τ detection and ν oscillation Gaston WILQUET Symposium in honorem Prof. Jean SACTON Brussels, 7 - 8 September 2000 Contents • The neutrino identity (brief) • Limits on neutrino masses • Neutrino oscillation phenomenology (brief) • Status of search for neutrino oscillation • OPERA : search for νµ⇔ντ oscillation in LBL accelerator experiment (refer to K.Niwa’s talk) • The neutrino mass and the mass budget of the Universe (if time allows) The Standard Model neutrino • The electron-neutrino νe is the massless, chargeless, colourless, spin 1/2, partner of the electron in the left handed SU(2) × U(1) lepton doublet • Neutrinos also exist in 3 families like the other fermions (measured at LEP from the width of the Z0; why? why 3?) νννe µτ −−− e LLL µτ • The lepton numbers Le , Lµ , Lτ are conserved independently B.R.(µγ− →<× e−− ) 51011 • Only ννLR and have CC and NC weak interactions (P conservation is fully violated) • If they exist, ννRL and are sterile Massless ν and ν are distinct by their observable helicity ≡ invariant chirality Massless neutrinos may not be overtaken and their spin cannot be flipped What if neutrinos have (even tiny) mass Dirac or Majorana neutrinos? − • νβ+ emitted in decay has vν <c may be overtaken and undergo spin flip → ν − + • Is it different from the νβ− emitted in decay? They only differ by L If yes: Dirac neutrinos, like other fermions, distinguished by L =±1 eigenvalues If no : Majorana neutrinos, νν≡ apparent distinction is artefact of - their V-A interactions - the difficulty to "flip spin" Limits on the neutrino masses: 1/ Direct measurements from decay kinematics • @%C.L.95 m..eVνe < 22− 2 3 33 − from end of E − spectrum in HHee→++ ν e e (Troitsk and Mainz experiments, 2000) • @%C.90L mkeVνµ <170 ++ from E + in πµν→ µ µ (Assagam et al, PSI, 1996) • @%C.L90 m.MeVντ <18 2 ±± from phase space in τπν→ 35() ττ()ν (LEP -ALEPH 2000) Limits on the neutrino masses: 2/ Big Bang cosmology 10 2 •≈×≈ At T210K2MeVm ν (t1kT0.25s) =() ≈ γνν and / decouple : ννγγ+↔+ nnνγ≈ TTνγ≈ • Taking account of - the thermodynamics of fermions/bosons - the adiabatic expension of the univers +− + − - γγγ and e/e decouple: e+↔+ e at T ≈ me ≈ 0.5eV 3 n03==n(03 412cm− ) = 113 cm− at present epoch ν 11 γ 4 1 T21o ===≈( )3 T0 ( 2.73 K ) 1.9 K × 0eV−4 ν 11 γ -4 • Yet undetected primeval neutrinos are non-relativistic if mν >10 eV 2 003H0 − 3 mνν×≈ n = ρρ ν<c = 4 . 5 keV cm 8πGN −−11 with H0 =±65 7 km s kpc meV≤ 60 ∑ νi i Limits on the neutrino masses: 3/ Supernovae SN1987A • February 23, 1987, 7h33 UT, 23 neutrino interactions in 3 underground experiments (Kamiokande, Japan; IMB, Ohio; Baksan, Caucase) in 12.3 s time gate • SN1987A: death of Sanduleak in LMC at 150 000 ly 46 58 SN-II models: 99% of ~2. 10 J released → ~10 ν with <Eν>=10 MeV after 150 000 y : 10s flash of ~4. 