for Kaluza-Klein Axions with DRIFT
The DRIFT-I (Directional Recoil Identification
From Tracks) detector, currently operating 1.1km
underground in the Boulby Mine is a 1m3 TPC using
CS2 at 40torr for dark matter detection. Although
optimised for nuclear recoil detection, it is
capable of low energy X-ray detection and thus
could be used for a KK axion search. Some
preliminary studies of its sensitivity to gagg
have therefore been performed.
Measurements of the neutron electric dipole
moment limit ? to lt10-9, but QCD gauge invariance
also allows ?O(1). As the quark masses arise in
the CP violating electroweak sector, ? gets
changed to ??QCD-?EW.
However, there is no reason why ?QCD and ?EW
should cancel so accurately, giving the strong CP
problem. Peccei Quinn (Phys. Rev. Lett. 38,
1440(1977)) proposed a possible solution in the
form of a spontaneously broken global U(1)
symmetry which gives rise to a pseudo-Nambu-Goldst
one boson, the axion. Non-perturbative QCD
effects dependent on ? induce a potential for the
axion field which dynamically drives ? to 0,
solving the strong CP problem. The mass of the
axion and its coupling to matter are inversely
proportional to the energy scale of the
Peccei-Quinn (PQ) symmetry breaking.
Considerations of the effect of axions on
cosmology, stellar energy loss and evolution
limit the axion mass to 10-2 eVgtmPQgt10-5eV, thus
axions are very light and very weakly interacting.
Figure 1 Axion to photon conversion via the
Primakoff process (left) and axion decay to two
photons via the triangle anomaly (right).
Whilst the axion remains hypothetical at present,
experimental searches for Big Bang relic and
solar axions are underway. These aim to detect
photons produced by axion conversion in the
presence of a strong electromagnetic field (the
Primakoff effect). Axions can also decay to two
photons with a lifetime determined by their mass
and the axion-photon coupling ga??. This a???
decay is however unobservable, as the current
axion mass limits mean that the lifetime is many
orders of magnitude greater than the age of the
universe.
However, in theories with n extra dimensions
beyond the usual (13)-dimensional Minkowski
space axions acquire a tower of Kaluza-Klein (KK)
excitations, an. The lowest excitation can be
identified as the usual
PQ axion, with the higher excitations having
masses spaced at 1/R, where R is the radius of
the compactified extra dimensions. Decay
lifetimes for these KK axions are much shorter
due to their higher mass, e.g. for
ga??10-11GeV-1 the lifetime of the PQ axion is
1027days, whereas a KK axion with mass 10keV has
a lifetime of 1012days (Phys. Rev. D. 62
125011(2000)). KK axions could be produced in hot
plasma inside the Sun, raising the possibility of
testing this model by searching for the an???
decays of solar KK axions trapped into orbits
around the Sun in a laboratory on Earth.
Solar Production of Kaluza-Klein Axions To test
the feasibility of searching for an??? decays,
the prodution rate of KK axions by the Primakoff
process, where the electromagnetic field is
provided by the Coulomb field of nuclei in the
solar plasma, and photon coalescence ???a. must
be determined. Production rates for modes of mass
m from both these processes have been calculated
in Di Lella, Pilaftsis, Raffelt and Zioutas
(Phys. Rev. D. 62, 125011(2000)), which shows the
mass spectrum to range from 0-20keV. An important
factor in these calculations is that the energy
carried away by axions should not exceed the
limit on exotic energy loss processes of lt0.2L?
set by helioseismology.
Future Work
Figure 2 Simulated orbits of KK axions around
the Sun(shaded disk) (left), and energy spectrum
of X-rays from a?gg decays (right), from
Astroparticle Phys. 19, 145(2003).
The resultant KK axions have a broad speed
spectrum from 0 to c, the majority streaming out
of the Sun. A small fraction have speeds below
the solar escape velocity and will become trapped
into orbits around the Sun, many of which will
intersect the orbit of the Earth. As the a???
lifetime is long compared with the solar age, a
population of particles will build up.
Calculations suggest that the KK axion density at
the Earth would be of order 1013-1014m3, giving
an a??? decay rate of 1m3day-1 for gagg10-13
GeV-1 (Astroparticle Physics 19, 145(2003)).
Although a very small rate, detection of the two
O(5keV) X-rays from these decays is potentially
within reach of the current generation of low
background dark matter detectors, allowing
stringent limits to be placed on gagg and hence
on the Kaluza-Klein model.
e-
z
e-
y
Decay Point
e- drift
x
Axion Searches with Gas Detectors As the axions
trapped into orbits around the Sun are
non-relativistic (v/clt10-2), the two X-rays from
a KK axion decay will be emitted back to back.
Since the primary X-ray interaction at keV
energies is photoelectric, the resultant
signature for a decay will be two electrons each
with energy ma/2 produced in coincidence (in the
readout time of the detector). The primary
background for these decays is therefore
coincident compton/photo electrons produced by
background X/g-rays from radioactive decay.
Work is now in progress to model detector
properties such as spatial/energy resolution and
gas diffusion. A Monte Carlo simulation of these
effects is being prepared, and will use the
output from the EGSnrc simulations to determine
the background coincidence rate in DRIFT. By
testing different gas mixtures, readout
resolutions and analysis cuts the sensitivity to
gagg will be optimised. Nevertheless, a
background rate of 1m-3day-1 should be
obtainable, potentially allowing a limit of
gagglt10-13GeV-1 to be set.
A gas Time Projection Chamber (TPC) is
particularly suited to searching for an??? as,
unlike a NaI or Ge detector, both electrons can
be seen by adjusting the gas pressure so that the
mean free path of X-rays is greater than the
spatial resolution of the readout. This ability
to detect both electrons helps in the suppression
of background coincidences, as both electrons
should have the same energy. A TPC therefore
enables the rate of background ?? coincidences to
be strongly suppressed, helping the search for
an???.
1 DRIFT Collaboration (University of Sheffield,
Rutherford Appleton Laboratory, Imperial College,
Temple University, Occidental College, LLNL. 2
University of Thessaloniki. 3 Technische
Universität Darmstadt. 4 University of
Frankfurt.