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

65Zn source given by Ray Davis

Axion SearchesSummary of Texono Coll. 2006

65Zn

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 4He

Total Flux constrained by luminosity: =( 2’s/26.7MeV) (L/4r2) ~ 6.6x1010/cm2/s.

Solar Neutrino Energy Spectrum

Birth of Solar Neutrino Experiments

1965-67: Davis builds 615 ton chlorine (C2Cl4) detector

Deep underground to suppress cosmic ray backgrounds.

Homestake Mine (4800 mwe depth)

Low background proportional detector for 37Ar decay.

37Cl + e -> 37Ar +e-

Detect 37Ar +e- -> 37Cl + e (t

1/2 ~ 37 d)

Detected ~1/3 of expected rate.

Chlorine Data 1970-1994

Neutrino Oscillations

The Solar Neutrino Problem was explained by neutrino oscillations, the possibility of which was first suggested by Pontecorvo in 1967. 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.

1985 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 8B 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 7Be neutrino provides a data point in the vacuum region

The Predicted Vacuum-Matter transition is being tested by Borexino.

p-p, 7Be, pep

8B

Non-Standard Neutrino-MatterInteractions?

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 oscillationsRed: Non-standard interactions tuned to agree with experiments.

Borexino Historical Highlights

1989-92: Prototype CTF Detector started 1995-96: Low background in CTF achieved 1996-98: Funding INFN,NSF, BMBF, DFG 1998-2002: Borexino construction August 16 2002: Accidental release of ~50 liter of

liquid scintillator shuts down Borexino and LNGS 2002-2005: 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 16 2002

Martin Deutsch January 29, 1917

August 16, 2002.

John Bahcall December 30, 1934 August 17, 2005

Borexino First Results Paper August 16 2007

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The Borexino Detector

Detection Principles

Detect -e scattering via scintillation light Features:

Low energy threshold (> 250 keV to avoid 14C) Good position recostruction by time of flight Good energy resolution (500 pe/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!7Be (12% difference) and CNO (50-60% difference)

Low Energy Neutrino Spectrum

Mono-energetic 7Be and pep neutrinos produce aBox-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 ~106 to ~1 muon/m2/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 (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 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 7Be 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-VacuumBefore Borexino

After Borexino

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

Background: 232ThAssuming secular equilibrium, 232Th is measured with the delayed

coincidence:

212Bi 212Po 208Pb

= 432.8 ns

2.25 MeV ~800 KeV eq.

From 212Bi-212Po correlated events in the scintillator: 232Th: < 6 ×10-18 g(Th)/g (90% C.L.)

Specs: 232Th: 1. 10-16 g/g 0.035 cpd/ton

Only fewbulk candidates

212Bi-212Po

Time (ns)

=423±42 ns

Events are mainly in the south vessel surface (probably particulate)

z (m

)

R (m) R(m)

Background: 238U Assuming secular equilibrium, 238U

is measured with the delayed

coincidence:

214Bi 214Po 210Pb

= 236 s

3.2 MeV ~700 KeV eq.

214Bi-214Po=240±8s

Time s

214Bi-214Po

z (m

)

Setp - Oct 2007

Specs: 238U: 1. 10-16 g/g

< 2 cpd/100 tons

238U: = 6.6 ± 1.7×10-18 g(U)/g

R(m)

Background: 210Po Big background!60 cpd/1ton

Not in equilibrium with 210Pb and 210Bi. But how???

210Po 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

85Kr came from a small leak during a short part of filling.

Important background to be removed in future purification.

Background: 85Kr

85Kr is studied through :

85Kr decay :(decay has an energy spectrum

similar to the 7Be recoil electron )

85Kr

85Rb

687 keV

= 10.76 y - BR: 99.56%

85Rb85Kr 85mRb

= 1.46 s - BR: 0.43%

514 keV

173 keV

Removal of 11C 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

11Cn 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 7Be 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 11C background.

Similar methods could be applied to neutrinoless decay and other low background exps..

Borexino Collaboration

Kurchatov Institute(Russia)

Dubna JINR(Russia) Heidelberg

(Germany)

Munich(Germany)

Jagiellonian U.Cracow(Poland)

Perugia

Genova

APC Paris

Milano

Princeton University

Virginia Tech. University

Rejection of 11C Background

Muon induced 11C Beta Background & pep neutrinos

PP Cycle: Branches 1 and 2

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PP cycle Branch 3

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

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Neutrino Mixing

€

e = cos(θ) ν 1 + sin(θ) ν 2

ν μ = −sin(θ) ν 1 + cos(θ) ν 2

Vacuum Oscillation Length for 2-state mixing: masses m1,m2

€

λ(E) = 4πEh /((m22 −m1

2)c 3)

=2.47E /MeV

(m22 −m1

2)c 3 /eV 2meters

≈ 30 km E /MeV

for (m22 −m1

2)c 4 = 8 ×10−5eV 2

Radius of sun's core where neutrinos are produced :

R ≈ 0.2Ro =1.4 ×105 km.

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