Deciphering the solar neutrino fluxand properties with Borexino
Gioacchino Ranucci
Istituto Nazionale di Fisica Nucleare
MilanoAugust 20, 2021

Formulation of the nuclear hypothesis on how the Sun and stars shine“If, indeed, the sub-atomic energy in the stars is being freely used to maintain their great furnaces, it
seems to bring a little nearer to fulfillment our dream of controlling this latent power for the wellbeing of the human race---or for its suicide” A. Eddington

1938 Von Weizsacker and Bethe→ independent
formulation of the CNO cycle
1938 Bethe → Formulation of the p-p chain andfull definition of the nuclear hypothesis

2G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

How to prove it ?
Hypothesis : there are nuclear
reactions occurring in the coresummarized as
4 H -> He + 2e +heat +2 neCan it be proved experimentally?
Yes, neutrinos coming from the reactions are the smoking gun! They pass undisturbed through thesolar matter and if detected at Earth they would prove unambiguously the nuclear hypothesis;
possibility debated in the context of the discussions about neutrino detection just after the world war II(Pontecorvo ‘47) From this trigger the Solar Neutrino Saga - a five decades long successful

experimental drama culminated with the Borexino results

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino3

Where we are today theoretically: SSM Solar
neutrinos production and spectrum predictions
Achieved withpersistentdedication anddevotion byJohn Bahcall
over morethan 40 yearsof enduring
efforts

CNO cycle
Dominant inmassivestars

pp chain∼99% of the energy in the Sun

Latest SSM
spectral prediction
A. Serenelli
EPJA, volume 5,
id 78 (2016)
N. Vinyoles et al.
The AstrophysicalJournal, 835:202(16pp), 2017

the remaining ∼1% ?

MeVControversy about the surface
metallicity composition of theSun: predictions differ up to28% for the CNO ν flux using
lower (LZ) or higher Z (HZ)
modelsG. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

4Borexino

Timeline from May 2007 up to now – three phases May 07 – August 10 I / December 11 – June 16 II / July 16– Feb 20 III – still ongoing for a few weeks – approaching the end of a great story

Summary of the achievements
 After the latest CNO result - achieved the full solar neutrino spectroscopy in a single experiment through theindividual real time detection of each ν spectral component – definite proof of the nuclear hypothesis
Unique Validation in the low energy regime of the LMA-MSW ν oscillation paradigm through the experimental
determination of the Pee electron neutrino survival probability of solar neutrinos
5G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

The Borexino
detector @Gran Sasso
Activevolume 280
tons of liquid
scintillator
Detection principle

ν x + e →ν x + e

Elastic scattering off theelectrons of the scintillator
threshold at ~ 60 keV(electron energy)
18 m high
20 m diameter
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

6

7

Borexino inner detector

Borexino
Inner vessel installation
May 3, 2004
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

89Detection principle

ν x + e →ν x + e
Elastic scattering off the electron of the scintillator
threshold at ~ 60 keV (electron energy)
Capabilities of the experiment : (in read tasks already accomplished)
7Be flux (862 keV),

8B with a lower threshold down to 3 MeV,

and hep limit
pep (1.44 MeV) coupled to a tight limit on CNO,
Geo-antineutrinos (beyond the scope of the talk, see talk of S. Zavatarelli in the yesterday neutrino session)
pp neutrinos
Supernovae neutrinos
CNO flux (for details see talk of N. Rossi in the yesterday neutrino session)
full solar ν-spectroscopy in one experiment ! Initial task only 7Be
all requiring ultra-low background especially the solar measurements → the big
10challenge of the experiment! → turned into an incredible success!!
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

Results made possible bya) Ultra-low backgroundb) Thorough calibration of the detector with internal and externalsourcesc) A detailed MC able to reproduce accurately the calibration resultsd) High statisticse) Threefold coincidence to remove the in-situ cosmogenic 11Cf) Pulse shape discrimination (property of LS) to cope with α
background and a series of effective cuts to remove muons andexternal background
Extraction of the fluxes through a data-to-model fit
Phase I May 2007 – May 2010
Phase II December 2011 - July 2016
Purification in between and Phase III from July 2016
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino11The
backgroundsaga → thequest forthe ultimatepurityFinal inphase IMay2010

