Recent results on pp-chain solar neutrinos with the Borexino detector

TL;DR

This paper reports a precise measurement of solar neutrinos from the pp-chain using the Borexino detector, providing insights into solar composition, neutrino oscillation parameters, and testing the MSW-LMA mechanism.

Contribution

It presents the first simultaneous precision spectroscopic measurement of all components of the solar pp-chain with the Borexino detector, addressing key questions in solar and neutrino physics.

Findings

Measurement of all pp-chain neutrino fluxes.

Implications for solar metallicity models.

Insights into neutrino oscillation parameters.

Abstract

Measuring all neutrino components is the most direct way to test the standard solar model (SSM). Despite the great results obtained so far, important questions such as the solar metallicity remain open. A precise measurement of the solar pp chain and the CNO cycle would settle this controversy between high (HZ) and low (LZ) metallicity compositions of the Sun. Solar neutrinos allow the determination of oscillation parameters, in particular the $\theta_{12}$ mixing angle and, to a lesser degree the $\Delta m^2_{12}$ mass splitting. Furthermore the measurement of the electron neutrino survival probability Pee as a function of neutrino energy allows one to directly probe the MSW-LMA mechanism of neutrino oscillations In this work I will report the first simultaneous precision spectroscopic measurement of the complete pp-chain and its implications for both solar and neutrino physics with the Borexino detector.

Figures

Full text

SNSN-323-63

Recent results on pp-chain solar neutrinos

with the Borexino detector

Lino Miramonti on behalf of the Borexino Collaboration111 M. Agostini, K. Altenmuller, S. Appel, V. Atroshchenko, Z. Bagdasarian, D. Basilico, G. Bellini, J. Benziger, D. Bick, I. Bolognini, G. Bonfini, D. Bravo,, B. Caccianiga, F. Calaprice, A. Caminata, S. Caprioli, M. Carlini, P. Cavalcante, F. Cavanna, A. Chepurnov, K. Choi, L. Collica, S. Davini, A. Derbin, X.F. Ding, A. Di Ludovico, L. Di Noto, I. Drachnev, K. Fomenko, A. Formozov, D. Franco, F. Gabriele, C. Galbiati, M. Gschwender, C. Ghiano, M. Giammarchi, A. Goretti, M. Gromov, D. Guffanti, C. Hagner, T. Houdy, E. Hungerford, Aldo Ianni, Andrea Ianni, A. Jany, D. Jeschke, V. Kobychev, D. Korablev, G. Korga, T. Lachenmaier, M. Laubenstein, E. Litvinovich, F. Lombardi, P. Lombardi, L. Ludhova, G. Lukyanchenko, L. Lukyanchenko, I. Machulin, G. Manuzio, S. Marcocci, J. Maricic, J. Martyn, E. Meroni, M. Meyer, L. Miramonti, M. Misiaszek, V. Muratova, B. Neumair, L. Oberauer, B. Opitz, V. Orekhov, F. Ortica, M. Pallavicini, L. Papp, O. Penek, L. Pietrofaccia, N. Pilipenko, A. Pocar, A. Porcelli, G. Raikov, G. Ranucci, A. Razeto, A. Re, M. Redchuk, A. Romani, N. Rossi, S. Rottenanger, S. Schönert, D. Semenov, M. Skorokhvatov, O. Smirnov, A. Sotnikov, L.F.F. Stokes, Y. Suvorov, R. Tartaglia, G. Testera, J. Thurn, M. Toropova, E. Unzhakov, A. Vishneva, R.B. Vogelaar, F. von Feilitzsch, S. Weinz, M. Wojcik, M. Wurm, Z. Yokley, O. Zaimidoroga, S. Zavatarelli, K. Zuber, G. Zuzel,

Dipartimento di Fisica, Università degli Studi e INFN, 20133 Milano, Italy

Measuring all neutrino components is the most direct way to test the standard solar model (SSM). Despite the great results obtained so far, important questions such as the solar metallicity remain open. A precise measurement of the solar pp chain and the CNO cycle would settle this controversy between high (HZ) and low (LZ) metallicity compositions of the Sun. Solar neutrinos allow the determination of oscillation parameters, in particular the $\theta_{12}$ mixing angle and, to a lesser degree the $\Delta m^{2}_{12}$ mass splitting. Furthermore the measurement of the electron neutrino survival probability Pee as a function of neutrino energy allows one to directly probe the MSW-LMA mechanism of neutrino oscillations In this work I will report the first simultaneous precision spectroscopic measurement of the complete pp-chain and its implications for both solar and neutrino physics with the Borexino detector.

