Andreas Knecht, Marcin Kuzniak, Paul Scherrer Institut
The idea of a mirror world consisting of mirror particles – even mirror planets and mirror stars – has inspired many fantasies and works of fiction. However, the roots of this idea are standing on a solid particle physical model, which goes back to 1956 and the work of Lee and Yang  on the possible violation of mirror symmetry (also called parity) and its discovery only a year later by Wu et al  and Garwin et al . This violation is manifest in the weak force, which interacts only with left-handed particles but cannot couple to right-handed ones. Already then, Lee and Yang proposed the concept of introducing mirror particles with right-handed interaction. In this broader sense, the breaking of mirror symmetry could be restored.
As each of the two worlds has its own force mediating particles for the electromagnetic, strong and weak interaction, the only way for any interplay is due to gravity. Actually it turns out that mirror matter is a viable candidate for the explanation of the Dark Matter puzzle. Apart from gravitational interactions, subtle effects could lead to oscillations between neutral particles from the two worlds such as photon to mirror photon, neutrino to mirror neutrino or neutron to mirror neutron. Experimentally investigated are the possibility of photon to mirror photon oscillations by a dedicated effort  and neutrino to mirror neutrino by many neutrino-oscillation searches. Recently neutron to mirror neutron oscillations received a lot of interest, as they could possibly also provide a mechanism for cosmic rays to arrive at the earth with highest energies .
On a very short time scale, two experiments have been performed at the Institut Laue-Langevin (Grenoble, France) to search for such oscillations and to set a lower limit on the oscillation time τnn’ in the case of its nonobservation. These measurements used their respective apparatus built for a Neutron Electric Dipole Moment experiment. The neutrons, that one uses, are so called ultra cold neutrons (UCNs). The kinetic energy of UCNs is so low (in the order of 100 neV, corresponding to mK - hence the name) that they are totally reflected by certain materials and can therefore be stored for hundreds of seconds. The experimental principle requires a measurement under two different conditions. In both, one fills UCNs into the storage chamber, stores them for a given time and counts afterwards the remaining UCNs in a detector. In one case however, one applies a moderate magnetic field (a few μT) over the storage chamber. In the other case, the 4 layer magnetic shield is demagnetised and the magnetic field generating coil switched off, thus effectively creating a zero field region (in the order of 10 nT) at the storage chamber.
The neutron couples to the magnetic field B due to its magnetic moment μ, thereby changing its energy by ±μB. The mirror neutron cannot couple to the ordinary magnetic field and it is assumed that no mirror magnetic field is present at the site of the experiment. In the case of an applied field over the storage chamber, the oscillation from neutron to mirror neutron is suppressed due to the separation of their energy levels, whereas in the zero field setting the oscillation can take place (see Fig. 1). The relevant time scale for the oscillation to happen is the free flight time tf of the UCNs between wall collisions as every wall collision probes whether an oscillation has taken place. If such oscillations occurred the mirror neutrons would leave the storage chamber resulting in a decreased UCN count at the end of the storage time. Due to the fact that many wall collisions take place during storage, it is possible to search for oscillation times τnn’ much larger than tf.
In the analysis, one calculates the ratio of average counts for field off and field on. Any reduction of this ratio below one could be an indication of neutron to mirror neutron oscillations. No deviation from one within the error had been observed in the two experiments. The first experiment set a lower limit on the oscillation time at 95% confidence level (C.L.) of
τnn’ > 103 s (95% C.L.) 
Only a few weeks later, the second experiment reported a limit at 90% C.L. of
τnn’ > 414 s (90% C.L.) 
using a 10 times larger storage chamber. The use of a more powerful UCN source (such as being constructed at the Paul Scherrer Institut (Villigen, Switzerland) ) combined with a large storage chamber would allow for measurements in the order of 103 s. A dedicated effort and the construction of a new apparatus is required to reach 104 s.
 T. D. Lee and C. N. Yang. Phys. Rev. 104, no. 1, 254 (1956).
 C. S. Wu et al. Phys. Rev. 105, no. 4, 1413 (1957).
 R. L. Garwin, L. M. Lederman and M. Weinrich. Phys. Rev. 105, no. 4, 1415 (1957).
 A. Badertscher et al. Phys. Rev. D 75, 032004 (2007).
 Z. Berezhiani and L. Bento. Phys. Rev. Lett. 96, 081801 (2006).
 G. Ban et al. Phys. Rev. Lett. 99, 161603 (2007).
 A. P. Serebrov et al. arXiv 0706.3600 (2007).
 ucn.web.psi.ch .
[Released: January 2008]