Production and Detection of Axion-Like Particles at the VUV-FEL: Letter of Intent

TL;DR

The paper interrogates whether the PVLAS anomaly can be explained by a light axion-like particle coupled to photons. It proposes a photon-regeneration (light-shining-through-walls) experiment at DESY's VUV-FEL using a transverse magnetic field and a linear magnet assembly to enable oscillations between photons and a spin-zero state, with a simplified pseudoscalar coupling $g \phi (E \cdot B)$ (scalar coupling $g \phi (E^2 - B^2)$) to distinguish the two. The predicted regeneration rate at $\omega = 38.7$ eV and $\dot N_0 = 6.5\times 10^{16}\ \text{s}^{-1}$, under a $BL \approx 27.66\ \text{Tm}$ magnet setup, is $\dot N_f \approx 1\times 10^{-4}\ \text{s}^{-1}$ times couplings and form-factor factors $F(q\ell)$ with $q = m_\phi^2/(2\omega)$, enabling sensitivity to PVLAS-preferred masses via coherence $F(q\ell)$; varying $\omega$ allows mass determination. If no signal is observed, the experiment could set a 95% C.L. limit of $g < 8.8\times 10^{-7}\ \text{GeV}^{-1}$ for $m_\phi \lesssim 3\times 10^{-3}$ eV, improving existing laboratory bounds in relevant regions. Overall, the work argues that a VUV-FEL-based test could quickly confirm or exclude the PVLAS interpretation with a competitive, low-background approach and lay groundwork for larger-scale photon-regeneration programs.

Abstract

Recently, the PVLAS collaboration has reported evidence for an anomalously large rotation of the polarization of light generated in vacuum in the presence of a transverse magnetic field. This may be explained through the production of a new light spin-zero particle coupled to two photons. In this Letter of Intent, we propose to test this hypothesis by setting up a photon regeneration experiment which exploits the photon beam of the Vacuum-UltraViolet Free-Electron Laser VUV-FEL, sent along the transverse magnetic field of a linear arrangement of dipole magnets of size B L ~ 30 Tm. The high photon energies available at the VUV-FEL increase substantially the expected photon regeneration rate in the mass range implied by the PVLAS anomaly, in comparison to the rate expected at visible lasers of similar power. We find that the particle interpretation of the PVLAS result can be tested within a short running period. The pseudoscalar vs. scalar nature can be determined by varying the direction of the magnetic field with respect to the laser polarization. The mass of the particle can be measured by running at different photon energies. The proposed experiment offers a window of opportunity for a firm establishment or exclusion of the particle interpretation of the PVLAS anomaly before other experiments can compete.

Figures (4)

• Figure 1: Schematic view of (pseudo-)scalar production through photon conversion in a magnetic field (left), subsequent travel through a wall, and final detection through photon regeneration (right).
• Figure 2: Two photon coupling $g$ of the (pseudo-)scalar versus its mass $m_\phi$. The upper limits from BFRT data Cameron:1993mr on polarization (rotation and ellipticity data; 95 % confidence level) and photon regeneration (95 % confidence level) are displayed as thick dots. The preferred values corresponding to the anomalous rotation signal observed by PVLAS Zavattini:2005tm are shown as a thick solid line. The projected 95 % confidence level upper limit which can be obtained with the proposed experiment (see text) is drawn as a dashed-dotted line.
• Figure 3: Exclusion region in mass $m_\phi$ vs. coupling $g$ for various current and future experiments. The laser experiments Cameron:1993mrZavattini:2005tm aim at (pseudo-)scalar production and detection in the laboratory. The galactic dark matter experiments Eidelman:2004wy exploit microwave cavities to detect pseudoscalars under the assumption that these pseudoscalars are the dominant constituents of our galactic halo, and the solar experiments search for axions from the sun Andriamonje:2004hi. The constraint from horizontal branch (HB) stars Raffelt:1985nk arises from a consideration of stellar energy losses through (pseudo-)scalar production. The predictions from two quite distinct QCD axion models, namely the KSVZ Kim:1979if (or hadronic) and the DFSZ Zhitnitsky:1980tq (or grand unified) one, are also shown.
• Figure 4: The second power of the form factor $F$, Eq. (\ref{['formfactor']}), as a function of the laser frequency $\omega$, for fixed length $\ell =6$ m of the magnetic field region and different values of the pseudoscalar mass $m_\phi$, corresponding to the central value, $m_\phi =1.35$ meV (solid), the lower value, $m_\phi=0.7$ meV (short dashed), and the upper value, $m_\phi=2.0$ meV (long dashed), of the range (\ref{['PVLAS_mass']}) suggested by PVLAS.