Searching for energetic cosmic axions in a laboratory experiment

Astrophysical sources of energetic gamma rays provide the right conditions for maximal mixing between (pseudo)scalar (axion-like) particles and photons if their coupling is strong enough. This is independent of whether or not the axion interaction is standard at all energies or becomes suppressed in the extreme conditions of the stellar interior. The flux of such particles through the Earth could be observed using a metre long, Tesla strength superconducting solenoid. The rate of events in CAST caused by axions from the Crab pulsar is also estimated. PACS. 14.80.Mz; 98.70.Rz; 95.85.Ry Recently, the interest in axion-like particles has been reignited due to the PVLAS experiment reporting an observation of a rotation of the plane of polarization of a laser beam passing through a magnetic field [1] which was claimed to be compatible with the existence of a new (pseudo)scalar particle with a mass of m ∼ 10−3 eV and an inverse coupling to the photon of M ∼ 10 GeV. This was unexpected since experiments such as CAST [2] have seemingly ruled out this region of parameter space. Attempts to explain this discrepancy resulted in alternative models for the pseudoscalar in which its effective coupling to photons is suppressed in the relatively extreme conditions of the stellar interior [3–12]. Alternative explanations of the effect by means of particles carrying very small electric charge [13, 14] were disfavoured [15] by preliminary PVLAS data and severely constrained by existing limits on the millicharged particles [16, 17]. Though the original results are not supported by further PVLAS studies [18] (see also early discussion in [19]), the theoretical work has demonstrated that an axion with such a strong coupling to the photon may be consistent with the CAST limits, provided the coupling is somehow suppressed at high temperature (such scenarios also depend on uncertainties in the model of the solar interior in parameters such as the magnetic field). More model-independent tests both in laboratory experiments [15, 20–22] and in gamma-ray astronomy [23–26] have been proposed. Here we suggest that if new axion-like particles exist which have a strong coupling to two photons, there should be a flux of these para e-mail: [email protected] ticles through the Earth coming from conversion of energetic gamma-rays emitted by astrophysical sources to axions in the magnetic fields of the sources themselves. Since the conditions (temperature, density and average momentum transfer) in such typical sources are much closer to those in the laboratory rather than the stellar interior, such a flux would be compatible with CAST limits and with bounds from stellar astrophysics. We argue that this flux can be detected in a laboratory experiment by using a superconducting solenoid surrounded by an electromagnetic calorimeter. For definiteness, let us consider the Lagrangian density of the photon-pseudoscalar system,