The results of a new search for positronium decays into invisible final states are reported. Convincing detection of this decay mode would be a strong evidence for new physics beyond the Standard Model (SM): for example the existence of extra-dimensions, of milli-charged particles, of new light gauge bosons or of mirror particles. Mirror matter could be a relevant dark matter candidate.In this paper the setup and the results of a new experiment are presented. In a collected sample of about (6.31 ± 0.28) × 10 6 orthopositronium decays, no evidence for invisible decays in an energy window [0,80] keV was found and an upper limit on the branching ratio of orthopositronium o − P s → invisible could be set:Our results provide a limit on the photon mirror-photon mixing strength ǫ ≤ 1.55 × 10 −7 (90% C.L.) and rule out particles lighter than the electron mass with a fraction Q x ≤ 3.4 × 10 −5 of the electron charge. Furthermore, upper limits on the branching ratios for the decay of parapositronium Br(p − P s → invisible) ≤ 4.3 × 10 −7 (90% C.L.) and the direct annihilation Br(e + e − → invisible) ≤ 2.1 × 10 −8 (90% C.L.) could be set.

In this paper, we present measurements of the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range 50-400 K. The sample with smaller pore size exhibits a higher minimal energy (73 ± 5 meV), compared to the sample with bigger pores (48 ± 5 meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-Ps being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.

The reemission yield of ortho-positronium (o-Ps) into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40% are found. The intensity of the 142ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2γ and 3γ energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in surfactant-templated mesoporous silica films.

Measurement result and performance parameters are presented for fast neutron detectors exploiting the scintillation of natural helium at high pressure. This detection medium has a very low electron density, minimizing the sensitivity to gamma radiation and thus enabling neutron detection also in high gamma radiation environment. Contrary to proportional counters, scintillation detection enables fast (nanosecond) timing and pulse shape discrimination, a technique that enables a lower neutron detection threshold. In this work, the basic principles of the detector are described, followed by a study of gamma rejection capabilities. Methods to calibrate the detector are discussed. Finally, a brief description of a 4 He scintillation based detector system including data acquisition electronics is given.

Shielding, coincidence, and time-of-flight measurement techniques are employed to tag fast neutrons emitted from an (241)Am/(9)Be source resulting in a continuous polychromatic energy-tagged beam of neutrons with energies up to 7MeV. The measured energy structure of the beam agrees qualitatively with both previous measurements and theoretical calculations.

The understanding of the origin of dark matter has great importance for cosmology and particle physics. Several interesting extensions of the standard model dealing with solution of this problem motivate the concept of hidden sectors consisting of SU (3) C × SU (2) L × U (1) Y singlet fields. Among these models, the mirror matter model is certainly one of the most interesting. The model explains the origin of parity violation in weak interactions, it could also explain the baryon asymmetry of the Universe and provide a natural ground for the explanation of dark matter. The mirror matter could have a portal to our world through photon-mirror photon mixing (ε). This mixing would lead to orthopositronium (o − Ps) to mirror orthopositronium oscillations, the experimental signature of which is the apparently invisible decay of o − Ps. In this paper, we describe an experiment to search for the decay o − Ps → invisible in vacuum by using a pulsed slow positron beam and a massive 4π BGO crystal calorimeter. The developed high efficiency positron tagging system, the low calorimeter energy threshold and high hermiticity allow the expected sensitivity in mixing strength to be ε ≃ 10 −9 , which is more than one order of magnitude below the current Big Bang Nucleosynthesis limit and in a region of parameter space of great theoretical and phenomenological interest. The vacuum experiment with such sensitivity is particularly timely in light of the recent DAMA/LIBRA observations of the annual modulation signal consistent with a mirror type dark matter interpretation.

Depth profiling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex-templated SiO 2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that ortho-positronium (o-Ps) escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps re-emission yield decreases from ∼0.25 to ∼0.11 as the pore size increases from 32 to 75 nm. The o-Ps re-emission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9.1 ± 0.4 g cm −2 . For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2) ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1.4 ± 4 nm) and from macropores with large effective size (∼32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to 7 2 be 19 ± 2 ns. Our results imply that o-Ps efficiently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores becomes small), this capture into the micropores assists the capture into the macropores.

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