GEONEUTRINO DETECTION AND
OTHER NON-SOLAR PHYSICSACHIEVEMENTS OF BOREXINOSANDRA ZAVATARELLI (INFN – SEZIONE DI GENOVA – ITALY)

20th Lomonosov Conference, August 19-25 2021, Moscow State University

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TALK LAYOUT• The Borexino detector layout• Antineutrinos detection techniques withliquid scintillators• Borexino anti-n studies:- Geoneutrinos- Diffuse supernovae neutrino background- Solar flares n• Perspectives
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THE BOREXINO DETECTOR
Laboratori Nazionali del Gran Sasso (Italy)• the world’s radio-purest LS detector:<5.7×10-19 g(Th)/g LS,<9.5×10-20 g(U)/g LS at 95% C.L.

• ~500 p.e. / MeV• energy reconstruction: 5% @ 1 MeV• position reconstruction: 10 cm @ 1 MeV• pulse shape identification (α/β, e+/e-)

Operating since 20073

THE BOREXINO DETECTOR
Laboratori Nazionali del Gran Sasso (Italy)
Main electronics (LABEN): developed for solar nphysics• 14 racks/158 channels: the front-end (FE) electronic
generates two signals, i.e. the timing and charge
information of the PMT pulse. The charge
integrated ~ 80 ns, followed by a 60 ns dead time forwriting.
• Good linearity up to few MeV
FADC system• For higher energies, up to ~ 50 MeV, a system wasdeveloped consisting of 96 fast waveform digitizers(CAEN v896, 8 bit, FADC - Flash ADC), each ofthem reading-in the signal summed from up to 24PMTs, with the sampling rate of 400 MHz.

Operating since 2007
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MEV ANTI-NEUTRINO DETECTION WITH LIQUIDSCINTILLATORSElastic scattering on electrons

n+e→n+e

Single events, no threshold, all flavours

Inverse beta decay

ne + p → n + e+

Charge current, electron flavour onlyDelayed coincidence →clean signature!
Energy threshold = 1.8 MeV
t ~ 255 µs

Eprompt = Evisible~ Te+ + 2・511 keV~ Eantinu – 0.784 MeV

sIBD at few MeV: ~10-42 cm2 (~100 x more than scattering)5

GEO-NEUTRINOS AS
PROBES FOR DEEP EARTH• Geoneutrinos are the mostabundant component of anti-n
flux at Earth

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EARTH’S ENERGETICS
Sources: unclear picture…

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GEO-NEUTRINOS AS PROBES FOR DEEP EARTH
232Th

à 208Pb + 6 a + 4 e- + 4 ne + 42.8 MeV

à 206Pb + 8 a + 8 e- + 6 ne + 51.7 MeV
235U à 207Pb + 7 a + 4 e- + 4 n + 46.4 MeVe238U

HeatProducing
Elements

HPE’s40K

à 40Ca + e- + 1 ne + 1.32 MeV (89.3%)

40K+ e à 40Ar + e+ + 1 ne + 1.505 MeV (10.7%)
Geo-n fluxes => HPE’s abundances => Earth energetics
Crust + mantle geo-n signal (U+Th)

The Earth shines in anti-n(Fn~ 106 cm-2 s-1)

Mantle geo-n signal in the TOMO model

Fluxes nothomogeneous=> needs for
multi-sitemeasurements!!
8EXPECTED SIGNAL AT GRAN SASSO
Expected signal at Gran Sasso

Ingredients:1) Geo-neutrino energy spectra

1 TNU (Terrestrial Neutrino Unit) = 1 event/ 1032 target protons (~1kton LS)/ year (100% eff.)

S (U+Th) TNU

S(U)/S(Th)

Local Crust ( R 500 km)

9.2 + 1.2

0.24

Far Field Lithosphere16.7 +3.8-3.1

0.29

Mantle (from Bulk silicate
Earth model – lithosphere)

2.5 – 19.6

0.26

Total

28.4 - 45.5

0.27 (chondritic)

˜2) Local and global geological informations

40K signal

U & Th abundances and distribution

3) Propagation effects (oscillations..): Pee ˜ 0.5+interaction cross sections

belowthreshold
9IBD SELECTION CUTS :154 GOLDENCANDIDATES
M. Agostini et al PRD 101(2020) 012009• December 9, 2007 to April
28, 2019 : 3262.74 days ofdata taking• Average FV = (245.8 ±8.7) ton , Exposure = (1.29 ±0.05) x 1032 proton x year