PRESENTED AT

NuPhys2018, Prospects in Neutrino Physics Barbican Centre, London, UK, December 19–21, 2018

1 First Simultaneous Precision Spectroscopy of pp, 7Be, and pep

The Borexino $PhaseII$ started after an extensive purification campaign consisting in six cycles of closed-loop water extraction, during which the radioactive contaminants were significantly reduced to: 238U $<$ 9.4 $\times$ 10*-20* g/g (95% C.L.), 232Th $<$ 5.7 $\times$ 10*-19* g/g (95% C.L.), 85Kr, reduced by a factor $\sim$ 4.6, and 210Bi, reduced by a factor $\sim$ 2.3.

For each event, the energy, the position and the pulse shape are reconstructed by exploiting the number of detected photons and their detection times. The energy resolution is $\sim$ 50 keV at 1 MeV. The hardware energy threshold is $N_{p}$ $>$ 20, (total number of triggered PMTs) which corresponds to $\sim$50 keV.

Events are selected removing internal (external) muons and applying a 300 (2) ms veto to suppress cosmogenic backgrounds. These vetos led to a total dead-time of about 1.5%.

The 214Bi -214Po fast coincidences from the 238U chain and unphysical noise events are removed; the fraction of good events removed is $\sim$ 0.1% and it is estimated using MonteCarlo (MC) simulations and calibration data [1].

A Fiducial Volume (FV) cut is defined in order to reduce background from sources external to the scintillator in particular from the nylon vessel, from the SSS, and from PMTs. Thanks to this FV the innermost region of the scintillator is selected (71.3 t), contained within the radius R $<$2.8 m and the vertical coordinate -1.8 $<$ z$<$ 2.2 m.

After these cuts the main background is due to radioactive isotopes in the scintillator itself: 14C ($\beta^{-}$ decay, Q = 156 keV), 210Po ($\alpha$ decay, E = 5.3 MeV quenched by a factor $\sim$10), 85Kr ($\beta^{-}$ decay, Q = 687 keV), and 210Bi ($\beta^{-}$ decay, Q = 1160 keV) from 210Pb. An additional background is also due to the pile-up of uncorrelated events coming mostly from 14C, external background, and 210Po  [1]. Other important contributions to the background are the residual external background, mainly due to $\gamma$’s from the decay of 208Tl, 214Bi, and 40K and the cosmogenic isotope 11C ($\beta^{+}$ decay, $\tau$ = 29.4 min) that is continuously produced by muons through spallation on 12C. The Collaboration has developed a method called Three-Fold Coincidence (TFC) by which it it possible to tag events correlated in space and time with a muon and a neutron (11C is often produced together with one or even a burst of neutrons). Furthermore, in order to better disentangle 11C events, a $e^{+}/e^{-}$ pulse-shape discrimination is applied [2, 3]. The TFC algorithm has (92 $\pm$ 4)% 11C-tagging efficiency.

In order to extract the interaction rates of the solar neutrinos and the background species we maximize a binned likelihood function (through a multivariate approach) built as the product of 4 different factors; the TFC-subtracted energy spectrum, the TFC-tagged energy spectrum, the PS-$\mathcal{L}_{\rm{PR}}$ and the radial distributions of the events.

In the fit procedure the neutrinos signal and the background reference spectral shapes are obtained with two complementary strategies; a first one based on the analytical description of the detector response function, and a second one fully based on MC simulations.

The interaction rates of pp, 7Be, and pep neutrinos are obtained from the fit together with the decay rates of 85Kr, 210Po, 210Bi, 11C, and external backgrounds due to $\gamma$ rays from 208Tl, 214Bi, and 40K.

Because the degeneracy between the CNO $\nu$ and the 210Bi spectral shapes we have constrained the CNO $\nu$ interaction rate to the HZ-SSM predictions, including MSW-LMA oscillations to 4.92 $\pm$ 0.55 cpd/100 t [4] [5], (3.52 $\pm$ 0.37 cpd/100 t in case of LZ-SSM). The contribution of 8B $\nu$’s has been fixed to the HZ-metallicity rate 0.46 cpd/100 t.

The 7Be solar $\nu$ flux is the sum of the two mono-energetic lines at 384 and 862 keV. The corresponding rate for the 862 keV line is 46.3 $\pm$ 1.1${}^{+0.4}_{-0.7}$ cpd/100 t, and it is compatible with the Borexino $phaseI$ measurement. The total uncertainty of 2.7% for 7Be solar $\nu$ represents a factor of 1.8 improvement with respect $phaseI$ result and is two times smaller than the theoretical error.

The pp interaction rate is compatible with precedent results and its uncertainty is reduced by about 20%.

To extract the pep neutrino flux we constrain the CNO one. With our sensitivity the 7Be and pp $\nu$ interaction rates are not affected by the hypothesis on CNO (i.e. $\nu$’s HZ hypothesis vs LZ hypothesis). However, the pep $\nu$ interaction rate depends on it, being 0.22 cpd/100 t higher if the LZ hypothesis is assumed. In both cases the absence of pep reaction in the Sun is rejected at more than 5 $\sigma$.

