Summary of the Reactor/  13 Meeting At College de France, Paris April 22-23, 2003 Thierry Lasserre On Behalf the reactor/  13 “european” working group CEA/Saclay Low energy Neutrino Workshop University of Alabama, Tuscaloosa May

European momentum Working group:  PCC & APC (from CHOOZ), CEA/Saclay  MPI Heidelberg  TU Munchen,  Kurchatov Institute  INFN/Bologna  First meeting in December 2002  Second meeting in April 2003  Next around the end of the summer ? Goal: Is it possible to build a set of 2 detectors to measure/constrain  13 with a new reactor experiment before ? Where ? What the optimum detector design ? Preliminary answer should come this year T. Lasserre

European momentum 22-23/04/03 Meeting : List of Participants  H. de Kerret (PCC+APC)  M. Obolinski (PCC+APC)  O. Dadoun (PCC+APC)  D. Vignaud (PCC+APC)  J. Lamblin (PCC+APC)  S. Schoenert (MPIK)  T. Knoepfle (MPIK)  L. Oberauer (TUM)  F. Von Feilitzsch (TUM)  C. Hagner (Virginia Polytechnic Institute)  T. Schwetz (TUM)  M. Selvi (INFN, Bologna)  M. Cribier (Saclay+APC)  C. Cavata (Saclay)  T.L (Saclay)  … T. Lasserre

Reactor/  13 meeting, Paris, 22-23/04/03 Tuesday 22 April 14h – 14h15 : Introduction 14h15 – 15h15 : Reactor Neutrino Experiments compared to Superbeams (Thomas Schwetz, TUM – 45’+15’) 15h15 – 16h00 : The CHOOZ experiment & the |Ue3|2 measurement – review of systematic errors. How and where to improve ? (H. de Kerret – 30’+15’) 16h00 – 16h30 : Coffee break 16h30 – 17h30 : Review of the current proposals (Kr2Det, Kashiwasaki, etc …) – Potential experiment sites in France ? (T. L – 45’+15’) 17h30 – 18h00 : Discussion Wednesday 23 April 9h00 - 9h30 : Analysis methods to account for near and far detectors – Systematic error handling (T. Schwetz, TUM, 30’) 09h30 – 10h15 : Discussion – Backgrounds: Accidental – Correlated – In-situ measurements (chaiperson: Stefan Schoenert) 10h15 – 10h45 : Coffee break 10h45 – 11h30 : Discussion - Detector design (chairperson: Lothar Oberauer) 11h30 – 12h30 : Conclusions

Parameter degeneracy in LBL experiments LBL  disappearance gives: sin 2 (2  23 )  2 solutions :  23 &  /2-  23 |  m 2 13 |  2 solutions m 1 >m 3 or m 3 >m 1 LBL appearance probability given by: K 1,K 2,K 3 : known constants (within experimental error) dependence on sin(2  23 ), sin(  23 )  2 solutions dependence on sign(  m 2 31 )  2 solutions  -CP phase can run in [0,2  ]  Interval of solutions in general P(   e ) ~ K 1 sin 2 (  23 ) sin 2 (2  13 ) + K 2 sin(2  23 ) sin(  13 ) sign(  m 2 31 ) cos(  )  K 3 sin(2  23 ) sin(  13 ) sin (  ) T. Lasserre |U e3 | 2 measurement with reactors Few MeV e  disappearance experiments 1-P( e  e ) = sin 2 (2  13 )sin 2 (  m 2 31 L/4E) + O(  m 2 21 /  m 2 31 ) Few MeV e + very short baseline  No matter effect contribution (O(10 -4 ) relative effect) |U e3 | 2  measurement independent of sign(  m 2 13 ) |U e3 | 2 measurement independent of the  -CP phase sin 2 (2  13 ) P(   e )

Achievable constraint on  13 with a reactor experiment (hep-ph/ , P. Huber et. al.) T. Schwetz

Achievable constraint on  13 with a reactor experiment (hep-ph/ , P. Huber et. al.) T. Schwetz

