pep Neutrinos

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]  suggests that this flavour change is the result of "MSW"-type matter enhanced neutrino oscillations [4]  .

Shown in Figure 2, below, is the survival probability for solar electron neutrinos (i.e. the probability that 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 2: The electron neutrino survival probability in the MSW

                   neutrino oscillation 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 have not yet been any 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 [5] , non-standard neutrino-matter interactions [6] , or a large value for Θ13.

From the solar neutrino spectrum, shown below in Figure 3, we can see that the best way to probe this interesting transition region is to measure the flux of pep neutrinos which, at 1.4MeV, sit right in the middle of the region (the 7Be neutrinos might also work, but they have a much larger uncertainty in their predicted flux; the CNO neutrinos, shown in blue in Figure 3, have an even greater uncertainty).

                                 Figure 3: The theoretical solar neutrino spectrum [7]  .

Thanks to the depth of SNOLAB, which shields the experiments from the interfering effects of cosmic rays, SNO+ will be by far the best (and perhaps only) experiment in the world for the measurement of the pep neutrino flux. Neutrinos are detected in a scintillator experiment through the "elastic scattering" interaction, νe + e- → νe + e-, which gives a "smeared out" version of the neutrino spectrum. Figure 4 shows the spectra of recoil electrons that we expect to be produced by the neutrino fluxes of interest to SNO+.

                    Figure 4: 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.

By looking for the pep recoil electron energy shape, SNO+ should be able to make a good measurement of the pep solar neutrino flux and hence the electron neutrino survival probability at 1.4 MeV. This should, in turn, make it possible to search for new physics by probing our understanding of solar neutrino oscillations.

CNO Neutrinos

The nuclear reactions that power the sun are based around the creation of helium nuclei from hydrogen nuclei (individual protons) [8] . 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 5. 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 possible, however, that a small fraction of the sun's energy comes from the "CNO" fusion cycle, shown on the right hand side of Figure 5. The actual CNO contribution is very poorly known, as it is difficult to predict theoretically [9] . 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.

                             Figure 5: 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] Smirnov, A., Lunardini, C., and Pen͂a-Garay, C., Phys. Lett. B. 594 347 (2004).

[6] Friedland, A., Lunardini, C., and Pen͂a-Garay, C., Phys. Lett. B. 594 347 (2004).

[7] Bahcall, J.N., Serenelli, A.M., and Basu, S., ApJ, 621 L85 (2005).

[8] Bethe, H.A., Phys. Rev. Lett. 55 434 (1939).

[9] Bahcall, J.N., Gonzalez-Garcia, M.C., and Pen͂a-Garay, C., Phys. Rev. Lett. 90 131301 (2003).


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