• Generated by “weak” force during   p+ n + e+ +
• “Massless” particles which interact poorly with matter
– In that first respect, similar to photons
– Can pass through sun without being absorbed
– Same property makes them very hard to detect
• Davis experiment at Homestake Mine in Black Hills
– 100,000 gallon tank of C2Cl4 dry cleaning fluid
– in Cl nuclei   n + p+ + e-   so Cl (Z=17) becomes Ar (Z=18)
– Physically separate out the Ar, then wait for it to radioactively decay
– Saw only 1/3 the neutrinos predicted