CHOOZ Double Chooz ralit mythe

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CHOOZ ?Double Chooz réalité ? mythe ?
Questions (sur le bruit) de fond
Yves Déclais , IPNL (CNRS-IN2P3/UCBL)
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In these slides the naming convention recommended
in the Neutrino News is followed The CHOOZ
reactor neutrino experiment was usually
capitalized, not because it was an acronym, but
because people thought it was one. The Double
Chooz experiment, which met in the U.S. for the
first time in February, has chosen a convention
for its name, which is Double Chooz, without a
dash and without all caps.

• All numbers used in these slides can be found in
  
• CHOOZ proposal
  http//duphy4.drexel.edu/chooz_pub/
• CHOOZ publi
  hep-ex/0301017
• Palo-Verde
  hep-ex/0107009
• Double Chooz LOI hep-ex/0405032
• Double Chooz US proposal hep-ex/0410081

I will limit the discussion on only one point
 the accidental background  many others could
be discussed
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CHOOZ ? Double Chooz
Active volume 26 m3 40 m3
2 tons StainlessSteel vessel
Inner 192 PMTs , 15 coverage Outer 24 PMTs
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Radiopurity
Used 192 24
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Individual contributions to single rates
Double Chooz NPMTsx 2.6 Joker gravel ? Iron
shot . but threshold .5 MeV but
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Accidental Background
Raccidental Rpositron x Rneutron x ?t (100
µsec) x dcut
Where is the mistake ?
Double Chooz will not use it ?

• Rneutron
• neutron capture on Gd from spontaneous fissions
  lt 6 10-3 /day
• the dominant source of the U is located inside
  the PMTs
• neutron capture on Gd from spallation neutrons
  outside the detector ltlt 1 /hour
• high energy ? from neutron capture (an
  reactions) in the surrounding rock ltlt 1 /hour
  in Region 12
• beware to neutron capture on heavy nuclei inside
  the detector

The remaining hypothesis is bremstrahlung ?
associated to cosmic muons It would be sensitive
to the overburden as well as the correlated
background
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Conclusion (1)

• the price to pay for increasing the target mass
  of the Double Chooz experiment within boundaries
  conditions (using the existing setup) is to
• 
  
• worsen the background
• So the S/B improvement as claimed by the Double
  Chooz authors is not realistic
• the accidental background will be strongly
  correlated with the overburden and so the near to
  far detector comparison will be affected by
  systematical uncertainties which have not been
  taken into account
• in the Double Chooz proposal there are no
  convincing arguments to prove that a CHOOZ type
  reactor neutrino detector can be efficiently
  operated at small overburden
• ? Palo Verde data could be used to understand
  where are the difficulties (bkgd 300/day,
  efficiency corrected, for 1000 ? )
• ? a (cheap) demonstrator is mandatory before
  spending 10th M

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Conclusion (2)
1.00
At such a small L/E Accidental signal
substraction can mimic or cancel an oscillation
signal  remember Bugey I 
For a safe oscillation search the oscillation
signal should be localised where there is no
background structure which could mimic it
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Conclusion (3)
T13 search _at_ reactors ? disappearance experiment

hard with Gd Loaded scintillator
Low level of systematics long and exhacting
task incompatible with  expeditiously deployed
multi detector reactor experiment  internal
cross check for the validity of the result
L/E should be large enough ( d2km ) for an
independant analysis of the energy spectrum
distortion the disappearance effect on the
rate