1043 ν cross the Earth • From measured and model energy spectrum model emission time dispersion measured arrival time dispersion Confirmation of the SN-II models meV≤∼ 25 ν e • Identical fluxes of all ν and ν species expected − Events expected to be ννee (much larger +→+nep cross-section) Limit on the neutrino masses: 4/ 0νββ decay experiments • The nuclear 0νββ decay experiment is THE ν spin-flip experiment (A,Z)→++ (A,Z2 ) 2 e− ∆ L =2 Process possible if the − - emitted right-handed ν e together with e - _ e e ν ν − e e absorbed as left-handed ν e to produce e W- Massive Majorana neutrino : d u W- ννee≡ and spin flipped d u T21090(Ge76→+> 76 See)− 25 y @ %C.L. d u 1 2 32 34 d u m.eV≤ 046 ν e B.Pontecorvo Massive neutrinos: 1957 Mixing & Neutrino oscillations B.Pontecorvo, V.N. Grimov 1967 ν l le,,= µτ family eigenstates ν k k,= 1 3 mass eigenstates 3 ννe 1 ννllkk= ∑U k,=1 ν ==U νµτle,, µ 2 3 2 νν U = 1 τ 3 ∑ αk k,=1 4 (6) parameters 3 U lk Dirac (Majorana) 1 (3) phases Straightforward extension to more than 3 neutrinos families, e.g. sterile neutrinos Oscillation cannot distinguish Dirac from Majorana neutrinos Propagation phase -i( E t - p L ) − i( m2 /( 2E ))L E ≈>>⇒p m e kk ≈e k + 3 u µ τ − + π + W ντ :Uτk νµµ:U k ν k ∑ + W k1= d d u n ddp u u Coherent propagation of different mass eigenstates over long L Oscillation probability (in practical units) P ((ν llllL0= )→=−νδ'' ()) L 13, ∆ meVLkm22[][] 4UUUU127Re(** sin2 . kk ' ∑ lk'' lk ' lk ' lk EGeV[] kk,'> k Mixingsdefine Maximum L/ E Oscillation term probability Sum on pairs of mass eigenstates 222 222 ∆∆∆∆mmmkk'' = k− k mmm0 12+ 23 + 31 = Neutrinos oscillate if massive and masses are non degenerate and if mixing between mass and weak eigenstates One mixing negligible : effective 2-family approximation cosθθeeµµ sin e.g. ντ ≈ν3 U ≈ 1 mixing angle, no phase − sinθθeeµµ cos ∆mL2 P2127()sinsin(.)νν→=22 θ 12 eeµµ E All mixings small : effective 2-family approximation 1 θθeeµτ U1≈−θθ all νl ≈νk eµµτ −−θθeτµτ1 ∆mL2 P2127()()sin(.)νν→= θ 2 kk ' lllll''≠ E Strong mass hierarchy : effective 2-family approximation if mm,m312 like quarks and charged leptons ν3 22222 ∆mmmmm=−≈−31 32 222 2 δ mmm=−21 ∆m ∆δmm22 ν δm2 2 ⇓ ν1 L/ E region where ∆ m2 L/ E causes oscillation 2 mν and δ mL/E2 ≈ 0 ⇓ 22eff 2 P(ννl→≈ l'≠ l ) sin2127 θ ll' sin ( . ∆ m E / L) 2 2 eff sin24θll'= U l33 U l' Physics governed by: • ∆m2 • family composition of ν3 only Example of 2-family approximation: large mixing and strong mass hierarchy oscillation in phase (2-family like) from source composition from first process E = 1GeV strong mass hierarchy ∆λmeVkm232=×310− → = 825 δΛmeVkm272=×110− → = 2.5 ×10 7 large mixing -0.567 0.820 -0.0782 U = 0.515 0.279 -0.811 0.643 0.500 0.580 E 2 2 sin 1.27 ⇒ oscillation damping for large ∆m 2 ∆mL 1 dispersion and resolution in L/E 2 Matter effect on neutrino oscillations • Propagation phase in matter for weakly interacting particles ipx−−iEt in px iEt ee⇒ e e nfE=+12πρ (0)/ Z EMeV=<−=1 : 0 | n 1 | 6.10−−19ρ [ gcm 3 ] 1 ν A • ννννes,µτ , , have different interact ions thus nes,,µ τ , : −− ννee,,µ ττ+ eq,→+ ,,µ eq , (NC) −− ννee+→ee (CC) ν s no interaction • Mass eigenstates have different family eigenstates composition ⇒ Coherence of mass eigenstates propagation is affected by matter Matter effect on neutrino oscillations Oscillation enhancement Oscillation can be enhanced by matter and is maximum for 2 given electron density ρ∆θR (Em | , ) where mixing is full even if mixing in vacuum is extremely small MSW effect If neutrinos travel through medium where ρρeR varies and crosses slowly (e.g. through the Sun): ν e created in the Sun core may disappear totally into ν µ by reaching the Sun surface. Energy spectrum distortion ρ R = ρ R ()E Solar neutrino oscillation experiments Firsts experimental hints of neutrinos oscillation dates back from 1968 The solar neutrino deficit problem Radiochemical Experiments − ν e +→(Z,A)e +(Z + 1,A) τ (Z,A+≈ 1) 10 days Water Cerenkov −− Counting experiment νν+→+ee ν + ee−−→+ν Low E threshold ee High E threshold Ga Cl nuclear fusion pp cycle Measure E, θ,t 1 4 + 4 H→ He + 2 e + 2 νe + 25 MeV 38− 1 Φν(Sun)=× 1.s 8 10 e 10-- 2 1 ν νBe Φν(Earth)= 6. 5 10 e cm s pp KAMIOKANDE CHLORINE SuperK • ν pp 99.75% of flux: bound by Sun luminosity ν very low energy B very difficult to detect extremely low rate GALLEX SAGE GNO •− Strong correlation ννBe B fluxes Eν(MeV) Measured event rates v.z. SSM predictions GNO +10. 8 65. 8−10. 2 Overall flux deficit 03./.≤≤ΦΦmeas pred 06 +78. 75. 4−74. +0. 015 0.. 465± 0 005−0. 013 A Crude solution meas pred ΦΦννpp≈ pp bound by Luminosity meas pred ΦΦννBB≈ 0.5 not well known meas Φν Be ≈ 0 not well known pred pred Contradiction with strong ΦΦννBe− B correlation No astrophysical explanation νe→νx (νµ,ντ, νs) Oscillation Signals? Inside the Sun? Between Sun-Earth? χ2/NDF = 13.7/17 - Total flux too low by factor 0.3-0.5 in all 6 experiments - E spectrum distortion: Eν SuperK and SSM spectra agree (En >6.5 MeV): - Seasonal effects: Effect of Sun-Earth distance variation (besides 1/L2): SuperK flux time dependence agrees with 1/L 2(t) - Day/Night effect: matter effect inside Earth ? SuperK flux time dependence compatible with no effect D-N =−0.. 034 ± 0 022+0. 013 (D+N)/2 −0. 012 Flux deficit is the only smoking gun Y.Suzuki Neutrino 2000 Y.Susuki Vietnam 2000 Global fit for ν eactive→ ννν (µ or τ ) Allowed by SK All measurements Excluded by SK Day/Night spectrum only Allowed by Ga+Cl+SK Flux measurements only Oscillation in Sun matter @ 95% C.L. Oscillation in vacuum 2 Y.Suzuki Neutrino 2000 tan θ Y.Susuki Vietnam 2000 Note Global fit for ν eactive→ ννν (µ or τ ) 2 solutions at sin2 2θ ≈ 1 ∆meV2452≈−10−− 10 ∆meV272≈ 10− @ 95% C.L. νe→νs disfavoured at 95% C.L. 2 Y.Suzuki Neutrino 2000 tan θ Y.Susuki Vietnam 2000 Note Solar neutrino oscillation: Summary and Future The solar neutrino deficit can be explained by νe oscillation to active neutrinos @ 95% C.L.