Borexino: Low energy Solar neutrinos and beyond

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BorexinoLow energy Solar neutrinos and beyond

• Oleg Smirnov
• (JINR, Dubna)
• on behalf of Borexino collaboration

XXIèmes Rencontres de Blois - Windows on the
Universe 21 - 26 Jun 2009 - Blois
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BOREXINO Collaboration
Genova
Princeton University
APC Paris
Virginia Tech. University
Munich (Germany)
Dubna JINR (Russia)
Kurchatov Institute (Russia)
Jagiellonian U. Cracow (Poland)
Heidelberg (Germany)
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- Borexino goal, 5
50 events/d/100t expected Low energy-gtno
Cherenkov light-gtNo directionality, no other
tags-gt extremely pure scintillator is needed
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Concept of graded shielding"
Cosmic muons (rocks, 3200 m.w.e.)
Neutrons and external gammas (ultrapure water
layer, 2.15 m, 2400 tones)
?-s from construction materials (PC buffer, 700
tones)
?-s from construction materials (outer layer of
scintillator, 1.25 m or 200 tones)
Software-defined active volume of
scintillator (fiducial volume, 3m, 100 tones)
Position reconstruction needed
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BOREXINO

• 278 t of liquid organic scintillator PC PPO
  (1.5 g/l)
• (?,e)-scattering with 200 keV threshold
• Outer muon detector

13.7m
18m
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LS radiopurity in Borexino
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Borexino technical data

• 1.Light yield gt500 p.e./MeV/2000 PMTs (31 of
  4p)
• 2.Energy resolution (1s) 5 _at_ 1 MeV
• 3.Mass full 278 t FV mass 78.5 tones (used in
  7Be analysis)
• 4.Practical threshold on the electrons recoil is
  180 keV (corresponds to 380 keV neutrino)
• 5.Muons registering efficiency close to 100
• 6.Triggers rate 11 cps (mainly 14C, 2.7 0.6 x
  10-18 g/g 14C/12C )
• 7.Spatial resolution 14 cm _at_ 1 MeV
• 8.Radiopurity of scintillator see previous slide

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Active shielding effectively suppress external
gamma background
210Po (not in equilibrium with 210Pb)
14C
??Kr?Be ??
No ?s
11C
Rlt3.0 m (100 t)
214Bi-214Po
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Spectral components in the experimental spectrum
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Energy scale

• Calibrated using internal sources taking into
  account the CTF calibration experience 14C
  (ß-,E0156 keV), 11C (ß decay), 210Po (a, Ea5.3
  MeV)
• Monoenergetic line of 210Po has been used to fit
  the detectors response width and shape
  (non-gaussian shape is used)
• Careful modeling of the Birks ionization
  quenching at low energies (worked out with the
  CTF data) kB0.017 cm/MeV
• Two quasi-independent energy variables are used
  the total number of registered p.e. (Q) and the
  number of triggered PMTs (Npm)

A first calibration campaign with on axis and off
axis radioactive sources has been performed (Oct
08 on axis, Jan-Feb 09 off axis). 115 points
inside the sphere ?,?,a,n sources. The model
used is in a good agreement with
measurements. Also the position reconstruction
has been tuned (source is localized within 2 cm
precision through red laser light and CCD
camera).
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Model used to fit the experimental data (7Be
analysis)

• Normalization of main backround components are
  free
• 14C (with fixed form-factor a)
• 85Kr free in principle can be bounded
  (correlated with 7Be)
• 210Po (in another approach is removed using a/ß
  statistical subtraction)
• 210Bi 11C
• 214Pb fixed at the number of registered events of
  222Rn (anyway negligible).
• Other background sources (40K isotopes from
  decay chains of 238U and 232Th in secular
  equilibrium) are found to give negligible
  contributions.
• Electrons recoil spectra for solar neutrino are
  calculated assuming MSW(LMA) scenario
• 7Be
• CNO fixed _at_ SSMMSW(LMA) (strongly correlated
  with free 210Bi component)
• pp and other solar neutrino fluxes are fixed _at_
  SSMMSW(LMA)
• Energy scale parameters
• Light yield 1 energy resolution parameter vT
  210Po peak position
• Two other parameters pt0.13 and gc0.105 (found
  using MC simulation) for Npm variable are fixed
• For Q variable calibration parameter c is free
  parameter feq is fixed (calculated) for both
  variables
• Birks parameter kB fixed at the value found
  with CTF

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Direct Measurement of the 7Be Solar Neutrino
Flux with 192 Days of Borexino Data PRL 101,
091302 (2008).
493stat4syst cpd/100 t
Fit to the spectrum with a-subtraction gives
consistent results
Main source of systematic uncertainty in this
measurent is error in FV definition.
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210Po and a/ß - discrimination
Optimal Gatti filter E. Gatti, F. De Martini, A
new linear method of discrimination between
elementary particles in scintillation counters,
in Nuclear Electronics, vol. 2, IAEA, Wien,
1962, pp. 265276.
H.O. Back et al. / NIM A 584 (2008)
98113 Pulse-shape discrimination with the
Counting Test Facility
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Physics results, 7Be

