The first year of Borexino data LIP July 9, 2008 – Lisbon Davide Franco Milano University & INFN

Outline The role of neutrinos in particle physics and astro-particle The physics of Borexino The Borexino detector The “radio-purity” challenge The reached goals (7Be and mn) Near and far future goals Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Neutrinos: cosmic messengers High energy protons 5·10Mpc Astrophysical source neutrinos High energy gammas 10 Mpc Low energy protons deflected Ideal messenger: neutral stable weakly interacting Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Messengers from the Sun’s core Core (0-0.25 Rs) Nuclear reactions: T~1.5 107 °K energy chains pp e CNO (neutrino production) Radiative region (0.25-0.75 Rs) Photons carry energy in ~ 105 y Convective region (0.75-1 Rs) Strong convection and turbulence Complex surface phenomena Corona (> 1 Rs) Complex magneto-hydrodynamic phenomena Gas at T~ 106 °K Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

The Standard Solar Model It is an evolution model f the Sun, starting from the origin up to now: Includes: Hydrostatic equilibrium equations Energetic and mass balance Nuclear Reactions Energy transport Depends on several parameters and/or assumptions: Initial chemical composition Nuclear cross sections Magnetic field, rotation The model predicts the actual temperature and hence neutrino production Main uncertainties due to chemical compound and nuclear cross sections Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Solar nuclear reactions pp chain Fusion of 4 protons in a 4He nucleus with emission of 27 MeV energy Main chain for “small-medium size” stars, like the Sun CNO cycle Carbon-Nitrogen-Oxygen or CNO cycle converts hydrogen to helium It is dominant in population I “hot” stars In the Sun, the CNO cycle contributes to 1-2 %, but there is no any experimental evidence Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Neutrino Production In The Sun pp chain: pp, pep, 7Be, and 8B n CNO cycle: 13N, 15O, and 17F n Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Solar Neutrinos Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Solar Neutrino Spectra Gallex GNO Homestake Sage SNO SuperK (real time) Borexino (real time) Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

The Standard Solar Model before 2004 One fundamental input of the Standard Solar Model is the metallicity of the Sun - abundance of all elements above Helium: The Standard Solar Model, based on the old metallicity derived by Grevesse and Sauval (Space Sci. Rev. 85, 161 (1998)), was in agreement within 0.5 in % with the solar sound speed measured by helioseismology. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Reconstructed speed of sound The Sun Structure The study of the Sun structure is supported by the “helium-seismology” Pressure wave propagation in the Sun Sun’s vibration modes depend on the internal structure The Standard Solar Model can be tested by looking at the p-mode (acoustic waves), g-mode (density or gravity waves) and f-mode (surface density waves) Reconstructed speed of sound Surface waves Reconstruction Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

The Standard Solar Model after 2004 Latest work by Asplund, Grevesse and Sauval (Nucl. Phys. A 777, 1 (2006)) indicates a metallicity lower by a factor ~2. This result destroys the agreement with helioseismology [cm-2 s-1] pp (1010) pep (1010) hep (103) 7Be (109) 8B (106) 13N (108) 15O (108) 17F (106) BS05 AGS 98 6.06 1.45 8.25 4.84 5.69 3.07 2.33 5.84 BS05 AGS 05 5.99 1.42 7.93 4.34 4.51 2.01 3.25 D -1% -2% -4% -12% -23% -42% -47% -57% Solar neutrino measurements can solve the problem! Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Borexino goals: solar physics First ever observations of sub-MeV neutrinos in real time Check the balance between photon luminosity and neutrino luminosity of the Sun CNO neutrinos (direct indication of metallicity in the Sun’s core) pep neutrinos (indirect constraint on pp neutrino flux) Low energy (3-5 MeV) 8B neutrinos Tail end of pp neutrino spectrum? Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Borexino goals: neutrino physics Test of the matter-vacuum oscillation transition with 7Be, pep, and low energy 8B neutrinos 7Be pep 8B Check of the mass varying neutrino model (Barger et al., PRL 95, 211802 (2005)) Limit on the neutrino magnetic moment by analyzing the 7Be energy spectrum and with Cr source Moreover: geoneutrinos and supernovae Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Huge active masses and extremely low background are required Detecting neutrinos Solar neutrino flux on Earth F ~ 1010 cm-2 s-1 with energy [0.1-10] MeV Weakly interacting : s ~ 10-46 – 10-43 cm2 At 1 MeV (F ~ 108 cm-2 s-1 ) in water (s ~2 x10-46 cm2) in 30 days (T = 2.5 106 s) to see 1 event we need: Huge active masses and extremely low background are required Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

