1. The Borexino Solar Neutrino Experiment1
The Borexino Solar Neutrino Experiment
Good afternoon. It’s a pleasure for me to be here at the Neutrino 2010 Conference and in Greece, I want to thank the organizing committee for the opportunity to speak about Borexino. I am pleased to do this on behalf of the excellent people who worked on Borexino for many years and made it a success.
Frank Calaprice
for the
Borexino Collaboration
June 14, 2013
RENO Workshop Seoul Korea

2. Borexino Collaboration
Milano
Princeton University
Genova
APC Paris
Perugia
Virginia Tech. University
Here is our collaboration of groups from Italy, France, Germany, Russia, Poland, and the United States.
Univ. Massachusetts
Dubna JINR
Kurchatov
Institute
Jagiellonian U.
Cracow
MPI Heidelberg
Tech. Univ. Munich
June 14, 2013
RENO Workshop Seoul Korea

3. The Borexino Detector (Mostly Active Shielding)
Shielding Against Ext. Backgnd.
Water: m
Buffer zones: m
Outer scintillator zone: m
Main backgrounds: in Liq. Scint.
14C/12C
10-18 g/g. cf g/g in air CO2
U, Th impurities
Dirt g/g
Needed: g/g
Obtained: g/g
Radon daughters (210Pb, 210Bi, 210Po)
Light yield (2200 PMT’s)
Emitted: 11,000 photons/Mev
Detected: pe/MeV (~4%)
Pulse shape discrimination.
Alpha-beta particle separation
June 14, 2013
RENO Workshop Seoul Korea

4. Solar Nuclear Fusion Cycles
The pp cycle
The CNO cycle
June 14, 2013
RENO Workshop Seoul Korea

5. RENO Workshop Seoul Korea
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 E < 3 MeV.
Made possible by development of new low-background methods.
I like to call it a major breakthrough in experimental physics.
June 14, 2013
RENO Workshop Seoul Korea

6. 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 cm.
Threshold energy is about 60 keV.
Calorimetric measurements- no directional sensitivity
We detect solar neutrinos by elastic scattering on electrons.
This has changed and neutral current interactions. The tau and muon neutrino interact with neutral current and have a smaller cross section so the effect of oscillation it a lower detector rate.
June 14, 2013
RENO Workshop Seoul Korea

7. Solar Neutrino Spectra
Neutrino Energy Spectrum
Neutrino-Electron Elastic Scattering Energy Spectrum
June 14, 2013
RENO Workshop Seoul Korea

8. Borexino Measurements 2007-2012
Solar Neutrinos
✓7Be cpd/100t ± 5%. PRL
8B (> 3 MeV) cpd/100t ± 19% PRD 2010
Pep cpd/100t ± 22% PRL 2012
CNO limit < cpd/100t PRL 2012
✓7Be day/night asy. A = ± PLB 2012
✓7Be annual mod. PLB 2012
Geo-neutrinos
Geo-neutrinos ± 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
Calibrations
Development of extensive Monte Carlo simulations
Muon tracking: neutron & cosmogenic studies.
Mitigation of small leak in Inner Vessel.
June 14, 2013
RENO Workshop Seoul Korea

9. RENO Workshop Seoul Korea
June 14, 2013
RENO Workshop Seoul Korea

10. RENO Workshop Seoul Korea
General Comments
Backgrounds
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 g/l PPO
Buffer zones
Pseudocumene 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
~ 1 MeV
Event position determination
photon time-of-flight.
Resolution: ~12 1 MeV
Muon flux: 1.1 mu/m2/hr.
Alphs/beta separation: pulse shape
June 14, 2013
RENO Workshop Seoul Korea

11. RENO Workshop Seoul Korea
2011 spectrum 7Be with 210Po a’s
210Po
210Bi
85Kr
CNO
June 14, 2013
RENO Workshop Seoul Korea

12. 7Be: fit of the energy spectrum
5 s evidence of oscillation
ne flux reduction
electron neutrino survival probability
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)
G. Bellini et al., Borexino Collaboration, Phys. Lett. B707 (2012) 22.
June 14, 2013
RENO Workshop Seoul Korea

13. RENO Workshop Seoul Korea
The first pep n measurement : multivariate analysis and background subtraction
Expected pep interaction rate: 2-3 cpd/100t
Background: 11C Bi external g
210Bi and CNO spectra: very similar
11C
G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012)
210Bi
pep
CNO
Three Fold Coincidence: 11C reduction
Novel pulse shape discrimination: e+ from 11C decay form Positronium
live time before annihilation in liquid: few ns
delayed scintillation signal
(Phys. Rev. C (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
June 14, 2013
RENO Workshop Seoul Korea

14. RENO Workshop Seoul Korea
Physics implication of the solar n Borexino results: the Neutrino Survival Probability Pee(E) Confirms MSW Vacuum to Matter Enhanced Oscillations
Before the Borexino results
G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012)
First solar pep neutrino detection
G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 107 (2011)
High precision 7Be solar neutrino measurement
Combined analysis
Borexino&solar
G. Bellini et al., Borexino Collaboration, Phys. Rev. D82 (2010)
8B flux with a threshold of 3MeV (e- recoil)
June 14, 2013
RENO Workshop Seoul Korea

15. 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

16. geon results: evidence of the signal
Nreactor
Expected with osc.
Expected
no osc.
Others back.
Ngeo
measured
events
Events
TNU
33.3±2.4
60.4±2.4
0.70±0.18
14.3±4.4
38.8±12.0
No geon signal: rejected at 4.5 s C.L.
Unbinned likelihood fit
reactor
geon
June 14, 2013
RENO Workshop Seoul Korea

17. geon results: U and Th separation
Chondritic U-Th ratio
Best fit
S(238 U)= ± TNU
S(232 T) = ± TNU
Fit with weight of 238U and 232Th spectra free
June 14, 2013
RENO Workshop Seoul Korea

18. 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
June 14, 2013
RENO Workshop Seoul Korea

19. 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 51Cr 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

20. 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 = (GS98)  (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
RENO Workshop Seoul Korea

21. 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.
June 14, 2013
RENO Workshop Seoul Korea

22. Before Re-purification of L.S. 210Bi = 38 ± 2.9 cpd/100t
Lower Backgrounds after Recent Scintillator Purification by Water Extraction and N2 Stripping
After Re-purification: Bi = 21 ± 4 cpd/100t
85Kr < 5 cpd/100t
Before Re-purification of L.S Bi = 38 ± 2.9 cpd/100t
85Kr = 28 ± 5 cpd/100t
June 14, 2013
RENO Workshop Seoul Korea

23. Short distance ne Oscillations with Borexino (SOX)
June 14, 2013
RENO Workshop Seoul Korea

24. SOX Expected Sensitivity (51Cr)
June 14, 2013
RENO Workshop Seoul Korea

25. RENO Workshop Seoul Korea
June 14, 2013
RENO Workshop Seoul Korea

26. RENO Workshop Seoul Korea
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
June 14, 2013
RENO Workshop Seoul Korea