0.99992

2.7 x10-18 14 C/12C

With thispurity firstindividual,
separatemeasurementsof 7Be and,pep fluxes,
and first low
thresholddetection of8B flux
→ Outcomeof phase I

5.35 ± 0.5×10−18
3.8 ± 0.8 × 10−18g/g

20 times better
than the design
value

Bismuth-21041.0±1.5±2.3
c/d/100t

12

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

Purification between phase I and II
Further achievements based on improved backgrounds after the purificationPurification (water extraction and nitrogenstripping) astonishingly effective in further
reducing the already ultralow background
Evaluated through the delayed coincidence tag

Th< 5.7 10-19 g/g 95% C.L.
U < 9.4 10-20 g/g 95% C.L.
Kr< 7.1 cpd/100 tons 95% C.L.
Only sizable residual
backgrounds:
210Po factor 100 less than at

the beginning of data taking
210Bismuth (the mostrelevant) factor 2 less than inphase I

Just after the purification - later
210Po further decreased as effect of
decay and of the subsequent
thermal stabilization which stopped
the recontamination from thevessel surface
Achievement following these data
purity improvement →Nature, Volume 512, Issue
7515, pp. 383-386 (2014)
Nature, Volume 562, Issue
7728, pp. 505-510 (2018)
The measurement of the fundamental
pp flux (Nature 2014) andsimultaneous high precision low
energy spectroscopy of the pp chain(Nature 2018) Outcome of phase II
Furthermore set the stage for the CNOquest (phase III)

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino13Low energy range (0.14-2 MeV) calibration

Energy scale-Resolution
R(m)

5%
E

@ MC tuned on γ source resultsfrom 200 keV to 2 MeV

Beyond 2 MeV: γ from n capture on Cand H@ Determination of Light yield and of the Birks parameter kB
L.Y.
obtained from the γ calibration sources with MC: ~ 500 p.e./MeV
left as free parameter in the total fit in the analytical approach@ Precision of the energy scale global determination: max deviation 1.5%
@ Fiducial volume uncertainty:+ 0.5−1.3 % ( 1 σ)(radon sources)
14

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

MC prediction ofsignal + intrinsic
Background
Calibration runs
spatial distribution

14C

7Be edge

Main visible features

11C

cosmogenic

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

15

Actual data histogram
The same features
noted in the MC
spectrum plus the 210Po
out of equilibrium peak210Po

an ubiquitous isotope
in all low backgroundexperiments always
accompanied by theprecursor 210Bi

Even at the Borexino very high radiopurity conditions, we still have background events
contaminating our solar neutrino signal and we need to apply software cuts to data, in order toremove as much background as possible. Furthermore, we need a powerful tool to separate thesignal from the residual background components.
16

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

Reduction of the 11C via the threefold coincidence

11C can be tagged and removed exploiting the space and time correlation

between the parent muon the spallation neutron(s) and the 11C β+ decay

17

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

Phase II data simultaneous low energy spectroscopy
data-to-model fit

Nature, Volume 562, pp. 505-510 (2018) and PhysicalReview D, Volume 100, Issue 8, id.082004 (2019)

In particular, more than 5σ evidenceof the tiny pep signal

18

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

Nature, Volume 562, pp. 505-510 (2018)
and Physical Review D, Volume 100, Issue 8, id.082004 (2019)

Summary of the low energy pp chain flux results from the previous fit

19

Beginning of the precision era in the study of low energy solar neutrinos
7Be precision 2.7%

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

pp study completed with the 8B determination with phase II data
Background subtraction and spectral analysis in two energy ranges
PHYSICAL REVIEW D 101, 062001 (2020)

