Any Light Particle Search II - Status Overview
AAny Light Particle Search II - Status Overview
No¨emie Bastidon for the ALPS II collaboration,
University of Hamburg, Hamburg, Germany
DOI: w ill be assigned The Any Light Particle Search II (ALPS II) experiment (DESY, Hamburg) searches forphoton oscillations into Weakly Interacting Sub-eV Particles (WISPs). This second gen-eration of the ALPS light-shining-through-a-wall (LSW) experiment approaches the final-ization of the preparation phase before ALPS IIa (search for hidden photons). In the lastyears, efforts have been put for the setting up of two optical cavities as well as charac-terization of a single-photon Transition-Edge Sensor (TES) detector. In the following, weput some emphasis on the detector development. In parallel, the setting up of ALPS IIc(search for axion-like particles), including the unbending of 20 HERA dipoles, has beenpursued. The latest progress in these tasks will be discussed.
The Any Light Particle Search II (ALPS II) experiment (DESY, Hamburg) searches for photonoscillations into light fundamental bosons (e.g., axion-like particles, hidden photons and otherWISPs) by shining light through a wall [1]. The aimed sensitivity increase for the couplingstrength of axion-like particles to photons of the experiment is of 3000 compared to ALPS I.Such improvements are due to the increase of the magnets length, to two optical cavities as wellas to the replacement of the single-photon detector. Indeed, the ALPS experiment sensitivityto the conversion of photons into axion-like particles depends on various parameters and isexpressed as S ( g aµ ) ∝ ( 1 BL )( DCT ) ( 1 η ˙ N Pr β PC β RC ) with a strong dependency on the magnetic length L and field B . The effect of the opticalsetup depends on ˙ N Pr , the number of injected photons as well as on β PC and β RC , the powerbuild-ups of the production (PC) and regeneration cavities (RC). Finally, the reached sensitivitydepends on the chosen detector’s detection efficiency η and dark current ( DC ). The data-takingtime is expressed as T . In the last years, preparation work has demonstrated the basics of thesetup. The ALPS IIa (search for hidden photons) optical setup includes two 10 meters optical cavitiesseparated by a ligth-tight barrier. A 30 W 1064 nm laser is injected inside the first cavity (Fig.
Patras 2015 a r X i v : . [ phy s i c s . i n s - d e t ] S e p igure 1: ALPS IIa experiment.1). Such a system is technically challenging for two reasons: First, an alignment of both cavitiestowards each other is necessary to provide a larger spatial overlap of the modes resonating inboth cavities. Second, high power buildups (PB) are required for both cavities in order toreach the ALPS IIa foreseen sensitivity. The aimed PB of the production cavity is of 5 000and the regeneration cavity PB is of 40 000. In order to maximise this characteristic, the PCand RC need to be in the same modal phase with a mode-overlap of 95 %. The regenerationcavity is locked via an auxiliary green beam obtained via second harmonic generation (KTPcrystal) of the PC infrared beam [2]. Latest tests showed a lower PB than required for theproduction cavity. Possible sources of such issues are the mirrors coating, cleanliness of themirrors, alignment of the cavity as well as a clipping in the beam pipes. Usage of a cavityring-down technique demonstrated a good quality of the mirrors [3]. Measurements will berepeated with a larger beam radius in order to enlarge the tested region on the mirrors surface. The regeneration cavity will be connected via a fiber to a single-photon detector in order todetect possible regenerated photons. Efficient coupling of a 4 .
