STAX. An Axion-like Particle Search with Microwave Photons

J. Ferretti

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Dipartimento di Fisica and INFN, ‘Sapienza’ Università di Roma, P.le Aldo Moro 5, 00185 Roma, Italy

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 $\approx 1$ ; 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 $≲ 0.02$ meV.

1 Introduction

Axions [1] are between the most serious dark matter candidates. They are light neutral scalar or pseudoscalar bosons, with mass $m_{a} \approx μ$ eV $- m$ eV, coupled to the electromagnetic field via

L_{I} = \frac{1}{4} G a F^{μ ν} {~ F}_{μ ν}

(1)

In QCD axion models (DFSZ [2] and KSVZ [3]), the axion-photon coupling constant $G$ is directly related to $m_{a}$ ; thus, $G$ is the only free parameter of the theory. In axion-like particle (ALP) searches, the parameter space is extended: $G$ and $m_{a}$ 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 $G$ 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 $G$ 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 $\approx 1$ , and high quality factor Fabry-Perot cavities in the microwave domain ( $Q \approx 10^{4} - 10^{5}$ ), both on the photon-axion conversion and photon regeneration sides.

Figure 1: Experimental configuration of the STAX LSW experiment. Fig. from Ref. [15]; Elsevier B.V. copyright.

2 LSW experiments

In a LSW experiment [6, 11, 12, 13, 14], a coherent photon beam traverses an intense magnetic field, $H$ . Here, some photons can convert into axions via the Primakoff effect. Photons exchange 3-momentum $q$ with $H$ , the energy is conserved. If the $^x$ axis is chosen in the direction of the propagating photon beam, then the external magnetic is assumed to be uniform in the volume $L_{x} L_{y} L_{z} = L_{x} S$ .

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 $ϵ_{γ} ≫ m_{a}$ limit, the photon to axion (axion to photon) conversion probability is given by [6]

P_{γ \to a} = P_{a \to γ} = G^{2} H^{2} \frac{{sin}^{2} (q_{x} L_{x} / 2)}{q_{x}^{2}} \frac{ϵ_{γ}}{\sqrt{ϵ_{γ}^{2} - m_{a}^{2}}}

(2)

where $ϵ_{γ}$ is the photon (axion) energy and $m_{a}$ the axion mass. In the limit $ϵ_{γ} \approx m_{a}$ , which is relevant for the STAX experiment, the previous expression for the conversion probability has to be regulated [15]

P_{γ \to a} = P_{a \to γ} = G^{2} H^{2} \frac{{sin}^{2} (q_{x} L_{x} / 2)}{q_{x}^{2}} \frac{ϵ_{γ}}{\frac{1}{L_{x}} + \sqrt{ϵ_{γ}^{2} - m_{a}^{2}}}

(3)

The photon-axion-photon rate reads

\frac{d N_{γ}}{d t} = Φ_{γ} η P_{γ \to a}^{2}

(4)

where $Φ_{γ}$ [s $^{- 1}$ ] is the initial photon flux and $η$ 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 $Q$ , 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.

Figure 2: $90 %$ CL exclusion limits that STAX and STAX 2 may achieve in case of a null result for axions with $m_{a} ≲ 0.02$ meV. An exposure time of one month and zero dark counts are considered. “STAX” and “STAX 2” configurations correspond to a 100 kW and 1 MW gyrotron sources, respectively. Picture from Ref. [15]; Elsevier B.V. copyright.

Our goal is to develop a new generation LSW experiment and improve the limits on $G$ 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 $10^{10}$ 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 $\approx 10$ mK.

In this way, we computed that present laboratory exclusion limits on axion-like particles might be improved by at least four orders of magnitude for axion masses $≲ 0.02$ meV [15]. The limits that STAX experiment may achieve are compared to previous experimental results in Fig. 2.

References

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