A low background Micromegas detector has been operating in the CAST experiment at CERN for the search of solar axions during the first phase of the experiment (2002-2004). The detector, made out of low radioactivity materials, operated efficiently and achieved a very low level of background rejection (5 x 10^-5 counts/keV/cm^2/s) without shielding.

P. Abbon S. Andriamonje S. Aune T. Dafni M. Davenport E. Delagnes R. de Oliveira G. Fanourakis E. Ferrer Ribas J. Franz T. Geralis M. Gros Y. Giomataris I. G. Irastorza K. Kousouris J. Morales T. Papaevangelou J. Ruz K. Zachariadou K. Zioutas

We present a brief overview of the ongoing searches for the axion particle via its coupling to photons. Both the classical QCD axions and more recently proposed Axion-Like-Particles are considered. Astrophysical bounds on the axion-photon coupling come from considerations of stellar energy loss during Helium burning, in both low- and high-mass stars. Helioscopes look for back-conversion of solar axions into x-ray photons in strong laboratory magnetic fields. Finally, haloscopes aim to detect dark matter axions in our galactic halo. Both types of searches are expecting significant advances in the future, which will enable them to probe large, well-motivated parts of the parameter space below the stellar cooling bounds.

G. Carosi A. Friedland M. Giannotti M. J. Pivovaroff J. Ruz J. K. Vogel

Curved thin sheets are ubiquitously found in nature and manmade structures. Within the framework of classical thin plate theory, the stiffness of thin sheets is independent of its bending state. This assumption, however, goes against intuition. Simple experiments with a cantilever sheet made of paper show that the cantilever stiffness largely increases with the transversal curvature. We here demonstrate by using simple geometric arguments that thin sheets subject to two-dimensional bending necessarily develop internal stresses. The coupling between the internal stresses and the bending moments can increase the stiffness of the plate by several times. We develop a theory that describes the stiffness of curved thin sheets with simple equations in terms of the longitudinal and transversal curvatures. The theory perfectly fits experimental results with a macroscopic cantilever sheet as well as numerical simulations by the finite element method. The results shed new light on plant and insect wing biomechanics and provide an easy route to engineer micro- and nanomechanical structures based on ultrathin materials with extraordinary stiffness tunability.

V. Pini J. J. Ruz P. M. Kosaka O. Malvar M. Calleja J. Tamayo

Dark (hidden) photons are widely recognised as well motivated candidates for physics beyond the standard model, and have been invoked for the solution of several outstanding problems, including to account for the dark matter in the universe. In this paper, we consider a simple model for dark photons, which is coupled to ordinary matter only through kinetic mixing with ordinary photons. Within this framework, we calculate the flux of solar dark photons on Earth and revise the potential to detect it with the next generation of axion helioscopes, particularly with the International AXion Observatory (IAXO). This paper extends on previous theoretical analyses in two main ways. Firstly, it includes a more complete analysis of the possible sources of dark photons from the sun, including the contribution of the solar magnetic field and of nuclear processes, and secondly it includes predictions on the parameter space accessible in the gas-filled phase of IAXO.

T. O'Shea M. Giannotti I. G. Irastorza L. M. Plasencia J. Redondo J. Ruz J. K. Vogel

A finite axion-nucleon coupling, nearly unavoidable for QCD axions, leads to the production of axions via the thermal excitation and subsequent de-excitation of Fe-57 isotopes in the sun. We revise the solar bound on this flux adopting the up to date emission rate, and investigate the sensitivity of the proposed International Axion Observatory IAXO and its intermediate stage BabyIAXO to detect these axions. We compare different realistic experimental options and discuss the model dependence of the signal. Already BabyIAXO has sensitivity far beyond previous solar axion searches via the nucleon coupling and IAXO can improve on this by more than an order of magnitude.

Luca Di Luzio Javier Galan Maurizio Giannotti Igor G. Irastorza Joerg Jaeckel Axel Lindner Jaime Ruz Uwe Schneekloth Lukas Sohl Lennert J. Thormaehlen Julia K. Vogel

Noether symmetry of F(R) theory of gravity in vacuum or in matter dominated era yields three-half power law of R. We show that this particular curvature invariant term is very special in the context of isotropic and homogeneous cosmological model as it makes the first fundamental form cyclic. As a result, it allows a unique power law solution, typical for this particular fourth order theory of gravity, both in the vacuum and in the matter dominated era. This power law solution has been found to be quite good to explain the early stage but not so special and useful to explain the late stage of cosmological evolution. The usefulness of Palatini variational technique in this regard has also been discussed.

