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DOI 10.1393/ncc/i2020-20021-8
Colloquia: IFAE 2019
IL NUOVO CIMENTO 43 C (2020) 21
A strategy for the detection of CNO solar neutrinos
with the Borexino experiment
D. Basilico on behalf of the Borexino Collaboration(∗)
Dipartimento di Fisica, Universit`a degli Studi e INFN, Sezione di Milano - 20133 Milano, Italy
received 8 June 2020
Summary. — Borexino is a large liquid scintillator detector with unprecedented
intrinsic radiopurity levels, located at the LNGS laboratory in Italy. Its primary
goal is to perform a real-time solar neutrinos spectroscopy, and the most recent
result consists in the simultaneous measurement of the fluxes of neutrinos from the
pp, 7Be and pep reactions, from the pp chain. The current goal of the Borexino
Collaboration consists in the detection of the solar neutrinos from the CNO cycle
reactions: a general strategy to pursue this measurement will be outlined in the
following.
1. – Solar neutrinos
Solar neutrinos are generated in the innermost layers of the Sun by means of two
sequences of nuclear fusion reactions. The main contribution to the solar luminosity
(∼99%) comes from the so-called pp chain reactions, while the CNO cycle, according to
the Standard Solar Models predictions, should play a secondary role [1, 2].
(∗) D. Basilico, M. Agostini, K. Altenm¨uller, S. Appel, V. Atroshchenko, Z. Bagdasarian,
G. Bellini, J. Benziger, D. Bick, G. Bonfini, D. Bravo, B. Caccianiga, F. Calaprice, A. Caminata,
L. Cappelli, P. Cavalcante, A. Chepurnov, K. Choi, D. D’Angelo, S. Davini, A. Derbin, A. Di
Giacinto, V. Di Marcello, 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, E. Hungerford, Aldo Ianni, Andrea Ianni,
A. Jany, D. Jeschke, V. Kobychev, G. Korga, S. Kumaran, T. Lachenmaier, M. Laubenstein,
E. Litvinovich, P. Lombardi, I. Lomskaya, 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, M. Nieslony, L. Oberauer, V. Orekhov, F. Ortica,
M. Pallavicini, L. Papp, ¨
O. Penek, L. Pietrofaccia, N. Pilipenko, A. Pocar, G. Raikov, M. T.
Ranalli, G. Ranucci, A. Razeto, A. Re, M. Redchuk, A. Romani, N. Rossi, S. Rottenanger,
S. Sch¨onert, D. Semenov, M. Skorokhvatov, O. Smirnov, A. Sotnikov, Y. Suvorov, R. Tartaglia,
G. Testera, J. Thurn, E. Unzhakov, A. Vishneva, R. B. Vogelaar, F. von Feilitzsch, M. Wojcik,
M. Wurm, O. Zaimidoroga, S. Zavatarelli, K. Zuber and G. Zuzel.
Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 1
2D. BASILICO on behalf of the BOREXINO COLLABORATION
Neutrinos provide a unique and direct way to study the interior regions of our star.
The study of solar neutrinos is important from a double point of view. From the solar
physics side, it allows to investigate the Standard Solar Model (SSM) predictions; from
the particle physics side, it has been crucial in order to discover and understand the
neutrino flavour oscillations. The main goal of the solar neutrinos spectroscopy is the
determination of the contribution of the several reactions involved (pp, 7Be, pep, CNO,
8B), measuring the associated fluxes.
Neutrinos emitted in the CNO cycle are the only undetected piece of the solar fusion
mechanisms. Even though it is expected to contribute only for ∼1% to the solar lumi-
nosity, the CNO cycle is believed to be the main engine of very massive stars. Observing
neutrinos from the CNO cycle reactions would have therefore a striking importance in
astrophysics, since it would provide the first experimental proof of the existence of this
important source of energy in the core of the stars. Moreover, since the CNO cycle is
catalyzed by elements heavier than helium, the related neutrino flux is very sensitive to
the metal abundance in the Sun: a precise experimental measurement of the CNO-νflux
could help to shed light on the solar metallicity abundance problem [1,2].
