Abstract
The axion emerges in extensions of the Standard Model that explain the absence of CP violation in the strong interactions. Simultaneously, it can provide naturally the cold dark matter in our universe. Several searches for axions and axion-like particles (ALPs) have constrained the corresponding parameter space over the last decades but no unambiguous hints of their existence have been found. The axion mass range below 1 meV remains highly attractive and a well motivated region for dark matter axions. In this White Paper we present a description of a new experiment based on the concept of a dielectric haloscope for the direct search of dark matter axions in the mass range of 40 to 400 upmu hbox {eV}. This MAgnetized Disk and Mirror Axion eXperiment (MADMAX) will consist of several parallel dielectric disks, which are placed in a strong magnetic field and with adjustable separations. This setting is expected to allow for an observable emission of axion induced electromagnetic waves at a frequency between 10 to 100 GHz corresponding to the axion mass.
Highlights
In scenarios in which the PQ symmetry is broken after inflation, the realignment mechanism along with decaying topological defects provides a cold dark matter (DM) axion density that matches the observed value if the axion mass ma is of the order of 100 μeV [22,23,24,25,26,27]
The results of measurements at a test setup are presented, which lead us to the conclusion that it should be realistic to build an experiment that can cover a large fraction of the parameter space including the unexplored one predicted for DM axions in the post inflationary PQ symmetry breaking scenario
The axion mass range above 40 μeV, which is predicted by theoretical models where the PQ symmetry breaking occurs after inflation, is not yet experimentally explored
Summary
In the standard model (SM) of particle physics, violation of CP in the strong interactions is controlled by just one parameter, the θ angle This angle appears as the sum of two contributions with a-priori unrelated origins: the angle defining the vacuum of QCD, θQCD, and the common phase of the quark mass matrix, Arg Det Mq , related to the Yukawa couplings of the Higgs sector. This is an amazingly small upper limit, especially if we consider that the only other CP violating phase in the SM, the CKM angle δ13 = 1.2 ± 0.08, is not small and comes from the quark mass matrix. The smallness of CP violation gives us a hint that some dynamical mechanism could be at work to suppress the effects of the θ term in Eq (2.1)
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