Kaushik Sarkar Nayem Sk. Soumendranath Ruz Subhra Debnath Abhik Kumar Sanyal

The International Axion Observatory (IAXO) is a new generation axion helioscope aiming at a sensitivity to the axion-photon coupling of a few 10$^{12}$ GeV$^{-1}$, i.e. 1 - 1.5 orders of magnitude beyond the one currently achieved by CAST. The project relies on improvements in magnetic field volume together with extensive use of x-ray focusing optics and low background detectors, innovations already successfully tested in CAST. Additional physics cases of IAXO could include the detection of electron-coupled axions invoked to solve the white dwarfs anomaly, relic axions, and a large variety of more generic axion-like particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics. This contribution is a summary of our paper [1] to which we refer for further details.

I. G. Irastorza F. T. Avignone G. Cantatore S. Caspi J. M. Carmona T. Dafni M. Davenport A. Dudarev G. Fanourakis E. Ferrer-Ribas J. Galan J. A. Garcia T. Geralis I. Giomataris S. Gninenko H. Gomez D. H. H. Hoffmann F. J. Iguaz K. Jakovcic M. Krcmar B. Lakic G. Luzon A. Lindner M. Pivovaroff T. Papaevangelou G. Raffelt J. Redondo A. Rodrıguez S. Russenschuck J. Ruz I. Shilon H. Ten Kate A. Tomas S. Troitsky K. van Bibber J. A. Villar J. Vogel L. Walckiers K. Zioutas

Solar axions could be converted into x-rays inside the strong magnetic field of an axion helioscope, triggering the detection of this elusive particle. Low background x-ray detectors are an essential component for the sensitivity of these searches. We report on the latest developments of the Micromegas detectors for the CERN Axion Solar Telescope (CAST), including technological pathfinder activities for the future International Axion Observatory (IAXO). The use of low background techniques and the application of discrimination algorithms based on the high granularity of the readout have led to background levels below 10$^{-6}$ counts/keV/cm$^2$/s, more than a factor 100 lower than the first generation of Micromegas detectors. The best levels achieved at the Canfranc Underground Laboratory (LSC) are as low as 10$^{-7}$ counts/keV/cm$^2$/s, showing good prospects for the application of this technology in IAXO. The current background model, based on underground and surface measurements, is presented, as well as the strategies to further reduce the background level. Finally, we will describe the R&D paths to achieve sub-keV energy thresholds, which could broaden the physics case of axion helioscopes.

J. G. Garza S. Aune D. Calvet J. F. Castel F. E. Christensen T. Dafni M. Davenport T. Decker E. Ferrer-Ribas J. Galán J. A. García I. Giomataris R. M. Hill F. J. Iguaz I. G. Irastorza A. C. Jakobsen D. Jourde H. Mirallas I. Ortega T. Papaevangelou M. J. Pivovaroff J. Ruz A. Tomás T. Vafeiadis J. K. Vogel

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of $^{235}$U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

V. Geppert-Kleinrath F. Tovesson J. S. Barrett N. S. Bowden J. Bundgaard R. J. Casperson D. A. Cebra T. Classen M. Cunningham D. L. Duke J. Gearhart U. Greife E. Guardincerri C. Hagmann M. Heffner D. Hensle D. Higgins L. D. Isenhower J. King J. L. Klay W. Loveland J. A. Magee B. Manning M. P. Mendenhall J. Ruz S. Sangiorgio K. T. Schmitt B. Seilhan L. Snyder A. C. Tate R. S. Towell N. Walsh S. Watson L. Yao W. Younes H. Leeb

Gas time projection chambers (TPCs) with Micromegas pixelated readouts are being used in dark matter searches and other rare event searches, due to their potential in terms of low background levels, energy and spatial resolution, gain, and operational stability. Moreover, these detectors can provide precious features,such as topological information, allowing for event directionality and powerful signal-background discrimination. The Micromegas technology of the microbulk type is particularly suited to low-background applications and is being exploited by detectors for CAST and IAXO (solar axions) and TREX-DM (low-mass WIMPs) experiments. Challenges for the future include reducing intrinsic background levels, reaching lower energy detection levels, and technical issues such as robustness of detector, new design choices, novel gas mixtures and operation points, scaling up to larger detector sizes, handling large readout granularity, etc. We report on the status and prospects of the development ongoing in the context of IAXO and TREX-DM experiments, pointing to promising perspectives for the use of Micromegas detectors in directdark matter searches

K. Altenmueller . Antolin D. Calvet F. R. Candon J. Castel S. Cebrian C. Cogollos T. Dafni D. Diez Ibanez E. Ferrer-Ribas J. Galan J. A. Garcia H. Gomez Y. Gu A. Ezquerro I. G Irastorza G. Luzon C. Margalejo H. Mirallas L. Obis A. Ortiz de Solorzano T. Papaevangelou O. Perez E. Picatoste J. Porron M. J. Puyuelo A. Quintana E. Ruiz-Choliz J. Ruz J. Vogel