Nowadays researchers are interested not only in extremely heavy particles, which have to be generated by means of large high-energy accelerators; very light particles at the lower end of the energy scale can also point the way toward unknown physics phenomena. The Any Light Particle Search (ALPS) experiment, which is tiny compared to the vast LHC and ILC facilities, is helping DESY researchers to track down such lightweights of the subatomic world.

Looking for WISPs

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), for example SUSY particles, but 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.

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. The mechanism which gives rise to the axion can be generalized to generic (pseudo-) scalars a coupled weakly to two-photons, so-called  axion like particles (ALPs) 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. 

The ALPS I already provides the best constraints which would be even further improved at ALPS II or, even better lead to the discovery of Dark Energy particle candidates in a laboratory.

 

Light-shining-through a wall

Light-shining-through a wall (LSW) experiments search for WISPs. Potential WISP candidates are axion-like particles or hidden sector photons. The ALPS experiment exploits resonant laser power build-up in a large-scale optical cavity to boost the available power for the WISP production. Light, typically from a strong laser, is shone into a magnetic field. Laser photons can be converted into a WISP in front of a light-blocking barrier (production 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.

For master/bachelor/diploma or Ph.D theses in connection with ALPS please contact Axel Lindner.