1 June 14, 2013

The Borexino Solar Neutrino Experiment

Frank Calaprice

for the Borexino Collaboration RENO Workshop Seoul Korea 1

Borexino Collaboration

Princeton University Genova Milano APC Paris Perugia Virginia Tech. University Univ. Massachusetts Dubna JINR Kurchatov Institute

June 14, 2013

Jagiellonian U.

Cracow MPI Heidelberg

RENO Workshop Seoul Korea

Tech. Univ. Munich

2

• • • •

The Borexino Detector (Mostly Active Shielding)

Shielding Against Ext. Backgnd.

– Water: 2.25m – Buffer zones: 2.5 m – Outer scintillator zone: 1.25 m Main backgrounds: in Liq. Scint.

– 14 C/ 12 C • 10 -18 g/g. cf. 10 -11 g/g in air CO 2 – U, Th impurities • Dirt • Needed: • Obtained: 10 -6 g/g 10 -16 g/g 10 -18 g/g – Radon daughters ( 210 Pb, 210 Bi, 210 Po) Light yield (2200 PMT’s) – Emitted: 11,000 photons/Mev – Detected: 500 p e /MeV (~4%) Pulse shape discrimination.

– Alpha-beta particle separation June 14, 2013 RENO Workshop Seoul Korea 3

Solar Nuclear Fusion Cycles

The pp cycle The CNO cycle

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Historical Note

• • • • Chlorine experiment: – First solar neutrino detector was the chlorine radiological experiment.

– Technique avoids the intense source of radiological backgrounds by producing 37 Ar by the reaction 37 Cl( n ,e) 37 Ar.

Gallium radiochemical experiment – Used simliar technique to measure pp neutrinos Kamiokande, Super-K, and SNO – Detected high enegy 8 B neutrinos (> 5 MeV ) to avoid radiological backgrounds Borexino – First experiment to directly detect neutrinos in the midst of soup of radiological background @ E < 3 MeV.

– Made possible by development of new low-background methods.

– I like to call it a major breakthrough in experimental physics.

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Neutrino Detection

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Solar Neutrino Spectra

Neutrino Energy Spectrum Neutrino-Electron Elastic Scattering Energy Spectrum Ev e nt s / (d a y x 1 0 0 to n s x 1 0 p . e. )

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Borexino Measurements 2007-2012

Solar Neutrinos ✓ 7 Be 46.0 cpd/100t ± 5%.

PRL ✓ ✓ 8 B (> 3 MeV) Pep 0.22 cpd/100t 3.1 cpd/100t ± 19% PRD ± 22% PRL ✓ CNO limit < 7.9 cpd/100t ✓ 7 Be day/night asy. ✓ 7 Be annual mod.

A = 0.001 ± 0.014

PRL PLB PLB Geo-neutrinos  Geo-neutrinos 14.3 ± 3.4 eV/(613 t-yr) PLB Rare Processes  Test of Pauli Exclusion Principle in Nuclei  Solar axion upper limit June 14, 2013 RENO Workshop Seoul Korea PRC PRD 2011 2010 2012 2012 2012 2012 2013 2010 2012 8

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General Comments

• • • • Backgrounds Long-lived Cosmogenic: – 14 C in hydrocarbon liq. Scint.

– Use material from deep site Short-lived Cosmogenic – Need deep site & active shielding.

Radiogenic (U, Th, K, 222 Rn, 210 Pb. 210 Bi, 210 – Po) Rock (room background) • Active shielding – Detector materials • Self shielding • Scintillation Pulse shape Discrimination rejects ’s in scintillator Radon daughters 210 Bi, 210 Po are serious background.

• • • • • • • • Specifications.

Liquid scintillator – Pseudicumene + 1.5 g/l PPO Buffer zones – – Pseudocumene + 2.5 g/l DMP Scintillation light is quenched.

Photomultipliers: – – 2200 8“ PMTs with concentrators.

Coverage: ~ 34% Light yield: – – 11,000 photons/ MeV 500 pe/MeV with 28% QE PMTs Energy resolution – ~ 7% @ 1 MeV Event position determination – – photon time-of-flight.

Resolution: ~12 cm @ 1 MeV Muon flux: 1.1 mu/m 2 /hr.

Alphs/beta separation: pulse shape June 14, 2013 RENO Workshop Seoul Korea 10

2011 spectrum

7

Be with

210

Po

a

’s

210

Po

210

Bi

85

Kr CNO

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7

Be: fit of the energy spectrum

R

 46 ± 1 .

5 (

stat

)   1 1 .

5 .

6 (

syst

)

R no oscillatio n

 74 ± 5 .

2

cpd

/ 100

t cpd

/ 100

t

5 s evidence of oscillation  n e flux reduction 0.62 +- 0.05

 electron neutrino survival probability 0.51 +- 0.07

• Search for a day night effect: • not expected for 7 Be in the LMA-MSW model • Large effect expected in the “LOW” solution (excluded by solar exp+Kamland)

A DN

 (

N N



D



D

) / 2  0 .

001 ± 0 .

012 (

stat

) ± 0 .

007 (

sys

) G. Bellini et al., Borexino Collaboration, Phys. Lett. B707 (2012) 22.

