T HE D AVIS E XPERIMENT AT H OMESTAKE Physics of Atomic Nuclei Peggy Norris,

H OW DOES THE S UN S HINE ?

N UCLEAR F USION ! At 15 million degrees Celsius in the center of the star, fusion ignites ! 4 ( 1 H) --> 4 He + 2 e neutrinos + energy Where does the energy come from ? Mass of four 1 H > Mass of one 4 He E = mc 2

H OW MUCH E NERGY 4 ( 1 H) --> 4 He + 2 e neutrinos + energy Energy released = 25 MeV = 4 x Joules = 1 x Calories But the sun does this times a second ! Sun has H atoms to burn !

D O WE H AVE TO W ORRY ? Sun has H atoms / 4 x times a second = 2.5 x second / 3 x 10 7 sec /year = years = 10 billion years 10 – 4.5 = 5.5 billion years left

N EUTRINOS FROM THE S UN ( ) photons/cm 2 /sec Based on solar models, John Bahcall predicted that our Sun puts out /cm 2 /sec

S OLAR F USION : S TEPS 1-3

S OLAR F USION : S TEPS 4-7

S OLAR F USION : S TEPS 8-11

R AY D AVIS HAD A PLAN … to measure the neutrinos coming to Earth from the Sun and prove John Bahcall’s prediction But how to detect such an elusive particle? Go very deep and very large!

C OSMIC RAYS ARE THE MAIN BACKGROUND High energy protons and other particles from the Sun and from outside the solar system are constantly bombarding the top of the atmosphere, making cascades of muons, electrons, neutrinos and other particles.

S O R AY D AVIS WENT DEEP : to Homestake Mine Ray Davis and John Bahcall at 4850’ 110,000 gallons of dry cleaning fluid

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H OW HE DETECTED NEUTRINOS Ray Davis was a chemist so he looked to nuclear chemistry for the answers:  -decay: 37 Ar  37 Cl +  - + e Davis looked for the inverse reaction: 37 Cl + e  37 Ar (t 1/2 =35 d) +  - Davis ran He gas through the tank to purge out any gases and collected Ar in a charcoal filter. Then he looked for

T HE I NVESTIGATION After years of collecting data, he found conclusively that the number of neutrinos was 1/3 that expected No one believed him! Scientists everywhere thought they understood how the Sun worked and so the experiment had to be wrong Davis stood by his data!

B UT WAS HE WRONG ? Did scientists really understand neutrinos? They assumed that the neutrinos from the Sun weren’t changed by the journey The family of leptons is part of the Standard Model of particle physics, and that model predicted neutrinos had zero mass In the 1980s, Bruno Pontocorvo and others suggested that if neutrinos did have mass, they could ‘oscillate’ from one flavor to another. This might explain Davis’ results because his experiment was only sensitive to electron neutrinos. e  

A SECOND GENERATION OF NEUTRINO EXPERIMENTS TOOK PLACE In Canada SNO In Japan superKamiokanda 50,000 tons H 2 0

R AY D AVIS WAS PROVEN RIGHT Neutrinos really do oscillate from one flavor to another John Bahcall was right in how the Sun works but 2/3 of the electron neutrinos made in the Sun have changed flavor by time they get to Earth Neutrinos do have mass and the Standard Model is wrong! The Nobel Prize in Physics 2002 "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos" And Homestake Mine had its first scientific gold!

‘F IXING ’ THE S TANDARD M ODEL In order to refine the standard model, scientists need to understand more about neutrinos 1. What is the mass ‘hierarchy’ of neutrinos, that is which is the heaviest and which the lightest (actually measuring their absolute mass is probably impossible) 2. Are neutrinos their own antiparticles? 3. What process leads to the mass of neutrinos? Experiments are planned for the DUSEL Laboratory at Homestake to answer both questions

M AJORANA : S EARCHING FOR 0 2  DECAY