• Borexino exp. result
• 49 3(stat) 4 (syst) cpd/ 100t
• High metallicity Solar model , MSW/LMA
• 48 4 cpd / 100t
• Low metallicity Solar model , MSW/LMA
• 44 4 cpd / 100t
• Solar model, nonoscillating neutrino
  (inconsistent
• with measurement at the 4 s C.L.)
• 74 4 cpd / 100t

The survival probability of the 0.862 MeV 7Be
neutrinos (assuming the BS07(GS98) SSSM) is
0.560.10.
A. M. Serenelli talk at PHYSUN-2008 workshop.
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Neutrino magnetic moment
From the theoretical point of view, there is no
magnetic moment for Dirac massless neutrino, as
well as for Majorana neutrino, massive or
massless. Massive Dirac neutrino should have
small m.m.
M.m. can be searched for by studying the
deviations from the weak shape
flat
1/T behaviour
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Limits on effective solar neutrino magnetic moment

• with 192 days of live-time statistics the 90
  c.l. limit is
  µefflt5.410-11 µB
• stronger limits with the same statistics can be
  obtained bounding some spectral contributions
  (i.e. 85Kr)
• The limit is model-independent, defined only by
  the shape of the spectra, also no systematics is
  attributed to the uncertainty of the FV.
• The best up-to-date existing limit comes from the
  measurements with high purity 1.5 kg Ge detector
  at Kalinin Nuclear Power Plant, GEMMA experiment
  (arXiv0906.1926)
• 
  µlt3.210-11 µB
• For flavour components one can write

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8B neutrinos
Energy spectrum after statistical 208Tl
subtraction.

Measurement of the solar 8B neutrino flux with
246 live days of Borexino and observation of the
MSW vacuum-matter transition by Borexino coll.
arXivastro/ph 0808.2868v1 0.260.04stat0.02
syst cpd/100 t
The 8B mean electron neutrino survival
probability, assuming the BS07(GS98) SSM, is
0.350.10 at the effective energy of 8.6
MeV. The ratio between the measured survival
probabilities for 7Be and 8B neutrinos is
1.600.33, 1.8s different from 1. First LS
experiment observing 8B neutrinos.
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Borexino provided measurement of electron
neutrino survival probability in two different
energy ranges
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Time variations of 7Be neutrino flux
3.5 variations due to the seasonal variation of
Earth-Sun distance need more statistics,
feasibility of measurement depends on stability
of backgrounds and strategy chosen for (possible)
repurification. For the moment no statistically
significant measurement is available.
Preliminary negative result on day/night
assimetry (see G.Testeras talk at Neutrino
Telescopes in March 2009) with 400 days
statistics
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Detecting antineutrino

• Inverse beta-decay high c.s. 10-42 cm2
• Evisible En 0.78MeV Engt1.8 MeV

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Geoneutrinos study is promising due to the
location of the Borexino far away from the
European reactors.
Emax(U) 3.26 MeV Emax(Th) 2.25 MeV Emax(K)
1.3 MeV
Energy window 1.81-3.26 MeV
28 April 2009 Milan
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Reactor antineutrino
in Borexino
15 ev/yr are expected for 100
reactors duty cycle. 5 ev/yr in the geoneutrino
region.
15 ev/yr
207 Nucl. power plants in 17 countries. 13
Plants give 40 of total signal. 3 most powerful
power plants in France give 13 of the total
signal.
28 April 2009 Milan
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Expected antineutrino signal for 1 yr of the
data taking
no FV cut (278 t)
For reactor neutrino 0.8 duty cycle has been
used. 13C(a,n)16O background is
negligible. Other (from random) backround sources
are muon-induced ?-n decaying isotopes (8He9Li)
and fast neutrons induced by muons missed by MVS.
28 April 2009 Milan
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Borexino potential on supernovae neutrinos
Borexino plans to enter SNEWS (Super Nova Early
Warning System)
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Summary/Whats next?

• Borexino operates at purity levels never achieved
  before, it demonstrated the feasibility of the
  neutrino flux measurement in sub-MeV region,
  under the natural radioactivity threshold (4.2
  MeV)
• Solar 7Be-n flux has been measured with 10
  accuracy
• a first measurement of 8B-n in LS with threshold
  below 5 MeV (2.8 MeV)
• Borexino results are compatible with MSW/LMA
• strong limit on neutrino effective magnetic
  moment is obtained
• extremely high sensitivity to electron
  antineutrino has been experimentally confirmed,
  waiting for more statistics.
• Further calibration and reduction of the error on
  the 7Be flux down to 5 (further improvements if
  constraining 85Kr, in this case also the limits
  on the effective magnetic moment will be
  improved)
• Seasonal variations of the neutrino fluxes
  (detector stability, more statistics) other time
  variations
• More precise measurement of the oscillation
  probability in the transition region (either due
  to the higher statistics or due to increase of
  the FV)
• The CNO and pep-neutrino fluxes measurement
  (requires cosmogenic 11C tagging)
• The feasibility of the pp-neutrino flux
  measurement is under study (better understanding
  of the detector at low energies and the precise
  spectral shape of 14C is needed)
• Antineutrino studies geo, reactor, supernova.