First Results from the Borexino Solar Neutrino Experiment Celebrating F.Avignone, E.Fiorini & P. Rosen University of South Carolina May 16, 2008 Frank Calaprice

First Contact with Frank Avignone 65 Zn source given by Ray Davis

Axion Searches Summary of Texono Coll Zn

Science with Borexino The Neutrino The Sun The Earth Supernovae

Basic Neutrino Facts Postulated in 1931 by Pauli to preserve energy conservation in  -decay. First Observed by Cowan and Reines in 1950’s by inverse beta decay: e +p->n+e +. Electric charge: 0; Spin: 1/2; Mass: very small Like other fermions, comes in 3 flavors: e, ,  Interactions: only via the weak force (and gravity)

Solar Neutrino Production Occurs in two cycles: pp and CNO (mostly pp) In each pp cycle: 26.7 MeV released 2 neutrinos created 4 protons are converted to 4 He Total Flux constrained by luminosity:  =( 2 ’s/26.7MeV) (L/4  r 2 ) ~ 6.6x10 10 /cm 2 /s.

Solar Neutrino Energy Spectrum

Birth of Solar Neutrino Experiments : Davis builds 615 ton chlorine (C 2 Cl 4 ) detector Deep underground to suppress cosmic ray backgrounds. Homestake Mine (4800 mwe depth) Low background proportional detector for 37 Ar decay. 37 Cl + e -> 37 Ar +e - Detect 37 Ar +e - -> 37 Cl + e (t 1/2 ~ 37 d) Detected ~1/3 of expected rate.

Chlorine Data

Neutrino Oscillations The Solar Neutrino Problem was explained by neutrino oscillations, the possibility of which was first suggested by Pontecorvo in An electron neutrino that oscillates into a muon neutrino would not be detected in the chlorine reaction. Experimental proof of oscillations came decades later from experiments on atmospheric neutrinos (SuperK), solar neutrinos (SNO), and reactor anti-neutrinos (Kamland).

Neutrino Vacuum Oscillations In 1967 Pontecorvo showed that non-conservation of lepton charge number would lead to oscillations in vacuum between various neutrino states. In 1968 Gribov and Pontecorvo suggested this could explain the low result of Davis. The neutrino rate is 2 times smaller if the oscillation length is smaller than the region where neutrinos are formed. The vacuum oscillation length is smaller than the sun’s core for the observed mass value. Matter enhancement was needed to get the full deficit

Matter Enhanced Oscillations 1978 Wolfenstein shows that neutrino oscillations are modified when neutrinos interact with matter Mikhaev and Smirnow show that neutrinos may undergo a resonant flavor conversion if the density of matter varies, as in the sun. The MSW theory describes the enhanced oscillation in matter.

The Sudbury Neutrino Observatory (SNO) SNO is water Cherenkov detector with heavy (deuterated) water. Detects 8 B neutrinos Two reactions enable charged and neutral currents to be observed e + d -> p + p +e - (only e detected) x + d -> p + n + x (all ’s; x = e,  ) Observed that e oscillated to x Total rate of neutrinos agrees with predictions Oscillations proven to be cause of deficit!

SNO Results Clinch Neutrino Oscillations SNO First Results: 2001 Neutral current interactions (sensitive to all neutrinos equally) Elastic scattering interactions (sensitive to all neutrinos, enhanced sensitivity for electron neutrinos) Charged current interactions (sensitive only to electron neutrinos)

The SNO Mixing Parameters

The Kamland Detector

Kamland Results 2003

KamLAND Results 2005 Neutrinos from 53 Reactors

The Vacuum-Matter Transition Above about 2 MeV solar neutrino oscillations are influenced by interactions with matter, the MSW effect. Below ~ 2 MeV neutrino oscillations are vacuum-like. The 0.86 MeV 7 Be neutrino provides a data point in the vacuum region The Predicted Vacuum- Matter transition is being tested by Borexino. p-p, 7 Be, pep 8B8B

Non-Standard Neutrino-Matter Interactions? Exploring the vacuum-matter transition is sensitive to new physics. New neutrino-matter couplings (either flavor-changing or lepton flavor violating) can be parametrized by a new MSW- equivalent term ε Where is the relative effect of new physics the largest? At resonance! Friedland, Lundardini & Peña-Garay Blue: Standard oscillations Red: Non-standard interactions tuned to agree with experiments.

Borexino Historical Highlights : Prototype CTF Detector started : Low background in CTF achieved : Funding INFN,NSF, BMBF, DFG : Borexino construction August : Accidental release of ~50 liter of liquid scintillator shuts down Borexino and LNGS : Legal and political actions: Princeton 2005 Borexino Restarts Fluid Operations August 16, 2007 First Borexino Results on Web.

