M. Misiaszek (Instytut Fizyki UJ, Kraków)

Rekonstrukcja oddziaływań neutrin w detektorze BOREXINO

Kurchatov Inst.(Russia)

Dubna JINR (Russia)

Heidelberg(Germany)

Munich (Germany)Jagiellonian U.

Cracow(Poland)

PerugiaGenova

APC Paris

Milano

Princeton University

Virginia Tech.University

Seminarium neutrinowe IFT, Wrocław 16 listopada 2009

Borexino aims to measure low energy solar neutrinos in real time by elastic neutrino-electron scattering in a volume of highly purified liquid scintillator

Mono-energetic 0.862 MeV 7Be ν is the main target

Pep, CNO and possibly pp ν

Geoneutrinos

Supernova ν

Detection via scintillation light Very low energy threshold Good position reconstruction Good energy resolution

Drawbacks: No direction measurements

ν induced events can’t be distinguished from β-decay due to natural radioactivity

Extreme radiopurity of the scintillator

Typical rate

(SSM+LMA+Borexino)

The physics goals and detection principles of Borexino

Detector design and layout

Water Tank: and n shield water Ch

detector208 PMTs in water2100 m3

20 legs

Carbon steel plates

Scintillator:270 t PC+PPO in a 125 m thick nylon vessel

Stainless Steel Sphere:

2212 photomultipliers 1350 m3

Nylon vessels:Inner: 4.25 mOuter: 5.50 m

Design based on the principle of graded shielding

CNO

7Be11C

10C

14C

pp+pep+8B

238U + 232Th

The expected signal and the irreducible background

Borexino is continuously taking data since 13/05/2007

• Algorytmy do rekonstrukcji pozycji zdarzeń oparte są o metodę największej wiarygodności, którą poszukuje się najbardziejprawdopodobnego miejsca emisji fotonów.

x0

t4

t5

t6

t1

t2t3

ti = const + tofi + t'i

tofi = n/c * di(xi,yi,zi)

• Zakładamy próbną pozycję zdarzenia x0

• Obliczamy tof (czas przelotu) dla każdego fotonu

• Odejmujemy tof od każdego ti

• Porównujemy otrzymany rozkład t'i z oczekiwanym rozkładem fotonów emitowanych ze scyntylatora

• Algorytm przeszukuje inne pozycje x0 dopóki nie znajdzie pozycji dla której dopasowanie jest najlepsze

ti t'i

(xi,yi,zi)

Rozkład przestrzenny zdarzeńOdrzucenie zdarzeń tła (głównie pr. gamma)R < 3.3 m (100 t masy scyntylatora)

Rozkład radialny

R2

gauss

2 2 2R x y z 2 2cR x y

z vs Rc scatter plot

FV

238U and 232Th

212Bi 212Po 208Pb = 432.8 ns

2.25 MeV ~800 KeV eq.

Only 3bulk candidates (47.4d)

214Bi-214Po

212Bi-212Po

212Bi-212Po

214Bi 214Po 210Pb = 236 s

3.2 MeV ~700 KeV eq.

238U: < 2. 10-17 g/g232Th: < 1. 10-17 g/g

Kształt impulsu w detektorze BOREXINO

[ns]

/ discrimination

particles

Small deformation due to averageSSS light reflectivity

particles

250-260 pe; near the 210Po peak 200-210 pe; low energy side of the 210Po peak

2 gaussians fit 2 gaussians fit

Full separation at high energy

ns

Gatti parameter Gatti parameter

Final spectrum after all cuts

Kr+Be shoulder

14C

210Po (only, not in eq. with 210Pb!)

11C

Understanding the final spectrum: main components

Last cut: 214Bi-214Po and Rn daughters removal

No s

Afterfiducial volume cut(“100 tons”)

Konwersja liczby zmierzonych

fotoelektronów do energii zdarzenia

Fit parametrów do kształtu elektronów z 14C

~ 500 pe /MeV

Monitoring stabilności detektora

Liczba fotoelektronów Date

N

100 Hz 14C+222Rn source diluted in PC: 115 points inside the sphere b : 14C, 222Rn diluted in scintillator a : 222Rn diluted in scintillator g : 54Mn, 85Sr, 222Rn in airN : AmBe

Source localization within 2 cmthrough red laser light and CCD camera

Accurate handling and manipulationof the source and of the materialsinserted in the scintillator

The Borexino calibrationA first calibration campaign with on axis and off axis radioactive sources has been performed (Oct 08 on axis, Jan-Feb09 off axis)

accurate position reconstruction

precise energy calibration

detector response vs scintillation position

The measured energy spectrum: May07 - Oct08

Records in the radiopurity achieved by Borexino

Material Typical conc. Borexino level

in the scintillator14C scintillator 14C/12C<10-12

238U,232Th equiv. - Hall C dust

- stainless. steel

- nylon

~1 ppm

~1ppb

~1ppt

Knat Hall C dust ~1 ppm

222Rn - external air.

