Present and Future of ALPS II

The ALPS Collaboration started its first “Light Shining through a Wall” experiment to search for photon oscillations into WISPs in 2007. Results were published in 2009 and 2010. The ALPS I experiment at DESY set the world-wide best laboratory limits for WISPs in 2010, improving previous results by a factor of 10. After its completion the ALPS collaboration decided to continue looking for WISPs by designing the ALPS II experiment for probing further into regions where there are strong astrophysical hints for their existence.

The ALPS II general design.

Experimental stages of the ALPS II

The ALPS II experiment is going to be conducted in two steps with increasing requirements reaching its full sensitivity in ALPS IIc.

1). ALPS II-a:

Schematic view of the ALPS IIa stage with two 10 m long cavities without a HERA superconducting magnet. 

This stage aims for demonstrating all features of the optical system and the superconducting transition-edge sensor (TES) detector in a dedicated laser laboratory without magnet system.

In a contract to ALPS I, ALPS II plans to set up optical cavities both on the production and the regeneration side of the experiment (10m each) with a power buildup of 5000 and 40000, respectively. Both cavities together with a light-tight "wall" in between are planned to be in one common vacuum tank.

2). ALPS II-b:

ALPS IIb stage has been skipped.

3). ALPS II-c:

Schematic view of the ALPS IIc stage with two 100 m long cavities using  the HERA superconducting  dipole magnets. 

This is a final stage of the ALPS II, which will be realized with full length cavities (100 m on each side) and  straightened HERA dipole magnets (10 magnets on each side). This stage will be performed in the HERA tunnel.

On the picture below one can see a straight section of the HERA tunnel equipped with HERA dipoles. The middle part, accommodating the central breadboard including the “wall” is highlighted.

Atistic's view of the ALPS IIc final stage. The pictue shows a straight section of the HERA tunnel equipped with 20 HERA dipoles.

Sensitivity of ALPS II

The sensitivity gains for the ALPs particles compared to ALPS I are achieved by increasing the magnetic length, introducing a regeneration cavity and an improved detector system. The main sensitivity gain is due to the enhanced magnet lenght arising from 2 times 10 HERA-dipole magnet string. A sizable additional gain arises from the installation of the regeneration cavity.

The numbers given in this table are for a transition edge sensor detector, as this is the detector which is expected to be used in the final version of all stages of ALPS II. For hidden photons, there is no gain from the magnetic field. Thus the sensitivity gain follows as above except for the factor coming from BL and amounts to 147 for hidden photons.

Parameters of the ALPS I experiment in comparison to the ALPS II. The second column shows the dependence of the reachable ALP-photon coupling on the experimental parameters. The last column lists the approximate sensitivity gain for axion-like particle searches compared to ALPS I. 

Schematic overview of the sensitivity reach of the final stage of ALPS II, ALPS IIc depicting its scientific impact.

The expected sensitivity of ALPS II experiment for the ALPs and hidden-photon search are compared to exclusion limits set by the other experiments. As a yellow band, generic QCD axion models are indicated. For comparison, the ALPS I results (in green, max. sensitivity  7 × 10−8 GeV−1 ) and the currently most stringent bounds on ALPs in this mass region from the CAST helioscope (in blue) are shown.

As visible, ALPS-IIc (in orange, max. sensitivity 2 × 10−11 GeV−1 ) will surpass the CAST bounds in the lower mass region and tackle parameter regions in which ALPs are motivated by fundamental particle physics questions (cold Dark Matter candidates, in gray, and predictions from string theory around  10−12 GeV−1 , not colored) as well as astrophysical hints: TeV transparency (red, fiducial region) and WD cooling (light red band)). In addition the parameter region excluded from the 1987a super-nova burst is indicated. The predictions from theory and astrophysical considerations are generally certain within about an order of magnitude. Note that the parameter range between µeV < m < 1meV is particularly interesting in the context of axion Dark Matter, and currently most successfully probed by ADMX (black line). ALPS, on the other hand, can be Dark Matter in a much wider mass range.


ALPS II tentative schedule