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The first pep n measurement : multivariate analysis and background subtraction 210 Bi  Expected pep interaction rate: 2-3 cpd/100t  Background: 11 C 210 Bi external g  210 Bi and CNO spectra: very similar 11 C G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302..

pep CNO  Three Fold Coincidence: 11 C reduction  Novel pulse shape discrimination: e + from 11 C decay form Positronium live time before annihilation in liquid: few ns delayed scintillation signal (Phys. Rev. C 015522 (2011)) Multivariate analysis:  fit of the energy spectra   fit the radial distribution of the events ( external background is not uniform) June 14, 2013 fit the pulse shape parameter RENO Workshop Seoul Korea 13

Physics implication of the solar n Borexino results: the Neutrino Survival Probability P ee (E) Confirms MSW Vacuum to Matter Enhanced Oscillations Before the Borexino results G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302..

First solar pep neutrino detection G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 107 (2011) 141362.

High precision 7Be solar neutrino measurement Combined analysis Borexino&solar G. Bellini et al., Borexino Collaboration, Phys. Rev. D82 (2010) 033006.

8B flux with a threshold of 3MeV (e- recoil) June 14, 2013 RENO Workshop Seoul Korea 14

Terrestrial and Reactor Neutrinos

• • • Terrestrial neutrinos are produced by long-lived radioactive elements, U, Th, K.

– Energy is confined to < 3 MeV – Radioactive decay accounts for significant part of known heat produced inside earth Reactor neutrinos are produced by the decay of fission fragments in nuclear reactors.

– Energies of reactor neutrinos are higher than geo neutrinos, but they can be an important background.

– No nuclear power reactors in Italy; background is small.

Both neutrinos are seen together at low comparable rate. June 14, 2013 RENO Workshop Seoul Korea 15

geo

n

results: evidence of the signal

N reactor Expected with osc.

events 33.3±2.4

N reactor Expected no osc.

Events 60.4±2.4

Others back.

N geo measured N reactor measured N geo measured N reactor measured

events 0.70±0.18

events 14.3±4.4

events 31.2

-6.1

+7 TNU TNU 38.8±12.0 84.5

+19.3

-16.9

No geo n signal: rejected at 4.5 s C.L.

Unbinned likelihood fit geo n reactor June 14, 2013 RENO Workshop Seoul Korea 16

geo

n

results: U and Th separation

Chondritic U-Th ratio Fit with weight of 238 U and 232 Th spectra free June 14, 2013 RENO Workshop Seoul Korea Best fit S( 238 S( 232 U)= 26.5 ± 19.5 TNU T) = 10.6 ± 12.7 TNU 17

Borexino Phase 2 Solar Neutrino Program

• •

Technical goals:

– Reduce scintillator backgrounds with loop purification • 210 Bi ( 210 Pb) • 85 Kr by nitrogen stripping

Measurement goals

– pp neutrino observation – CNO neutrinos detection or lower limit – Improve pep, 7 Be, 8 B measurement June 14, 2013 RENO Workshop Seoul Korea 18

Phase-2 Borexino Program Scientific Goals

• •

The Metallicity Problem:

– Measurement of CNO neutrinos will shed light on the controversial abundance of heavy elements.

Sterile Neutrinos:

– The “SOX” Source Experiment will place a 10 MCi 51 Cr source under Borexino to search for short baseline beutrino oscillations. • Motivated by reactor, gallium, and Miniboone neutrino anamolies June 14, 2013 RENO Workshop Seoul Korea 19

The Solar Metallicity Problem

• • • In 1998 the metallicity (abundance of elements heavier than – 4 He) determined from line spectra in Sun’s atmosphere agreed well with other data.

Standard solar model based on uniform composition.

– Helioseismology data – Solar neutrino data ( 8 B by SNO) Improvements were made in the analysis of solar atmospheric spectra over next 10 years (3D model,etc.) – A 2009 assessment of data resulted in a lower metallicity.

• Z /X = metal/hydrogen ratio = 0.024 (GS98)  0.018 (AGSS09).

– The new resukts are in conflict with helioseismic data that probe the composition at greater depths in the sun. This is a serious problem for stellar models because it implies that the chemical composition is not uniform.

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Re-Purification of the Liquid Scintillator for Lower Background

• • • • Reducing backgrounds is essential for Phase 2 solar program.

– 210 Bi obscures CNO and pep neutrinos.

– 85 Kr interferes with 7 Be neutrinos Purification of the scintillator by “water extraction” and “nitrogen stripping” was carried out recently.

– – Backgrounds were reduced significantly.

Lower background is still necessary.

Refinements in water extraction are being developed.

Discussion of purification in my next talk.

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Lower Backgrounds after Recent Scintillator Purification by Water Extraction and N

2

Before Re-purification of L.S. 210 Bi = 38 ± 2.9 cpd/100t 85 Kr = 28 ± 5 cpd/100t

Stripping

After Re-purification: 210 Bi = 21 ± 4 cpd/100t 85 Kr < 5 cpd/100t

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Short distance

n e

Oscillations with Borexino (SOX)

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SOX Expected Sensitivity (

51

Cr)

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Conclusions

• • • Borexino was started in the early 90’s to determine if the low energy 7 Be solar neutrinos exhibit neutrino oscillations.

Twenty years later, the evidence for oscillations with the peculiar energy dependence in matter predicted in MSW theory is convincing.

The new data were made possible with innovations in low background methods that are relevant for new rare event challenges: – Direct detection of dark matter WIMPS – Neutrinoless double beta decay June 14, 2013 RENO Workshop Seoul Korea 26