Improved measurement of ${}^8\mathrm{B}$ solar neutrinos with $1.5\mathrm{kt}·\mathrm{y}$ of Borexino exposure

We report on an improved measurement of the ${}^8\mathrm{B}$ solar neutrino interaction rate with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. Neutrinos are detected via their elastic scattering on electrons in a large volume of liquid scintillator. The measured rate of scattered electrons above 3 MeV of energy is ${0.223}_{-0.016}^{+0.015}(\mathrm{stat}){}_{-0.006}^{+0.006}(\mathrm{syst})\mathrm{cpd}/100\mathrm{t}$, which corresponds to an observed solar neutrino flux assuming no neutrino flavor conversion of ${\mathrm{\Phi }}_{{}^8\mathrm{B}}^{\mathrm{ES}}=2.57_{-0.18}^{+0.17}(\mathrm{stat}){}_{-0.07}^{+0.07}(\mathrm{syst})\times {10}^6{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. This measurement exploits the active volume of the detector in almost its entirety for the first time, and takes advantage of a reduced radioactive background following the 2011 scintillator purification campaign and of novel analysis tools providing a more precise modeling of the background. Additionally, we set a new limit on the interaction rate of solar hep neutrinos, searched via their elastic scattering on electrons as well as their neutral current-mediated inelastic scattering on carbon, ${}^{12}\mathrm{C}(\nu ,{\nu }^{\prime }){}^{12}{\mathrm{C}}^{*}$ ($E_{\gamma }=15.1\mathrm{MeV}$).

Figure 1

Relative variation [%], from Monte Carlo simulations, of the light yield with respect to the detector center, as a function of the event reconstructed position (z vs ${\mathrm{R}}_{xy}=\sqrt{x^2+y^2}$). The correspondent systematic error was estimated in 1.9%, using ${}^{241}\mathrm{Am}–{}^9\mathrm{Be}$ calibration data.

Figure 2

Detection efficiencies and associated uncertainties (due to the electron energy scale determination) of the HER-I [(1650, 2950) p.e., red line] and HER-II [(2950, 8500) p.e., blue line] ranges as a function of electron energy. The detection efficiency equals 1 up to the 8500 p.e. upper edge ($\sim 17\mathrm{MeV}$) of the HER-II range.

Figure 3

Energy spectrum of residual events after selection cuts in the HER-I (red) and HER-II (blue) ranges. A z-cut at 2.5 m is applied to the HER-II region (see the text) causing the step in the number of selected events visible at 2950 pe. No events are found in the HER-II subrange [6000, 8500] p.e.

Figure 4

Radial distributions of simulated ${}^8\mathrm{B}$ $\nu$ events (black) and ${}^{208}\mathrm{Tl}$ decays (red), with [1650, 2950] p.e. energy cut. The two spectra are normalized to the number of reconstructed events. The difference is due to ${}^{208}\mathrm{Tl}$ emission $\gamma$ rays, escaping the scintillator.

Figure 5

Radial distribution of ${}^{212}\mathrm{Bi}$ events occurring in the scintillator, selected with the fast ${}^{212}\mathrm{Bi}$-Po coincidences, with no energy degradation for the $\alpha$ decay (black dots). The distribution is fitted with a bulk (red) component and one from emanation and diffusion from the nylon vessel (green). The radial distribution of ${}^{208}\mathrm{Tl}$ (blue) emanated from the vessel is derived from the latter, as described in the text.

Figure 6

Radial distribution of ${}^8\mathrm{B}$ candidates (black dots) compared with the Monte Carlo (red) one for events with $\mathrm{Q}>2950\mathrm{pe}(\sim 5\mathrm{MeV})$. The excess at large radii is an indication for the additional high energy external background component induced by radiogenic neutron captures on detector materials.

Figure 7

Predicted energy spectra and fluxes of neutrons produced via ($\alpha$, n) reactions and spontaneous fissions, from ${}^{238}\mathrm{U}$, ${}^{235}\mathrm{U}$, and ${}^{232}\mathrm{Th}$ contaminations in SSS and in PMT glasses.

Figure 8

Reconstructed radial distributions of simulated events with energy falling in the HER-I range and generated by neutron captures on carbon in the buffer (red), and on iron in the SSS (black).

Figure 9

Comparison of the radial distributions from unoscillated and oscillated neutrinos in the HER-I and HER-II ranges. Oscillated and unoscillated spectra are almost indistinguishable, confirming the negligible dependence of the radial distribution on the energy spectrum. The MSW-LMA parameters used for simulating oscillating neutrinos are $\mathrm{\Delta }{\mathrm{m}}^2=7.50\times {10}^{-5}{\mathrm{eV}}^2$, ${\sin }^2({\theta }_{12})=0.306$, and ${\sin }^2({\theta }_{13})=0.02166$ [29].

Figure 10

Fit of the event radial distribution in the HER-II range, [2950, 8500] p.e.

Figure 11

Fit of the event radial distribution in the HER-I range, [1650, 2950] p.e.

Figure 12

FADC energy spectrum of selected events above 11 MeV, compared with the expected background spectrum.

Figure 13

Response functions from Eq. (a2) for $i=e$, $x$ corresponding to the HER (green), HER-I (red), and HER-II (blue) energy ranges, respectively. The black line is the ${}^8\mathrm{B}$ neutrino spectrum, for reference.