- air underground

~20 Bq/m3

~40-100 Bq/m3

85Kr39Ar

in N2 for stripping ~1.1 Bq/m3

~13 mBq/m3

- 222Rn

- 238U,232Th equiv.

LNGS - Hall C water ~50 Bq/m3

~10-10 g/g

14C /12C 210 18

10 17 10 18g /g

10 14 g /g

1 Bq /m3

~ 0.16 mBq /m3

~ 0.5 mBq /m3

~ 30 Bq /m3

~ 10 14 g /g

•Fit between 100-800 p.e.;

•Light yield: a free fit parameter;

•Ionization quenching included (Birks’ parametrization);

• 210Bi, 11C and 85Kr free fit parameters;

•Others v fixed

•Fit to the spectrum without and with subtraction is performed giving consistent results

R7Be= 49 ± 3stat ± 4sys cpd/100 tons

The measurement of the 7Be flux (192 days of live time)

Borexino Collaboration Phys. Lett. B 658 (2008) : after 2 months of data takingBorexino Collaboration PRL 101 (2008) : 192 days of live time

Expected rate (cpd/100 t)

No oscillation 75 ± 4

BPS07(GS98) HighZ 48 ± 4

BPS07(AGS05) LowZ 44 ± 4

No-oscillation hypothesis rejected at 4 level

7Be: (49 ± 3stat ±4sys ) cpd/100 tons (192 days)

The analysis of the calibration datais in progress

The measurement of the 7Be flux (192 days of live time)

Survival probability of the e

Before Borexino

After Borexino

Survival probability of the e

First measurement of the ratio between the survival probabilities in vacuum and in matter

Limits obtained by Borexino after 200 days of data taking -the best in the literature

1- Limits on pp e CNO solar fluxes; with the Luminosity constraint:

2- Limit on the neutrino magnetic moment:

5.4 10 11B (90%C.L.)

pp (Borexino data) /pp (SSM) 1.00 0.0200.008

CNO (Borexino data) /CNO (SSM) 3.8 (90%C.D.)

The low threshold measurement of the 8B solar neutrinos

2.6 MeV ’s from 208Tl on PMT’sand in the buffer Borexino threshold: 2.8 MeV

Expected (MSW-LMA) count rate due to 8B neutrinos above 2.8 MeV:0.26±0.03 c/d/100 tons

Borexino energy spectrum after muon subtraction: 246 days of live time

The low threshold measurement of the 8B solar neutrinos

Major background sources:1) Muons;2) Gammas from neutron capture;3) Radon emanation from the nylon vessel; 4) Short lived (t < 2 s) cosmogenic isotopes;5) Long lived (t > 2 s) cosmogenic isotopes (10C);6) Bulk 232Th contamination (208Tl);

The Borexino 8B spectrum

tons/100counts/day )0.02 0.04 0.26( sysstat8.2 MeVRate

7Be and 8B flux measured with the same detector

Borexino 8B flux above 5 MeV agrees with existing data

Neutrino oscillation is confirmed by the 8B of Borexinoat 4.2 sigma

Results already achieved in Borexino1. First direct experimental evidence of the vacuum regime and of the transition region in the neutrino

oscillation at very low energy: measurement of the 7Be flux (0.2-0.8 MeV) and strong limit on the pp flux.

2. First determination of the ratio between the e survival probability in vacuum and in matter: 1.6 ± 0.33 (from the 7Be flux and the 8B flux, measured with a threshold down to 2.8 MeV).

3. Measurements of the day/night effect for at very low energy:

4. First validation of the MSW-LMA model in the vacuum regime and in the transitionregion within the error (10% for the 7Be flux measurement: stat.+ syst.).

5. Best limits for CNO flux, magnetic moment, Pauli principle violation.

ADN N DN D

0.02 0.04

What nextA. Measurement of the 7Be flux with a total error final validation of the MSW-LMA model;

important insight for the Standard Solar Model metallicity puzzle and stronger limitson the pp flux.

B. Determination of the survival probability ratio, day/night effect, etc. with strongly reduced errors.C. Study of the pep and CNO region (energy spectrum in the range 0.9-1.5 MeV) with the

suppression of the 11C muon produced.D. Measurements of the geoneutrinos (the Gran Sasso region is especially favoured due to the

absence of the main background: reactor ).

e

Observatory• Borexino is a Supernova observatory in the network SNEWS .

Literatura

First real time detection of 7Be solar neutrinos by Borexino(Phys. Lett. B 658 (2008) 101-108)

New results on solar neutrino fluxes from 192 days of Borexino data(Phys. Rev. Lett. 101 (2008) 091302 )

Measurement of the solar 8B neutrino flux with 246 live days of Borexino and observation of the MSW vacuum-matter transition

(arXiv:0808.2868v1)