• Including systematics on
position reconstruction andmuon veto loss, for 100%detection eff
Exposure : a factor 2 increase
respect to 2015 analysis

10SOURCES OF BACKGROUNDS
We need to estimate different contributions and then to extract the number of measured
geo-neutrinos by fitting the Eprompt energy spectrum;
Antineutrino backgrounds:(a)Reactor antineutrinos(b)Atmospheric neutrinos

Backgrounds mimicking inverse
beta decay reaction:(a)Cosmogenic nuclides(b)(a,n) reactions(c) Accidental coincidences

Prompt signal in
13C(a,n)16O

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BACKGROUNDS
Antineutrino backgrounds

Non antineutrino backgrounds

Expected signal at LNGS evaluated with dedicated codes
Reactor antineutineutrinos• For all ~440 world reactors (1.2 TW
total power): info on thermal powers,
laod factors.. from IAEA and PRIS
databases• Propagation effects included• Interaction cross section• Detection efficiency = 0.8955 ±0.0150

Atmospheric neutrinos••

Energy window> 1 MeV ( Q>408 p.e)

Events9.2 + 4.6

Atmospheric neutrino fluxes from HKKM2014 (>100 MeV) andFLUKA (<100 MeV)
Matter effects included, Simulation of detector response + selection
cuts as for real data•
Accidental coincidences;
Estimated from OFF-time coincidences: IBD-like events in Dt = 2 -20 s•(a, n) reactions: 13C(a, n)16OPrompt: scattered proton, 12C(4.4 MeV) & 16O (6.1 MeV)
Estimated from 210Po(a) and 13C contaminations, cross section.
•Cosmogenic background- 9Li and 8He (t1/2 = 119/178 ms)
decay: b(prompt) + n (delayed);- fast neutrons
Prompt :unscattered protons (prompt)
Estimated by studying coincidences detected AFTER muons.

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GEO-NEUTRINO SIGNAL : SPECTRAL FIT OF EPROMPT
Unbinned likelihood fit of charge spectrum of 154 prompts• Fixed: S(Th)/S(U) = 0.27 corresponding to chondritic Th/U
mass ratio of 3.9• 9Li, accidentals, and (a, n) bgr constrained according toexpectations• Reactor signal unconstrained and result compatible withexpectations
Ngeo =Prompt charge [p.e.]: 1 MeV ~500 photoelectrons

Systematic uncertainties

Nrea = 93.4+11.3-10.4

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TH/U RATIOSPECTRAL FIT with Th and U fit independently :50.4 events +46.8 -44.05% total precision
In agreement with the fit with Th/U fixed

From MC studies: no sensitivity to measure the Th/U ratio with the present statistics

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GEO-NEUTRINO SIGNAL FROM THE MANTLE
Bulk Silicate Earth’sModelsCosmochemical (CC)based on the enstatinechondrites

Geochemical (GC)based on mantle samples
compared withcarbonaceous chondrites

Geodinamical (GD)

Prompt charge [p.e.]: 1 MeV ~500 photoelectrons

Constraining the contribution from the bulk
lithosphere (28.8 ± 5.6 events with
S(Th)/S(U) = 0.29 ), the extracted
mantle events are 23.7+10.7-10.1
Sensitivity study using log-likelihood ratio method:
Null mantle signal hypothesis rejected with 99.0%
C.L.

Mantle radiogenic heat from U+Th:

Hmantle (U+Th) = 24.6 +11.1 -10.4 TW

based on balancing mantle
viscosity and heatdissipation

FR =Full radiogenic

Compatible with predictions, in tension at 2.4s
with the CosmoChemical models (CC)15

EARTH RADIOGENIC HEAT
Reminder: Htot =(47± 2)TW
CC = continental crustassuming 18% 40K mantle contribution + contribution of lithosphere

H(U+Th+K) =Convective Urey ratio:

URcv= 0.78 +0.41 −0.28

At 90% C.L., mantle characteristics:a(Th) >48 ppb & a(U) >13ppb URCV >0.13

Borexino estimates a high probability that the radioactive decays produce more than half of the total Earth’s heat.