First requirement: underground Second requirement: ultra high purity materials and proper design to minimize contaminations Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Borexino Collaboration Genova Milano Princeton University Perugia APC Paris Virginia Tech. University Munich (Germany) Dubna JINR (Russia) Kurchatov Institute (Russia) Jagiellonian U. Cracow (Poland) Heidelberg (Germany) Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e Abruzzo 120 Km da Roma Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e Laboratori esterni Borexino – Rivelatore e impianti Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Detection principles and n signature Borexino detects solar n via their elastic scattering off electrons in a volume of highly purified liquid scintillator Mono-energetic 0.862 MeV 7Be n are the main target, and the only considered so far Mono-energetic pep n , CNO n and possibly pp n will be studied in the future Detection via scintillation light: Very low energy threshold Good position reconstruction Good energy resolution BUT… No direction measurement The n induced events can’t be distinguished from other b events due to natural radioactivity Extreme radiopurity of the scintillator is a must! Typical n rate (SSM+LMA+Borexino) Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

n-spectrum in Borexino NW-1 = 0.25 – 0.8 MeV (7Be) NW-2 = 0.8 – 1.4 MeV (pep & CNO) LMA –BP2004 – LUNA 3 years statistics 100 tons Events/3years/100 tons/0.05 MeV NW-1 NW-2 Source Number of events in 3 y NW-1 NW-2 7Be 28470 pep 1095 986 pp 8B 44 33 13N 1260 66 15O 1533 593 All 33497 1634 Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Borexino Background Expected solar neutrino rate in 100 tons of scintillator ~ 50 counts/day (~ 5 10-9 Bq/Kg) Just for comparison: Natural water ~ 10 Bq/kg in 238U, 232Th and 40K Air ~ 10 Bq/m3 in 39Ar, 85Kr and 222Rn Typical rock ~ 100-1000 Bq/kg in 238U, 232Th and 40K BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth! - Low background nylon vessel fabricated in hermetically sealed low radon clean room (~1 yr) - Rapid transport of scintillator solvent (PC) from production plant to underground lab to avoid cosmogenic production of radioactivity (7Be) - Underground purification plant to distill scintillator components. - Gas stripping of scintlllator with special nitrogen free of radioactive 85Kr and 39Ar from air - All materials electropolished SS or teflon, precision cleaned with a dedicated cleaning module Throughout the experiment we developed special methods to achieve extremely low levels of background in the detector. [See text in slide.] Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Detector layout and main features Stainless Steel Sphere: 2212 PMTs 1350 m3 Scintillator: 270 t PC+PPO in a 150 mm thick nylon vessel Nylon vessels: Inner: 4.25 m Outer: 5.50 m Water Tank: g and n shield m water Č detector 208 PMTs in water 2100 m3 Carbon steel plates 20 legs Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Nylon vessel installation PMTs: PC & Water proof Nylon vessel installation Installation of PMTs on the sphere Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Storage area and Plants Water Plant Storage area and Plants Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Counting Test Facility CTF è un prototipo su piccola scala di Borexino: ~ 4 tons of scintillator 100 PMTs Buffer of water Muon veto Vessel radius: 1 m CTF ha dimostrato la fattibilità di Borexino Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

