RENO Workshop Seoul Korea

The Borexino Solar Neutrino Experiment

Frank Calapricefor the

Borexino Collaboration

June 14, 2013

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Borexino Collaboration

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Kurchatov Institute

Dubna JINR

MPI Heidelberg Tech. Univ. Munich

Jagiellonian U.Cracow

Perugia

Genova APC ParisMilano

Princeton University

Virginia Tech. University

Univ. Massachusetts

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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.– 14C/12C

• 10-18 g/g. cf. 10-11 g/g in air CO2

– U, Th impurities• Dirt 10-6 g/g• Needed: 10-16 g/g• Obtained: 10-18 g/g

– Radon daughters (210Pb, 210Bi, 210Po)• Light yield (2200 PMT’s)

– Emitted: 11,000 photons/Mev – Detected: 500 pe/MeV (~4%)

• Pulse shape discrimination.– Alpha-beta particle separation

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Solar Nuclear Fusion CyclesThe 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

37Ar by the reaction 37Cl(n,e)37Ar.• Gallium radiochemical experiment

– Used simliar technique to measure pp neutrinos• Kamiokande, Super-K, and SNO

– Detected high enegy 8B 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

Neutrino-electron elastic scattering

• Contributions from charged and neutral currents.• Measure energy of recoil electron by number of detected

scintillation photons. • With 500 pe/MeV, energy resolution is about 5% at 1 MeV.

• Position of event is measured by photon time-of-flight.• Position resolution is 10-15 cm.

• Threshold energy is about 60 keV.• Calorimetric measurements- no directional sensitivity

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Solar Neutrino Spectra

Neutrino Energy SpectrumNeutrino-Electron Elastic Scattering

Energy Spectrum

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Borexino Measurements 2007-2012

Solar Neutrinos✓7Be 46.0 cpd/100t± 5%. PRL 2011✓ 8B (> 3 MeV) 0.22 cpd/100t± 19% PRD 2010✓ Pep 3.1 cpd/100t ± 22% PRL 2012✓ CNO limit < 7.9 cpd/100t PRL 2012 ✓7Be day/night asy. A = 0.001 ± 0.014 PLB 2012 ✓7Be annual mod. PLB 2012

Geo-neutrinos Geo-neutrinos 14.3 ± 3.4 eV/(613 t-yr) PLB 2013

Rare Processes Test of Pauli Exclusion Principle in Nuclei PRC 2010 Solar axion upper limit PRD 2012

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General CommentsBackgrounds• Long-lived Cosmogenic:

– 14C in hydrocarbon liq. Scint.– Use material from deep site

• Short-lived Cosmogenic– Need deep site & active shielding.

• Radiogenic (U, Th, K, 222Rn, 210Pb. 210Bi, 210Po)– Rock (room background)

• Active shielding– Detector materials

• Self shielding• Scintillation Pulse shape

Discrimination rejects a’s in scintillator

• Radon daughters 210Bi, 210Po 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/m2/hr.• Alphs/beta separation: pulse shape

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2011 spectrum 7Be with 210Po a’s

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210Po

210Bi

85Kr

CNO

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7Be: fit of the energy spectrumtcpdR syststat 100/5.146 )(

5.16.1)(

tcpdR noscillationo 100/2.574

5 s evidence of oscillation

ne flux reduction 0.62 +- 0.05electron neutrino survival probability 0.51 +- 0.07

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

)(007.0)(012.0001.02/)(

sysstatDNDNADN

G. Bellini et al., Borexino Collaboration, Phys. Lett. B707 (2012) 22.June 14, 2013

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The first pep n measurement : multivariate analysis and background subtraction

Expected pep interaction rate: 2-3 cpd/100tBackground: 11C 210Bi external g210Bi and CNO spectra: very similar

Three Fold Coincidence: 11C reductionNovel pulse shape discrimination: e+ from 11C 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) fit the pulse shape parameter

pep

CNO

210Bi

11CG. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302..

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Physics implication of the solar n Borexino results:the Neutrino Survival Probability Pee(E)

Confirms MSW Vacuum to Matter Enhanced Oscillations

Before the Borexino resultsG. 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

G. Bellini et al., Borexino Collaboration, Phys. Rev. D82 (2010) 033006.8B flux with a threshold of 3MeV (e- recoil)

Combined analysisBorexino&solar

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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.

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geon results: evidence of the signal

NreactorExpected with osc.

NreactorExpected no osc.

Others back. Ngeomeasured

Nreactormeasured

Ngeomeasured

Nreactormeasured

events Events events events events TNU TNU

33.3±2.4 60.4±2.4 0.70±0.18 14.3±4.4 31.2-6.1+7 38.8±12.

084.5+19.3

-16.9

reactorgeon

No geon signal: rejected at 4.5 s C.L.Unbinned likelihood fit

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geon results: U and Th separation

Chondritic U-Th ratio

Best fitS(238 U)= 26.5 ± 19.5 TNUS(232 T) = 10.6 ± 12.7 TNUFit with weight of 238U and 232Th spectra free

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Borexino Phase 2 Solar Neutrino Program

• Technical goals:– Reduce scintillator backgrounds with loop

purification• 210Bi (210Pb)• 85Kr by nitrogen stripping

• Measurement goals– pp neutrino observation– CNO neutrinos detection or lower limit– Improve pep, 7Be, 8B measurement

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Phase-2 Borexino ProgramScientific 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

51Cr source under Borexino to search for short baseline beutrino oscillations. • Motivated by reactor, gallium, and Miniboone neutrino

anamolies

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The Solar Metallicity Problem• In 1998 the metallicity (abundance of elements heavier than 4He)

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 (8B 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.June 14, 2013

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Re-Purification of the Liquid Scintillator for Lower Background

• Reducing backgrounds is essential for Phase 2 solar program.– 210Bi obscures CNO and pep neutrinos.– 85Kr interferes with 7Be 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 N2 Stripping

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

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

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Short distance ne Oscillations with Borexino (SOX)

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SOX Expected Sensitivity (51Cr)

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Conclusions• Borexino was started in the early 90’s to determine if the

low energy 7Be 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

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