Search for weakly interacting sub-eV particles with the OSQAR laser-based experiment: results and perspectives

Recent theoretical and experimental studies highlight the possibility of new fundamental particle physics beyond the Standard Model that can be probed by sub-eV energy experiments. The OSQAR photon regeneration experiment looks for"Light Shining through a Wall"(LSW) from the quantum oscillation of optical photons into"Weakly Interacting Sub-eV Particles"(WISPs), like axion or axion-like particles (ALPs), in a 9 T transverse magnetic field over the unprecedented length of $2 \times 14.3$ m. No excess of events has been detected over the background. The di-photon couplings of possible new light scalar and pseudo-scalar particles can be constrained in the massless limit to be less than $8.0\times10^{-8}$ GeV$^{-1}$. These results are very close to the most stringent laboratory constraints obtained for the coupling of ALPs to two photons. Plans for further improving the sensitivity of the OSQAR experiment are presented.

Particle and astroparticle physics beyond the Standard Model (SM) is not restricted to the high energy frontier. Many extensions of the SM, in particular those based on supergravity or superstrings, predict not only massive particles like the WIMPS (weakly interacting massive particles), but also weakly interacting sub-eV particles (WISPs). Whereas the formers can be searched for at TeV colliders such as the Large Hadron Collider (LHC) at CERN, signature of WISPs seems most likely to be detected in low energy experiments based on lasers, microwave cavities, strong electromagnetic fields or torsion balances [1,2]. As the first and paradigmatic examples of WISPs, there is the axion and other axion like particles (ALPs) [3]. Axion remains not only one of the most plausible solutions to the strong-CP problem [4]. It constitues a fundamental underlying feature of the string theory in which a great number of axions or ALPs is naturally present [5]. The interest for axion search extends beyond particle physics, since such a hypothetical light spin-zero particle is considered as a serious dark-matter candidate [6], the only non-supersymmetric one.
The OSQAR proposal is at the forefront of this emerging low energy frontier of particle/astroparticle physics. It combines the simultaneous use of high magnetic fields with laser beams in two distinct experiments. In the first one, the photon regeneration effect is looked for as a Light Shining through the Wall (LSW), whereas in the second one, ultra-fine magnetic birefringence of the vacuum is aimed to be measured for the first time [7]. This letter focuses on the OSQAR LSW experiment and is in the same line as the pioneering work in the early 1990 that excluded WISPs with a di-photon coupling constant g Wγγ > 7 × 10 −7 GeV −1 for a WISP mass below 1 meV [8]. A strong revival of interest in this type of experiment has been triggered by the announcement in 2006 of possible positive WISP signal [9], but all recent LSW worldwide experiments have excluded such a possibility [10] and the present reference result, obtained by the ALPS collaboration, excludes WISPs with g Wγγ > 6.5 × 10 −8 GeV −1 in the massless limit [11]. In this work we report the last OSQAR results approaching this exclusion limit for WISP search and we present the next upgrade phases of the OSQAR LSW experiment.
LSW experiment is the simplest and most unambiguous laboratory experiment to look for WISPs. It combines the photon-to-WISP and WISP-to-photon double quantum oscillation effect in a transverse magnetic field with the weakness of the coupling of WISPs to matter [12,13]. When a linearly polarized laser light beam propagating in a transverse magnetic field is sent through an optical barrier, only photons converted to WISPs due to the mixing effect will not be absorbed. Weakly inter- acting particles will propagate freely through the barrier before being reconverted in photons of same energy that can be easily detected and identified ( Figure 1).
