The Solar Neutrino Problem

1. Richard Kass The sun only produces electron neutrinos (ne)! M&S 11.1.2 The Solar Neutrino Problem • Since 1968 R.Davis and collaborators have been measuring the cross section of: • ne + 37Cl ® e- + 37Ar Their measured rate is significantly lower than what is expected from the “standard solar model” Measured: 2.55±0.17±0.18 SNU Calculated: 7.3±2.3 SNU SNU=standard solar unit SNU=1 capture/s/1036 target atoms Data from the Homestake Gold Mine (South Dakota) There is a long list of other experiments have verified this “problem”. Too few neutrinos from the sun!

2. Richard Kass Figure by J. Bahcall The Solar Neutrino Energy Spectrum Homestake: Chlorine ne + 37Cl ® e- + 37Ar SAGE/GALLEX: Gallium ne + 71Ga ® e- + 71Ge SuperK: nX + e-® nX + e- nmt + e-® 1/6(ne + e-)

3. Richard Kass The Solar Neutrino Problem

4. Richard Kass Located in a mine in Sudbury Canada Uses “Heavy” water (D2O) Detects Cerenkov light like SuperK The SNO Detector SNO=Sudbury Neutrino Observatory Nucl. Inst. and Meth. A449, p172 (2000)

5. Richard Kass Charged Current interaction (CC): ne + d ® e- + p + p (ne + n ® e- + p ) Deuterium has neutrons! Only electron neutrinos can cause this reaction Neutral Current Interactions (NC): nemt+ d ®nemt+ n + p D2O has twice as many nucleons as H2O all neutrino flavors contribute equally energy threshold for NC reaction is 2.2 MeV Elastic Scattering interactions (ES): nemt + e-®nemt + e- mostly electron neutrinos (NC and CC) Why Use “Heavy” Water? Neutrons are captured by deuterium and produce 6.25 MeV g SuperK only has protons! SNO measures several quantities (fCC, fNC,fES) and from them calculates the flux of muon and tau neutrinos (fm+ft): The quantities can be compared with the standard solar model. They also measure the total 8B solar neutrino flux into NC events and compare it with the prediction of the SSM.

6. Richard Kass +1.01 Fssm = 5.05 -0.81 +0.44 +0.46 -0.43 -0.43 Results from SNO neutral current results: • Fsno = 5.09 Best fit to data gives: Flux of 8B solar neutrinos Fmt=0 if no oscillations. “SSM”=Standard Solar Model Strong evidence for Neutrino Flavor Mixing at 5.3s (5.5s if include SuperK). Total active neutrino flux agrees with standard solar model predictions. Believe that the mixing occurs in the sun (“MSW effect”)

7. Richard Kass Neutrino oscillations can be enhanced by traveling through matter. Origin of enhancement is very similar to a “birefringent” medium where different polarizations of light have different indexes of refraction. When polarized light passes through a birefringent medium the relative phase of each polarization component evolves differently and the plane of polarization rotates. The neutrino “index of refraction” depends on its scattering amplitude with matter: sun is made of protons, neutrons, electrons ®up/down quarks, electrons All neutrinos can interact through neutral currents equally. Only electron neutrino can interact through CC scattering: ne+ e-®ne + e- The “refractive index” seen by electron neutrinos is different than the one seen by muon and tau neutrinos. The MikheyevSmirnov Wolfenstein Effect The MSW effect gives for the probability of an electron neutrino produced at t=0 to be detected as a muon neutrino: The MSW effect is very similar to “K-short regeneration” M&S 10.2.4 Here Ne is the electron density. For travel through vacuum Ne=0 and the MSW result reduces to our previous result.

8. Richard Kass There are only a few allowed regions in (q, Dm2) space that are compatible with MSW effect: LMA= Large Mixing Angle region favored. The MSW Effect SNO Day and Night Energy Spectra Alone Combining All Experimental and Solar Model information From A. Hamer, APS Talk, 4/2002