May 15, 2007 Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Strategy for Reduction Borexino background RadioIsotope Concentration or Flux Strategy for Reduction Name Source Typical Required Hardware Software Achieved m cosmic ~200 s-1 m-2 ~ 10-10 Underground Cherenkov signal <10-10   at sea level Cherenkov detector PS analysis (overall) Ext. g rock Water Tank shielding Fiducial Volume negligible Int. g PMTs, SSS Material Selection Water, Vessels Clean constr. and handling 14C Intrinsic PC/PPO ~ 10-12 ~ 10-18 Old Oil, check in CTF Threshold cut 238U Dust ~ 10-5-10-6 g/g < 10-16 g/g Distillation, Water Extraction ~ 2 10-17 232Th Organometallic (?) (dust) (in scintillator) Filtration, cleanliness ~ 7 10-18 7Be Cosmogenic (12C) ~ 3 10-2 Bq/t < 10-6 Bq/ton Fast procurement, distillation Not yet measurable ? 40K Dust, ~ 2 10-6 g/g < 10-14 g/g scin. Water Extraction PPO < 10-11 g/g PPO Distillation 210Pb Surface contam. Cleanliness, distillation from 222Rn decay (NOT in eq. with 210Po) 210Po Spectral analysis ~ 14 a/b stat. subtraction ~ 0.01 c/d/t 222Rn air, emanation from ~ 10 Bq/l (air) < 1 c/d/100 t Water and PC N2 stripping, Delayed coincidence < 0.02 c/d/t materials, vessels ~100 Bq/l (water) (scintillator) cleanliness, material selection 39Ar Air (nitrogen) ~17 mBq/m3 (air) Select vendor, leak tightness 85Kr ~ 1 Bq/m3 in air Spectral fit = 25±3 (learn how to measure it) fast coincidence = 29±14 Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

“Few” details Detector & Plants All materials carefully and painfully selected for: Low intrinsic radioactivity Low Rn emanation Good behaviour in contact with PC Pipes, vessels, plants: electropolished, cleaned with detergent(s), pickled and passivated with acids, rinsed with ultra-pure water down to class 20-50 The whole plant is vacuum tight Leak requirements < 10-8 atm/cc/s Critical regions (pumps, valves, big flanges, small failures) were protected with additional nitrogen blanketing PMTs (2212) Sealing: PC and water tolerant Low radioactivity glass Light cones (Al) for uniform light collection in fiducial volume Time jitter: 1.1 ns (for good spatial resolution, mu-metal shielding) 384 PMTs with no cones for m id Nylon vessels Material selection for chemical & mechanical strength Low radioactivity to get <1 c/d/100 t in FV Construction in low 222Rn clean room Never exposed to air Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Expected Spectrum How did Borexino do? Let’s first state what is the ultimate goal. The ideal experiment looking at low energy solar neutrinos with organic liquid scintillators faces two main sources of irreducible background. 14C - in yellow - is an intrinsic low energy contaminant for the scintillator and determines the lowest possible threshold of sensitivity to solar neutrinos. 11C - in green - is a cosmogenic radionuclide. It is produced in situ by the residual muon flux. It was long thought to be irreducible background, covering the region between 1 and 2 MeV. Some recent on cosmogenics will allow a very significant reduction of this background. All other source of background - including 85Kr, 210Po, 210Bi, that impede observation of Solar Neutrinos in other contexts - can be removed. In between 14C and 11C is the expected signal of low energy solar neutrinos: 7Be - in red - and CNO + pep - in pink -. The 7Be neutrinos produce a box-like spectrum of recoil electrons with end point at 667 keV. I remark that this is the ideal spectrum: what you get if you are able to remove all possible contaminants that can be removed. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

