STAX. An Axion-like Particle Search with Microwave Photons
We discuss an improved detection scheme for a light-shining-through-wall (LSW) experiment for axion-like particle searches. We propose to use: gyrotrons or klystrons, which can provide extremely intense photon fluxes at frequencies around 30 GHz; transition-edge-sensors (TES) single photon detectors in this frequency domain, with efficiency ; high quality factor Fabry-Perot cavities in the microwave domain, both on the photon-axion conversion and photon regeneration sides. We compute that present laboratory exclusion limits on axion-like particles might be improved by at least four orders of magnitude for axion masses meV.
Axions  are between the most serious dark matter candidates. They are light neutral scalar or pseudoscalar bosons, with mass eVeV, coupled to the electromagnetic field via
In QCD axion models (DFSZ  and KSVZ ), the axion-photon coupling constant is directly related to ; thus, is the only free parameter of the theory. In axion-like particle (ALP) searches, the parameter space is extended: and are the free parameters .
Axions and ALPs experimental searches can be divided into two main categories: 1) Axions from astrophysical and cosmological sources; 2) Laboratory searches. In the former case, exclusion limits on the axion-photon coupling constant are provided by estimates of stellar-energy losses [4, 5], helioscopes [6, 7, 8] and haloscopes [6, 9]. In the latter case, limits on are given by photon polarization  and Light-Shining-Through-Wall (LSW) [6, 11, 12, 13, 14] experiments.
In this contribution, we will focus on LSW experiments. After a brief description of the standard LSW experimental apparatus, we will discuss how to improve present ALPs laboratory limits on by at least four orders of magnitude . We are willing to do this by using extremely intense photon fluxes from gyrotron sources at frequencies around 30 GHz, TES single photon detectors with efficiency , and high quality factor Fabry-Perot cavities in the microwave domain (), both on the photon-axion conversion and photon regeneration sides.
2 LSW experiments
In a LSW experiment [6, 11, 12, 13, 14], a coherent photon beam traverses an intense magnetic field, . Here, some photons can convert into axions via the Primakoff effect. Photons exchange 3-momentum with , the energy is conserved. If the axis is chosen in the direction of the propagating photon beam, then the external magnetic is assumed to be uniform in the volume .
The photons which do not convert into axions are stopped by an optical barrier, “the wall”. while axions can cross the wall, due to their negligible cross-section with ordinary matter. On the other side of the wall there is a second magnetic field, which can convert axions back to photons. Reconverted photons may be detected via a single-photon detector.
In the limit, the photon to axion (axion to photon) conversion probability is given by 
where is the photon (axion) energy and the axion mass. In the limit , which is relevant for the STAX experiment, the previous expression for the conversion probability has to be regulated 
The photon-axion-photon rate reads
where [s] is the initial photon flux and the single-photon-detector efficiency. The rate can be increased by introducing a Fabry-Perot cavity in the magnetic field area before the wall by a factor of , which is the quality factor of the cavity. Moreover, as discussed in Ref. , the rate can be further increased with the addition of a second Fabry-Perot cavity in the magnetic field region beyond the wall. See Fig. 1.
3 STAX experimental configuration and calculated exclusion limits
The best laboratory limits for the axion-photon coupling constant have been provided by the ALPS Collaboration . The second stage of ALPS, ALPS-II , will improve the previous limits mainly by increasing the magnetic field length as well as introducing a second cavity in the magnetic field region behind the wall. ALPS-II configuration is very similar to that of Fig. 1, but in this case the photon flux is provided by an optical laser.
Our goal is to develop a new generation LSW experiment and improve the limits on by using sub-THz photon sources. Sub-THz sources, like gyrotrons and klystrons, can provide very high powers (up to 1 MW) at small photon frequencies, resulting in photon fluxes up to more intense than those from optical lasers, used in previous LSW experiments. We will also use high Q-factor Fabry-Perot cavities for microwave photons and single-photon detectors for light at these frequencies, with almost zero dark count, based on the (Transition-Edge-Sensor) TES technology. The TES detector will be coupled to an antenna and operated at temperatures mK.
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