The photon-to-WISP (γ → W) conversion probability, as well as the WISP-to-photon (W → γ) one, in vacuum over a length L permeated by a transverse magnetic field B are given by [12,13]: using Heaviside-Lorentz units (h = c = 1). g Wγγ is the WISP di-photon coupling constant and q = k γ − k W the momentum transfer, with k W = (ω 2 − m 2 W ) 1/2 and k γ = ω where ω is the energy for photons and WISPs. The form factor of the conversion probability is at maximum for qL = 0, which corresponds to the limit m W ≪ ω otherwise incoherent effects of the WISPs-to-photon oscillation reduce the conversion probability. With pseudoscalar WISPs, which includes the axion and the ALPs, (resp. with scalar WISPs) the probability is maximum when the linear polarization of light is parallel (resp. perpendicular) to the magnetic field and vanishes when this polarization is rotated in the perpendicular direction. The photon flux after the optical barrier ( Figure 1) is given by: where P is the optical power, ω the photon energy and η the efficiency of the detector. Equation (2) shows that the regenerated photon flux scales as (BL) 4 , highlighting the interest of using high field magnets over the longest optical path length. With this respect, the LHC dipole magnets cooled down to 1.9 K with superfluid He and each of them being able to produce a transverse magnetic field of 9 T over 14.3 m constitute nowadays the state of the art. The present experimental setup of the OSQAR photon regeneration experiment is using two LHC dipole magnets separated by an optical barrier as schematized in Figure 1. Each dipole magnet is connected to a cryogenics feedbox. The total length of the experiment is about 53 m. Magnet apertures is pumped typically down to 10 −6 − 10 −7 mbar with two turbomolecular pumping groups. An ionized Ar + laser has been used to deliver in multi-line mode 3.31 ± 0.03 W of optical power in average with approximately 2/3 of the optical power at 514 nm (2.41 eV) and 1/3 at 488 nm(2.54 eV). Its beam has a well defined linear polarization parallel to the magnetic field optimized for the search of new pseudoscalar/axion particles. To look for scalar particles, a λ/2 wave plate with antireflective coating layers is oriented at 45 • and inserted at the laser exit to align the polarization perpendicularly to the magnetic field. The laser beam divergence has been reduced with a specially developed beam expander telescope. By combining converging and diverging lenses, the optical Gaussian beam with a waist equal to 0.708 mm at the level of the output coupler, was shaped to obtain a spot size not exceeding 6mm of diameter at 53m of distance. To minimize spherical aberrations, lenses were mounted with planar surfaces face-to-face and thermal effects on lenses were minimized by developing proper supports. For photon counting, a liquid nitrogen (LN2) cooled CCD detector from Princeton Instrument (model LN/CCD-1024E/1) has been used. The CCD chip of 26.6 × 6.7 mm 2 is composed by a 2D array of 1024 × 256 square pixels of 26 µm size. The quantum efficiency of the detector is equal to 30% for the Ar + laser wavelengths. Dark current and readout noise at 20 kHz are typically equal to 0.5 e − /pixel/hour and 3.4 e − rms respectively. An optical lens with a focal length of 100 mm has been installed just in front of the detector to focus the laser beam on the CCD chip.
Both LHC dipole magnets used for OSQAR have been installed on their horizontal cryogenic bench and precisely aligned with a Laser Tracker LTD 500 from Leica before being thoroughly tested at 1.9 K and used in routine operation to provide the 9 T magnetic field over 2 × 14.3 m. In particular, the field strength and field errors of both LHC dipoles used have been precisely characterized. The long term stability of the alignment of the laser beam traveling through the aperture of both LHC dipoles has been also carefully checked and controlled periodically with the LN 2 cooled CCD detector.
A typical experimental run starts and ends with laser beam alignments using absorptive filters to reduce the optical intensity below the saturation level of the CCD detector. An example of raw signals recorded with the CCD detector in 2D mode is given in Figure 2. The photons regenerated from WISPs can be efficiently and unambiguously identified from the shape and the precise localization of the known expected signal. Due to the double binning at the read out step, i.e. summing the recorded entries of four (2 × 2) neighbor pixels into a single super-pixel, the CCD spectra is composed of a 2D array of 512 × 128 counted entries. The central peak corresponding to the focused and attenuated laser beam has a much broader width compared to parasitic signals such as those coming from cosmic rays. Table I gives a summary of the duration of data taking as a function of various configurations of the experiment including the effective optical power, i.e. after subtracting all losses due to parasitic reflexions of light. A dedicated procedure has been developed to filter signals coming from cosmic rays and has been validated on laser beam profile data to ensure that it does not impact some broader signal distributions compatible with a regenerated photon beam. In a first step, this cosmic cleaning procedure was applied systematically to each 2D recorded spectrum of 900 s exposure time. In addition, the average recorded count-rate per super-pixel was treated as a constant bias or offset and therefore subtracted from each spectrum. Alternatively the subtraction of the background acquisition frame to each spectrum acquired for WIPs search gives no significant difference. In a second step, all spectra corresponding to the search of either scalar or pseudoscalar particles have been added separately. The cosmic cleaning procedure has been applied a second time in order to identify hot cells, which were rejected from further study. As a last step, the final data spectrum has been clustered to optimize the experimental sensitivity. The optimal size of the clusters obtained corresponds to a rectangle of 4 × 5 super-pixels and contains 30.3 % of the signal in the pointing zone of the laser beam. The integrated signal of each cluster is defined by the sum of all recorded counts of the corresponding 20 super-pixels. Resulting histograms for the search of pseudoscalar and scalar particles respec- tively are shown in Figure 3. They can be accurately fitted with Gaussian distributions and the best parameters obtained are listed in Table II Figure 3) and deduced sensitivity.
