KamLAND, a culmination of half century of reactor neutrino studies.

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KamLAND, a culmination ofhalf century of reactor
neutrinostudies.

• Petr Vogel, Caltech

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Selected references to this lecture

• Kamland papers
• K.Eguchi et al., Phys. Rev. Lett.90, 021802
  (2003),
• K.Eguchi et al., Phys. Rev. Lett.92, 071301
  (2004),
• T. Araki et al., hep-ex/0406035 .
• General review
• C. Bemporad, G. Gratta, and P. Vogel., Rev. Mod.
  Phys. 74, 297 (2002).

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• Pontecorvo already in 1946 suggested to
• use nuclear reactors in order to perform
• neutrino experiments.
• Indeed, in 1953-1959 Reines and Cowan
• showed that neutrinos are real particles
• using nuclear reactors as a source.
• Since then, reactors, powerful sources
• with 6x1020 /s electron antineutrinos
• emitted by a modern 3.8 GWthermal reactor,
• have been used often in neutrino studies.
• The spectrum is well understood.

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Electron antineutrinos are produced bythe b
decay of fission fragments
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• Reactor spectrum
• 1) Fission yields Y(Z,A,t), essentially all known
• 2) b decay branching ratios bn,i(E0i) for decay
  branch i,
• with endpoint E0i , some known but some
  (particularly
• for the very short-lived and hence high Q-value)
• unknown.
• 3) b decay shape, assumed allowed shape, known
• P(En,E0i,Z) or for electrons Ee E0 En.
• dN/dE Sn Yn(Z,A,t) Si bn,i(E0i) P(En,E0i,Z)
• and a similar formula for electrons.
• If the electron spectrum is known, it can be
  converted
• into the antineutrino spectrum.

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Spectrum Uncertainties
Theory only Klapdor and Metzinger, 1982
Beta calibrated Schreckenbach, 1985 Hahn, 1989
Bemporad, Gratta, and Vogel, RMP 74, 297 (2002)
Results of Bugey experiment (1996)
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Reactor spectra
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Detecting reactor antineutrinoslow detection
threshold required
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Detector reaction ne p -gt e n, positron
spectrum measured
Cross section known to 0.2, see Vogel
Beacom, Phys. Rev. D60,053003
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To study oscillations, use the disappearance test

• The survival probability of electron
• antineutrinos of energy En produced
• at the distance L from the detector is
• Pee(En,L) 1 sin2(2q)sin2(Dm2L/4En)
• The experiment become sensitive to
• oscillations if Dm2L/En 1,
• proof of oscillations is Pee(En,L) lt 1.

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History of reactor neutrino oscillation search
Probability of oscillations is proportional
to sin2(Dm2L/4E). Since for the reactors
E4MeV, the sensitivity to Dm2 is
inversely proportional to the distance L.
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• Discovery of oscillations of atmospheric
• neutrinos implies Dm2 (2-3)x10-3 eV2,
• thus indicating that reactor experiments
• with L (1-3) km should be performed
• (Chooz and Palo Verde).
• Also, the preferred solution to the solar
• neutrino deficit implies Dm2 (5-10)x10-5 eV2,
• thus indicating that reactor experiments
• with L 100 km should be performed
• (KamLAND)

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180 km
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80 GW 6 of world nuclear power
25GW most powerful station in the world
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KamLAND Collaboration
13 institutions 93 members
Collaborator
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300
antineutrinos from the Sun . . .
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A brief history of KamLAND
Dates Live time (days)
Start data taking Jan 2002 -
Run A (data-set of 1st paper) Mar 9 Oct 6 2002 145.4
Electronics upgrade 20 PMT commissioning Jan/Feb 2003 -
Run B Oct - Jan 11 2004 369.7
Data-set presented here Mar 9, 2002 Jan 11, 2004 515.1
Was 145.1 with old analysis
T.Araki et al, arXivhep-ex/0406035 Jun 13, 04
submitted to Phys Rev Lett
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A limited range of baselines contribute to the
flux of reactor antineutrinos at Kamioka
Over the data period Reported here Korean
reactors 3.40.3 Rest of the world JP research
reactors 1.10.5 Japanese spent fuel 0.040.02
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DT (T-Tm ), DL
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Tagged cosmogenics can be used for calibration
t29.1ms Q13.4MeV
12B
12N
t15.9ms Q17.3MeV
µ
Fit to data shows that 12B12N 1001
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Radioactivity in liquid scintillator
238U 214Bi ? 214Po ? 210Pb
ß? a t28.7 m
t237 µs E3.27 MeV E7.69 MeV
232Th 212Bi ? 212Po ? 208Pb
ß? a t87.4 m
t440 ns E2.25 MeV E8.79
MeV
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238U (3.50.5)10-18 g/g needed
10-14 g/g 232Th (5.20.8)10-17 g/g
t(21929) µs Expected 237 µs
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• Note The best background in 76Ge bb decay
• detectors is at present 0.2 counts/(keV kg y).
• Expressing the background in the liquid
  scintillator
• in KamLAND in the same units, and for
• energies 2-3 MeV, one finds value 10 times
• smaller going out to 5.5 m radius and 20 times
• smaller for 5 m radius