The starting point: no cut spectrum How did Borexino do? The black curve shows the all data collected in Borexino, with no cuts. You can notice background from 14C at low energy and a peak around 400 keV due to alphas from 210Po, which are heavily quenched in organic liquid scintillator. The portion of the spectrum above 400 keV is dominated by external gammas and muons. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Detecting (and rejecting) cosmic muons m pulses m crossing the buffer only m crossing the scintillator m are identified by ID and OD OD eff: ~ 99% ID based on pulse shape analysis Rejection factor > 103 (conservative) A muon in OD m track ID efficiency Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Detecting (and rejecting) cosmogenic neutrons t = 255 ± 5 ms n + p d + g t ~ 250 ms A dedicated trigger starts after each muon opening a gate for 1.6 ms. An offline clustering algorithm identifies neutron in high multiplicity events A muon in OD Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Residual background : << 1 c/d/100 t Muon and neutron cuts No cut m cut Residual background : << 1 c/d/100 t Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Position reconstruction Position reconstruction algorythms (we have 4 codes right now) time of flight fit to hit time distribution developed with MC, tested and validated in CTF cross checked and tuned in Borexino with 214Bi-214Po events and 14C events z vs Rc scatter plot Resolution 214Bi-214Po (~800 KeV) 14±2 cm 14C (~100 KeV): 41±4 cm Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Vessel Radius and Shape Vessel radius and origin calibrated exploiting fast coincidences of 222Rn and 220Rn chain segments emanated from vessel nylon 212Bi-212Po 214Bi-214Po t = 432.8 ns t = 236 ms The two plots confirms the CCD camera calibration results: vessel origin is shifted along positive z-axis by about 5 cm. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

7Be energy region (mainly 210Po) Spatial resolution 22 cm @ ~400 keV FV 220Rn-216Po 7Be energy region (mainly 210Po) 214Bi-214Po (~800 KeV) 14±2 cm 14C (~100 KeV): 41±4 cm m-induced neutrons FV 15 cm @ ~2200 keV Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Spectrum after FV cut (100 tons) The blue curve shows what happens when applying a fiducial cut to select the innermost 100 tons as the fiducial mass of the detector. The peak at 400 keV becomes more prominent, because 210Po is intrinsic to the scintillator. The background above 400 keV is reduced very substantially. A first evidence of the 7Be shoulder appears. You will also notice that the background above 1 MeV is limited to 11C. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

238U content 214Bi 214Po 210Pb b a t = 236 ms 3.2 MeV ~700 keV eq. Assuming secular equilibrium and looking in the FV only: 0.02 cpd/tons corresponding to 238U = (1.9 ± 0.3)×10-17 g(U)/g 214Bi 214Po Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

232Th content 212Bi 212Po 208Pb b a t = 432.8 ns 2.25 MeV ~1000 keV eq. t = 442 ± 57 ns Events are mainly on the south vessel surface (probably particulate) Assuming secular equilibrium and looking in the FV only : 0.00256 cpd/ton corresponding to 232Th = (6.8±1.5)×10-18 g(Th)/g Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

a/b discrimination Full separation at high energy a particles Small deformation due to average SSS light reflectivity a particles b particles ns 250-260 pe; near the 210Po peak 200-210 pe; low energy side of the 210Po peak 2 gaussians fit 2 gaussians fit a/b Gatti parameter a/b Gatti parameter Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

210Po contamination Not from 210Pb FV (R < 3 m) No radial cut Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

a/b statistical subtraction The red curve shows what happens after application of pulse shape discrimination to remove alphas from 210Po. The 210Po peak disappears. The 7Be shoulder now appears in its full glory. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Energy calibration and stability We have not calibrated with inserted sources (yet) Planned for the near future So far, energy calibration determined from 14C end point spectrum Energy stability and resolution monitored with 210Po a peak Difficult to obtain a very precise calibration because: 14C intrinsic spectrum and electron quenching factor poorly known Light yield determined from 14C fit Light yield monitored with 210Po peak position Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Energy scale 14C MC vs data comparison of photoelectron time distributions from 14C MC-G4Bx Data 11C m-induced neutrons LY = 500 (1%) p.e./MeV kB = 0.017 (15%) cm/MeV Ph.Y. ~ 9000 photons/MeV Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

New results with 192 days of statistics Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