23 mm beam inside a 8 . µ msingle-mode fiber is feasible but still needs to be demonstrated to remain stable over longertime-scales.The coupling of the beam inside a fiber setup includes two mirrors as well as an asphericlens (Fig. 2). In the test setup, a class 1 λ = 1064 nm laser is shone to a mirror setup beforebeing focused inside a standard single-mode fiber. It has been shown that the efficiency of thecoupling depends highly on the alignement of the setup and on the focal length of the usedlens (Fig. 2). During the preliminary tests, an efficiency higher than 80 % was reached. Thehighest value for the final setup which has been currently obtained is of 53 % for a focal lengthof 35 mm. This value is lower than what was expected for such a lens. In near future, thebeam quality will be studied with a knife-edge unit. Such a device allows characterization andadjustement of the beam on micrometer-scale before it enters the fiber. The detection of a low rate (one event every few hours) of low energetic (1.17 eV) photonsrequires both, a high detection efficiency as well as a dark count rate. Additionally, the ALPS2
Patras 2015 igure 2: Coupling of the beam. On the left, a drawing of the coupling of the beam test setup.On the right, the theoritical efficiency of the coupling values η for different levels of alignment∆ x tot and for different focal length f.II detection system is required to have a good energy and time resolution as well as a good long-term stability. To meet all of these criteria, the ALPS II setup includes a cryogenic detectorof the transition edge type (TES) developed by NIST (National Institute of Standard andTechnology) [4].Transition-Edge Sensors are superconductive microcalorimeters measuring the temperaturedifference ∆T induced by the absorption of a photon. The positioning of the detector withinits superconductive transition (30% of its normal resistance) is induced through a thermal linkto a heat bath at T b = 80 mK and through a constant bias voltage. In order to obtain thecool-down of the detector, it is placed in an adiabatic demagnetization refrigerator (ADR) [5].The ALPS detector module includes two TESs inductively coupled to a SQUID (Super-conducting Quantum Interference Device). The ALPS detectors are optimized for 1064 nmphotons. The sensitive area of each chip measures 25 x 25 µm for a thickness of 20 nm. Thesubstrate is surrounded by a standard fiber ceramic sleeve allowing connection of a single modefiber ferrule [6].NIST has demonstrated that such a detector can reach quantum efficiency higher than 95 %[7]. Latest measurements of the ALPS II detector efficiency led to a first approximation of30 %. Optimization work is currently under progress. The ALPS IIc experiment will allow the search for axion-like particles (ALPs). It is constitutedin the same way as ALPS IIa with two 100 meters cavity and the addition of 20 HERA (Hadron-Electron Ring Accelerator) dipoles [1] to allow the conversion of photons into ALPs and re-conversion. The HERA dipoles were all bent during their design, leading to a small aperture
Patras 2015
3f 35 mm. It was foreseen to unbend all of the dipoles by applying a force in their middle (coldmass). The deformation of the first magnet was successful, yielding to an aperture of 50 mmallowing to set up the 100 m long cavities without any aperture limitations. The magnet isworking according to its specifications with a slight increase of its quench current. Effort tostraighten further magnets are on-going.
The ALPS II experiment aims at an improvement of sensitivity by a factor of 3 000 comparedto ALPS I for the coupling of axion-like particles to photons. This improvement is achievedmainly by implementing a regeneration cavity and a larger magnetic length. Basics of theoptics setup have been demonstrated but not all of the specifications have been reached yet. ATungsten Transition-Edge Sensor operated below 100 mK has been successfully used to detectsingle-photons in the near-infrared.
The author would like to thank all the members of the ALPS collaboration. The author alsothanks the PIER Helmholtz Graduate School for their financial travel support.
References [1] R. B¨ahre et al. , “Any light particle search II Technical Design Report,” JINST , (2013) [arXiv:1302.5647v2[hep-ex]].[2] R. Hodajerdi, “Production Cavity and Central Optics for a Light Shining through a Wall Experiment,”ISBN 1435-8085 (2015)[3] T. Isogai et al. , “Loss in long-storage-time optical cavities,” Optics Express , 24 (2013) [arXiv:1310.1820v2 [hep-ex]][4] N. Bastidon, D. Horns, A. Lindner, “Characterization of a Transition-Edge Sensor for the ALPS II Exper-iment,” These proceedings, ISBN 978-3-935702-99-7 (2015)[5] G. K. White, P. J. Meeson, “Experimental techniques in low-temperature physics,” Fourth Edition (2002).[6] J. Dreyling-Eschweiler et al. , “Characterization, 1064 nm photon signals and background events of a tung-sten TES detector for the ALPS experiment,” J. Mod. Opt. , 14 (2005) [arXiv:1502.07878 [hep-ex]].[7] A. E. Lita, A. J. Miller and S. W. Nam, “Counting near-infrared single-photons with 95% efficiency,” Opticsexpress , 5 (2008).4