No excess of accumulated counts can be detected for the cluster located at the pointing zone of the laser beam whether for the search of pseudoscalar or scalar particles. Conservatively the 95% confidence interval for pseudoscalar particle can be set from Gaussian fitting parameters of Table II to 41 cluster counts. The predicted CCD quantum efficiency of 30 % corresponds to 137 cluster counts and gives a limit to the sensitivity of 6.32 photon/h or 1.76 × 10 −3 photon/s. A very close sensitivity is obtained for scalar particle search and both these results are reported in Table II. By inserting this photon flux into Equation (2), the exclusion limits for WISPs parameter can be determined and are compared in Figure 4 to the last results from ALPS collaboration [11]. The values of coupling constants of possible new light scalar and pseudo-scalar particles that can couple to two photons is constrained in the massless limit to be less than 8.0 × 10 −8 GeV −1 . This result constitutes the first precise confirmation of the present most stringent exclusion limit for WISPs obtained from a purely laboratory experiment [11].
The resonantly-enhanced photon regeneration effect can significantly improve LSW experiments [14] allowing in principle to compete and even exceed the limits provided by solar telescopes [15] or haloscope [16]. It should be emphasized that experiments based on potential extra-terrestrial WISP sources can be considered as providing today the best constraints on the coupling of WISPs to photons but they are based on strong assumptions. For example, in solar experiments the role of the solar magnetic field allowing the conversion of WISPs into photons at the surface of the sun, which can lead to " a solar axion problem "is neglected [17] whereas for haloscope, it is usually assumed that axions saturate the Milky Way's halo [16]. Purely laboratory experiments for WISP search offer a complementary approach but their sensitivity needs to be significantly further improved. In this line an ambitious upgrade of the ALPS experiment is ongoing [18].
The next steps of the OSQAR LSW experiment will be based on three main improvements. First, the axion source will be upgraded with a more powerful laser source of 25 W and by adding a Fabry-Perot cavity. Second, the amplification of the photon regeneration effect from the resonantly-enhanced conversion scheme [14] will be implemented. Third, the possibility of further increasing the number of LHC dipoles for OSQAR is currently studied, starting from a configuration using 2 + 2 dipoles for the coming years.
The implementation of the resonantly-enhanced conversion scheme is challenging with one of the main technical difficulties resulting from the design and realization of active locking systems for two high-finesse Fabry-Perot cavities of long length [19]. An intermediate step considered as being necessary is currently under study. It is based on the development of a 20 m long extended laser cavity to profit from the large laser intracavity optical power for the WISP source. In that way, the locking of the high-finesse Fabry-Perot cavity of the photon regeneration zone to the laser in extended cavity mode will become a more affordable operation as a first step. First experimental trials with Ar + laser operating in extended cavity mode have been successful for linear as well as for Z-fold configurations of the cavity, the latter allowing better adjustments of the laser beam directions ( Figure   FIG. 5. Ar + laser working in a Z-fold extended cavity mode of 10 m long. The output coupler of the laser is clearly visible close to one of the guiding mirrors inside the cavity. FIG. 6. Mirror of 99.55 % reflectivity, 2 inch of diameter and 10 m of curvature radius mounted on the mole supporting structure, which is inserted inside the magnet aperture. 5). For both these configurations, the output coupler of the Ar + laser has been replaced by a mirror with a reflectivity of 99.55 % and a radius of curvature of 10 m. The latter has been mounted on a mole type support that can be inserted and translated inside the magnet aperture ( Figure 6).
A stable beam with an intracavity optical power of few hundred watts has been obtained. A precise measurement method of this intracavity optical power based on Rayleigh scattering [20] is under development. Among further improvements that need to be achieved it can also be emphasized the use of a mirror with higher reflectivity and a curvature radius of 20 m allowing the laser cavity to be extended over the whole length of one LHC dipole connected to its cryogenic feed box.
To conclude the obtained limits for the di-photon coupling constants of scalar and pseudo-scalar particles have confirmed the results obtained by ALPS collaboration. The mechanical stability achieved for the present 53 meter long OSQAR experiment using two spare superconducting dipole magnets of the LHC gives full confidence on the feasibility in the near future of the resonantlyenhanced scheme proposed by P. Sikivie et al. [14].
We would like to express our thanks to the CERN Directorate for his continuous support, the support teams