Direct Experimental Limit on Neutron–Mirror-Neutron Oscillations

Abstract

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 n ′ , could potentially mix and undergo nn ′ oscillations. The interaction of an ordinary magnetic field with the ordinary neutron would lift the degeneracy between the mirror partners, diminish the 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 τ nn ′ > 103 s (95% C.L.) is derived.PACS numbers: 11.30. Er, 11.30.Fs, 14.20.Dh, The concept of a mirror world, as an attempt to restore global parity symmetry, has attracted interest since the 1950's, started by the famous paper of Lee and Yang [1] and significantly expanded in the work of Kobzarev, Okun, and Pomeranchuk [2]. The mirror matter idea was first applied to the Standard Model of particle physics in [3]. More recent overviews can be found in [4,5]. The mirror world could hold a copy of the particle spectrum of our ordinary world. Matter and mirror matter would interact via gravity and present a viable explanation to the dark matter problem [6,7,8,9,10]. Besides gravity, other (new) interactions could show up in minute mixings of neutral matter particles -such as neutrinos, pions, kaons, or positronium (see [11] for e + e − ) -and their degenerate mirror partners making oscillations between them possible. Recently it was pointed out [12] that no direct experimental limits exist on the oscillation time τ nn ′ [13] between ordinary matter neutrons (n) and the speculative mirror neutrons (n ′ ). An indirect limit of the order τ nn ′ ≥ 1 s has been derived in [12] based on the search for neutron -antineutron (nn) oscillations [14]. Fast nn ′ oscillations with τ nn ′ ∼ 1 s, or at least much shorter than the neutron β-decay lifetime, could explain [12,15] the origin of ultra-high energy cosmic rays above the Greisen-Zatsepin-Kuzmin (GZK) cutoff [16,17]. The viability of models and implications have been further discussed in [18].Possible approaches to nn ′ oscillation experiments with sensitivities of several hundred seconds have been discussed in [19]. One approach is to search for nn ′ oscillations by comparing the storage of ultracold neutrons (UCN) in vacuum in a trap in the presence and the absence, respectively, of a magnetic field. The essential idea is that the neutron and mirror neutron states would be degenerate in the absence of a magnetic field and nn ′ transitions could occur. (The absence of mirror magnetic fields at the location of the experiment is assumed throughout this paper [20].) The interaction of the neutron with a magnetic field would lift the degeneracy and suppress the transition into a mirror neutron which, of course, does not interact with the ordinary magnetic field, nor with the trap via the ordinary strong interaction. Thus...

Comparison of ultracold neutron sources for fundamental physics measurements

“…The emergent discussion for dark matter and dark energy triggered further use of UCNs to search for exotic physics beyond the Standard Model of particle physics, like searches for mirror matter [34][35][36], for Lorentz violation effects [37], for exotic interactions induced by axionlike particles [38][39][40][41][42], dark matter [43,44], or probing dark energy [45][46][47]. The sensitivities of all such UCN experiments depend directly on the total UCN statistics available in the measurement, hence on high UCN densities in sizeable storage vessels.…”

The Mirror Symmetry of Matter and Antimatter

“…There have been proposals that "mirror matter", which is related to matter by the conventional P operator, may exist after all [7]. Some evidence seems to be consistent with this theory [8], but recent neutron experiments have yielded null results [9,10]. While it is impossible to completely rule out the existence of a particle with particular physical properties, the existence of such particles is not a correct prediction of the hypothesis of mirror symmetry.…”

Cold and Ultra-cold Neutrons as Probes of New Physics

“…The best limits for nn oscillations stem from bound neutrons inside nuclei [25,26]. Present limits for nn and nn oscillations from free neutrons are from 1994 [27] respectively the late 2000s [28,29,30]. New experiments are proposed at the SNS [31], PNPI [32], and at the ESS [14,33].…”