Neutrinos

Federico Sánchez

For more than a decade IFAE has been contributing to several key experiments in this field, such as K2K, which obtained the first measurement of neutrino oscillations with a neutrino beam from an accelerator, and T2K, that presented in 2011 the first indication of the transformation of muon neutrinos into electron neutrinos, thereby demonstrating a non-zero value for the third mixing angle.

T2K scientific results in 2017

T2K acquired more data in 2017 in neutrino and antineutrino modes. T2K has accumulated $1.65×10^21$ protons on target, comprising approximately 20% of the total requested exposure during the first phase. The data taken has been shared between neutrino (57.14%) and antineutrino modes (42.86%). The preliminary results in 2017 constrain the $δ_{CP}$ phase to negative values with more than 95% The collaboration also published the current world best constrain on $θ_{23}$ see Figure 4. These results are consequence of the increased statistics but also on the improvement of systematic errors and the inclusion of new neutrino interaction samples in the analysis.

Figure 1: Preliminary 2017 results on the $δ_{CP}$ and $θ_{13}$ angles combining the T2K and the reactor neutrino results.

Figure 2: Marginalized posterior probability for the δCP angle, the results show more that 95.4% probability for non-zero CP violation.

IFAE Contribution to T2K

IFAE continued with its contribution to reduce the systematic errors associated to the neutrino cross-sections. IFAE group adapted the neutrino cross-section model based on Local Fermi Gas into the official T2K Monte Carlo generator NEUT. IFAE also improved the data analysis by enlarging the near detector angular acceptance and setting the bases of a new generation of analysis based on 4π acceptance. The first results obtained by the IFAE group, see Figure 5, have been approved for publication.

Figure 3: Number of νμ detected at the far detector compared to the MC prediction with maximal mixing.

Figure 4: Results for the atmospheric parameters using the full T2K statistics until 2017. This is the most precise result up to date on $θ_{23}$.

Figure 5: The flux-integrated, double-differential cross section per nucleon for NEUT (continuous red line), for GENIE (dashed red line), and the unfolded-data result using as prior either NEUT or GENIE. The bin of highest momentum is scaled by the factor shown in each plot to make it visible. χ2 values are computed with unfolded-data result using as prior NEUT.