The lowest stationary quantum state of neutrons in the Earth's gravitational field is identified in the measurement of neutron transmission between a horizontal mirror on the bottom and an absorber/scatterer on top. Such an assembly is not transparent for neutrons if the absorber height is smaller than the ''height'' of the lowest quantum state.

We present a new constraint for the axion monopole-dipole coupling in the range of 1 µm -a few mm, previously unavailable for experimental study. The constraint was obtained using our recent results on the observation of neutron quantum states in the Earth's gravitational field. We exploit the ultimate sensitivity of ultra-cold neutrons (UCN) in the lowest gravitational states above a material surface to any additional interaction between the UCN and the matter, if the characteristic interaction range is within the mentioned domain. In particular, we find that the upper limit for the axion monopole-dipole coupling constant is gpgs/( c) < 2 10 −15 for the axion mass in the "promising" axion mass region MA ∼ 1 meV.A vanishing value for the neutron electric dipole moment motivated the introduction of hypothetical light (pseudo) scalar bosons (commonly called axions), as an extension of the Standard Model [1,2,3,4]. According to the suggested theories the axion mass could be in the range of 10 −6 < M A < 10 −1 eV, while its coupling to photons, leptons and nucleons is not fixed by the existing models (though it is extremely weak). Following the theoretical predictions mentioned intensive searches for axions have been performed over recent decades. These studies include testing the astrophysical consequences of the axion theories, QED effects (axion-two photon coupling) and macroscopic forces (spin-matter coupling). They put severe constraints on axion-matter coupling in different axion mass ranges. A detailed review of axion studies can be found in [5,6].The recently reported positive results of the PVLAS experiment on light polarization rotation in a vacuum in the presence of a transverse magnetic field [7] may be seen as evidence of the long sought axion [8]. According to [7], the mass of neutral boson possibly responsible for the observed signal is 1 < M A < 1.5 meV.The value of the axion-photon coupling strength obtained from the PVLAS experiment is in contradiction with recent CAST observations [9]. Several ideas have been discussed recently, in [10,11], capable of explaining this discrepancy.This intriguing result makes it particularly important to carry out independent testing on the axion-matter coupling in the corresponding distance range of 130 < λ < 200 µm.In the present Letter we report on constraints for axion monopole-dipole coupling. Such coupling results in a spin-matter CP violating Yukawa-type interaction potential [12]between spin and matter, where g p g s is the product of couplings at the scalar and polarized vertices and λ is the force range. Here r is the distance between a neutron and a nucleus, n = r/r is a unitary vector, and m is the nucleon mass. Only a few experiments for distances below 100 mm have placed upper limits on the product coupling in a system of magnetized media and test masses [6]. One experiment [13] had peak sensitivity at ∼ 100 mm and two other ones [14,15] had peak sensitivity at ∼ 10 mm.The constraint for the g s g p presented in this article is competitive in the dista...

We study quantum reflection of antihydrogen atoms from matter slabs due to the van der Waals/Casimir-Polder (vdW/CP) potential. By taking into account the specificities of antihydrogen and the optical properties and width of the slabs we calculate realistic estimates for the potential and quantum reflection amplitudes. Next we discuss the paradoxical result of larger reflection coefficients estimated for weaker potentials in terms of the Schwarzian derivative. We analyze the limiting case of reflections at small energies, which are characterized by a scattering length and have interesting applications for trapping and guiding antihydrogen using material walls.

Quantum states in the Earth's gravitational field were observed, when ultra-cold neutrons fall under gravity. The experimental results can be described by the quantum mechanical scattering model as it is presented here. We also discuss other geometries of the experimental setup which correspond to the absence or the reversion of gravity. Since our quantum mechanical model describes, particularly, the experimentally realized situation of reversed gravity quantitatively, we can practically rule out alternative explanations of the quantum states in terms of pure confinement effects.

The GBAR project (Gravitational Behaviour of Anti hydrogen at Rest) at CERN, aims to measure the free fall acceleration of ultracold neutral anti hydrogen atoms in the terrestrial gravitational field. The experiment consists preparing anti hydrogen ions (one antiproton and two positrons) and sympathetically cooling them with Be + ions to less than P. Pérez et al.10 μK. The ultracold ions will then be photo-ionized just above threshold, and the free fall time over a known distance measured. We will describe the project, the accuracy that can be reached by standard techniques, and discuss a possible improvement to reduce the vertical velocity spread.