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The idea of a hidden sector of mirror partners of elementary particles has attracted considerable interest as a possible candidate for dark matter.Recently it was pointed out by Berezhiani and Bento that the present experimental data cannot exclude the possibility of a rapid oscillation of the neutron n to a mirror neutron n′ with oscillation time much smaller than the neutron lifetime. A dedicated search for vacuum transitions n → n′ has to be performed at weak magnetic field, where both states are degenerate. We report the result of our experiment, which compares rates of ultracold neutrons after storage at a weak magnetic field well below 20 nT and at a magnetic field strong enough to suppress the seeked transitions. We obtain a new limit for the oscillation time of n-n' transitions, τ osc (90% C.L.) > 414 s.The corresponding limit for the mixing energy of the normal and mirror neutron states is δm (90% C.L.) < 1.5×10 -18 eV.

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Our experiment using gravitationally trapped ultracold neutrons (UCN) to measure the neutron lifetime is reviewed. Ultracold neutrons were trapped in a material bottle covered with perfluoropolyether. The neutron lifetime was deduced from comparison of UCN losses in the traps with different surface-to-volume ratios. The precise value of the neutron lifetime is of fundamental importance to particle physics and cosmology. In this experiment, the UCN storage time is brought closer to the neutron lifetime than in any experiments before: the probability of UCN losses from the trap was only 1% of that for neutron β decay. The neutron lifetime obtained, 878.5 ± 0.7 stat ± 0.3 sys s, is the most accurate experimental measurement to date.A. P. SEREBROV et al. PHYSICAL REVIEW C 78, 035505 (2008) 035505-3 A. P. SEREBROV et al. PHYSICAL REVIEW C 78, 035505 (2008)

Neutron lifetime is one of the most important physical constants which determines parameters of the weak interaction and predictions of primordial nucleosynthesis theory. There remains the unsolved problem of a 3.9σ discrepancy between measurements of this lifetime using neutrons in beams and those with stored neutrons (UCN). In our experiment we measure the lifetime of neutrons trapped by Earth's gravity in an open-topped vessel. Two configurations of the trap geometry are used to change the mean frequency of UCN collisions with the surfacesthis is achieved by plunging an additional surface into the trap without breaking the vacuum. The trap walls are coated with a hydrogen-less fluorine-containing polymer to reduce losses of UCN. The stability of this coating to multiple thermal cycles between 80 K and 300 K was tested. At 80 K, the probability of UCN loss due to collisions with the trap walls is just 1.5% of the probability of beta-decay. The free neutron lifetime is determined by extrapolation to an infinitely large trap with zero collision frequency. The result of these measurements is which is consistent with the conventional value of 880.2±1.0s presented by the Particle Data Group. Future prospects for this experiment are in further cooling to 10 K which will lead to an improved accuracy of measurement. In conclusion we present an analysis of currently-available data on various measurements of the neutron lifetime.

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We report Neutrino-4 experiment results of measurements of reactor antineutrinos flux and spectrum dependence on the distance in range 6-12 meters from the center of the reactor core. The fit of experimental dependence with the law 1 L 2 ⁄ , where L is the distance from the reactor center, gave satisfactory result with goodness of fit 81%. However, we discovered that the experimental neutrino spectrum is different from the calculated one. Using experimental spectrum, we performed the model independent analysis of restrictions on oscillation parameters ∆m 14 2 and sin 2 2 14 . The results of this analysis exclude area of reactor and gallium anomaly at CL more than 99.7% (> 3 ) for values ∆m 14 2 < 3eV 2 and sin 2 2θ 14 > 0.1 However, we observed an oscillation effect at CL 2.8 in vicinity of ∆m 14 2 ≈ 7.34eV 2 and sin 2 2θ 14 ≈ 0.39. The method of coherent addition of results of measurements, which allows us to directly observe the effect of oscillations, is proposed. The analysis of that effect is presented. In general, it seems that the effect predicted in gallium and reactor experiments is confirmed but at sufficiently large value of ∆m 14 2 . Future prospects of the experiment are discussed.