P780.02 Spring 2002 L18 The Solar Neutrino Problem The sun only produces electron neutrinos (ne)! Richard Kass M&S 11.1.2 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” SNU=standard solar unit Measured: 2.550.170.18 SNU SNU=1 capture/s/1036 target atoms Calculated: 7.32.3 SNU 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! P780.02 Spring 2002 L18 Richard Kass The Solar Neutrino Energy Spectrum Homestake: Chlorine ne + 37Cl e- + 37Ar SAGE/GALLEX: Gallium ne + 71Ga e- + 71Ge Figure by J. Bahcall SuperK: nX + e- nX +enmt + e- 1/6(ne +e-) P780.02 Spring 2002 L18 The Solar Neutrino Problem Richard Kass P780.02 Spring 2002 L18 The SNO Detector Located in a mine in Sudbury Canada Uses “Heavy” water (D2O) Detects Cerenkov light like SuperK Richard Kass SNO=Sudbury Neutrino Observatory Nucl. Inst. and Meth. A449, p172 (2000) P780.02 Spring 2002 L18 Richard Kass Why Use “Heavy” Water? 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 Neutrons are captured by all neutrino flavors contribute equally deuterium and produce energy threshold for NC reaction is 2.2 MeV 6.25 MeV g Elastic Scattering interactions (ES): nemt + e- nemt + emostly electron neutrinos (NC and CC) SNO measures several quantities (fCC, fNC, fES) and from them calculates the flux of muon and tau neutrinos (fm+ftf): f CC ne f NC fn e + fn m + fn t f ES fn e + 0.154(fn m + fn t ) SuperK only has protons! 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. P780.02 Spring 2002 L18 Results from SNO Richard Kass +0.44 +0.46 neutral current results: Fssm = 5.05 +1.01 F = 5.09 sno -0.81 -0.43 -0.43 Best fit to data gives: Flux of 8B solar neutrinos 6 2 1 F mt 3.41 0.45 00..45 48 10 cm s 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”) P780.02 Spring 2002 L18 Richard 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: ne+ e- ne + 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: The MSW effect is sin 2 2 xW P(n e n m ) sin 2 m sin ( ) osc 2 2 sin 2 2 m W2 W 2 sin 2 2 + ( D cos2 2 ) 2 D 2GF N e 2 En m 2 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. P780.02 Spring 2002 L18 The MSW Effect Richard Kass There are only a few allowed regions in (, m2) space that are compatible with MSW effect: LMA= Large Mixing Angle region favored. SNO Day and Night Energy Spectra Alone Combining All Experimental and Solar Model information From A. Hamer, APS Talk, 4/2002
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