Any Light Particle Search – ALPS II Natali Kuzkova Ph.D. student, DESY PIER PhD seminar 20 th January, 2015

First and Last Name | Title of Presentation | Date | Page 2 Fundamental questions: physics beyond the Standard Model > Standard Model (SM) of particle physics describes basic properties of known matter and forces. > SM not a complete and fundamental theory:  No satisfactory explanation for values of its many parameters. Why is the neutron dipole moment so tiny? → charge parity (CP) conservation in quantum chromodynamics (QCD);  No quantum theory of gravity;  No explanation of the origin of the dark sector of the universe. What constitutes dark matter and dark energy? > Astrophysical riddles:  Why is the universe transparent to TeV photons?  Why do white dwarfs cool so fast?  Why is there a soft X-ray excess from galaxy clusters? [NASA]

First and Last Name | Title of Presentation | Date | Page 3 Fundamental questions: physics beyond the Standard Model Well-motivated SM extensions provide dark matter candidates:  Neutralinos and other Weakly Interacting Massive Particles (WIMPs);  Axions and other very Weakly Interacting Slim (ultra-light) Particles (WISPs). Axion like particles (ALPs), light spin particles called "hidden sector photons" or light minicharged particles. The axion provides the most elegant solution to the strong CP problem; ALPs are embedded in string theory inspired Standard Model extensions; ALPs, axions (and other WISPs) could explain dark matter; They would be a good explanation for several astrophysical phenomena (TeV transparency, white dwarf cooling). [Kim, Carosi 10]

First and Last Name | Title of Presentation | Date | Page 4 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. 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. [Manuel Meyer, PATRAS Workshop 2011]

First and Last Name | Title of Presentation | Date | Page 5 Light-Shining-Through-a-Wall experiment > How to detect ALPs:  Light from a strong laser is shone into a magnetic field;  Laser photons can be converted into a ALPs (WISPs) in front of a light-blocking barrier (generation region) in a magnetic field and pass the wall;  Behind the wall, the ALPs reconverted into photons back in a magnetic field;  Light is detected by a detector. 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.

First and Last Name | Title of Presentation | Date | Page 6 > The ALPS Collaboration started its first Light-Shining-Through-a-Wall experiment to search for photon oscillations into WISPs in Results were published in 2009 and 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. Present and future of the ALPS

First and Last Name | Title of Presentation | Date | Page 7 Stages of the experiment ALPS I ALPS-IIa ALPS-IIc  ALPS-IIa: with two 10m long production and regeneration cavities, without HERA superconducting dipole magnets;  ALPS-IIc: with two 100m long cavities using magnets. Parameters of the long cavity of ALPS-IIa: 20 m length; Two 750 ppm curved mirrors (250 m ROC); Finesse 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.

First and Last Name | Title of Presentation | Date | Page 8 > 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. Conceptual design > 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 photons is to be achieved.

First and Last Name | Title of Presentation | Date | Page 9 > Schematic of the ALPS-II injection stage including the production cavity (PC): Production and regeneration cavities > Schematic of the ALPS-II regeneration cavity (RC) including control loop:

First and Last Name | Title of Presentation | Date | Page 10 Optical setup of the ALPS II The resonant enhancement of the production and regeneration process is a key feature of the experiment. WISP flux from the production region is increased by a factor equal to the power buildup of the production cavity (PB PC = 5 000) and likewise is the reconversion probability on the right-hand side of the wall enhanced by the power buildup of the regeneration cavity (PB RC = ) when both cavities resonate on the same optical mode.

First and Last Name | Title of Presentation | Date | Page 11 Laser system > 35 W, 1064 nm laser power; > Single mode; > Single frequency; > High intrinsic frequency stability; > Frequency modulation with PZT. An end-pumped laser design was chosen to achieve a well defined Gaussian mode and an efficient amplification with excellent beam quality.

First and Last Name | Title of Presentation | Date | Page 12 Present status of the experiment > 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 with 0,025 m (2,5 cm) available effective mirror diameter. Behind the wall, the regeneration cavity increases the production probability with which photons are created from the axion field by a factor of > Calculated dependence of the power buildup vs effective mirror diameter to achieve a RC buildup 40000:

First and Last Name | Title of Presentation | Date | Page 13 ALPS II in the HERA tunnel Straightening magnets > To 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 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. Such an aperture would limit ALPS II to 8 magnets. ALPS II schedule