~30-50 tons detectors Complementarity Reactor/Superbeam (hep-ph/ , P. Huber et. al.) Reactor experiment slightly less sensitive to “non optimal”  m 2 31 LBL (JHF) rather sensitive to  m 2 21 (especially if LMA-II) T. Schwetz

Complementarity Reactor/Superbeam (hep-ph/ , P. Huber et. al.) Systematics Correlations & Degeneracies Reactor: dominated by systematics LBL: dominated by correlations and degeneracies T. Schwetz

The past: CHOOZ (H.D.K) Site: CHOOZ reactor, Ardennes (France) 2 cores: 2x4200 MWth Depth: 300 mwe 5 tons of liquid scintillator (gadolinium loaded) ~ 1 km Exclusion   e :  m 2 sol < 7x10 -4 eV 2 (90% CL) (slightly lower limit obtained at Palo-Verde) Best constraint on sin 2 (2  13 ) < 0.14 Spectre des positrons e + p  e + + n

CHOOZ Systematics (H.D.K) T. Lasserre Systematics From Error originCHOOZ 2 non-identical detectors 2 identical detectors 2 id. detectors + low accidentals Remarks Reactor Complex cross section/fission1.9%--- Power0.7%--- E/Fission0.6%---  2.1%<< 1% Detector Scint. Density0.1%---+difficult with Gd Target volume0.3%No cancellation- (  V) % H1.2%---+difficult with Gd Spill in/out1.0%No cancellation--Scint. buffer  2.5%<< 1% Analysis Cuts Ee+<8 Mev 6<En (MeV)<12 de+-geode<30cm dn-geode<30cm de+-n < 100 cm 2 < n delay < 100  s n multiplicity = 1 e+ energy0.8%No cancellation ?-No threshold  0%Scint. buffer e+ pos. cut / vessel (30cm)0.1%No cancellation ?-- n capture1.0%No cancellation ?--Scint buffer n energy0.4%No cancellation ?--Gd 8 MeV  ’s n pos. cut / vessel (30 cm)0.1%No cancellation ?-- (e+-n) distance0.3%No cancellation ?- No Distance cut  0% (e+-n) time delay0.4%No cancellation ?--No Gd  ~0% n multiplicity0.5%No cancellation ?-Much better  1.5%<< 1 % ? Be carfull: It is also possible to increase CHOOZ systematics (scintillating buffer for exemple) Detector design with 2 identical low background detectors  Overall systematics controlled at < 1%

Energy threshold effect  If E th > E min  systematics due to threshold  Lower threshold  lower backgrounds (accidental+correlated)  Advantage if E th < E min :  No systematics on energy threshold (0.8% in CHOOZ, xx% in KamLAND)  Start of the spectrum provides calibration point between near and far detctors  Allow to understand & measure background at low energy (<1 MeV) Positron Detection (H.D.K) T. Lasserre Edge effects: Interaction of close to the target volume  No scintillating buffer : One  ’s can escape without being detected  Energy calibration !  Scintillating buffer (CHOOZ case) : full energy of e+ always detected within the target  BUT e+ efficiency non zero outside the target volume  to control !  Spill in/out : compensation of loss and gain of efficiency near the vessel Cancel if near and far detector are identical Identical detector  No absolute energy scale needed To check: Light propagation around the vessel

Neutron Detection (H.D.K) T. Lasserre Gd loaded scintillator: To be or not to be ?  Gd  8 MeV  ’s  H 2  2.2 MeV  ’s Edge effect: H 2 scintillator + non-scintillating buffer  spill out  n-capture with target : decrease efficiency (  ’s escape)  spill in  n-capture outside target : increase efficiency (  ’s come back in target)  partial compensation = spill in/out (MC, 1% error in CHOOZ)  Cancel if near and far detector are identical Other reasons to have 2 identical detectors  Ratio Gd/H 2 capture (~80% on Gd) – Error will depend on detector geometry  Time capture on Gd: the tail has no reason to be exponential  Energy window for n capture on Gd/H 2

(e + - n) Tag (H.D.K) T. Lasserre Distance Cut: d(e+ - n)<100 cm  Position reconstruction is not a technique at the % level (tails) !  Position reconstruction was not in CHOOZ design  Not mandatory if accidental background very low  Lower accidental background: 1 systematic error less ! Time Cut: neutron capture on  Hydrogen: exponential behavior of neutron time capture (can be demonstrated)  Gadolinium: exponential behaviour ? Increase systematics  0.4% systematics in CHOOZ  Lower accidental background: no need of Gd ?