3 MeV electron threshold
5 MeV electron en. boundary between LE and HE
Moreover limit on the hep flux < 1.8×105 cm−2 s−1Completion of the investigation of the pp chainError halved

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

Phase I
Phase II20

Closing on the pp chain burning mechanism experimental investigation
The complete spectroscopy from pp to 8B represents the first and unique full
determination of the pp cycle → final crowing of the experimental quest forthe burning mechanism fueling the Sun!
Quantitative probeof the pp solar fusion
long advocated byJohn Bahcall
Nature, Volume 562, pp. 505-510 (2018)

Borexino completes and closes
the experimental quest of the pp
mechanism powering the Sun

Moreover, using Borexino results we can calculate the neutrino solar luminosity which is found to be well inagreement with the measured photon luminosity
Further confirmation of the nuclear origin of the solar power. It proves that the Sun has been inthermodynamic equilibrium over the last 105 years, the time required for radiation to flow from the center tothe surface of the Sun
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

21

The global oscillation picture: survival probability of the electron
neutrinos contrasted with the improved Borexino results of phase II
Nature, Volume 562, pp. 505-510 (2018)
Vacuum regime

LMA prediction
Matter regime

FROM BOREXINO ALONEVALIDATION OF THE LMAMSW OSCILLATIONSOLUTION OVER THE FULLSOLAR NEUTRINO SPECTRUM

Reinforced by theimproved precision of thephase II data

Before the Borexino results“Although historicallyby measuring ∆m221 KamLAND
has uniquely selected the LMA
7Be 2.7%
solution, now the solar
neutrino experiments alone
Simultaneous low energy
can do this due to new
spectroscopy
measurements by Borexino,
whichPee curve (magenta band) as expected from ν oscillation+Matter effect (LMA-MSW) validated the solution at low
energies, and due to higher
Day-Night asymmetry of 7Be
accuracy of other results.“neutrinos consistent with 0M. Maltoni and A.Yu. Smirnov
in agreement with the LMAEPJA 52 , 87 2016
MSW expectation
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

22

The next and last step of theBorexino journey throughout thenuclear reactions powering theSUN : detection of solar neutrinos
from the CNO cycle
Phase III dataJuly ‘16 – Feb ‘20
Main issue: 210Bi background indistinguishable from the CNO spectrum 210Bi/CNO degeneracy
Handle: 210Bi determination via intrinsic (in the LS) 210Po successor
Requirement: stop of the extra 210Po from the surface of the vessel due to convective motions of the scintillator
Action on the detector: thermal stabilization to maintain the liquid as still as possible - very challenging task

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

23

Multiple approaches to monitor, understand,and suppress the temperature variationsThermal insulation &Active GradientStabilization System

Temperature monitoring probes

Fluid dynamical simulation
Very good agreement withmeasured temperatures

time54 temperature probes

•••
•

6/04
2015

10/06
2015

14/08
2015

18/10
2015

22/12
2015

Double layer of mineral wool (thermal conductivity down to 0.03 W/m/K) & Active Gradient Stabilization
System (2014-2016)
Temperature Probes (2014-2015)
V. di Marcello et al., NIM A 964, id. 163801
Fluid dynamical simulations
Hall C Temperature Stabilization (2019)

Enduring effort over the past seven years

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

24Temperature evolution from the probes
Global view of stabilization from 2015
Probes sensing
the outer buffer

Snapshot of the last year
Achieved excellent temperaturestability with the establishmentof a clear temperature gradient
Probes resolution 0.07 °C
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino25

A 2D detailed view - Polonium data spatial mapping vs. time
Crucialobservation:“Low PoloniumField” 20 tons
size from which
we can infer
the intrinsic
210Po andhence the 210Bi- agreementwithsimulations
Convective
condition before
insulation

Quiet situation
after insulation

Stabilization measures were very
effective at reducing the 210Po motion

1. Beginning of the Insulation Program
2. Turning off the water recirculation
system in the Water Tank
3. Start of the active temperature control
system operations
4. Change of the active control set points
5. Installation and commissioning of theHall C temperature control system.

