Neutrinos arise in three flavours: electron, muon and tau. The discovery of neutrino oscillations between these flavours implies that neutrinos are massive particles. The SM predicts massless neutrinos, and therefore neutrino oscillations imply physics beyond the SM. This motivates the study of neutrino processes beyond the SM framework, for instance, non-standard interactions.
Solar neutrinos were crucial to discover and understand neutrino oscillations. Only electron-neutrinos are produced in the Sun, and by using a model for the interior of the Sun (the standard solar model), the flux of solar neutrinos on Earth is predicted. However, there was a discrepancy between the theoretical predictions and the measured solar neutrino flux, dubbed the so-called solar neutrino problem.
Helioseismology, the study of seismic waves within the Sun, has provided a valuable tool to understand the Sun’s structure and improve the solar standard model. In turn, measurements of neutrino fluxes can help in understanding of the Sun’s structure. Today there is a good level of agreement between the predictions of the standard solar model and measurements of neutrino fluxes. Future experiments will allow precise measurement of the neutrino fluxes and of the neutrino energy spectra of reactions within the Sun, thereby providing insight into the Sun’s core, and sensitivity to new physics in the neutrino sector.
Predictions of the solar model include identifying the regions where particular fusion processes take place. Neutrinos produced in these regions will then propagate through the solar matter. Figure 1 shows the regions where various reactions take place within the sun, where(r)is comparable to the flux at those positions. The reactions labelled with an asterisk are spectral lines with minimal energy spread.
The solar plasma is dominated by electrons, protons and neutrons. Figure 2 shows the number densities of electrons (red curve), up quarks (blue quarks) and down quarks (green curve), according to the solar model. The ratio between up and down quarks is also shown (black curve).
As the electron-neutrinos traverse the solar interior, their flavour state changes.
This variation can be described by a survival probability, which is the probability for an electron-neutrino to reach the Earth in the same flavour state in which it was produced. Non-standard interactions could also affect this survival probability.
An effective potential for standard interactions, is expressed as a sum of effective charged-current and neutral-current potentials. In the case of non-standard interactions, the effective potential for neutral currents acquires a more general form, introducing two classes of non-standard terms that can either preserve or change flavour.
As the solar plasma contains protons (two up quarks and a down quark), neutrons (an up quark and two down quarks) and electrons, only the interactions with up and down quarks are taken into account, as well as the electrons. These couplings between an electron-neutrino and a quark can be parametrised by a set of 2 independent parameters (ENu, EDu) and (ENd, Edd) for the interactions with up and down quarks, respectively.
It has been previously shown that only a small subset of parameter combinations (ENu, EDu, ENd, Edd) are compatible with current solar neutrino flux measurements. Here, two such models were studied, each corresponding to non-standard interactions with either the up- or down- quark.
Figure 3 shows the shape of the solar neutrino spectra observed on Earth from the pp-chain and the CNO-cycle after accounting for standard oscillation effects only (in red), and for including non-standard interactions with up-quarks (in blue) or with down-quarks (in green). Although for some reactions (PP, 13N), very little change in the spectra is predicted, for the HeP and 8B reactions, clear differences are seen between the standard and non-standard interaction cases. In addition, it is possible to distinguish between the two non-standard interactions models.
High precision measurements of neutrino spectra will therefore have sensitivity to non-standard interactions in the Sun’s interior. In particular, the Low Energy Neutrino Astronomy (LENA) experiment will perform a precise measurement of the 8B neutrino spectrum. Figure 4 shows the estimated uncertainty on the 8B neutrino spectrum after 5 years of LENA measurements (black lines). After a relatively short measurement time, it should be possible to discover if neutrinos do have non-standard interactions with quarks, and depending on the location and magnitude of the possible spectrum distortion, particular models could be specified.
References:  I. Lopes, New neutrino physics and the altered shapes of solar neutrino spectra (2017)
Text by Chloe Ransom and Laura Paulina Šinkūnaitė