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The observation of neutrons turning into antineutrons would constitute a discovery of fundamental importance for particle physics and cosmology. Observing the n−n transition would show that baryon number (B) is violated by two units and that matter containing neutrons is unstable. It would provide a clue to how the matter in our universe might have evolved from the B = 0 early universe. If seen at rates observable in foreseeable next-generation experiments, it might well help us understand the observed baryon asymmetry of the universe. A demonstration of the violation of B − L by 2 units would have a profound impact on our understanding of phenomena beyond the Standard Model of particle physics.Slow neutrons have kinetic energies of a few meV. By exploiting new slow neutron sources and optics technology developed for materials research, an optimized search for oscillations using free neutrons from a slow neutron moderator could improve existing limits on the free oscillation probability by at least three orders of magnitude. Such an experiment would deliver a slow neutron beam through a magnetically-shielded vacuum chamber to a thin annihilation target surrounded by a low-background antineutron annihilation detector. Antineutron annihilation in a target downstream of a free neutron beam is such a spectacular experimental signature that an essentially background-free search is possible. An authentic positive signal can be extinguished by a very small change in the ambient magnetic field in such an experiment. It is also possible to improve the sensitivity of neutron oscillation searches in nuclei using large underground detectors built mainly to search for proton decay and detect neutrinos. This paper summarizes the relevant theoretical developments, outlines some ideas to improve experimental searches for free neutron oscillations, and suggests avenues both for theoretical investigation and for future improvement in the experimental sensitivity.

We have measured invariant mass spectra of electron-positron pairs in the target rapidity region of 12GeV p+A reactions. We have observed a significant difference in the mass spectra below the ω meson between p+C and p+Cu interactions. The difference is interpreted as a signature of the ρ/ω * Present Address: Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan, email: ozawa@cns.s.u-tokyo.ac.jp † Present Address: ICEPP, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan ‡ Present Address: Physics Department, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan § Present Address: Fujitsu Corporation, 4-1-1, Kamikodanaka, Nakahara, Kawasaki, Kanagawa 211-8588, Japan * * Present Address: RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan † † Present Address: Xaxon Corporation,1-3-19, Tanimachi, Chu-ou, Osaka, Japan ‡ ‡ Present Address: Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 , Japan 1 modification at normal nuclear-matter density. 24.85.+p,25.30.-c Typeset using REVT E X 2 Recently, chiral property of QCD in hot(T = 0) or dense(ρ = 0) matter attracts wide interests in the field of hadron physics. The dynamical breaking of chiral symmetry in the QCD vacuum induces an effective mass of quarks, known as constituent quark mass. In hot and/or dense matter this broken symmetry is subject to restore partially or completely, and hence the properties of hadrons can be modified. To observe such an effect, measurements of in-medium decay of vector mesons, especially in the lepton-pair channel, are highly desirable to obtain directly the meson properties in matter [1].Although many heavy-ion experiments were carried out in CERN-SPS and BNL-AGS to study hot and dense matter, there was only an experiment which could address the mass modification of vector mesons. The CERES/NA45 collaboration measured low-mass electron pair productions in Pb-Au collisions at 158 A GeV [2], and observed an enhancement of e + e − pair yield in the mass range 0.3 < m ee < 0.7 GeV/c 2 over the expected yield from the known hadronic sources in pp collisions. This striking effect could be understood as a consequence of the mass modification of the ρ and ω meson in hot matter.In QCD the mass of vector mesons, mainly determined by the effective mass of quarks, is closely related toqq condensates ( qq ) which is an order parameter of the chiral symmetry of the QCD vacuum. In this framework a significant decrease of qq is expected not only at high temperature but also at normal nuclear density [3]. Using the QCD sum rule, Hatsuda and Lee theoretically predicted in-medium modification of the vector mesons [4]. According to this model, mass decrease at normal nuclear density is 120 ∼ 180 MeV/c 2 for the ω and ρ mesons and 20 ∼ 40 MeV/c 2 for the φ meson. Thus the measurements of such mesons, which are produced and decayed in a nucleus, are of great interest. The present experiment is one of the several experimental efforts [1...

The invariant mass spectra of e+e- pairs produced in 12 GeV proton-induced nuclear reactions are measured at the KEK Proton Synchrotron. On the low-mass side of the meson peak, a significant enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.

The violation of baryon number, B , is an essential ingredient for the preferential creation of matter over antimatter needed to account for the observed baryon asymmetry in the Universe. However, such a process has yet to be experimentally observed. The HIBEAM/NNBAR program is a proposed two-stage experiment at the European Spallation Source to search for baryon number violation. The program will include high-sensitivity searches for processes that violate baryon number by one or two units: free neutron–antineutron oscillation ( n → n ̄ ) via mixing, neutron–antineutron oscillation via regeneration from a sterile neutron state ( n → [ n ′ , n ̄ ′ ] → n ̄ ), and neutron disappearance (n → n′); the effective Δ B = 0 process of neutron regeneration ( n → [ n ′ , n ̄ ′ ] → n ) is also possible. The program can be used to discover and characterize mixing in the neutron, antineutron and sterile neutron sectors. The experiment addresses topical open questions such as the origins of baryogenesis and the nature of dark matter, and is sensitive to scales of new physics substantially in excess of those available at colliders. A goal of the program is to open a discovery window to neutron conversion probabilities (sensitivities) by up to three orders of magnitude compared with previous searches. The opportunity to make such a leap in sensitivity tests should not be squandered. The experiment pulls together a diverse international team of physicists from the particle (collider and low energy) and nuclear physics communities, while also including specialists in neutronics and magnetics.