P Spring 2002 L18Richard Kass The Solar Neutrino Problem M&S Since 1968 R.Davis and collaborators have been measuring the cross section of:  e + 37 Cl  e Ar 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/10 36 target atoms There is a long list of other experiments have verified this “problem”. Too few neutrinos from the sun! Data from the Homestake Gold Mine (South Dakota) The sun only produces electron neutrinos ( e )!

P Spring 2002 L18Richard Kass The Solar Neutrino Energy Spectrum Figure by J. Bahcall Homestake: Chlorine e + 37 Cl  e Ar SAGE/GALLEX: Gallium e + 71 Ga  e Ge SuperK: X + e -  X  e -  + e -  1/6( e  e - )

P Spring 2002 L18Richard Kass The Solar Neutrino Problem

P Spring 2002 L18Richard Kass The SNO Detector Nucl. Inst. and Meth. A449, p172 (2000) Located in a mine in Sudbury Canada Uses “Heavy” water (D 2 O) Detects Cerenkov light like SuperK SNO=Sudbury Neutrino Observatory

P Spring 2002 L18Richard Kass Why Use “Heavy” Water? Charged Current interaction (CC): e + d  e - + p + p ( e + n  e - + p ) Deuterium has neutrons! Only electron neutrinos can cause this reaction Neutral Current Interactions (NC): e   + d  e  + n + p D 2 O has twice as many nucleons as H 2 O all neutrino flavors contribute equally energy threshold for NC reaction is 2.2 MeV Elastic Scattering interactions (ES): e  + e -  e  + e - mostly electron neutrinos (NC and CC) SNO measures several quantities (  CC,  NC,  ES ) and from them calculates the flux of muon and tau neutrinos (   +   ): They also measure the total 8 B solar neutrino flux into NC events and compare it with the prediction of the SSM. SuperK only has protons! Neutrons are captured by deuterium and produce 6.25 MeV  The quantities can be compared with the standard solar model.

P Spring 2002 L18Richard Kass Results from SNO Strong evidence for Neutrino Flavor Mixing at 5.3  (5.5  if include SuperK). Total active neutrino flux agrees with standard solar model predictions. Believe that the mixing occurs in the sun (“MSW effect”)  ssm =  sno = “SSM”=Standard Solar Model Flux of 8 B solar neutrinos neutral current results: Best fit to data gives:   =0 if no oscillations.

P Spring 2002 L18Richard Kass The Mikheyev Smirnov Wolfenstein Effect 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: e + e -  e + e - The “refractive index” seen by electron neutrinos is different than the one seen by muon and tau neutrinos. The MSW effect gives for the probability of an electron neutrino produced at t=0 to be detected as a muon neutrino: Here N e is the electron density. For travel through vacuum N e =0 and the MSW result reduces to our previous result. The MSW effect is very similar to “K-short regeneration” M&S

P Spring 2002 L18Richard Kass The MSW Effect SNO Day and Night Energy Spectra Alone Combining All Experimental and Solar Model information From A. Hamer, APS Talk, 4/2002 There are only a few allowed regions in ( ,  m 2 ) space that are compatible with MSW effect: LMA= Large Mixing Angle region favored.