2. – Borexino detector
Borexino is a large volume liquid scintillator detector, whose primary purpose is the
real-time spectroscopy of low-energy solar neutrinos [3]. It is located deep underground
(approximately 3800 meters of water equivalent) in the Hall C of the Laboratori Nazionali
del Gran Sasso, in Italy. The Gran Sasso mountain natural shielding, combined with the
detector structure, allows extremely high muon flux suppression. Borexino has been
taking data for more than ten years, achieving crucial results for what concerns the solar
neutrino spectroscopy [4], such as detecting and then precisely measuring the flux of the
7Be solar neutrinos and of the other pp-chain components [5], ruling out the day-night
asymmetry of their interaction rate [6], and setting the tightest upper limit so far on the
flux of CNO solar neutrinos [5].
The Borexino design is driven by the principle of graded shielding: an inner scin-
tillating ultra-pure core is found at the center of shielding concentric shells, with de-
creasing radio-purity from inside to outside. The extremely low intrinsic radioactivity
achieved in Borexino, the strong cosmic ray shielding, the high photon yield have made
a sensitive search for neutrinos in the sub-MeV and MeV energy range possible, measur-
ing their energy and position through the elastic scattering with scintillator electrons.
The scintillator is a solution of PPO (2,5-diphenyloxazole) in pseudocumene (PC, 1,2,4-
trimethylbenzene) with a concentration of 2.5 g/l. The scintillator mass is ab out 278 ton
and is contained in a 125 μm thick spherical nylon inner vessel (IV) of approximately
4.25 m radius. The detector is instrumented with 2112 photomultiplier tubes (PMTs)
that measure the intensity and the arrival time of this light, allowing the reconstruction
of the energy, position and time of the events.
Neutrino-induced events are intrinsically indistinguishable on an event-by-event ba-
sis from the most of backgrounds due to βor γdecays. Many selection cuts can
be applied to partially remove several categories of backgrounds on an event-by-event
basis: muons and muon-induced events, noise events, radioactive decays from de-
layed coincidences. The fiducial volume (FV) cut selects events in a central scin-
tillator region reducing the external background. The cosmogenic events can be
isolated via dedicated tagging techniques, exploiting the decay peculiarities of their
products.
A STRATEGY FOR THE DETECTION OF CNO SOLAR NEUTRINOS ETC. 3
Even after the application of the selection cuts, background is still present. To disen-
tangle the neutrino signal components from the background ones, a multivariate analysis
is performed. It involves the simultaneous fit to the distributions of three physical quan-
tities of interest: the event reconstructed energy, the radial and the β+/β−pulse-shape
parameter distributions. The reference distributions for these three quantities are built
using either analytical models or Monte Carlo simulations. Then, the model is fitted
against data to extract the interaction rates of each background and neutrino signal.
The Phase-II data taking (December 2011 - May 2016), which followed a purifica-
tion campaign, allowed to perform the complete spectroscopy of solar neutrinos from
the pp-chain, as described in ref. [5]. The most relevant result is the simultaneous mea-
surement of the interaction rates of pp-ν,7Be-ν, pep-νin an extended energy range
(0.19–2.93) MeV. All the pp-chain flux precisions have been improved with respect to
the Phase-I analysis (2007–2010), reaching 2.7% for 7Be-νand 9.5% for pp-ν. The pep-ν
discovery is claimed, rejecting at >5σsignificance the hypothesis of absence of the pep
reaction in the Sun.
3. – A strategy for the CNO neutrinos detection with Borexino
The current goal of Borexino is a measurement of the CNO-νsignal, analyzing the
data taken from half 2016 on. The preliminary step towards the detection of a CNO-ν
signal is to study via Monte Carlo simulations the sensitivity of Borexino under realistic
conditions: this allows to define which are the elements playing the most relevant roles
and therefore to elaborate a strategy for the analysis.
The Borexino sensitivity to a CNO-νsignal is driven by two main factors. The first is
the low signal/background ratio for CNO-νinteraction rate in Borexino. According to the
SSM predictions [1], indeed, the CNO-νreactions are expected to play a secondary role
in the solar reaction framework: this translates into a faint neutrino flux, and therefore
into a very low counting rate in Borexino. The expected CNO-νinteraction rate in
Borexino, according to the high-metallicity (HZ) and low-metallicity (LZ) scenarios, is
RHZ(CNO-ν)=4.92±0.56 cpd/100t or RLZ (CNO-ν)=3.56±0.37 cpd/100t, respectively.
