Borexino results and future

Borexino results and future

Davide Franco CNRS-APC

GdR Neutrino 2012October 30-31, 2012

Davide Franco – APC CNRS – GdR Neutrino 2012

Neutrino Production In The Sun

pp chain:pp, pep, 7Be, hep ,and 8B

CNO cycle:13N, 15O, and 17F


Solar Neutrino Spectra

SNOSuperK

(real time)

HomestakeGallexGNOSage

Borexino (real time)


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.


The Standard Solar Model after 2004

Latest work by Asplund, Grevesse and Sauval (Nucl. Phys. A 777, 1 (2006)) indicates a lower metallicity 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)

GS 98 5.97 1.41 7.91 5.08 5.88 2.82 2.09 5.65

AGS 09 6.03 1.44 8.18 4.64 4.85 2.07 1.47 3.48

-1% -2% -3% -9% -18% -27% -30% -38%

Solar neutrino measurements could solve the problem!


Borexino goals

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? Test of the matter-vacuum oscillation transition with 7Be, pep, and low energy 8B neutrinos SNEWS network for supernovae First evidence (>3) of geoneutrinos Neutrino speed of light Limit on the neutrino magnetic moment by analyzing the 7Be energy spectrum and with Cr source Sterile neutrinos with sources


Detector layout and radiopurity

Water Tank: and n shield water Č detector208 PMTs in water2100 m3

Carbon steel plates

Scintillator:270 t PC+PPO in a 150 m thick nylon vessel

Stainless Steel Sphere:2212 PMTs 1350 m3

Nylon vessels:Inner: 4.25 mOuter: 5.50 m


Detection principles and signature Borexino detects solar via their elastic scattering off electrons in a volume of

highly purified liquid scintillator

Mono-energetic 0.862 MeV 7Be are the main target, and the only considered so far

Mono-energetic pep , CNO and possibly pp 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 induced events can’t be distinguished from other events due to natural radioactivity

Extreme radiopurity of the scintillator is a must!

Typical rate (SSM+LMA+Borexino)


Borexino background

RadioIsotope Concentration or Flux Strategy for Reduction

Name Source Typical Required Hardware Software Achieved

cosmic ~200 s-1 m-2 ~ 10-10Underground Cherenkov signal <10-10

at sea level Cherenkov detector PS analysis (overall)

Ext. rock Water Tank shielding Fiducial Volume negligible

Int. PMTs, SSS Material Selection Fiducial Volume negligible

Water, Vessels Clean constr. and handling

14C Intrinsic PC/PPO ~ 10-12 ~ 10-18 Old Oil, check in CTF Threshold cut ~ 10-18

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 Not yet measurable ?

PPO (dust) < 10-11 g/g PPO Distillation

210Pb Surface contam. Cleanliness, distillation Not yet measurable ?

from 222Rn decay (NOT in eq. with 210Po)

210Po Surface contam. Cleanliness, distillation Spectral analysis ~ 14 from 222Rn decay 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) < 1 c/d/100 t Select vendor, leak tightness Not yet measurable ?

85Kr Air (nitrogen) ~ 1 Bq/m3 in air < 1 c/d/100 t Select vendor, leak tightness Spectral fit = 25±3 (learn how to measure it) fast coincidence = 29±14


Detector Calibration

May 2007

Detector response vs position:100 Hz 14C+222Rn in scintillator in >100

positions

Quenching and energy scale:Beta: 14C, 222Rn in scintillatorAlpha: 222Rn in scintillatorGamma: 139Ce, 57Co, 60Co, 203Hg, 65Zn, 40K, 85Sr, 54MnNeutron: AmBe

MC-G4BxData

Pulse shape of 14C events

LY ~ 500 p.e./MeV Ph.Y. ~ 12000 photons/MeV

Data - MC


7Be neutrino (862 keV) rate @ 4.6%Phys. Rev. Lett. 107, (2011) 141302

740 live days

MC Fit

Analytical Fit

Systematics


Implication of the 7Be measurement

No oscillation excluded @ 5.0 Assuming MSW-LMA:

f7Be= 0.97± 0.09

fpp = 1.013 +0.003 -0.010

fCNO < 1.7% (95 % C.L.)

Pee = 0.51 ± 0.07 @ 867 keV no power to resolve low/high metallicity problem

< 5.4 10-11 b

Neutrino magnetic moment limit:


The Day-Night Asymmetry

Adn = 0.001 ± 0.012 (stat) ± 0.007 (syst) Adn = 0.001 ± 0.012 (stat) ± 0.007 (syst)

LOW ruled out at 8.5 LOW ruled out at 8.5

A neutrino “regeneration” is expected only in the LOW

solution

A neutrino “regeneration” is expected only in the LOW

solution


8B neutrinos with the lowest threshold: 3 MeV

2.6 MeV ’s from 208Tl on PMT’s and in the buffer

All volume

R < 3 m (100 tons)

Energy spectrum in Borexino (after subtraction)

Expected 8B rate in 100 tons of liquid scintillator above 2.8 MeV:0.26±0.03 c/d/100 tons

> 5 distant from the 2.6 MeV peak S/B ratio < 1/6000!!!

