1. CHOOZ  Double Chooz réalité  mythe ? Yves Déclais, IPNL (CNRS-IN2P3/UCBL) Questions (sur le bruit) de fond

2. 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 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. I will limit the discussion on only one point « the accidental background » many others could be discussed

3. CHOOZ  Double Chooz Inner : 192 PMTs, 15% coverage Outer : 24 PMTs Inactive volume Inner : 507 PMTs, 13% coverage Outer : 256 PMTs 2 tons StainlessSteel vessel Active volume 26 m 3 40 m 3

4. Radiopurity ThUK EMI 9351 CHOOZ proposal100 ppb70 ppb150 ppm EMI 9351 CHOOZ used (glass :B53 )20 ppb10 ppb60 ppm Double Chooz30 ppb 60 ppm Used : 192 + 24

5. Individual contributions to single rates Measured @ CHOOZ ( E e > 1.3 MeV, d géode > 30cm ) OK within a factor 2 65/sec 18.3 65/sec B53 glass Double Chooz : N PMTs x 2.6 Joker : gravel  Iron shot …. but threshold.5 MeV … but …

6. Accidental Background R accidental = R positron x R neutron x Δt (100 μsec) x d cut proposalmeasured R positron 100 /sec65 /sec R neutron 1 /hour45 /hour d cut 1.0.12 R accidental 0.24 /day0.42 /day Double Chooz will not use it ? Where is the mistake ? R neutron : neutron capture on Gd from spontaneous fissions : < 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 : << 1 /hour high energy γ from neutron capture (αn reactions) in the surrounding rock : << 1 /hour in Region 1+2 beware to neutron capture on heavy nuclei inside the detector … The remaining hypothesis is bremstrahlung γ associated to cosmic muons It would be sensitive to the overburden as well as the correlated background

7. 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 ν )  a (cheap) demonstrator is mandatory before spending 10 th M€

8. Conclusion (2) 1.00 Sin 2 (2θ 13 )=0.05 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 …

9. Conclusion (3) Θ 13 search @ 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 ( d~2km ) for an independant analysis of + the energy spectrum distortion + the disappearance effect on the rate