The SNO Measurement
of a Neutrino Mass and a Mixing Angle
An important determination of two fundamental parameters has just been announced (in September 2003). The Sudbury Neutrino Observatory (SNO) has measured a neutrino mass difference Delta mnu and a neutrino mixing angle thetanu. The results when combined with other experiments are
Delta mnu = 0.0084 electron-volts/c2 ,
thetanu = 32.5 degrees .
The above numbers are accurate to a little better than 10%. Given the extreme difficulty in detecting neutrinos, this is an amazing result. If the masses of the two neutrinos are very different, then Delta mnu is due to the heavier neutrino and its mass would be very close to 0.0084 electron-volts/c2. An electron-volt/c2 is 1.78 x 10-33 grams.
The non-zero value for thetanu means that the electron-neutrino combines with another neutrino to produce a massive neutrino; this massive neutrino is comprised of approximately cos(thetanu) = 0.84 of electron-neutrino and sin(thetanu) = 0.54 of another neutrino type, which, according to the rules of quantum mechanics, means that the probability of detecting an electron-neutrino (respectively, the other neutrino type) in this massive neutrino is cos2(thetanu) = 0.71 (respectively, sin2(thetanu) = 0.29).
A consequence of neutrino mixing is that an electron-neutrino can change into another neutrino type and that type can change back into an electron-neutrino. This extraordinary quantum mechanical effect is called neutrino oscillations. In certain circumstances, the presence of electrons in the Sun can enhance the conversion of electron-neutrinos into another type as they stream out of the Sun. This is known as the MSW effect, named after three physicists, S. P. Mikheev, A. Yu. Smirnov and L. Wolfenstein, who uncovered the phenomenon.
SNO detects neutrinos through three processes:
(i) deuteron disintegration by the charged-current interactions: nue + d –> p + p + e
(ii) deuteron disintegration by the neutral-current interactions: nux + d –> p + n + nux
(iii) elastic scattering of neutrinos off electrons: nux + e –> nux + e
In the above, d stands for deuterium (an isotope of hydrogen in which the nucleus consists of a proton and a neutron), e represents an electron, p stands for proton, n stands for a neutron, nue is an electron-neutrino, and nux is a neutrino of electron, muon, or tau type.
The exchange of the electrically charged W boson occurs in Process (i) (hence the name charged-current interactions), and the electron-neutrino causes a neutron to be converted into a proton and an electron. Process (ii) involves the exchange of the neutral Z boson (hence the name neutral-current interactions) and the neutrino splits the deuteron into its component proton and neutron. Process (iii) can proceed both through the charged- and neutral-current interactions; in elastic scattering, the neutrino bounces off an electron causing the two to fly off at high speeds.
For information about neutrinos, see Jupiter Scientific's first report on SNO results. For more on W's, Z's and the standard model of particle physics, see Jupiter Scientific's report entitled "God Particle" Possibly Discovered.
SNO consists of a spherical chamber of about 1000 tons of heavy water (D2O instead of H2O, where D is deuterium) surrounded by almost 10,000 photomultiplier tubes that detect photons (particles of light). When any of the above three processes occur, energy is generated that eventually produces a type of electromagnetic radiation called Cerenkov light. During the past year, salt (NaCl) was added to the heavy water. This doubled the sensitivity of SNO because neutrons generated in Process (ii) can be captured by the Chlorine in the salt to produce an isotope of Chlorine and gamma rays (very energetic photons). SNO is located 6800 feet (2070m) below the ground in a mine to shield it from cosmic rays that would create a background of events that would mask the signal produced by solar neutrinos.
Because, Process (i) is only sensitive to electron-neutrinos, while processes (ii) and (iii) are sensitive to all neutrino types, SNO is able to measure the overall flux of neutrinos emanating from the Sun and its electron-neutrino fraction. If neutrinos had no masses then all of the neutrinos coming out of the Sun would be of the electron type. However, SNO finds that only about 1/3 of the solar neutrinos are of the electron type and 2/3 of them are of a different type. This means that neutrinos must have mass. It is believed that 2/3 of the electron-neutrinos that are produced in the core of the Sun are converted into another type through the MSW effect. A useful consistency check is that SNO finds that the total flux of neutrinos is in agreement with standard solar models.
In short, an important measurement has been accomplished. However, only two of many fundamental parameters have been determined. In nature, there are three massive neutrinos, and they are comprised of various amounts of electron-, muon- and tau-neutrinos. This means that there are two more neutrino masses and many mixing angles that scientists would eventually like to measure.
To the original report on the existence of neutrino masses
To the Jupiter Scientific's Science Information Page
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