Abstract
We use $\bar p p$ and $e^+e^-$ annihilation data to further strengthen lower bounds on the partial lifetimes for the baryon-number-violating dinucleon decays $nn \to e^+ e^-$ and $nn \to \mu^+\mu^-$.
Highlights
In Ref. [1], lower limits on the partial lifetimes τ=BR ≡ Γ−1 for a number of ΔB 1⁄4 −2, ΔL 1⁄4 0 dinucleon decays were presented, including nn → eþe−, nn → μþμ−, nn → νlνl, and np → lþνl, where l 1⁄4 e, μ, τ. (Here, for the decay of an initial state to a given final state, Γ and BR denote the decay rate and branching ratio, and τ denotes the mean life of the initial state.) The lower bounds obtained in [1] were substantially stronger than limits from direct experimental searches
We briefly review some background and basic notation
Because the operators that contribute to baryon-numberviolating decays of individual nucleons are four-fermion operators with coefficients of mass dimension −2, while the operators that contribute to n − ntransitions and the associated dinucleon decays are six-quark operators with coefficients of mass dimension −5, it follows that, if the physics responsible for baryon number violation were characterized by a single mass scale, MBNV, nucleon decays would be much more important than n − noscillations as a manifestation of baryon number violation
Summary
In Ref. [1], lower limits on the partial lifetimes τ=BR ≡ Γ−1 for a number of ΔB 1⁄4 −2, ΔL 1⁄4 0 dinucleon decays were presented, including nn → eþe−, nn → μþμ−, nn → νlνl, and np → lþνl, where l 1⁄4 e, μ, τ. (Here, for the decay of an initial state to a given final state, Γ and BR denote the decay rate and branching ratio, and τ denotes the mean life of the initial state.) The lower bounds obtained in [1] were substantially stronger than limits from direct experimental searches. A number of dedicated experiments have been carried out since the early 1980s to search for proton decay (and the decay of neutrons bound in nuclei). These experiments have obtained null results and have set stringent lower limits on the partial lifetimes for such ΔB 1⁄4 −1 baryon-number-violating nucleon decays. The presence of a nonzero transition amplitude hnjHeffjni means that a physical neutron state jniphys: 1⁄4 cos θmjni þ sin θmjni in a nucleus has an admixture of jni. This admixture has a very small coefficient, sin θm.
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