Recent results from Borexino Phase-II solar

neutrino program

A. VishnevaJINR

16th Baksan School on Astroparticle PhysicsTerskol, 12 Apr 2019

Production of neutrinos in the Sun

2H+p→3He+γ

3He+3He→4He+2p

3He+4He→7Be+γ

7Be+p→8B+γ

7Li+p→24He

8Be*→24He

pp-νν pep-νν

99.6% 0.4%

hep-νν

7Be-νν

8B-νν

85%

15%

0.13%

2×10-5 %

99.87%

pp chain

pp-I

pp-II

pp-III

7Be+e-→7Li+νe

3He+p→4He+e++νe

p+e-+p→2H+νep+p→2H+e++νe

8B→8Be*+e++νe

12C+p→13N+γ

13C+p→14N+γ

14N+p→15O+γ

15N+p→4He+12C

CNO cycle

CNO-ν

15N+p→16O+γ

16O+p→17F+γ

17O+p→14N+4He

99.96% 0.04%

17F→17O+e++νe15O→15N+e++νe

13N→13C+e++νe

99% of solar energy dominant in heavier stars

What we can get from solar neutrino measurements

Neutrino energy [MeV]1−10 1 10

]-1

s-2

Sol

ar n

eutr

ino

flux

[cm

210

310

410

510

610

710

810

910

1010

1110

1210

1310

30%]±hep [

0.6%]±pp [

12%]±B [8

15%]±N [13

17%]±O [15

20%]±F [17

6%]±Be [7

1%]±pep [

B16 - SSMSolar physics:

Solar neutrino fluxes

test of the SSMtest of the Sun stability

Solar metallicity (abundanceof heavy elements)

Neutrino physics:

Neutrino oscillations inmatter (MSW-effect)

oscillation parametersday-night effect

Non-standard neutrinoproperties

Solar metallicity problem

3D hydrodynamical models to determine abundances near the solarsurface (assumed to be the same as in the core):

GS98 (Grevesse & Sauval 1998)

AGSS09 (Asplund et al. 2009) — worse agreement withhelioseismology

Solar ν GS98-HZ [cm−2s−1] AGSS09-LZ [cm−2s−1]

pp 5.98 (1± 0.006)× 1010 6.03 (1± 0.005)× 1010

7Be 4.93 (1± 0.06)× 109 4.50 (1± 0.06)× 109

pep 1.44 (1± 0.009)× 108 1.46 (1± 0.009)× 108

CNO 4.88 (1± 0.11)× 108 3.51 (1± 0.10)× 108

8B 5.46 (1± 0.12)× 106 4.50 (1± 0.12)× 106

Direct metallicity measurement: solar neutrino fluxes(especially CNO)

Borexino detector

Location: LaboratoriNazionali del Gran Sasso(Italy)

Primary goal:measurements of solarneutrino fluxes at lowenergies

Energy threshold:∼ 150 keV (recoil electrons)

Energy resolution: ∼ 5%@ 1 MeV

Abundance of 238U and232Th: < 10−19 g/g

Data sets

Energy regions

Low Energy Region (LER)

0.19–2.93 MeV

Dec 2011 – May 2016(2191.51 days)

Fiducial mass 71.3 t

pp, 7Be, pep, CNO

High Energy Region (HER-I/II)

3.2–5.7/5.7–16 MeV

Jan 2008 – Dec 2016(2062.4 days)

Fiducial mass 227.8/266 t8B

LER spectrum composition

event-by-event selection is nearly impossible → spectral fit isneeded

LER analysis: background rejection

Muon veto:

2 ms after crossing the outer detector

300 ms after crossing the inner detector

Fiducial volume:

R < 2.8 m

−1.8 < z < 2.2 m11C cut: three-fold coincidence (TFC)

muon event

neutron capture

β+ decay of 11C

LER analysis: multivariate approach

Simultaneous fit of TFC-subtracted, TFC-tagged, pulse shape andradial distributions:

L = LTFC−sub · LTFC−comp · LPS · LRD

Energy (keV)

