First Results of the Full-Scale OSQAR Photon Regeneration Experiment
Recent intensive theoretical and experimental studies shed light on possible new physics beyond the standard model of particle physics, which can be probed with sub-eV energy experiments. In the second run of the OSQAR photon regeneration experiment, which looks for the conversion of photon to axion (or Axion-Like Particle), two spare superconducting dipole magnets of the Large Hadron Collider (LHC) have been used. In this paper we report on first results obtained from a light beam propagating in vacuum within the 9 T field of two LHC dipole magnets. No excess of events above the background was detected and the two-photon couplings of possible new scalar and pseudo-scalar particles can be constrained to be less than and respectively, in the limit of massless particles.
CERN, CH-1211 Geneva-23, Switzerland
LNCMI-G, CNRS-UJF-UPS-INSA, BP 166, 38042 Grenoble Cedex-9, France
Institut Néel, CNRS and Universit Joseph Fourier, BP 166, 38042 Grenoble Cedex-9, France
IMEP-LAHC, UMR CNRS 5130, Minatec-INPG, 3 parvis Louis Néel, BP 257, 38016 Grenoble Cedex-1, France
Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic
LSP, UMR CNRS 5588, Universit Joseph Fourier, BP 87, 38402 Saint-Martin d’Hères, France
Czech Technical University, Faculty of Mechanical Engineering, Prague, Czech Republic
Institute of Theoretical Physics, University of Warsaw, Poland
LASIM, UMR CNRS 5579, Université Claude Bernard Lyon-1, 69622 Villeurbanne, France
Technical University of Liberec, Czech Republic
The axion is a neutral pseudo-scalar particle predicted independently by S. Weinberg [T1] and F. Wilczek [T2] from the Peccei and Quinn symmetry breaking [T3]. It remains the most plausible solution to the strong-CP problem and constitutes a fundamental underlying feature of the string theory in which a great number of axions or Axion-Like Particles (ALPs) is naturally present [T4]. In addition, the interest in axion search lies beyond particle physics since such hypothetical light spin-zero particles are considered as one of the most serious dark-matter candidates [T5], and the only non-supersymmetric one. Within this scope and in agreement with previous measurement results excluding ”heavy” axions [T6], the allowed range for the ”invisible” axion mass is nominally .
Most of the experimental approaches to the search for ”invisible” scalar or pseudo-scalar particles, such as the axion, are based on the the concept of their coupling to two photons [T7]. The simplest and most unambiguous purely laboratory experiment to look for axion and ALPs is the so-called ”photon regeneration” or ”shining light through the wall” experiment [T8], which is of double oscillation type. A linearly polarized laser light beam propagating in a transverse magnetic field is sent through an optical absorber. When the linear polarization of the light is parallel to the magnetic field, photons of energy can be converted to axions with a maximum of probability due to the mixing effect. Such weakly interacting particles can then propagate freely through the absorber before being regenerated in the magnetic field on the other side of the absorber. For scalar particles the maximum probability corresponds to a light polarised perpendicularly to the magnetic field.
If a region of length is permeated by a transverse magnetic field , the photon-to-axion () conversion probability, as well as the axion-to-photon () one, are given in vacuum by [T8]:
with . Here is the axion or ALP velocity, the axion/ALPs diphoton coupling constant and the momentum transfer. The energy is the same for photons and axions, , and . The form factor of the conversion probability has its maximum for , which corresponds to the limit . Otherwise incoherence effects emerge from the axion-to-photon oscillation, reducing the conversion probability.
2 The OSQAR Experiment
The experimental setup of the photon regeneration experiment using two LHC dipole magnets equipped with an optical barrier at the end of the first magnet is schematised in Figure 1. To operate, the LHC dipole is cooled down to 1.9 K with superfluid He and provides in two apertures a transverse magnetic field which can reach 9.5 T at maximum over a magnetic length of 14.3 m.
As for all LHC superconducting magnets, the dipole used for OSQAR was thoroughly tested at . In particular, the field strength and field errors were precisely characterized. An ionized laser, able to deliver in multi-line mode up to 7 W of optical power is used as light source. The optical beam is linearly polarized with a vertical orientation. To align the polarization of the light in the horizontal direction, a wave-plate is inserted between the laser and the first LHC dipole. It introduces an optical power loss of 20% at the laser wavelengths. The laser was operated in multi-line mode with approximately 6.4W of the optical power at 514 nm (2.41 eV). The laser beam profile was measured at the location of the photon detector and can be well fitted with a gaussian distribution. For photon counting, a cooled CCD detector from Princeton Instrument is used. It is composed of an array of 1100 pixels of height densely packed over a length of 27 mm. The effective sensitive fraction area of the CCD was increase from 65% to 100% by using an optical lens with a focal length of 100 mm. The quantum efficiency of the detector is equal to for the Ar+ laser wavelengths and the number of dark counts per pixel is lower than 0.1 cts/mn.