In the CNO-νenergy region of interest, the leading contributions in Borexino are instead
provided by 7Be-ν,210Bi and 11C.
The CNO-ν, pep-νand 210 Bi spectral shape similarity is the second and most critical
factor preventing a straightforward CNO-νdetection in Borexino. Figure 1 shows the
simulated energy spectra of e−scattered by CNO-ν, pep-νand produced in the 210 Bi
decay (respectively, red line, blue line and green line), overlapped to Borexino Phase-II
data (black). The CNO-νevents energy shape in Borexino does not show any prominent
structure but, on the contrary, a smooth featureless profile, similar to the one of 210 Bi
background and pep-ν. This shape similarity prevents the multivariate fit from distin-
guishing and isolating separately the interaction rates of the three contributions and,
consequently, drastically reduces the sensitivity to a CNO-νsignal.
Dedicated sensitivity studies show that Borexino has the chance to isolate a CNO-ν
signal with relevant significance only if this 210 Bi-CNO-pep correlation is broken. The
only way to do this consists in the determination of the 210Bi background and pep-νrate
independently of the fit, to constrain them in the multivariate analysis itself.
The pep-νrate can be safely constrained to the Standard Solar Model predictions.
Along with the pp-νreaction, it starts the pp chain sequence; its neutrino flux, and
therefore the related interaction rate in Borexino, are estimated at the 1% precision level;
accounting also for the error on the flavour oscillation parameters, the expected rate in
4D. BASILICO on behalf of the BOREXINO COLLABORATION
−
−
−
ν
ν
Fig. 1. – Simulated energy spectra of e
−scattered by CNO-ν,bypep-νand produced in the
210Bi decay (respectively, red line, blue line and green line), overlapped with Borexino Phase-II
data (black). Injected rates for CNO-νand pep-νare matched to the HZ-SSM predictions, while
210Bi one has been selected from Phase-II results [5].
Borexino is RLZ(pep-ν)=2.78 ±0.04 cpd/100t or RHZ(pep-ν)=2.74 ±0.04 cpd/100t,
depending on the metallicity scenario.
The independent constraint on 210Bi is a crucial key of the CNO-νanalysis; in partic-
ular, its rate must be determined independently with a precision better than ∼10%–15%
in order to reach a median sensitivity for a CNO-νsignal of 3σor more. The proposed
strategy is based on the tagging with its daughter isotope 210Po, which decays emitting
a mono-energetic αof 5.3 MeV, quenched in the scintillator to an equivalent energy of
approximately 400 keV. αevents can be distinguished from βones by means of pulse-
shape discrimination techniques, which exploit the different time distributions of photons
emitted by αand βin the scintillator. In a nutshell, the signature of this class of events
in Borexino is a very clear Gaussian-like peak, which is easily recognizable with respect
to events originated in βdecays.
If the chain 210Pb →210 Bi →210Po is in secular equilibrium, the activity of 210Po is de
facto the same as the 210Bi one, making the measurement straightforward. Unfortunately,
this strategy is complicated by additional 210Po contributions out of equilibrium, which
are present in the detector. In particular, the most annoying 210Po contribution is given
by the contamination present on the inner vessel, which can detach from the nylon and
can be carried inside the innermost region of the scintillator (the fiducial volume in which
the analysis is done) by convective currents.
Analyzing the 210Po evolution in time, along with its spatial distribution in the fiducial
volume, it has been observed that these convective currents are triggered by variations
in the temperature profile of the scintillator, mainly due to the season changes. In
order to minimize this effect, the detector has been thermally insulated starting from
2015. A 20 cm thick layer of mineral wool have been installed around the detector; the
insulation was completed at the end of 2015. In addition, in 2016 a Temperature Active
Control System (TACS) has been installed on the top ring of the Water Tank. It provides
A STRATEGY FOR THE DETECTION OF CNO SOLAR NEUTRINOS ETC. 5
heat to compensate for the seasonal cooling in Fall and Winter time and it is useful to
further decouple the upper part of the detector from the Hall C temperature variations.
In spite of this, 210 Po motions due to convection have not stopped completely and a
dedicated study of the spatial profile of the 210Po distribution is needed to extract the
supported term which is directly related to 210Bi.
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