Solar and Geo Neutrinos in Borexino
Gemma Testera discussed a summary of “phase 1” results of the Borexino experiment. Borexino is a detector in the Gran Sasso mine built to study the low-energy spectrum of neutrinos from the Sun, up to the 7Be and pep neutrinos, and 8B neutrnios up to 3 MeV energy.
The measurement of solar neutrinos is still interesting; from an astrophysical point of view one wants to test the models for high- or low-metallicity options of solar models. The second reason is connected to neutrino oscillations: for solar neutrinos, the parametrization is via the 1-2 mixing, for which there is a squared mass difference of 7.54+-0.26 eV^2. There is a shape of the neutrino survival probability as a function of energy which can be investigated in the MeV range.
They divide the Borexino operation in two phases: phase 1 lasted until May 2010, when they did data taking and calibration; then they purified the scintillators and from October 2011 they entered in phase 2, when they are taking data with better performance. They are also studying geo-neutrinos.
The talk then focused on the evidence of a seasonal modulation of neutrinos from 7Be line. To see these neutrinos (0.862 MeV) they fit the energy spectrum. The signal is seen in the electron recoil spectrum, which has large background from Polonium alpha decay, and Krypton decays among other sources. The 7Be is measured by a fit to the spectrum, which includes a accurate modeling of the response function done with Monte Carlo and analytical models. The rate of 7Be neutrinos is seen and there is a 5-sigma evidence of the oscillation.
The pep solar neutrino detection is more challenging because of their smaller rate, few counts per day per hundred tons. Backgrounds are large, the carbon 11 background is the largest in that energy region. In Borexino a procedure was developed to reduce background from 11C producing positronium. The light emitted by 11C is delayed and they use this feature to build a pulse shape discriminant. 11C is produced when a muon hits a regular 12C atom, ejecting a neutron from it. Neutron is captured by hydrogen, emitting a photon. The detector region where this occurs is vetoed for long enough, reducing the background. This technique reduces background by a factor of 10.
For CNO neutrinos they can only place a limit on the number of counts per day. It does not allow to discriminate between high- or low-metallicity solar models.
Putting everything together, the neutrino survival probability looks much nicer, with the energy dependence being evidenced. The data instead can’t discriminate the different solar models in the plane of 8Be vs 7Be fluxes: their data is just in the middle of the two model predictions.
A new result was then discussed: the annual modulation of the 7Be flux. This dependence is foreseen because of the changing distance Sun-Earth. The variation is of 7% peak-to-peak. They selected a energy window where the 7Be is dominant, searching for a periodical component. The challenge is due to the amplitude of some untaggable backgrounds (like the one from 210Bi), which are not stable in time. It increases exponentially with time (the plot shown could be fit very well by a straight line, but I failed to ask why they model it with an exponential…). Another problem is the enlarging of the fiducial volume to search for the signal of 7Be: the shape of the vessel and its position also changes in time, so they needed to take this into account.
All together, the rate of 7Be can be seen to vary, if one believes the underlying model for the 210Bi background variation in time. They end up at about 2-sigma from expectation in this measurement.
Another measurement of this kind had been done in phase 1, when they searched for the annual modulation of the total neutrino signal in a Lomb-Scargle periodogram. They had rejected at more than 3-sigma significance the absence of a seasonal effect.
She showed also a preliminary result of the counting rate versus time, where the data is now much more stable (less background), and the oscillation can be seen more clearly.
Gemma then discussed geo-neutrinos, these probe the uranium and thorium content of the earth. of course another source is from reactors, which can be estimated knowing the operating conditions of the various sites.
They find 14.3+-4.4 candidates for geo-neutrinos. They tried to see if they could separate the U and Th components, based on the light yield of the two signals. They find values in agreements with the model.