Rekonstrukcja oddziaływań neutrin w detektorze BOREXINO

Seminarium neutrinowe IFT, Wrocław 16 listopada 2009 M. Misiaszek ( Instytut Fizyki UJ, Kraków)

Perugia Genova Milano APC Paris Princeton University Virginia Tech.University

Dubna JINR (Russia) Kurchatov Inst.

(Russia) Jagiellonian U.

Cracow (Poland) Heidelberg (Germany) Munich (Germany)

The physics goals and detection principles of Borexino

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 7 Be 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

Typical



rate (SSM+LMA+Borexino)

Drawbacks: No direction measurements ν induced events can’t be distinguished from β-decay due to natural radioactivity

Extreme radiopurity of the scintillator

Detector design and layout

Scintillator:

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

Stainless Steel Sphere:

2212 photomultipliers 1350 m 3

Nylon vessels:

Inner: 4.25 m Outer: 5.50 m

Carbon steel plates Water Tank:

g and n shield m water 2100 m 3 Ch detector 208 PMTs in water

20 legs Design based on the principle of graded shielding

The expected signal and the irreducible background

14 C 7 Be CNO pp+pep+ 8 B 238 U + 232 Th 11 C 10 C

Borexino is continuously taking data since 13/05/2007

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

t 4 t

• • • • • Zakładamy próbną pozycję zdarzenia

x 0

Obliczamy

tof (czas przelotu)

dla każdego fotonu Odejmujemy

tof

od każdego

t i

Porównujemy otrzymany rozkład

t' i

z oczekiwanym rozkładem fotonów emitowanych ze scyntylatora Algorytm przeszukuje inne pozycje

x 0

dopóki nie znajdzie pozycji dla której dopasowanie jest najlepsze

6 t 5 t i t 3 t 2 = const + tof tof i i 1 t x 0 + t ' i = n/c * d i (x i ,y i ,z i ) (x i ,y i ,z i ) t i t ' i

Rozkład przestrzenny zdarzeń

Odrzucenie zdarzeń tła (głównie pr. gamma)

R < 3.3 m

(100 t masy scyntylatora)

Rozkład radialny z vs R c scatter plot R 2 gauss FV

2   2 

x

2 

y

2

238

U and

232

Th

212 Bi 214 Bi

 

2.25 MeV = 432.8 ns 212 Po



208 Pb ~800 KeV eq.

 

3.2 MeV = 236

m

s 214 Po



210 Pb ~700 KeV eq.

214 Bi 214 Po 238 U: < 2. 10 -17 g/g 212 Bi 212 Po 212 Bi 212 Po Only 3 bulk candidates (47.4d) 232 Th: < 1. 10 -17 g/g

Kształt impulsu w detektorze BOREXINO [ns]



/



discrimination

Small deformation due to average SSS light reflectivity



particles



particles 250-260 pe; near the 210 Po peak ns 2 gaussians fit Full separation at high energy 200-210 pe; low energy side of the 210 Po peak 2 gaussians fit

/

Gatti parameter

/

Gatti parameter

14 C

Final spectrum after all cuts

Understanding the final spectrum: main components 210 Po (only, not in eq. with 210 Pb!)



Kr+



Be



shoulder 11 C No

m

s After fiducial volume cut (“100 tons”) Last cut: 214 Bi 214 Po and Rn daughters removal

N Konwersja liczby zmierzonych fotoelektronów do energii zdarzenia Fit parametrów do kształtu elektronów z 14 C Monitoring stabilności detektora ~ 500 pe /MeV Liczba fotoelektronów Date

The Borexino calibration

A 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 100 Hz 14 C+ 222 Rn source diluted in PC: 115 points inside the sphere b : 14 C, 222 Rn diluted in scintillator a : 222 Rn diluted in scintillator g : 54 Mn, 85 Sr, 222 Rn in air N : AmBe  Source localization within 2 cm through red laser light and CCD camera  Accurate handling and manipulation of the source and of the materials inserted in the scintillator

The measured energy spectrum: May07 - Oct08

Records in the radiopurity achieved by Borexino

14 C 238 U, 232 Th equiv.

K nat 222 Rn 85 Kr 39 Ar 222 Rn 238 U, 232 Th equiv.