John Bahcall-Martin Deutsch Borexino Mishap August Martin Deutsch January 29, 1917 August 16, John Bahcall December 30, 1934 August 17, 2005 Borexino First Results Paper August

The Borexino Detector

Detection Principles Detect -e scattering via scintillation light Features: Low energy threshold (> 250 keV to avoid 14 C) Good position recostruction by time of flight Good energy resolution (500 p e /MeV) Drawbacks: No directional measurements ν induced events can’t be distinguished from other β/γ due to natural radioactivity Experiment requires extreme ssuppression of all radioactive contaminants

Solar Neutrino Science Goals Test MSW vacuum solution of neutrino oscillations at low energy. Look for non-standard interactions. Measure CNO neutrinos- metallicity problem. Compare neutrino and photon luminosities

Neutrinos and Solar Metallicity A direct measurement of the CNO neutrinos rate could help solve the latest controversy surrounding the Standard Solar Model. 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)), is in agreement within 0.5% with the solar sound speed measured by helioseismology. 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 Can use solar neutrino measurements to help resolve! 7 Be (12% difference) and CNO (50-60% difference)

Low Energy Neutrino Spectrum Mono-energetic 7Be and pep neutrinos produce a Box-like electron recoil energy spectrum pep

The Underground Halls of the Gran Sasso Laboratory Halls in tunnel off A24 autostrada with horizontal drive-in access Under 1400 m rock shielding (~3800 mwe) Muon flux reduced by factor of ~10 6 to ~1 muon/m 2 /hr BX in Hall C ~20mx20mx100m To Rome ~ 100 km

Special Methods Developed 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 ( 7 Be) Underground purification plant to distill scintillator components. Gas stripping of scintlllator with special nitrogen, free of radioactive 85 Kr and 39 Ar from air. All materials electropolished SS or teflon, precision cleaned with a dedicated cleaning module Vacuum tightness standard: 10-8 atm-cc/s

Purification of Scintillator

Assembly of Distillation Column in Princeton Cleanroom 100

Assembly of Columns Installing sieve trays in distillation column Installing structured packing in stripping column

Fabrication of Nylon Vessel John Bahcall

Raw Spectrum- No cuts

Expected Spectrum

Data with Fiducial Cut (100 tons) Kills gamma background from PMTs

Data: α/β Statistical Subtraction

Data with Expected pep & CNO

Published Data on 7 Be Rate Phys Lett B 658 (2008) 101 Expected interaction rate in absence of oscillations: 75±4 cpd/100 tons for LMA-MSW oscillations: 49±4 cpd/100 tons Measured: 47± 7± 12 cpd/100ton

Matter-Vacuum Before Borexino

After Borexino

Future Possibilities? Borexino could possibly measure pep, 8 B, and pp

Background: 232 Th Assuming secular equilibrium, 232 Th is measured with the delayed coincidence: 212 Bi 212 Po 208 Pb   = ns 2.25 MeV ~800 KeV eq. From 212 Bi- 212 Po correlated events in the scintillator : 232 Th: < 6 × g(Th)/g (90% C.L.) Specs: 232 Th: g/g cpd/ton Only few bulk candidates 212 Bi- 212 Po Time (ns)  =423±42 ns Events are mainly in the south vessel surface (probably particulate) z (m) R (m)

Background: 238 U Assuming secular equilibrium, 238 U is measured with the delayed coincidence: 214 Bi 214 Po 210 Pb   = 236  s 3.2 MeV ~700 KeV eq. 214 Bi- 214 Po  =240±8  s Time  s 214 Bi- 214 Po z (m) Setp - Oct 2007 Specs: 238 U: g/g < 2 cpd/100 tons 238 U: = 6.6 ± 1.7× g(U)/g R(m)

Background: 210 Po Big background! 60 cpd/1ton Not in equilibrium with 210 Pb and 210 Bi. But how??? 210 Po decays as expected. Where it comes from is not understood at all! It is also a serious problem for other experiments- dark matter, double beta decay

85 Kr came from a small leak during a short part of filling. Important background to be removed in future purification. Background: 85 Kr 85 Kr is studied through : 85 Kr  decay : (  decay has an energy spectrum similar to the 7 Be recoil electron ) 85 Kr  85 Rb 687 keV  = y - BR: 99.56% 85 Rb 85 Kr 85m Rb  = 1.46  s - BR: 0.43% 514 keV  173 keV 

Removal of 11 C Produced by muons: 25 cpd/100ton Obscures pep (2 cpd/100ton) Muon rate too high and half-life too long to veto all events after each muon. Strategy suggested by Martin Dentsch Look for muon-neutron coincidence and veto events near where the neutron is detected.

μ Track 11 C n Capture

Conclusions Methods developed for Borexino successfully achieved for the first time, a background low enough to observe low energy solar neutrinos in real time. Preliminary results on 7 Be favor neutrino oscillations in agreement with the MSW Large Mixing Angle solution. Backgrounds may be low enough to measure pep and CNO neutrinos using the muon+neutron tag to reduce 11 C background. Similar methods could be applied to neutrinoless  decay and other low background exps..

Borexino Collaboration Kurchatov Institute (Russia) Dubna JINR (Russia) Heidelber g (Germany ) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech. University

Rejection of 11 C Background

Muon induced 11 C Beta Background & pep neutrinos

PP Cycle: Branches 1 and 2

PP cycle Branch 3

CNO Cycle: Neutrinos from  -decay of 13 N, 15 O and 17 F

Neutrino Mixing

Vacuum Oscillation Length for 2-state mixing: masses m 1,m 2

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