Accidental background rate: b acc ~ b p x b d x  x V coinc x V det  Goal: rate b acc < 1/year within a 20 tons PXE target detector  Case 1: with position reconstruction  V det = 1 m 3  Constraint b p.b d < s -2 m -6  CHOOZ systematics = 0.4%  Case 2: without position reconstruction  0% systematics  V det = V det  Constraint b p.b d < s -2 m -6 With Borexino material for estimation : b p b d = s -2 m -8 ! Position reconstruction no required but … at the limit … (Argument valid only for radioactivity in scint., buffer, material, etc …) b p, b d : specific prompt, delayed rate V det : detector volume 20m³  : coincidence time 1ms V coinc : coincidence volume 1m³ or V det Backgrounds from radioactivity Based on estimation done for the HLMA project, S.Schoenert, T.L, & L.Oberauer, Astropart.Phys. 18 (2003) Accidental background (S.S, L.O, T.L)

Detector Design: Scintillator T. Lasserre Unloaded scintillator provide best:  Optical properties (light yield, attenuation lenght)  Radiopurity  Stability  PSD !!! To fight fast neutron background Gd loaded scintillator  Shorten neutron capture time x~3  Helps only for accidental background  Increase neutron capture energy release to 8 MeV instead of 2.2 MeV on Hydrogen  Chemical stability (but CHOOZ, Palo-Verde, and LENS  > 5-8% loading)  Radiopurity ? More difficult  Gd/H neutron capture systematics  If Gd: same batch to be used for both detectors to avoid effect such as systematics on the Gd content of the near and far detectors … etc …

Detector Design: Buffer T. Lasserre Question: scintillating or non-sintillating buffer ? Scintillating  Help to get positron energy  No energy threshold  0% systematics !  Help to get neutron 2.2/8 MeV peak  BUT high activity in buffer due to PMTs 40 K  BUT high activity in buffer due to muons crossing the buffer (no shallow depth)  More expensive ?  Same fluor / wavelenght shifter? time constants ? Non-scintillating  Not the CHOOZ design  More difficult to understand positron + neutron spectrum ?  Increase of systematics : cut for energy threshold ! Light prop. around vessel !  Solution: scintillating buffer + encapsulated PMTs and deep detector site ?

Detector Design: Vessel(s) T. Lasserre Vessel(s) = separation between Target and Buffer Target Volume uncertainty:  Near detector : V near &  V near  Far detector: V far &  V far  Ideally V near = V far  systematics cancel but relative error ~ O(  V near -  V far ) Nylon Vessel (BOREXINO, KamLAND) : Should not be underestimated …  Volume & shape more difficult to control ?  Compatibility with PC  Buoyancy problems if slight density differences between target and buffer Plexigass Vessel  Volume & Shape well under control  Compatibility problems ?  Contains protons  act as a target Shape of the vessel : Spherical ? CHOOZ like ?

The 3-Volume detector (H.D.K) T. Lasserre CHOOZ: Gd-loaded scintillator + H scintillating buffer BOREXINO: H scintillator + Non-scintillating buffer 3V detector: H2/Gd loaded scintillator + Proton free scintillator + Non-scintillating buffer 3V detector: Gd Target + H2 sint. Buffer + Non-scintillating buffer A very nice detector, and easier to understand ?  See the start of the positron spectrum  No threshold effect for positron energy  n-capture peak very well defined  Target volume perfectly defined  No PMT activity seen (non scintillating buffer) Technically ?  Need to construct a 2-volume inner vessel  plexiglass  Proton free scintillator expensive  C 6 F 6 – Expensive ? – d = 1.6 (shielding, buoyancy problem)