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ANTINEUTRINOS FROM COSMOS
Several possible sources:• Diffuse supernovae neutrino background(*)• Solar flares(*)• GW events(**) and Gamma Ray Bursts(***)•…
*M. Agostini et al., Astroparticle Physics 125 (2021) 102509**M. Agostini et al. Astroparticle Physics 86, (2017) 1-17
SN remnant by HST***M. Agostini et al, The Astrophysical Journal (ApJ), 850-21, Nov. 201717

DIFFUSE SUPERNOVAE NEUTRINO BACKGROUND (DSNB)The Diffuse Supernova Neutrino Background is formed by the whole of the star collapsing during the evolution of theUniverse and consists of neutrinos and antineutrinos of all flavors.
•••

Analysis cuts : similar to geo-n analysis, but:- Smaller statistics sample : Dec 2007-Oct2017- Smaller FV : Dprompt from IV > 0.25 m- Stronger cosmogenic cut :RSN : supernova rate at adistance z ;Wm and WL : relative
densities of matter anddark energy
No unique model.

§ 2 s veto after ID muon
§ 2 ms veto after OD muon

Hüdepohl

Small flux → Large
underground detectors
particularly suited !!

Nakazato

Conservative limits on DSNB:
the minimal expected number of events
considered:•Detection channel : inverse beta decay (IBD)•Minimal Radiogenic Earth model for geo-n:
only radioactivity from the crust
Normalization to DayBay spectra forreactor-n since it gives the lower signal 18

DSNB: UPPER LIMITS

Data sample : Nov 2007- Oct 2017
Statistics : 2485 days (6.8 years)
Exposure : 1494 ± 6 tons year
Nprotons=(1.32 ± 0.06) 1031
Efficiency =85.0 + 0.15 %Model independent limits

101 candidates

Model dependent limits
90% C.L.

• Best limits below 8 MeV
KamLAND (8.3-31.8 MeV): F < 139 cm-2 s-1 (A. Gando 2012)
Super-K ( > 17.3 MeV):F < 2.9 cm-2 s-1 (K. Bays 2012)19

SOLAR FLARES
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Flares are caused by the restructuring of thesolar magnetic field which leads toacceleration of protons.
Neutrinos could be generated in the decaysof pions which are numerously produced inpp-, pα-collisions in the flare’s region.
The mean neutrino energy is expected to bearound 10 MeV
R. Davis attempted to explain an excess ofneutrino events in several Homestake runs bymeans of solar flares.
Studied by Kamiokande-II, LSD, SNO.

●Flares selection:lGOES databasel
M and X classes flares (most intensive)
l798 flares selected from December 2009
to October 2017, 472 with >95% data
coverage●

Both main DAQ and FADC system data => 115 MeV energy range & whole IV for FADC,
0.25-15 MeV and FV=145 tons for main DAQ(FV cut: 75 cm);

●Strategy: search for excess of single events
over background at a time of the flare

BX analysis: search for excess of single events from nx and nx (x = e,µ,t) over background at he time of the flareby looking for their elastic scattering on electrons in the Borexino scintillator
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SOLAR FLARES : LIMITS ON NEUTRINO FLUENCES
M. Agostini et al., Astroparticle Physics 125 (2021) 102509

Ne =9.2×1031 for the whole IV and Ne =4.8×1031 for the 145 tons FV
FADCMain DAQ• Limits obtained from the primary DAQ (En< 3.5 MeV) and the FADC DAQ (En > 3.5MeV).
• Strongest limits on fluences of all neutrino
flavors from the solar flares below 3–7MeV• Borexino’s data excludes an intense solar
flare occurred during run 117 of the Cl-Ar
Homestake experiment as a possible
source of the observed excess of events.

Events in coincidencewith flares
BX21

CONCLUSIONS• Thanks to the extreme radiopurity and the underground site
Borexino has an exceptional sensitivity to anti-n signals• Borexino updated the geoneutrino measurement with all available data up to April
2019 with optimized selection cuts & analysis• Null mantle signal excluded at 99.0% C.L.
• Estimates of mantle radiogenic heat and Urey convective ratio• Signal in agreement with geological predictions, with a preference for models predicting highU and Th abundances• Borexino has achieved the best upper limits on DSNB anti-ne flux for En< 8 MeV;
• No excess of events was observed in coincidence with solar flares, GRB’s and GW events;
All results will be updated at the end of data taking (Sep. 21)

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Thanks
for yourattention!!!!
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BACKUP

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GEOREACTOR
Hypothetical fission of Uranium deep in the Earth
235U : 238U = 0.76 : 0.23 (Herndon)

No sensitivity tooscillation pattern

Fit with reactor spectrum constrained

Upper limit (95% CL): 18.7 TNU• 2.4 TW in the Earth’s center• 0.5 TW near CMB at 2900 km• 5.7 TW far CMB at 9842 km
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