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Recent results of the Borexino experiment
Evgeny Litvinovich, RRC "Kurchatov Institute"
(on behalf of the Borexino collaboration)
14th Lomonosov Conference on Elementary Particle Physics Moscow, 19-25 August, 2009


Borexino Collaboration
Genova Milano
Princeton University

APC Paris

Perugia

Virginia Tech. University

Dubna JINR (Russia)

RRC "Kurchatov Institute" (Russia)

Jagiellonian U. Cracow (Poland)

Munich (Germany) Heidelberg (Germany)


Borexino Physics
Solar program:
Be neutrinos (E = 0.862 MeV); 8B neutrinos (E < 14.06 MeV); Possibly pp-, pep- and CNO-neutrinos.
7

Study of geo-neutrinos; Reactor antineutrinos; Supernovae (SNEWS); Beyond SM...


Borexino Physics
Solar program:
Be neutrinos (E = 0.862 MeV); 8B neutrinos (E < 14.06 MeV); Possibly pp-, pep- and CNO-neutrinos.
7

Study of geo-neutrinos; Reactor antineutrinos; Supernovae (SNEWS); Beyond SM...


Solar neutrinos energy spectrum
Bahcall & Serenelli 2005


Solar neutrinos energy spectrum
Bahcall & Serenelli 2005


Solar neutrinos energy spectrum
Bahcall & Serenelli 2005

+ BX


Borexino detector
...located at the Gran Sasso underground laboratory (3800 m.w.e.), central Italy. Detection principle is neutrino-electron elastic scattering in organic liquid scintillator, target mass is 278 tons. Energy threshold ~60 keV, counting rate ~30 Hz! Energy resolution 6% @ 1 MeV (14% FWHM). Spatial resolution 14 cm @ 1 MeV. Detector is fully operative since 15 May 2007.


Borexino detector
Scintillator: 278 t PC+PPO (1.5 g/l) in a 150 m thick inner nylon vessel (R = 4.25 m) Buffer region: PC+DMP quencher (5 g/l) 4.25 m < R < 6.75 m Stainless Steel Sphere: R = 6.75 m 2212 PMTs 1350 m3

Outer nylon vessel: R = 5.50 m (222Rn barrier)

Water Tank: and n shield water C detector 208 PMTs in water 2100 m3

Carbon steel plates

20 steel legs


Borexino detector

...30% optical coverage


Detector radiopurity
Source
14C/12C

Typical abundance 10
-12

Level achieved (2.7 ± 0.6) · 10 (1.6 ± 0.1) · 10 (6.8 ± 1.5) · 10
-18 -17

(cosmogenic) g/g

g/g g/g g/g

U (by 214Bi-214Po)
238 232Th

2 · 10-5 (dust) g/g 2 · 10-5 (dust) g/g

-18

(by

212Bi-212 222

Po) 100 atoms/cm3 (air) emanation from materials Surface contamination 2 · 10-6 (dust) g/g 1 Bq/m3 (air) 17 mBq/m3 (air) ~ 10-17 g/g (~1 cpd/100 tons) May 07: 70 cpd/ton Sep 08: 7 cpd/ton < 3 · 10
-18

Rn (by 214Bi-214Po)
210

Po K

40

(90%) g/g

85

Kr Ar

(28 ± 7) cpd/100 tons << 85Kr

39

...is a result of more than 15 years of work!


Solar 7Be neutrinos
C. Arpesella et al. (Borexino Collab.), Direct measurement of the 7Be solar neutrino flux with 192 days of Borexino data, Phys. Rev. Lett. 101, 091302 (2008).

Measured rate is: R(7Be) = 49 ± 3(stat) ± 4(sys) cpd/100 t

Expected rate (cpd/100 t) No oscillation High-Z SSM Low-Z SSM 78 ± 5 48 ± 4 44 ± 4


Solar 7Be neutrinos flux measured
No oscillation:
(7Be) = (2.74 ± 0.27) · 109 cm-2 s
-1

MSW-LMA:
(7Be) = (5.18 ± 0.51) · 109 cm-2 s
-1

SSM prediction: High-Z:
(7Be) = (5.07 ± 0.31) · 109 cm-2 s
-1

Low-Z:
(7Be) = (4.55 ± 0.27) · 109 cm-2 s
-1


Systematic uncertainties (%)

Calibration program is in progress, see below...


Solar 8B neutrinos
Energy spectrum before cosmogenic cut (5 s after each muon):

MSW-LMA prediction:
expected 8B neutrinos rate in 100 tons fiducial volume of BX scintillator above 2.8 MeV: R(8B) = 0.27 ± 0.03 cpd

2.6 MeV gammas (208Tl) from PMTs and in the buffer

Muon cut

Measured rate in 100 tons fiducial volume: R(8B) = 0.26 ± 0.04 ± 0.02 cpd

FV cut astro-ph > arXiv:0808.2868 Ethr= 2.8 MeV > 5 distant from the 2.6 MeV gamma peak


Solar 8B neutrinos


Solar 8B neutrinos flux
(8B) = (2.65 ± 0.44 ± 0.18) · 106 cm-2 s (from the 2.8 MeV threshold)
Main sources of systematic errors: 6% from the determination of the 100 tons fiducial mass 3% uncertainty in the 8B rate above 2.8 MeV from the determination of the detector energy response
-1


Electron neutrino survival probability
Borexino has performed first simultaneous spectral measurements both in vacuum-dominated (7Be) and matter-enhanced (8B) neutrino oscillation (LMA) regions.


Borexino calibration
We plan to achieve 5% error in determination of 7Be solar neutrino flux. Several calibration campaigns (on- and off-axis) have been performed starting from October 2008. Radioactive source position inside the Borexino is known within 2 cm error. Calibrations are completed by July 2009. Study of alpha-, gamma- and proton quenching in the Borexino liquid scintillator. Analysis of calibration data are in progress.


Borexino calibration
Am-Be source inside the cross above Borexino:


Borexino calibration
Several gamma sources used: - 57Co (122 keV) - 139Ce (166 keV) - 203Hg (279 keV) - 85Sr (514 keV) - 54Mn (835 keV) - 65Zn (1115 keV) - 60Co (1173 + 1332 keV) - 40K (1461 keV) Alpha source 222Rn Neutron source 241Am-9Be

Data analysis are in progress!


Conclusions
Borexino has performed first real-time measurement of 7Be solar neutrinos flux. Direct measurement of 8B solar neutrinos flux with the lowest threshold ever achieved by realtime detector (2.8 MeV) was done. Simultaneous spectral measurement in vacuumdominated (7Be-neutrinos) and matter-enhanced (8B-neutrinos) oscillation (LMA) regions was done for the first time by single detector. More results are coming...


Bonus


Bonus


Bonus
14 85

C free (with fixed shape factor); Po; free (in another approach removed by statistical / subtraction) Pb fixed at the value found for the number of Bi; free free normalization for MSW(LMA) parameters
210 222

Kr free; in principle can be bounded (correlated with 7Be) Rn events

210 214 210

7Be; 11

C; free Bi);
238

CNO fixed @ SSM+MSW(LMA) (highly correlated with Other contributions (still not completely excluded and
232 40

fix pp and other solar neutrino contributions @ SSM+MSW(LMA) K; isotopes from U Th decay chains in secular equilibrium) are found to be negligible

Light yield + one energy resolution parameter are free; Birks' quenching model with parameter kB fixed at the (best-fit) value found with CTF