live-time: 246 days


8B neutrino rate measurement

raw spectrum

cut

FV cut

cosmogenic, neutron, 214Bi and 10C cuts

208Tl

3.0-16.3 MeV 5.0-16.3 MeV

Rate [c/d/100 tons] 0.22±0.04±0.01

0.13±0.02± 0.01

ESexp [106 cm−2s−1] 2.4±0.4±0.1 2.7±0.4±0.2

ESexp/ES

th 0.88±0.19 1.08±0.23


First observation of pep neutrinos

+ 12C → + 11C + n

+ p → d +

→11B + e+ + e

Three Fold Coincidence (TFC): space-time veto removes 90% of 11C

paid with 50% loss of exposure

11C ~ 30 min


o-Ps in scintillators

Measurements of the o-Ps mean-life and formation

probability in scintillators with the PALS technique

Measurements of the o-Ps mean-life and formation

probability in scintillators with the PALS technique

D. Franco, G. Consolati, D. Trezzi, Phys. Rev. C83 (2011) 015504

Pulse shape induced distortion

Pulse shape induced distortion

MC

~ 3 nsp ~ 50%

ANR JCJC: NuToPsANR JCJC: NuToPs


Multivariate maximum likelihood fit

Energy spectral fitEnergy spectral fit

Radial fitRadial fitPulse shape fitPulse shape fit

Pulse shape testPulse shape test


pep neutrinos: results

pep neutrinos: Rate: 3.1 ± 0.6(stat) ± 0.3(sys) cpd/100 t Assuming MSW-LMA: Φpep = 1.6 ± 0.3 108 cm-2 s-1

No oscillations excluded at 97% C.L. Absence of pep solar ν excluded at 98%

CNO neutrinos: only limits, correlation with 210Bi; CNO limit obtained assuming pep @ SSM CNO rate < 7.1 cpd/100 t (95% c.l.) Assuming MSW-LMA: ΦCNO < 7.7 108 cm-2 s-1 (95% C.L.)

the strongest limit to date not sufficient to resolve metallicity problem

Δχ2 profile for fixed pep and CNO rates

Δχ2 profile for fixed pep and CNO rates

Borexino limit


The Borexino Solar neutrino spectroscopy


Status and future perspectives

Since July 2010, purification campaigns:

• Nitrogen stripping successful in 85Kr removal:85Kr < 8.8 cpd / 100 t (2007-2010: 31.2 ± 5)

• moderate success in 210Bi removal by water extraction:210Bi : (16 + 4) cpd / 100 t (2007-2010: 41.0 ± 2.8)

• unprecedented 238U and 232Th radio-purity: 238U < 9.7 10-19 g/g and 232Th < 2.9 10-18 g/g

• 210Po natural decreasing:

~5 cpd / t

Borexino phase II started:

• Solar neutrino program:

- Improve 7Be, 8B → test of MSW

- Confirm pep at more than 3 σ

- CNO neutrino measurement → probe metallicity

- Attempt direct pp measurement

• more statistics for an update of geo-neutrino measurement;

• sterile neutrinos with sources


CNO Neutrino Measurement

Main background: 210BiCriticality: similar spectral shapes

Strategy suggested by Villante et al. (Phys.Lett.B701:336-341,2011):

Constraining 210Bi rate looking at 210Po decay

Energy Energy + Decay Time 210Po R(t=0) = 2000 cpd/100 tonM = 100 tont = 1 year


SOX

ERC Advanced Granted(~3.5 Meuro)

Goal Borexino sterile neutrino oscillation measurement with a 51Cr (144Ce) source

5. Activity: several 1000 evts within 1 year

6. E >250 keV (14C background)7. Half-life ≥1 month

1. Limited heat2. Efficient shielding3. Low impurities level4. Compactness

Requirements


Sterile Neutrinos in Borexino

A

BC

A: underneath WT

D=825 cm

No change to present configuration

C: center

Major change

Remove inner vessels

At the end of Solar Neutrino physics

B: inside WT

D = 700 cm

Need to remove shielding water


The external 51Cr signature

Multivariate fit:• energy • radius• decay time


The Sensitivity

External source

activity=10MCi

Error on activity=1%

Error on FV=1%

External source

activity=10MCi

Error on activity=1%

Error on FV=1%

FV error better than 1% already achieved in BX (calibration)

Error of 1% on the source intensity is agressive – important effort to achieve it


Conclusions

Phys. Lett. B658:101-108, 2008Phys. Rev. Lett. 101, 091302, 2008Nucl. Instr. & Meth A600:568-593, 2009Nucl. Instr. & Meth A609:58-78, 2009Phys. Lett. B687:299-304, 2010Phys. Rev. D82, 033006, 2010Phys. Rev. C81, 034317, 2010Phys. Rev. Lett. 107, 141302, 2011JINST 6 P05005, 2011Phys. Lett. B707:22–26, 2012Phys. Rev. Lett. 108, 051302, 2012Phys. Rev. D85, 092003, 2012

• The Phase 1 of the Borexino experiment is over

• First detection and 5% measurement of 7Be line

• 8B at low energy, 7Be day-night, geo-nu at 99.997% C.L.

• First detection of pep solar neutrinos

• New limits on Pauli Principle Violation and Solar Axions

• Purification was successful, and phase 2 is starting

• A rich program on solar neutrino physics to be completed

• Probe MSW through 8B at low energy, pep and more precise 7Be

• Attempt to detect pp in real time and possible interesting upper limit on CNO

• SOX: Sterile Neutrino Search with neutrino and/or anti-neutrino source

Documents

Borexino results and future