Material Typical conc.

scintillator 14 C/ 12 C<10 -12 - Hall C dust - stainless. steel - nylon Hall C dust - external air.

- air underground in N 2 for stripping LNGS - Hall C water ~1 ppm ~1ppb ~1ppt  ~1 ppm ~20 Bq/m 3  ~40-100 Bq/m 3 ~1.1 Bq/m 3  ~13 mBq/m 3  ~50 Bq/m 3 ~10 -10 g/g  Borexino level in the scintillator 14

C

/ 12

C

 2  10  18 10  17  10  18

g

 10  14

g

/

g

 1 m

Bq

/

m

3 /

g

~ 0.16

mBq

/

m

3 ~ 0.5

mBq

/

m

3 ~ 30 m

Bq

/

m

3 ~ 10  14

g

/

g



The measurement of the

7

Be flux

(192 days of live time) •Fit between 100-800 p.e.; •Light yield: a free fit parameter; •Ionization quenching included (Birks’ parametrization); • 210 Bi, 11 C and 85 Kr free fit parameters; •Others v fixed •Fit to the spectrum

without

and

with

 subtraction is performed giving consistent results R 7Be = 49 ± 3 stat ± 4 sys cpd/100 tons Borexino Collaboration Phys. Lett. B 658 (2008) : after 2 months of data taking Borexino Collaboration PRL 101 (2008) : 192 days of live time

The measurement of the

7

Be flux

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

No oscillation BPS07(GS98) HighZ BPS07(AGS05) LowZ Expected rate (cpd/100 t) 75 ± 4 48 ± 4 44 ± 4

No-oscillation hypothesis rejected at 4 s level The analysis of the calibration data is in progress

Survival probability of the

 e

Survival probability of the

 e Before Borexino After Borexino 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: 

pp

(

Borexino data

) / 

pp

(

SSM

)  1.00

 0.008

 0.020



CNO

(

Borexino data

) / 

CNO

(

SSM

)  3.8 (90%

C

.

D

.) 2 Limit on the neutrino magnetic moment: 5.4

 10  11 m

B

(90%

C

.

L

.) 

The low threshold measurement of the 8B solar neutrinos

2.6 MeV g ’s from 208 Tl on PMT’s and 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 ( 10 C); 6) Bulk 232 Th contamination ( 208 Tl);

Rate

 2 .

8

MeV

 ( 0.26

 0.04

stat  0.02

sys ) counts/day /100 tons The Borexino 8 B spectrum  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 Borexino at 4.2 sigma

3.

4.

1.

2.

5.

A.

B.

C.

D.

Results already achieved in Borexino

First direct experimental evidence of the vacuum regime and of the transition region in the neutrino oscillation at very low energy: measurement of the 7 Be flux (0.2-0.8 MeV) and strong limit on the pp  flux. First determination of the ratio between the  e survival probability in vacuum and in matter: 1.6 ± 0.33 (from the 7 Be flux and the 8 B flux, measured with a threshold down to 2.8 MeV).

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

ADN



N N

 

D D

 0.02

 0.04

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

Best limits for CNO flux,  What next Measurement of the 7 Be flux with a total error  final validation of the MSW-LMA model; important insight for the Standard Solar Model metallicity puzzle and stronger limits on the pp flux.

Determination of the survival probability ratio, day/night effect, etc. with strongly reduced errors.

Study of the pep and CNO region (energy spectrum in the range 0.9-1.5 MeV) with the suppression of the 11 C muon produced.

Measurements of the geoneutrinos (the Gran Sasso region is especially favoured due to the 

e

Observatory

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

(arXiv:0808.2868v1)