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Total LS mass 2.1 Fiducial mass
ratio 4.1 Energy threshold 2.1 Tagging
efficiency 2.1 Live time 0.07 Reactor
power 2.1 Fuel composition 1.0 Time
lag 0.28 ne spectra 2.5 Cross section 0.2
of target protons lt 0.1 Total
Error 6.4
E gt 2.6 MeV
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Expect 1.5 n-12C captures
Very clean measurement
Accidental background
Second KamLAND paper
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2003 saw a substantial dip in reactor
antineutrino flux
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Good correlation with reactor flux
Fit constrained through known background ?22.1/4
Expected for no oscillations
90 CL
0.03 for 3TW hypothetical Earth core reactor
(But a horizontal line still gives a decent fit
with ?25.4/4)
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In CHOOZ it was possible to determine background b
y this effect.
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(766.3 tonyr, 4.7? the statistics of the first
paper)
Results
Background Events
Accidentals 2.690.02
8He/9Li 4.80.9
µ-induced n lt0.89
Total 7.51.3
Observed events 258 No osc. expected
36524(syst) Background 7.51.3
Inconsistent with simple 1/R2 propagation at
99.995 CL
(Observed-Background)/Expected
0.6860.044(stat)0.045(syst) Caveat this
specific number does not have an absolute meaning
in KamLAND, since, with oscillations, it depends
on which reactors are on/off
Second paper
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• Decay chain leading to 210Po
• 222Rn a(3.8d) 218Po a(3.1m) 214Pb b(27m) 214Bi,
• 214Bi b(20m) 214Po a(164ms) 210Pb b(22.3y) 210Bi,
• 210Bi b(5d) 210Po a(138d) 206Pb(stable)
• The long lifetime of 210Pb causes its
  accumulation.
• The a from 210Po decay then interact with 13C in
• the scintillator by 13C(a,n)16O making unwanted
• background. There is only 10-11g of 210Pb in
• fhe fiducial volume, enough however to cause
• 1.7x109a decays in 514 days.

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G.Fogli et al., PR D66, 010001-406, (2002)
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Energy spectrum now adds substantial information
Best fit to oscillations ?m28.310-5
eV2 sin22?0.83 Straightforward ?2 on the
histo is 19.6/11 Using equal probability
bins ?2/dof18.3/18 (goodness of fit is 42)
A fit to a simple rescaled reactor spectrum is
excluded at 99.89 CL (?243.4/19)
Second paper
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This result ?m28.310-5 eV2 sin22?0.83
First KamLAND result
Dm2 6.9 x 10-5 eV2 sin2 2 q 1.0
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Combined solar ? KamLAND 2-flavor analysis
Includes (small) matter effects
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KamLAND uses a range of L and it cannot assign a
specific L to each event Nevertheless the
ratio of detected/expected for L0/E (or 1/E) is
an interesting quantity, as it decouples the
oscillation pattern from the reactor energy
spectrum
no oscillation expectation
p/2
Hypothetical single 180km baseline experiment
p
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Conclusions
KamLAND reactor exposure 766.3 tonyr (470
increase)
Data consistent with large flux swings in 2003
Spectrum distortions now quite significant,
shape-only very powerful
Best KamLAND fit to oscillations ?m8.310-5 eV2,
sin22?0.83 LMA2 is now excluded
Together with solar ?
Welcome to precision neutrino physics !
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Whats next?
What next in reactor neutrino studies?

• Purification of liquid scintillator remove 85Kr
• and 210Pb (low energy background) and
• attempt to observe solar 7Be ne
• (feasibility studies under way).
• Determine or constrain the flux of solar
• antineutrinos and of the geoneutrinos. Study
• the neutron production by muons.
• In a different reactor experiment (two detectors,
• one close and another at 1-2 km) determine or
• constrain the unknown mixing angle q13. That
• is a whole different story.

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Future Reactor Measurements
Chooz reached at 1 km 2.8 statistical error
2.7 systematic error
Next generation search for Theta-13 needs to
achieve 1 errors
Apollonio et al., EPJ C27, 331 (2003)
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A high sensitivity search for ne from the Sun and
other sources at KamLAND
hep-ex/0310047, Phys. Rev. Lett. 92, 071301
(2004)
No events found between 8.3-14.8 MeV for
0.28kt-y exposure. Assuming the 8B spectrum
shape, this limits the antineutrino flux to
2.8x10-4 of the SSM 8B flux. This represents a
factor of 30 improvement over the best previous
limit.
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• Thanks to Atsuto Suzuki, Patrick Decowski,
• Gianni Fiorentini, Andreas Piepke and
• Giorgio Gratta who made some of the
• figures used in this talk.