STAX. An Axion-like Particle Search with Microwave Photons
Abstract
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.
1 Introduction
Axions [1] 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
(1) |
In QCD axion models (DFSZ [2] and KSVZ [3]), 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 [4].
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 [10] 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 [15]. 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 [6]
(2) |
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 [15]
(3) |
The photon-axion-photon rate reads
(4) |
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. [16], 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 [11]. The second stage of ALPS, ALPS-II [13], 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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