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

26

Determination of the 210Po minimum
by fitting its spatial distribution
Spline fit:

Paraboloidfit:

Account for complexity along the zaxis with a cubic spline modelusing a Bayesian nested samplingalgorithm

Both methods agree within systematics:
210

See next slides
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

See next slides
27

210Po and 210Bi final numerical assessment
Basis of the approach

210Po rate inferred from the Low Polonium Field with all errors
210

The 210Po evaluated rate still possibly contaminated with residual 210Po fromthe vessel surface → upper limit to the rate of 210Bi

R(210Bi) ≤ 11.5 ± 1.3 cpd/100t

Sought constraint essential to break the 210Bi/CNO degeneracy →Outcome of the relentless years-long effort to stabilize the detector
and understand the 210Po behavior in the Inner Vessel
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

28

CNO-ν analysis: Phase-III MV fit
TFC-subtracted spectrum

TFC-tagged spectrum

rate constrained → solar luminosity constraint
Multivariate fit (0.32-2.64 MeV) pep-ν210Bi rate constrained→ 210Bi-210Po tagging
July ‘16 – February ‘20→ free to vary
CNO rate
Maximization of a binned
likelihood 3 distributions
simultaneously:• Reconstructed energy for TFCtagged and TFC-subtracted
datasets (11C identification)• Radial position

Other ν and bkg rates→ free to vary

Result
CNO best fit 7.2 cpd/100t
asymmetric confidence interval -1.7 +2.9 cpd/100t(stat only) asymmetry ↔ 210Bi upper limit

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

29

Systematic sources and final CNO-ν result
Monte Carlo → simulate distorted datasets,
fit with un-distorted PDFs

1) Fit conditions→ negligible

4) 210Bi spectral
shape (diff.~18%)

2) 11C spectrum
deformation by noise cuts

Final syst:-0.5 +0.6
cpd/100t

3) Energy response function: energy scale (~0.23%),
non-uniformity (~0.28%), non-linearity (~0.4%) fromdetector calibration

Final CNO result 7.2 (-1.7 +3.0) cpd/100t stat + sys
corresponding to a flux of neutrinos on
Earth of 7.0 (−1.9 +2.9)×108 cm−2 s−1

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

30

Significance of CNO-ν detection
Likelihood ratio test
Determination of the q0 discovery test statistic fromG. Cowan et al., Eur.
the likelihood with and without the CNO signal

Phys. J. C, 71:1554,2011

Remember:
CNO provides a handle for
the Sun metallicity
Compatible with HZ and LZ
but comparing pp, 7Be, CNO,
8B fluxes measured byBorexino with SSM-LZ →rejected @ 2.1 σ

13.8 millions pseudo-datasetswith deformed PDFs and noCNO to determine the q0
reference distributionq0(data) from the real dataset
From the MC distributions p-value of q0 (greycurve) with respect to q0 (data) (black line) →correspondingly significance greater than 5 σ at
99% CL0Consistent with 5.1σ through the log-likelihoodfrom the fit folded with uncertainties

No CNO hypothesis disfavored at 5 σ

With these results Borexino marked thefirst detection ever of CNO solar neutrinos

Nature, Volume 587,
Issue 7835, p.577-582(2020)

G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

31ConclusionsThe long journey of Borexino to decipher the mysteries of the neutrinos from the Sun has produced the following
enduring scientific legacies
 Full solar neutrino spectroscopy in only one experiment which has enabled• Complete investigation the main pp fueling mechanism of the Sun• first detection ever of the CNO neutrinos with 5 σ significance which confirms the existence of this energy
generating cycle dominant in the more massive stars
 Validation at of the MSW-LMA ν oscillation solution by Borexino alone over the entire solar neutrino
spectrumWith these outcomes Borexino has accomplished its mission by completely unraveling the two processes powering
the Sun and the stars
the pp Chain and the CNO Cycle
G. Ranucci/INFN Milano - Deciphering the solar neutrino flux and properties with Borexino

32