The Borexino Experiment: Past, Present and Future
Nicola Rossi Laboratori Nazionali del Gran Sasso
Neutrino 2016 Conference London, 3-10 July 2016
The Borexino Collaboration
Location: “Laboratori Nazionali del Gran Sasso” – INFN - Italy Why the Borexino Experiment?
Neutrinos coming from the Sun (< 3 MeV region ) - Key information for accurate solar modeling (ASTRO-) - Study of neutrino oscillations (PARTICLE-)
νCNO νpp
Total Energy 99% pp chain 1% CNO bi-cycle
Borexino Milestones
Borexino is presently the only detector able to measure the solar neutrino interaction rate with threshold ∼150 keV and to reconstruct the energy spectrum of the events.
Solar physics results: First measurement of the interaction rate of the 7Be (862 keV) neutrinos (5%accuracy) Exclusion of any significant day-night asymmetry of the 7Be solar neutrino flux Annual modulation observation of the 7Be neutrino flux First direct observation of the monoenergetic 1440 keV pep solar neutrinos Set of the strongest upper limit of the CNO solar neutrinos flux Measure of the 8B solar neutrinos with an energy threshold of 3 MeV First spectroscopical observation of pp neutrinos
Other results: 5σ geo-netrinos detection Detailed study of the cosmogenics in liquid scintillator Search for rare processes (recently: electron lifetime > 6.6 x 10 28 y (90% CL)) Borexino Time Line
Phase II Solar (Dec 2011) Physics Activities pp flux
Intense Purification Campaign (2010 – 2011 ) Phase-I Record (2007-2010) Radiopurity Others: levels 7Be flux – Geoneutrinos 8B flux (Phase-I + Phase-II) pep flux – Rare processes CNO limit
The experiment set-up 13.7 m stainless sphere ~ 1000 tons of pseudocumene (PC+PPO) ~300 tons Inner Vessel
More than 2000 Photomultiplier tubes (PMTs)
34% Coverage
Light Yield = 500 PE/MeV
Cherenkov muon veto (~200 PMTs)
“Dynamic reconstruction of the nylon vessel” Very (very) low background: - Nitrogen stripping - Distillation - Water Extraction The Borexino Signal
The Borexino PMTs detect the scintillation light produced by electrons scattered by neutrinos
This signal is indistinguishable from the natural radioactivity (β- and γ components)
For α and β+ we can apply the pulse shape discrimination
Crucial point: Extreme low background required!!!
Beta-like spectrum
Contaminants: Phase-I Calibration Campaign
Calibration of the full energy scale with different standard sources deployed inside the detector with mechanical arms
Calibration of the position. Position is important for defining the fiducial volume and full reconstruction of the event
Energy resolution: ~5%√E [MeV] The Borexino energy spectrum
Basic data selection 1. Raw spectrum 2. Muon cut 3. Fiducial Volume cut
210Po (α)
7Be (ν) 11C (β++2γ) External 14 - C (β ) BG
α-β Statistical Subtraction (210Po)
Scintillation light in time Projection
α – β discrimination with Gatti's parameter
– Trained on 214Bi – 214Po coincidences
– Study of new high efficiency PSD based on neural networks 7Be spectrum fit α-subtracted spectrum →
← Full spectrum
Threefold coincidence
[1] e re μ + 12C → μ + 11C + n T on isi D: ec - S D [2] +/ P d te 11C → 11B + e+ + ν Β os e Bo (~30 min)
[3] H + n → D + γ 2.2 Mev (250 μs )
Multivariate Fit (Likelihood)
– radial distribution – β+/- pulse shape discrimination – TFC subtracted spectrum (~50% exposure) – Complementary spectrum pp energy spectrum
Same window: 1st event 2nd event
– Independent constraint on 14C (40 ± 1 Bq) – Synthetic pile-up events: 14C+14C, 14C+other...
The first direct observation of the low energy neutrinos coming from the "pp" fusion in the core of the Sun. Solar Neutrino: Published Results
Species Rate Flux [cpd/100t] [cm-2s-1] 7 +1.5 9 Be (863 keV) 46.0 ± 1.5 -1.6 3.1 ± 0.15 x 10 pep 3.1 ± 0.6 ± 0.3 1.6 ± 0.6 x 108 CNO < 7.9 (95% CL) 7.7 x 108 8B(> 3 MeV) 0.22 ± 0.04 ± 0.01 2.4 ± 0.4 ± 0.1 x 106 pp 144 ± 13 ± 10 6.6 ± 0.7 x 1010
Pee survival probability in the MSW-LMA scenario with Borexino data only!
Geo-neutrinos
Detection trough inverse beta dacay
Last update: ~ 2000 days exposure
Possibility to distinguish > 5 σ between evidence different geological out of null models observation
Geo- vs. reactor neutrinos Free chondritic ratio Borexino Phase II
Major contaminants: comparison 14C/12C ~ 2.7 x 10-18 cpd/100t Phase I Phase II 238 85 U (Bi-Po 214) Kr ~30 < 5 < 9.7 x 10-19 g/g (95% CL) 210Bi ~40 ~20 210Po >2000 <60 232Th (Bi-Po 212) < 1.2 x 10-18 g/g (95% CL)
6 cycles of Water Extraction (~1 year) reduced drastically the background 40K no evidence (TBD) contaminants 39Ar << 85Kr We have ~5 years of high quality data (Phase-II)!!
210Po decay and 210Bi constraint Expected CNO rate (MSW-LMA): – High metallicity: ~5.3 cpd/100t – Low metallicity: ~3.7 cpd/100t Picture must Stategy: be updated – Needs of an independent constraint on 210Bi rate (remove correlation with CNO spectrum):
210Pb (long lived) → 210Bi (β)→ 210Po (α)
– 210Po can be measured with high efficiency pulse shape discrimination Borexino Insulation (Picture: Yury Suvorov) – 210Bi can be extracted through the temporal fit
– Total rate 210Po homogeneity is required: – Unsupported 210Po 210 – Supported Po Thermal insulation for preventing convective motions (since May 2015)
17 Temperature evolution in the SSS
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2015 Dec 9yh: insulation stop 2015 Jul 1st: Water loop OFF 2015 May 15: first insulation ring 18 210Po in 60 cubes (r < 3 m) Spring 2014 Spring 2015
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time Autumn 2013 Autumn 2014 Insulation effect: 19 Summer 2015 Main future plans
1. Solars Improvement of the 7Be accurancy (<5%) + annual modulation Improvement of the pep evidence (> 3σ) New update of the 8B analysis n tio Possible update of the pp measurement ra ib 17 al ! 0 Improvement of CNO sensitivity C n 2 ig of ew a - Thermal insulation (ongoing) N p ng am ni C in - Further purification campaign eg B ~ 2. Update of Geo-neutrino flux 3. Short range neutrino oscillation project “SOX” (→ M. Pallavicini, in this conference)
Bruno Pontecorvo Prize 2016
JINR
“The Scientific Council of JINR approved the Jury’s recommendations on the award of the B.Pontecorvo Prize to Professor Gianpaolo Bellini (INFN and University of Milan, Italy) The award for his outstanding contributions to the development of low-energy neutrino detection methods, their realization in the Borexino detector, and the important solar and geo-neutrino results obtained in this experiment.” 21 Thank you for your attention
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