R. Bähre1, N. Bastidon2, B. Döbrich3, J. Dreyling-Eschweiler3, S. Gharazyan3, R. Hodajerdi1,2,3, D. Horns2, F. Januschek3, E.-A. Knabbe3, N. Kuzkova1,3, A. Lindner3, J. Põld3, A. Ringwald3, M. Schott4, J. E. von Seggern3, R. Stromhagen3, D. Trines3, C. Weinsheimer4, B. Willke1 1: Albert-Einstein-Institute Hannover2: Universität Hamburg3: Deutsches Elektronen-Synchrotron4: Johannes Gutenberg-Universität Mainz

Laser system

Light-shining-through-a-wall

ALPS II is planned to be realized in two stages:

ALPS-IIa: with two 10m long production and regeneration cavities, without HERA superconducting dipole magnets;

ALPS-IIc: with two 100m long cavities using magnets.

Optical setup of the ALPS II

Hints for axions and ALPs

The principle of a light-shining-through a wall experimentLight, typically from a strong laser, is shone into a magnetic field. Experiments particularly apt to look for WISPs with photons are of the "light-shining-through-a-wall" type. Laser photons can be converted into a WISP in front of a light-blocking barrier (generation region) and reconverted into photons behind that barrier (regeneration region). Depending on the particle type, these conversion processes are induced by magnetic fields or happen by kinetic mixing. The most sensitive LSW laboratory setup thus far is the first stage of the Any Light Particle Search (ALPS I) concluded in 2010. With major upgrades in magnetic length, laser power and the detection system, the proposed ALPS II experiment aims at improving the sensitivity by a few orders of magnitude for the different WISPs.

Conceptual design

Schematic of the ALPS-II regeneration cavity (RC) including control loop:

Straightening magnetsTo increase the sensitivity for the detection of axion-like particles, the ALPS-II collaboration plans to set up optical cavities both on the production and the regeneration side of the experiment with a power buildup of 5000 and 40000, respectively, and magnet strings of superconducting HERA dipoles as long as possible, as the sensitivity for the detection of axion-like particles scales with the product of magnetic field strength B and magnetic length L.

The maximal length is determined by the aperture of the beam tube, because clipping losses of the laser light are to be avoided. Now the inner diameter of the vacuum pipe in the superconducting HERA dipole is 55 mm, which is more than sufficient for an installation of 20 HERA dipole magnets for ALPS II. However, due to the curvature of the dipoles built for HERA, the free horizontal aperture is reduced to ~35 mm. Such an aperture would limit ALPS II to just 8 magnets.

35 W Laser systemAn end-pumped laser design was chosen to achieve a well defined Gaussian mode and an efficient amplification with excellent beam quality. An efficient amplification of a Nd:YAG single-frequency, fundamental mode laser with Nd:YVO4 as amplifier material is feasible. With a 2 W NPRO as seed laser source output power levels of more than 35 W were achieved. If the amplifier was seeded with higher power levels, for example with one of the currently used gravitational wave laser systems (10 W to 20 W) output power levels up to 65 W could be realized.

Stages of the experiment

Present and future ALPS II

There are many hints for the existence of axions and axion-like particles: theoretical (most elegant solution to the strong CP problem), them being a good dark matter candidate and several astrophysical observations. TeV photons may “hide” as ALPs.

TeV transparency One astrophysical hint pertains to the propagation of cosmic gamma rays with TeV energies. Even if no absorbing matter blocks the way of these high energy photons, absorption must be expected as the gamma rays deplete through electron-positron pair production through interaction with extragalactic background light. However, the observed energy spectra do not seem to match the absorption feature inferred from this argument. Axion-like particles could provide a resolution to this puzzle. Here, the anomalous transparency can be explained if photons convert into ALPs in astrophysical magnetic fields. The ALPs then travel unhindered due to their weak coupling to normal matter. Close to the solar neighborhood, ALPs could then be reconverted to high-energy photons.

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.

Any Light Particle Search II.

WISPs produced by laser light as well as reconverted photons originating from these WISPs have laser-like properties. This allows to: Guide them through long and narrow tubes inside accelerator

dipole magnets; To exploit resonance effects by setting up optical resonators.

