Introduction: Axions in Extra Dimensions The axion arises as a solution to the strong CP problem. The Lagrangian for QCD

1. Decay Properties and Background Events The separation S between the interaction points of the two decay X-rays, and hence between the two photoelectrons, can be evaluated analytically in terms of the mean free path of X-rays of energy E, l(E). Although the distribution of S values has a long tail, 99% of have electrons with S<6.6l(E). This allows an analysis cut on the spatial separation of coincident events, thereby reducing the volume in, and hence rate at, which coincident background events can occur. The S distribution also determines the fiducial volume of the detector, since one or both of the X-rays may escape the detector without interacting in the gas. A Monte Carlo simulation of the DRIFT-I volume shows: CS2 at 40torr: Fiducial Volume = 0.55m3. CS2 at 160torr: Fiducial Volume = 0.88m3. Thus the lower pressure of DRIFT-I significantly lowers the efficiency for detecting both X-rays. A pressure of 160torr has therefore been chosen as a starting point Sulphur K-Shell Peak Figure 2: Distribution of gg interaction point separations (black line) and event fractions with separations less than a given number of gamma mean free paths. a g Figure 1: Left - Simulated 10keV agg decay in a 20x20x20cm TPC with CS2 gas at 160torr, showing the hits on a 20x20 element readout. Right – Simulated voltage pulses on the x-axis readout elements from this event, showing the coincidence of the events within the readout time of the TPC (7700ms). for simulations of the electron background, although some further optimisation of the separation cut will will be necessary due to the dependence of the background rate on the gas pressure. Ze Ze X-Ray Background Simulations The EGSnrc package has been used to simulate the background electron event rate in DRIFT due to g-rays originating from U, Th and 40K decays in the NaCl cavern walls of the Boulby Mine laboratory. At present the simulation models the stainless steel vacuum vessel of the detector filled with CS2 gas at 160torr, and only the 1.46MeV g-rays from 40K decay have been included. g a Using the data from this simulation and requiring any coincidences to have a spatial separation <3l(E) (~15cm at 5keV in 160torr CS2) a background rate of axion-like events is derived: Background ~ 100m-3day-1 for E<25keV. Clearly, some Pb shielding will be required to reduce this rate to the predicted <1m-3day-1. Nevertheless, this background rate should be obtainable. The simulation is now being extended in order to determine the thickness of Pb required. U and Th decays will also be included, as well as extra detector components such as an inner Cu shield and the perspex field cage. g 80% of Events 60% of Events Figure 2: Background electron energy spectrum in DRIFT operating with CS2 at 160torr due to 40K decay gammas. Photon Coalescence Primakoff Effect 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 <10-9, 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-Goldstone 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 eV>mPQ>10-5eV, 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 (1+3)-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 <0.2L๏ 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 gagg~10-13 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/c<10-2), 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 gagg<10-13GeV-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 therate 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.