We discuss implications of open string theory in B-violating low energy physics. In particular, exotic instantons can dynamically generate effective the six quark operator, leading to a neutron-antineutron transition. The proton is not destabilized and flavor changing neutral currents are under control.

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We propose a leptobaryogenesis mechanism in which the non-zero B −L of the Universe is produced in out-of-equilibrium, lepton number and CP violating scattering processes that convert ordinary particles into particles of some hidden sector. In particular, we consider the processes lφ → l ′ φ ′ ,l ′φ′ mediated by the heavy Majorana neutrinos N of the seesaw mechanism, where l and φ are ordinary lepton and Higgs doublets and l ′ , φ ′ their hidden counterparts. Such a leptogenesis mechanism is effective even if the reheat temperature is much smaller than the heavy neutrino masses. In particular, it can be as low as 10 9 GeV. It is well known that a non-zero baryon asymmetry (BA) can be produced in the initially baryon symmetric universe if three conditions are fulfilled: B-violation, CP-violation and departure from thermal equilibrium [1]. These conditions can be satisfied in the decays of heavy particles of grand unified theories. On the other hand, the sphaleron processes, which violate B + L but conserve B − L, are effective at temperatures from about 10 12 GeV down to 100 GeV [2]. Thus, one actually needs to produce a non-zero B − L rather than just B, a fact that disfavors the simplest baryogenesis picture based on grand unification models like SU (5). When sphalerons are in equilibrium, the baryon number and B − L are related as B = a(B − L), where a is a model dependent order one coefficient [3]. Hence, in order to obtain the observed baryon to entropy density ratio B = n B /s = (0.6 − 1) × 10 −10

If there exists the mirror world, a parallel hidden sector of particles with exactly the same microphysics as that of the observable particles, then the primordial nucleosynthesis constraints require that the temperature of the cosmic background of mirror relic photons should be smaller than that of the ordinary relic photons, T ′ /T < 0.5 or so. On the other hand, the present experimental and astrophysical limits allow a rather fast neutron -mirror neutron oscillation in vacuum, with an oscillation time τ ∼ 1 s, much smaller than the neutron lifetime. We show that this could provide a very efficient mechanism for transporting ultra high energy protons at large cosmological distances. The mechanism operates as follows: a super-GZK energy proton scatters a relic photon producing a neutron that oscillates into a mirror neutron which then decays into a mirror proton. The latter undergoes a symmetric process, scattering a mirror relic photon and producing back an ordinary nucleon, but only after traveling a distance (T /T ′ ) 3 times larger than ordinary protons. This may relax or completely remove the GZK-cutoff in the cosmic ray spectrum and also explain the correlation between the observed ultra high energy protons and far distant sources as are the BL Lacs.

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