Looking for WISPs

The most famous WISP candidate is the axion, which has been introduced to explain the smallness of CP violation in QCD and which turned out to also be a prime candidate for a constituent of the dark matter in the universe. Similarly axion like particles (ALPs), light spin 1 particles called "hidden sector photons" or light minicharged particles seem to occur naturally in realistic embeddings of the standard model into string theory.

It is therefore an important and fundamental question whether any of these light particles exists. WISP scenarios gain support by recent astrophysical studies like the TeV-photon emission of active galactic nuclei or the properties of white dwarf stars, which hint at the existence of ALPs. Hence it is time to experimentally search for WISPs!

NASA

One of the most exciting quests in particle physics is the search for new particles beyond the standard model. Extensions of the standard model predict not only new particles with masses above the electroweak scale (about 100 GeV), like SUSY particles, also so-called WISPs (very Weakly Interacting Sub-eV Particles).

Such new particles arise naturally in many extensions of the Standard Model and might also explain observations that are not accounted for within the particle physics known today.

Courtesy of Manuel Meyer

Sensitivity of ALPS IIThe sensitivity gains compared to ALPS-I are achieved by increasing the magnetic length, introducing a regeneration cavity and an improved detector system.

To avoid disturbance of the single photon detector with spurious photons from optical readout of the regeneration cavity mode, an auxiliary green beam obtained via second harmonic generation from the infrared production field is fed into the regeneration cavity. The green light is then separated from the infrared signal field prior to detection. A production probability for 1064 nm from 532 nm photons of less than 10-21 photons is to be achieved. The total lateral and angular beam shift introduced to the 1064 nm beam by optical components between two cavities on the central breadboard has to be smaller than 1 mm and 10mrad, respectively, because any beam shift is not seen by particles traversing the wall and hence reproduced light would not match the RC Eigenmode.

The ALPS experiment utilizes a light-shining-through-a-wall setup. Strong light fields are send towards a wall that is opaque to photons but transparent to WISPs due to their vanishing interaction with ordinary matter. WISPs, which exhibit coupling to a photon field, can thus be produced in front of the wall and be reconverted afterwards and consequently be detected by a photon detector. ALPS-II experiment is going to be conducted in two steps with increasing requirements reaching its full sensitivity in ALPS-IIc.

Schematic of the ALPS-II injection stage including the production cavity (PC):

A major challenge of the ALPS II optical design is the stabilization of both optical cavities to ensure a decent overlap between the optical modes. This is achieved by actively controlling production and regeneration cavity length and alignment in a Pound-Drever-Hall (PDH) and Differential Wavefront Sensing (DWS) scheme.

ALPS II in the HERA tunnelALPS-IIc in the HERA tunnelThe final stage of the ALPS II will be realized with full length cavities and straightened HERA dipoles. The picture shows a straight section of the HERA tunnel equipped with HERA dipoles. The middle part, accommodating the central breadboard including the “wall” is highlighted.

https://ALPS.desy.de

Present status of the experiment

Intermediate step of ALPS-IIa:Parameters of the long cavity 20 m length; Two 750 ppm curved mirrors (250 m ROC); Finesse 4100 and power buildup of 1300.

Advantages of the long cavity More stable: G-factor 0,85 (for 10 m flat/curved: G-factor 0,96); Two mirrors from the same coating run; Impedance matched cavity.

Power buildupThe main goal of the optical design of ALPS-II is to make the electro-magnetic field provided by the laser beam on one side of the wall as large as possible and to detect a possibly regenerated field on the other side with a very high sensitivity. On the side in front of the wall, the ALPS-II production cavity can increase the optical power of the light beam directed towards the wall by a factor of 5000 compared to the power of the injected laser. Behind the wall, the regeneration cavity increases the production probability with which photons are created from the axion field by a factor of 40000.

Calculated dependence of the power buildup vs effective mirror diameter to achieve a buildup 40000:

ALPS II schedule

Closure of the LINAC tunnel of the European XFEL project under construction at DESY

Using a 532 nm green laser which was aligned inside the beam pipe a central position of a laser beam which will go through the PC and RC mirrors was set. The results proved that it will be possible to achieve a power buildup 40000 with 0,025 m available effective mirror diameter.