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Register a new userA direct experimental limit on neutron -- mirror neutron oscillations
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In case a mirror world with a copy of our ordinary particle spectrum would exist, the neutron n and its degenerate partner, the mirror neutron ${rm n}$, could potentially mix and undergo ${rm nn}$ oscillations. The interaction of an ordinary magnetic field with the ordinary neutron would lift the degeneracy between the mirror partners, diminish the ${rm n}$-amplitude in the n-wavefunction and, thus, suppress its observability. We report an experimental comparison of ultracold neutron storage in a trap with and without superimposed magnetic field. No influence of the magnetic field is found and, assuming negligible mirror magnetic fields, a limit on the oscillation time $tau_{rm nn} > 103$ s (95% C.L.) is derived.
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Present probes do not exclude that the neutron ($n$) oscillation into mirror neutron ($n$), a sterile state exactly degenerate in mass with the neutron, can be a very fast process, in fact faster than the neutron decay itself. This process is sensitive to the magnetic field. Namely, if the mirror magnetic field $vec{B}$ exists at the Earth, $n-n$ oscillation probability can be suppressed or resonantly amplified by the applied magnetic field $vec{B}$, depending on its strength and on the angle $beta$ between $vec{B}$ and $vec{B}$. We present the results of ultra-cold neutron storage measurements aiming to check the anomalies observed in previous experiments which could be a signal for $n-n$ oscillation in the presence of mirror magnetic field $Bsim 0.1$~G. Analyzing the experimental data on neutron loses, we obtain a new lower limit on $n-n$ oscillation time $tau_{nn} > 17$ s (95 % C.L.) for any $B$ between 0.08 and 0.17 G, and $tau_{nn}/sqrt{cosbeta} > 27 $s (95 % C.L.) for any $B$ in the interval ($0.06div0.25$) G.
We performed ultracold neutron (UCN) storage measurements to search for additional losses due to neutron (n) to mirror-neutron (n) oscillations as a function of an applied magnetic field B. In the presence of a mirror magnetic field B, UCN losses would be maximal for B = B. We did not observe any indication for nn oscillations and placed a lower limit on the oscillation time of tau_{nn} > 12.0 s at 95% C.L. for any B between 0 and 12.5 uT.
Mirror matter is considered as a candidate for dark matter. In connection with this an experimental search for neutron - mirror neutron (nn) transitions has been carried out using storage of ultracold neutrons in a trap with different magnetic fields. As a result, a new limit for the neutron - mirror neutron oscillation time has been obtained, tau_osc >= 448 s (90% C.L.), assuming that there is no mirror magnetic field larger than 100 nT. Besides a first attempt to obtain some restriction for mirror magnetic field has been done.
The neutron and its hypothetical mirror counterpart, a sterile state degenerate in mass, could spontaneously mix in a process much faster than the neutron $beta$-decay. Two groups have performed a series of experiments in search of neutron - mirror-neutron ($n-n$) oscillations. They reported no evidence, thereby setting stringent limits on the oscillation time $tau_{nn}$. Later, these data sets have been further analyzed by Berezhiani et al.(2009-2017), and signals, compatible with $n-n$ oscillations in the presence of mirror magnetic fields, have been reported. The Neutron Electric Dipole Moment Collaboration based at the Paul Scherrer Institute performed a new series of experiments to further test these signals. In this paper, we describe and motivate our choice of run configurations with an optimal filling time of $29~$s, storage times of $180~$s and $380~$s, and applied magnetic fields of $10~mu$T and $20~mu$T. The choice of these run configurations ensures a reliable overlap in settings with the previous efforts and also improves the sensitivity to test the signals. We also elaborate on the technique of normalizing the neutron counts, making such a counting experiment at the ultra-cold neutron source at the Paul Scherrer Institute possible. Furthermore, the magnetic field characterization to meet the requirements of this $n-n$ oscillation search is demonstrated. Finally, we show that this effort has a statistical sensitivity comparable to the current leading constraints for $n-n$ oscillations.
It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: $tau_{nn} > 352~$s at $B=0$ (95% C.L.), $tau_{nn} > 6~text{s}$ for all $0.4~mutext{T}<B<25.7~mutext{T}$ (95% C.L.), and $tau_{nn}/sqrt{cosbeta}>9~text{s}$ for all $5.0~mutext{T}<B<25.4~mutext{T}$ (95% C.L.), where $beta$ is the fixed angle between the applied magnetic field and the local mirror magnetic field which is assumed to be bound to the Earth. These new constraints are the best measured so far around $Bsim10~mu$T, and $Bsim20~mu$T.
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