The $e^{-}$ recoil spectrum induced by CNO neutrinos and the 210Bi spectrum are degenerate and this makes impossible to disentangle the two contributions with the spectral fit. Due to this spectrum degeneration, it is only possible to provide an upper limit on the CNO neutrinos contribution, and in order to extract this number, we have further to break the correlation between the CNO and pep contributions. We exploit the theoretically well known pp and pep flux ratio in order to indirectly constraint the pep $\nu$’s contribution. The interaction rate ratio R(pp/pep)  is constrained to  (47.8 $\pm$ 0.8) (HZ) [4], [5] (Constraining R(pp/pep) to the LZ hypothesis value 47.5 $\pm$ 0.8 gives identical results). We obtain an upper limit on CNO $\nu$ rate of 8.1 cpd/100 t (95 $\%$ C.L.).

It is possible to combine the Borexino results on pp and 7Be $\nu$ fluxes in order to measure experimentally the ratio $\mathcal{R}$ between the rates of the 3He-4He and the 3He-3He reactions occurring within the pp chain [6]. The value of $\mathcal{R}$ tell us the competition between the two primary modes of terminating the pp chain and for this reason represent a valuable probe of solar fusion. In first approximation we can neglect the pep and 8B $\nu$ contribution and $\mathcal{R}$ can be written as  2 $\Phi$(7Be)/$[\Phi$(pp)-$\Phi$(7Be)] . The measured value is $\mathcal{R}$ = 0.178 +0,027*-0,023*, in agreement with the predicted values for $\mathcal{R}$ = 0.180 $\pm$ 0.011 (HZ) and 0.161 $\pm$  0.010 (LZ) [4].

2 Improved measurement of 8B solar neutrinos with 1.5 kt$\cdot$y exposure

For what concern the analysis 8B the energy threshold is set at 1650 p.e., which correspond to 3.2 MeV electron energy. The analysis is based on data collected between January 2008 and December 2016 and corresponds to 2062.4 live days of data. Data collected during detector operations such as scintillator purification and calibrations are omitted. The dataset is split into a low energy range (LE), with [1650, 2950] p.e., including events from natural radioactivity, and a high energy range (HE), with [2950, 8500] p.e.. This high energy region is dominated by external $\gamma$-rays following neutron capture processes on the SSS. Results from the HE sample use data from the entire active volume, while the LE sample requires a spatial cut to remove the top layer of scintillator (the motivation is due to the presence of PPO from the scintillator leak in the upper buffer fluid volume).

The total exposure is 1,519 t$\cdot$y, and the time-averaged mass is 266.0$\pm$5.3 ton (assuming a scintillator density of 0.8802 g/cm3). For the LE sample the mass fraction, after the z-cut at 2.5 m, is 0.857$\pm$0.006.

The High Energy data sample is fitted with only two components, the 8B neutrinos and the external component from neutron captures, while the Low Energy sample requires three additional fit components, all due to 208Tl that is present in the bulk dissolved in the scintillator, at the surface intrinsic to the nylon vessel, and from emanation diffused from the nylon vessel into the outer edge of scintillator.

3 $P_{ee}$ and 7Be and 8B $\nu$ fluxes

We can write the electron neutrino survival probability as function of the neutrino energy as shown in figure 1 (left). The value for flavor conversion parameters from the MSW-LMA solution are ($\Delta$m${}_{12}^{2}$=7.50$\times$10*-5* eV2, tan${}^{2}\theta_{12}$=0.441, and tan${}^{2}\theta_{13}$=0.022 [7]). For the 8B neutrino source both the high-Z B16 (GS98) SSM and the low-Z B16 (AGSS09met) SSM are assumed [4, 8, 9]. Dots represent the Borexino results from pp (red), 7Be (blue), pep (azure), 8B neutrino measurements are in green for the LE+HE range, and grey for the separate sub-ranges. For the non mono-energetic pp and 8B dots are set at the mean energy of detected neutrinos, weighted on the detection range in electron recoil energy. The error bars include experimental and theoretical uncertainties

A first hint toward the solution of the solar metallicity problem could be obtained from the measurement of 7Be and 8B $\nu$ fluxes. We can define the reduced fluxes $f_{\rm Be}$ and $f_{\rm B}$ ($f_{\rm Be}$ = $\Phi$(7Be)/$\Phi$(7Be)HZ, $f_{\rm B}$ = $\Phi$(8B)/$\Phi$(8B$)_{\rm HZ}$). When we combine the new Borexino results on $\nu$ interaction rate with all the solar and KamLAND data we obtain the regions of allowed values. Figure 1 (right) shows the allowed contours together with the 1$\sigma$ theoretical predictions for high metallicity and low metallicity SSM. There is a weak hint towards the HZ hypothesis, which is however not statistically significant; the discrimination between the high and low metallicity solar models is largely dominated by the uncertainties of the theoretical models.