Neutrino Oscillation Experiments at Reactors and Accelerators Gaston Wilquet IIHE-Université Libre de Bruxelles IV èmes Rencontres du Vietnam, Hanoi, July 2000
Contents CHOOZ and PALO VERDE long baseline experiments Search for ν e disappearance at reactors. Large mixing angle, small ∆ m 2 (>10 -3 eV 2 ) KARMEN2 and LSND short baseline experiments Search for ν µ −ν e oscillation in low energy neutrino beams. High sensitivity, small mixing angle, large ∆ m 2 (few eV 2 ) CHORUS and NOMAD short baseline experiments Search for ν µ −ν τ oscillation in high energy neutrino beams. High sensitivity, small mixing angle, large ∆ m 2 (few tens eV 2 )
To-morrow Byron Lundberg will give a seminar at FermiLab on "Results from DONUT: First Direct Evidence of the Tau Neutrino” This was expected and did no happen explicitly in Sudbury at Neutrino 2000
Neutrino Oscillation Experiments at Nuclear Reactors Long base line experiments at nuclear power plants of Chooz and Palo Verde Motivation: ν e disappearance in ( ∆ m 2 , sin 2 2 θ) parameter space indicated by ν µ disappearance in atmospheric experiments ν µ → ν e ?
Neutrino Oscillation at Reactors: pros and cons ≈ E 3 MeV • E ν ≈ few MeV ⇒ Access to low ∆ m 2 at medium L ∆ ≈ ≈ 2 2 m 0.003 eV ≈ L 1000 m ν → ν ⇒ Below µ, τ thresholds: only disappearance e x • High flux, but small σ = ⇒ P 1 GW 3 . 3 GW elec therm 3 . 3 GW − ⇒ − ≈ ≈ 20 1 20 ν 1 Fission rate 10 s 6 10 s • 4 π source ⇒ detector mass ÷ L 2 e 200 MeV • Disappearance ⇒ good knowledge of absolute ν flux and e + energy spectrum ⇒ or multi-L experiment ( ≥ 2 detectors or reactors) ⇒ no sensitivity at high ∆ m 2 (not serious problem with ∆ m 2 ≤ 0.01 eV 2 • Cheep and well known ν source Calculated and measured ν flux ν Interaction spectrum and energy spectrum at L=0 known 6.5 10 -42 cm 2 to ~ 2% (Bugey 1995) thresh = E 1 . 8 MeV ν e < > ≈ E 3 MeV + e
Detection of neutrinos from nuclear reactors 1953 : F.Reines and C.L. Cowans discover the neutrino at Savannah River nuclear power plant -vessel filled with liquid scintillator Detectors doped with neutronphage -shielding (bunker, underground) + active veto: cosmic rays, reactor n, natural radioactivity ∑ → γ nuclear capture ' s ( E known) Signal γ Space and thresh = E 1 . 8 MeV ν + → + delayed time ν p e n e < > ≈ e E 3 MeV correlation + e γ s c i n t i l l a t i o n ' s Cerenkov light + − γ e - e a n n i h i l a t i o n : 2 o f 0 . 5 1 1 k e V
The Long Base Line CHOOZ Experiment Phys. Let. B466 (1999) 415
CHOOZ detector -1 detector - 2 reactors (8.5GW) : L= 998, 1114m ∆ L=116.7m -rock overburden: 300 m water equivalent 0.4 cosmic µ m -2 s -1 -5 tons Gd-doped liquid scintillatior (0.09%) 5t ∑ γ = E 8MeV 17t 90t -17 tons liquid scintillator : contain γ from n PMT radioactivity shield -90 tons active cosmic-ray muon veto
Event rates : full power: 24.7±0.7 eve nt s/day reactors off: 1.2 even ts /day Data taking: April 1997 - July 1998 Reactor 1 ON 2058.0 h 8295 GWh Reactor 2 ON 1187.8 4136 Reactors 1 & 2 ON 1543.1 8841 Reactors OFF 3420.4 Background estimates Response calibration: γ , n and γ -n radioactive sources ( 60 Co, 252 Cf, Am/Be) ∑ abs time dependence monitoring ( ) with n from cosmic : σ E = 0.5 MeV γ = E n E 8MeV
Reactor ON No event selection Reactor OFF n-like E (MeV) n-like E (MeV) Main background ν region ν region fast spallation n in rock + p from n scattering (e + like) + n capture e + -like E (MeV) e + -like E (MeV) ν selection @ > 30 cm from wall, n - e + distance < 100 cm n-like E (MeV) n-like E (MeV) n - e + delay in (2-100) µ s E(e + -like) in (1.3 - 8) MeV ν region E(n-like) in (6-12) MeV ν region 2991 candidates (287 reactors off) Efficiency: 69.8%
ν e flux known to 1.4% • daily evolution of core isotopic evolution • instantaneous fission rate from thermal power • ν yield from measured β spectra of main isotopes + E e+ spectrum • inverse β -decay cross-section • simulation of detector response reactor ON • data — MC R OFF E spectrum measured background + = R e E spectrum expected subtracted + e E e+ (MeV) E e+ (MeV) E e+ (MeV) = ± ± No oscillation signal R 1.010 0.028 (stat) 0.027 (syst)
