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Good morning everyone，my topic is about solar neutrino oscillation. In
order to explain the puzzle of solar neutron more clearly，firstly，I will give a short
overview of the Standard Solar Model，then I will talk about a certain experiment，the
Sudbury Neutrino Observatory experiment，which gave a first strong evidence of solar
neutrino oscillation in 2002.
Finally I’ll give a summary and outlook.
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In the Standard Solar Model (SSM), the Sun was treated as a spherical ball
of gas, and there are two combination reactions will occur in the Sun. One is the CNO
cycle, as shown the left figure, the other one is called PP chain as shown the right one.
Both of the two reactions only produce electron neutrinos. Since The Sun has a core
temperature of around 15.7×10^6 kaierwen, and PP chain starts occurring at temperature
about 4 x 10^6 kaierwen, CNO cycle starts occurring approximately 15×10^6 kaierwen, So the
PP chain is the dominant reactions in the sun.
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The PP chain consists of several steps, as figure 3 shows. There are totally 5
neutrino-producing reactions, the energy range from 0.1 to 20 MeV. The colored 3 reactions
are the most important reactions due to their energy spectrum,
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as shown in figure 4. neutrinos produced by the CNO cycle are shown in the
dashed lines. We listed the different features of the 3 most important reactions and their
relative experimental methods as shown in Table 1. The experiment I will talk about latter,
SNO, is only sensitive to Boron-eight neutrinos. Scine hep neutrinos flux is 1000 times
smaller boron- eight neutrinos, so, generaly, they are ignored,
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The first experiment to detect solar neutrino named Homestake which leaded by
Davis in the 1960s. The experiment found about one-third as many neutrinos as were
prediction.
In the last 50 decades, there were so many experiments taken into operate, but
they all got smaller flux than prediction of SSM. Two ways could explain those results: 1, SSM
is wrong, 2, electron neutrinos change into other flavor. Today the most popular opinion is the
second one.
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SNO is special than others in its use of heavy water as a detection medium.
So, it’s not just sensitive to the electron neutrino, but also sensitive to the nonelectron
neutrinos. All flavor neutrinos will interact with heavy water in the following three
reactions. Here mui x refers to any flavor. The charged current (CC) reaction is only
sensitive to electron neutrinos, while the neutral current (NC) reaction and elastic
scattering (ES) reaction are sensitive to all active neutrino flavors. That means: if SNO
detect non-zero flux of muon and tauno neutrinos, the hypothesis of flavor transformation
is true.
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The SNO detector located about 2 km underground, and consists of 3 parts
as shown in Figure 5. 1000 tonne heavy water was in a transparent acrylic vessel. In order
to remove the background from the radiactive elements in the vessel, only reconstructed
events in the radius less than 5.5 m from the detector center are valid. Geodesic
structure carrying 9438 inward-looking PMTs to detect signals produced after neutrinos
interact with heavy water. The whole barrel-shaped cavity is full of pure water to reduce
radiation in both the PMTs and the rocks.
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Figure 6 shows out the selected events spectra, the dashed line is the
summed result of three interactions, and the colored lines are MC simulation of each
interaction and background.
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The analysis results are listed here. Since charge current reaction is only
sensitive to electron neutrinos, so the red bar is perpendicular to fai-e axis. The slopes of
blue bar and green bar represent different reactions sensitivity to different reactions. The
NC reaction is equal sensitive to all flavors, and ES more sensitive to electron neutrinos.
The observed NC reactions flux is in agreement with Standard Solar model
prediction. In addition, the flux of nonelectron (ϕμ τ ) components is 3.41 with 5.3 sigma
than zero, providing a strong evidence of flavor transformation.
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At almost the same time, SNO reported day and night asymmetry. The
asymmetry ratio is defined by this formula. All 3 reactions results are non-zero
asymmetry ratio.
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After collecting and analysis whole three phases data, in the 2005, SNO
reported their new results, which consistent with the results published in 2002.
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Summary: We compared the SSM prediction and experiments results,
especialy SNO experiment. In the SSM, there are two combination reactions can produce
electron neutrinos in the Sun, and SSM predicted the total flux of the PP chain neutrinos.
SNO uses heavy water as detection medium, so it’s sensitive to all flavor neutrinos. They
detected non-zero flux of muon and tauno netrinos, and day and night asymmetry ratio is
non-zero too, those results provide strong evidence of solar neutrino flavor transformation
and matter enhanced oscillation.