New results with 192 days of statistics Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Systematic and Final Result Estimated 1σ Systematic Uncertainties* [%] *Prior to Calibration Expected interaction rate in absence of oscillations: 75±4 cpd/100 tons for LMA-MSW oscillations: 48±4 cpd/100 tons, which means: Total Scintillator Mass 0.2 Fiducial Mass Ratio 6.0 Live Time 0.1 Detector Resp. Function Cuts Efficiency 0.3 Total 8.5 Signal from 7Be neutrinos was extracted through a fit in the region 270-800 keV. We determined that the neutrino signal is 47±7±12 cpd/100 tons. The large systematic error quoted in the paper is due to the lack of calibrations at the time of publication of the results - 2 1/2 months after the detector was full! Let me remark that the Borexino calibration program is designed to reach an accuracy of ±2%. The systematic error will be strongly suppressed in a short time. The signal expected in absence of oscillations is 75±4 cpd/100 tons. The signal expected for LMA-MSW oscillations is 49±4 cpd/100 tons. Our result is consistent - within errors - with the LMA-MSW expectation. 7Be Rate: 49±3stat±4syst cpd/100 tons , which means Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Before Borexino Here is what was known about the survival probability of electron neutrinos before Borexino. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

After Borexino This is the improved situation after the initial release of the Borexino results, offering a clearer picture, and more consistent with the LMA-MSW paradigm. Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Constraints on pp and CNO fluxes Combining Borexino 7Be results with other experiments,the expected rate in Clorine and Gallium experiments is Survival Probability measured over predicted flux ratio where Ri,k and Pi,k are calculated in the hypothesis of high-Z SSM and MSW LMA Rk are the rates actually measured by Clorine and Gallium experiments f8B is measured by SNO and SuperK to be 0.87 ±0.07 f7Be =1.02 ±0.10 is given by Borexino results Plus luminosity constraint: best determination of pp flux! Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Neutrino Magnetic Moment Neutrino-electron scattering is the most sensitive test for mn search EM current affects cross section: spectral shape sensitive to μν sensitivity enhanced at low energies (c.s.≈ 1/T) Estimate Method 10-11 μB SuperK 8B <11 Montanino et al. 7Be <8.4 GEMMA Reactor <5.8 Borexino <5.4 A fit is performed to the energy spectrum including contributions from 14C, leaving μn as free parameter of the fit Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

What next? Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

BOREXino NA54 @ CERN: 100 and 190 GeV muon beams on a 12C target: 11C represents 80% of all the muon-induced contaminants and more than 99% in the CNO pep-n energy window Hagner et al., Astropart. Phys. 14, 33 (2000) KamLAND (from the Mitzui presentation at Neutrino06) 11C Rate (cts / day / 100 tons) All energy 0.8 – 1.4 MeV KamLAND 107 55 BOREXino 15 7.4 SNO+ 0.15 0.074 Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

11C production and decay m (+ secondaries) + 12C → m (+ secondaries) + 11C + n Coincidence among: cosmic muon: rate at LNGS (3700 mwe): 1.16 hr-1 m-2 average energy: 320 GeV gamma from neutron capture: energy: 2.2 MeV capture time: 250 ms positron from 11C decay: deposited energy between 1.022 and 1.982 MeV mean life: 30 min n + p → d + g 11C → 11B + e+ + ne Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Large scintillator detector potential m PC+PPO 11C n g Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Muon track reconstruction pep and CNO n fluxes software algorithm based on a three-fold coincidence analysis to subtract efficiently cosmogenic 11C background Muon track reconstruction 8B at low energy region (3-5 MeV) pp seasonal variations (?) High precision measurements systematic reduction calibrations geoneutrinos Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008

Conclusion Borexino opened the study of the solar neutrinos in real time below the barrier of natural radioactivity (4 MeV) Two measurements reported for 7Be neutrinos Best limits for pp and CNO neutrinos, combining information from SNO and radiochemical experiments Opportunities to tackle pep and CNO neutrinos in direct measurement Borexino will run comprehensive program for study of antineutrinos (from Earth, Sun, and Reactors) Borexino is a powerful observatory for neutrinos from Supernovae explosions within few tens of kpc Best limit on neutrino magnetic moment. Improve by dedicated measurement with 51Cr neutrino source Davide Franco – Università di Milano & INFN Lisbon – July 9, 2008