The Search of Axions Through Polarized Matter

The QCD axion is an hypothetical particle introduced to solve the strong CP problem of standard model of particle physics and are of interest as a possible component of cold dark matter. The allowed couplings between particles is determined by the vertices of the spin-0 bosons: the pseudoscalar interaction is always spin-dependent, while, in the non-relativistic limit, the scalar interaction can be treated as spin-independent. Thus, in a multipole expansion, the two fields are described by the \dipole" (pseudo-scalar coupling gp) and "monopole" (scalar coupling gs) moments, respectively. Aim of the experiment is to improve the measured limits for the gpgs product, for different values of the axion mass, this interaction arises between a nucleus N and the spin of an electron e-. Even if the coupling between single particles is weak, a macroscopic sample with the order of 1023 atoms, could produce a light coherent bosonic field that can be measured. In the axion scenario, J.E. Moody and F. Wilczek showed that a new macroscopic force, mediated by the exchange of axions, acts on electron spins, and that such force can be described in terms of the potential of a field. The effective field interacts with electron spins of matter, can be associated to an effective magnetic field and detected by measuring the induced changes of magnetization. The purpose of the experiment is to detect with a SQUID a magnetization signal which is not produced by a magnetic field but by a material with high nuclei density. The source consists of large unpolarized masses that provides the monopole part of the interaction gNs while the dipole part gep depends on the electron spins of the crystal. The interaction causes a change in the magnetization of the sample and induces a change of the magnetic ux collected by a coil surrounding the crystal. As the interaction potential is generated by pseudoscalar exchange rather than by vector gauge boson exchange, this field does not satisfy the Maxwell's equations, therefore it is possible to shield the apparatus from electromagnetic noise sources without affecting the signal. A rotating wheel with evenly spaced lead disks allows for a source mass with a variable distance, while a cryostat houses the detector in lHe, placed as close as possible to the moving source; this allows a periodic modulation of the signal. To further increase the sensitivity of the apparatus, the signal is amplified with a resonant RLC circuit, tuned at the signal frequency given by the rotating wheel. This circuit will be coupled with the pick-up of the SQUID and the resulting signal will be increased by its Q-factor. The main efforts to measure this interaction has been performed by Ni et al. and Adelberger et al., this experiment lowered this limits of one orders of magnitude. Considering an integration time of 4 h, the minimum detected signal is Beff;min ' 10^-17 T, with a resultant limit on the coupling gepgNs =~c . 10^-30. Using a resonant pick-up with Q ' 104 the limit becomes Beff;min ' 10^-22 T, therefore the limit on the coupling is gepgNs =~c . 10^-34.