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 \u3b812 mixing angle and, to a lesser degree the \u394m212 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

L. Miramonti

AIR Universita degli studi di Milano

SNSN-323-63
January 30, 2019
Recent results on pp-chain solar neutrinos
with the Borexino detector
Lino Miramonti on behalf of the Borexino Collaboration1
Dipartimento di Fisica, Universita` 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 θ12 mixing angle and, to a lesser degree the ∆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 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. Lom-
bardi, 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. Mura-
tova, 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. Red-
chuk, A. Romani, N. Rossi, S. Rottenanger, S. Scho¨nert, D. Semenov, M. Skorokhvatov, O. Smirnov,
A. Sotnikov, L.F.F. Stokes, Y. Suvorov, R. Tartaglia, G. Testera, J. Thurn, M. Toropova, E. Un-
zhakov, A. Vishneva, R.B. Vogelaar, F. von Feilitzsch, S. Weinz, M. Wojcik, M. Wurm, Z. Yokley,
O. Zaimidoroga, S. Zavatarelli, K. Zuber, G. Zuzel,
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1 First Simultaneous Precision Spectroscopy of pp,
7Be, and pep
The Borexino PhaseII started after an extensive purification campaign consist-
ing in six cycles of closed-loop water extraction, during which the radioactive con-
taminants were significantly reduced to: 238U < 9.4× 10−20 g/g (95% C.L.), 232Th
< 5.7× 10−19 g/g (95% C.L.), 85Kr, reduced by a factor ∼ 4.6, and 210Bi, reduced by
a factor ∼ 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 ∼ 50 keV at 1 MeV. The hardware energy threshold is Np> 20, (total
number of triggered PMTs) which corresponds to ∼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 ∼ 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 scintil-
lator itself: 14C (β− decay, Q = 156 keV), 210Po (α decay, E = 5.3 MeV quenched by a
factor ∼10), 85Kr (β− decay, Q = 687 keV), and 210Bi (β− 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 con-
tributions to the background are the residual external background, mainly due to γ’s
from the decay of 208Tl, 214Bi, and 40K and the cosmogenic isotope 11C (β+ decay, τ
= 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 ap-
plied [2, 3]. The TFC algorithm has (92± 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-LPR and the radial distributions of the events.
In the fit procedure the neutrinos signal and the background reference spectral
1
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 γ rays from 208Tl, 214Bi, and 40K.
Because the degeneracy between the CNO ν and the 210Bi spectral shapes we
have constrained the CNO ν interaction rate to the HZ-SSM predictions, including
MSW-LMA oscillations to 4.92 ± 0.55 cpd/100 t [4] [5], (3.52 ± 0.37 cpd/100 t in
case of LZ-SSM). The contribution of 8B ν’s has been fixed to the HZ-metallicity rate
0.46 cpd/100 t.
The 7Be solar ν 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± 1.1+0.4−0.7 cpd/100 t, and it is com-
patible with the Borexino phaseI measurement. The total uncertainty of 2.7% for
7Be solar ν 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 ν interaction rates are not affected by the hypothesis on CNO (i.e.
ν’s HZ hypothesis vs LZ hypothesis). However, the pep ν 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 5σ.
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 ν’s contribution. The interaction rate ratio R(pp/pep) is constrained to (47.8
± 0.8) (HZ) [4], [5] (Constraining R(pp/pep) to the LZ hypothesis value 47.5 ± 0.8
gives identical results). We obtain an upper limit on CNO ν rate of 8.1 cpd/100 t (95
% C.L.).
It is possible to combine the Borexino results on pp and 7Be ν fluxes in order to
measure experimentally the ratioR between the rates of the 3He-4He and the 3He-3He
reactions occurring within the pp chain [6]. The value of 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 ν contribution and R can be written as 2 Φ(7Be)/[Φ(pp)-Φ(7Be)] . The
measured value is R= 0.178+0,027−0,023, in agreement with the predicted values for
R= 0.180± 0.011 (HZ) and 0.161± 0.010 (LZ) [4].
2
2 Improved measurement of 8B solar neutrinos with
1.5 kt·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 γ-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·y, and the time-averaged mass is 266.0±5.3 ton (as-
suming 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±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 Pee and
7Be and 8B ν 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 (∆m212=7.50×10−5 eV2, tan2θ12=0.441, and tan2θ13=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 ν fluxes. We can define the reduced fluxes fBe
and fB (fBe = Φ(
7Be)/Φ(7Be)HZ, fB = Φ(
8B)/Φ(8B)HZ). When we combine the new
Borexino results on ν interaction rate with all the solar and KamLAND data we obtain
the regions of allowed values. Figure 1 (right) shows the allowed contours together
3
with the 1σ 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.
Figure 1: Left: electron neutrino survival probability as function of the neutrino
energy. Right: allowed contours in the fBe-fB parameter space (see text).
References
[1] Borexino Collaboration Nature 562, 505 (2018)
[2] G. Bellini et al. Physical Review Letters 108, 051302 (2012)
[3] G. Bellini et al. Physical Review D 89, 112007 (2014)
[4] N. Vinyoles, A. M. Serenelli, F. L. Villante, S. Basu, J. Bergstro¨m, M. C.
Gonzalez-Garcia, M. Maltoni, C. Pen˜a-Garay, and N. Song, The Astrophysical
Journal 835, 202 (2017).
[5] I. Esteban et al. Journal of High Energy Physics 01, (2017)
[6] C. Pen˜a-Garay, Journal of High Energy Physics 11 (2003)
[7] I. Esteban, M. C. Gonzalez-Garcia, M. Maltoni, I. Martinez-Soler, and T.
Schwetz, JHEP 01, 087 (2017)
[8] N. Grevesse and A. Sauval, Space Science Reviews 85, 161 (1998).
[9] M. Asplund, N. Grevesse, A. J. Sauval, and P. Scott, Ann. Rev. Astron. Astro-
phys. 47, 481 (2009),
4


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

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

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