) hE

vent

s / (

day

x 1

00 t

x N

3−10

2−10

1−10

1

10 C14

Po210

Bi210

Kr85

Total fit: p-value=0.7

C11

pile-upext bkg

pp

Be7

CNOpep B8

hN100 200 300 400 500 600 700 800 900

500 1000 1500 2000 2500Energy (keV)

) hE

vent

s / (

day

x 1

00 t

x N

3−10

2−10

1−10

1

10 C14

Po210

Bi210

Kr85

He6

Total fit: p-value=0.7

C11

C10

pile-upext bkg

pp

Be7

CNO pep B8

hN100 200 300 400 500 600 700 800 900

500 1000 1500 2000 2500

PRPS-L4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3

Eve

nts

/ ( d

ay x

100

t x

0.01

)

3−10

2−10

1−10

ElectronsPositrons

/NDF = 85.1/1072χBest Fit -

R (m)0 0.5 1 1.5 2 2.5 3

Eve

nts

/ ( d

ay x

100

t x

40 c

m )

3−10

2−10

1−10

Uniform componentExternal component

/NDF = 47.8/692χBest Fit -

HER analysis

Muon veto:

6.5 s after crossing the inner detector

Fiducial volume:

R < 2.5 m (HER-I)

Results: solar neutrino fluxes

Solar ν

Borexino experimental results B16(GS98)-HZ B16(AGSS09)-LZ

Rate Flux Flux Flux[cpd/100 t] [cm−2s−1] [cm−2s−1] [cm−2s−1]

pp 134± 10 +6−10 (6.1± 0.5 +0.3

−0.5)× 1010 5.98 (1± 0.006)× 1010 6.03 (1± 0.005)× 1010

7Be 48.3± 1.1 +0.4−0.7 (4.99± 0.13 +0.07

−0.10)× 109 4.93 (1± 0.06)× 109 4.50 (1± 0.06)× 109

pep (HZ) 2.43± 0.36 +0.15−0.22 (1.27± 0.19 +0.08

−0.12)× 108 1.44 (1± 0.009)× 108 1.46 (1± 0.009)× 108

pep (LZ) 2.65± 0.36 +0.15−0.24 (1.39± 0.19 +0.08

−0.13)× 108 1.44 (1± 0.009)× 108 1.46 (1± 0.009)× 108

CNO < 8.1 (95%C.L.) < 7.9× 108 (95%C.L.) 4.88 (1± 0.11)× 108 3.51 (1± 0.10)× 108

8B 0.223 +0.015−0.016

+0.006−0.006 (5.68 +0.39

−0.41+0.03−0.03)× 106 5.46 (1± 0.12)× 106 4.50 (1± 0.12)× 106

hep < 0.002 (90%C.L.) < 2.2× 105 (90%C.L.) 7.98 (1± 0.30)× 103 8.25 (1± 0.12)× 103

pep rate (cpd/100 t)0 1 2 3 4 5 6

2 χ∆

0

10

20

30

40

50

60

70

CNO fixed to LZ solar model

CNO fixed to HZ solar model

More than 5σ significance!

CNO rate (cpd/100 t)0 2 4 6 8 10 12 14 16

2 χ ∆

0

5

10

15

20

25

30

HZ solar modelpp/pep ratio fixed toHZ solar modelpp/pep ratio fixed to

Φ(pp)/Φ(pep) = 47.7± 1.2

Solar metallicity

Compatibility with the MSW-LMA solution (HZ assumed)

Summary

The first simultaneous analysis of pp, pep and 7Be solarneutrino fluxes

The first complete study of solar neutrinos

Φ7Be with unprecedentedly high precision (2.8%)

First 5σ evidence of pep neutrinos

updated limit on CNO neutrino flux and measurements of ppand 8 neutrinos

First hint towards high metallicity

See detail in our papers:

entire pp-chain: Nature 562 (2018) no. 7728, 505-5108B in details: arXiv:1707.09279 [hep-ex]

pp, 7Be, pep and CNO in details: arXiv:1709.00756 [hep-ex]