Low Energy Solar Neutrinos

Solar Neutrino Oscillations in Matter


The SNO experiment [1] proved that about two thirds of the electron neutrinos produced in the sun show up on earth as muon- or tau- type neutrinos. Combining the SNO result with those of other solar neutrino experiments [2] and the KamLAND reactor anti-neutrino experiment [3] demonstrated that this flavour change is the result of "MSW"-type matter enhanced neutrino oscillations [4].


Shown in Figure 1, below, is the survival probability for solar electron neutrinos (i.e. the probability that an electron neutrino produced in the sun is detected as an electron neutrino on earth) as a function of the neutrino energy based on our current understanding of neutrino oscillations. At high energies (at least high for solar neutrinos), matter effects are important and the survival probability is uniformly low. At lower neutrino energies, however, the matter effects gradually give way to vacuum oscillations, and the survival probability increases.



Figure 1. Solar ν e survival probability [5] as a function of neutrino energy including all solar (with Borexino) experimental results. The grey band is the prediction of the LMA solution in the frame of MSW model.


Although the electron neutrino survival probability at the "matter oscillation dominated" and "vacuum oscillation dominated" extremes of the solar neutrino flux have been studied, there is interest to make more precise measurements in the "transition region" between about 1 and 4 MeV. Directly observing the rise in the survival probability in the transition region would confirm the MSW mechanism. The transition region is also the most sensitive place to look for sub-dominant effects in neutrino oscillations, such as a sterile neutrino admixture [6], non-standard neutrino-matter interactions [7], or a large value for Θ13.


From the solar neutrino spectrum, shown below in Figure 2, we can see that the best way to probe this interesting transition region is to measure the flux of pep neutrinos and attempt to detect 8B solar neutrinos down to lower energies than previously studied.


Figure 2. The theoretical solar neutrino spectrum [8]



Thanks to the depth of SNOLAB, which shields the experiments from the interfering effects of cosmic rays, SNO+ will have the potential to measure the fluxes of low energy solar neutrinos. The pep neutrinos are interesting because their flux is predicted with small uncertainty. The 8B solar neutrinos span the transition energy region and their survival probability changes with energy. Neutrinos are detected in a scintillator experiment through the "elastic scattering" interaction, νe + e- → νe + e-. Figure 4 shows the spectra of recoil electrons that we expect to be produced by the neutrino fluxes of interest to SNO+. SNO+ will search for new physics by probing our understanding of neutrino-matter interactions in solar neutrino oscillations.


Figure 3. The expected spectra of recoil electrons from solar neutrinos that we expect to see in SNO+. The spectra of the radioactive contaminants that we expect are also shown.


Solar Metallicity and Solar Models


The nuclear reactions that power the sun are based around the creation of helium nuclei from hydrogen nuclei (individual protons) [9]. In the course of this transition, several neutrinos and a large amount of energy (which eventually becomes sunlight) are produced. In our sun, the majority of these hydrogen-helium transitions occur via the "pp" fusion cycle, which is shown on the left hand side of Figure 4. The solar neutrino measurements that have been made so far are based on the neutrinos produced by the reactions in the pp chain.

It is expected, however, that a small fraction of the sun's energy comes from the "CNO" fusion cycle, shown on the right hand side of Figure 4. The actual CNO contribution is very poorly known, as it is difficult to predict theoretically [10]. A measurement by SNO+ of the flux of neutrinos produced in the CNO cycle (these are shown in blue in Figure 3), would tell us the CNO contribution to solar energy generation, giving us interesting information about the inner workings of the sun. Measuring the CNO solar neutrino flux also reveals the chemical composition or "metallicity" in the core of the sun; this is important for understanding energy production and flow in solar models.

Detection of CNO solar neutrinos is particularly challenging due to their low rate and the presence of similar radioactive backgrounds. SNO+ will attempt this detection if backgrounds are low enough in the detector.



Figure 4. The p-p and CNO solar energy generation cycles.



 [1] Ahmed, S.N. et al, Phys. Rev. Lett. 92 181301 (2004).
[2] Abdurashitov, J.N. et al, J. Exp. Theor Phys., 95 181 (2002).....Hampel, W. et al, Phys. Lett. B., 447 127 (1999).....Altmann, M. et al, Phys. Lett. B., 490 16 (2000).....Fukuda, Y. et al, Phys. Rev. Lett., 77 1683 (1996).....Fukuda, S. et al, Phys. Lett. B., 539 179 (2002).
[3] Araki, T. et al, hep-ex/0406035, (2004).
[4] The SNO Collaboration, Phys. Rev. C 72, 055502 (2005).
[5] The Borexino Collaboration, Nature 562 505-510 (2018).
[6] Smirnov, A., Lunardini, C., and Pen͂a-Garay, C., Phys. Lett. B. 594 347 (2004).
[7] Friedland, A., Lunardini, C., and Pen͂a-Garay, C., Phys. Lett. B. 594 347 (2004).
[8] Bahcall, J.N., Serenelli, A.M., and Basu, S., ApJ, 621 L85 (2005).
[9] Bethe, H.A., Phys. Rev. Lett. 55 434 (1939).
[10] Bahcall, J.N., Gonzalez-Garcia, M.C., and Pen͂a-Garay, C., Phys. Rev. Lett. 90 131301 (2003).