Analysis Methods A - Compare unfolded E e+ absolute spectra of both reactors to expectation Systematic uncertainty on absolute normalisation: ~2% Two “independent” measurements B - Ratio of spectra Most systematic cancel No sensitivity at large ∆ m 2 C - Compare unfolded E e+ spectra shapes of both reactors to expectation Intermediate sensitivity
⇓ 0 . 1 1 δ m 2 (eV 2 ) ν e → ν x Chooz exclusion plot ∆ 2 m 2 ( eV ) -1 10 A — absolute spectra B — spectra ratio -2 10 C — spectra shape Kamiokande 90% -3 analysis A 10 − ⇐ 4 7 . 10 analysis B analysis C 90 % CL Kamiokande (multi-GeV) 90 % CL Kamiokande (sub+multi-GeV) -4 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 sin 2 (2 θ ) sin 2 θ 2
The Long Base Line Palo Verde Experiment G.Gratta Neutrino 2000 F.Boehm et al. hep-ex/000322
-1 detector - 3 reactors (11.6 GW) : L= 750, 890m ∆ L= 110m Palo Verde detector -rock overburden: 32 m water equivalent 22 cosmic µ m -2 s -1 -11.3 tons Gd-doped liquid scintillation (0.1%) ∑ γ = E 8 M eV - oil and 105 tons water buffer: γ and n shield shield PM radioactivity -optically segmented detector (900x12.7x25.4 cm 3 ) ⇒ background suppression
Difficulty : No period with all reactors off to measure simply the reactors off background. Analysis based on the knowledge of the flux form the known reactors power ⇒ True expected event number compared to Observed number of candidates corrected for detector efficiencies (MC) Unknowns : -Background -Overall normalisation within systematic uncertainty efficiency 0.075 0.077 0.112 0.111 R = 1.04 + 0.03 (stat) + 0.08 (sys) ⇒ No oscillation
Run till end Summer 2000 2 new reduced power periods Not likely to do better than Chooz
Three neutrinos families analysis Reactor experiments exclude two-family ν µ → ν e oscillation in parameters region where ν µ deficit in atmospheric experiments favours two-family ν µ → ν τ (or ν s ) (at least) 3-flavour analysis
3-flavour mixing parametrization 3 ∑ ν = ν U α α k k = k 1 , ν ν ∆ 2 2 m e 1 kk' 6 (8) parameters 3 U ∑ 2 ν = ν = α = µ τ U 1 e, , 3 U µ α α 2 k k Dirac (Majorana) 1 (3)phases = k 1 , ν ν τ 3 = 1 3 . ∑ ∆ 2 = m 0 ' kk ' ≠ k ,k k CKM-like matrix standard parametrization) − δ i c c s c s e = θ 12 13 12 13 13 c cos U ij ij = = s c i, j 1 3 genreation nunbers , 23 13 = θ s sin ij ij c c 23 13
3-family flavour at the strong mass hierarchy approximation m � m ,m if ν 3 1 2 3 ∆ 2 = 2 − 2 ≈ 2 − 2 -3 2 m m m m m e.g. 10 eV atmospheric neutrinos 3 1 3 2 ∆ 2 m δ = − 2 2 2 -6 2 m m m e.g. 10 eV solar neutrinos 2 1 ν ∆ δ 2 2 m � m δ 2 2 m ν ⇓ 1 2 m ν ∆ 2 L/E region where m E/ L causes oscillation ∆ 2 = -3 2 = (e.g. atmospheric neutrinos m 10 eV , E 1GeV, L=1000km) δ 2 ≈ and m E/ L 0 ⇓ 2 ν → ν ≈ ∆ Physics governed by: 2 2 P( ) 4 U U sin (1.27 m E/L) α β≠α α β 3 3 • ∆ m 2 2 θ = 2 eff sin 2 4 U U • flavour contents of ν 3 αβ α β 3 3 • effective 2-flavour like oscillation
Effective 2-family atmospheric ν µ disappearance in 3-family mixing E.Lisi, G.Fogli, ... θ = = θ θ 2 eff 2 2 2 2 sin 2 4 U U sin 2 sin µ µ e 3 e3 13 23 2 θ eff = 2 2 = 2 θ 4 θ sin 2 4 U U sin 2 cos µτ µ τ 3 3 23 13 2 U small (reactors) e3 < 2 U 0.1 ? ≈ ⇒ e3 2 2 4 U U 1 (full mixing atmospheric) µ τ 3 3 ≈ ≈ 2 2 U U 0.5 ? µ τ 3 3 2 + 2 ≈ U U 1 µ τ 3 3 E.Lisi, Neutrino 2000
H.Sobel Neutrino2000 Space is left e3 ≠ 0 for U 2
Conclusions: • No evidence for ν e disappearance in LBL reactor experiments • Reactor + Atmospheric neutrino experiments + in 3-flavour strong mass hierarchy model room left for a small ν e contents in ν 3 • No more constraining data to be expected from reactors in near future
Neutrino Oscillation Experiments at Low Energy Accelerators (Beam Stoppers) ν → ν ∆ > 2 2 Search for oscillation at rather large m ~ . eV 0 1 µ e Compare somehow conflicting results from two similar experiments: KARMEN2: no signal LSND: statistically significant signal LSND: G.Mills Neutrino 2000 KARMEN2: K.Eitel
Conceptual design: shielding p 800 Mev π ν p target ν detector L 30 m ∆ 2 ≈ ≈ 2 m 1 eV E <E ν > ~ 30 MeV
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