Enrique Fernández Univ. Autónoma Barcelona/IFAE

1. Neutrino oscillations: status and plans Enrique Fernández Univ. Autónoma Barcelona/IFAE Trobada de Nadal, Univ. Barcelona, Dec 21-22, 2005

2. Neutrino properties Neutrinos are somewhat special particles. This is mainly due to the fact that they only interact weakly. At low energies (MeV’s), the cross-section for interacting with 1 nucleon is very small, ~ 10-40 cm2. This implies that they are “invisible” in most cases. The charged current weak interaction is also very peculiar: it only acts on the left-handed chiral projection of particle spinors (right-handed, for antiparticles). It does not conserve P (nor CP). Spin effect are thus very strong. They also have a very small mass, compared with that of the other elementary particles.

3. Neutrino properties Quark and Lepton masses in units of MeV/c2 From CMB anisotropies:

4. Neutrinos in the SM In the SM there are 3 lepton families, each containing a charged lepton and a neutrino. Neutrinos are massless particles and each family lepton number (as well as global lepton number) is conserved. The neutrino has three states (weak eigenstates): ne, nm, nt. By definition these are the states that couple to the W together with the corresponding charged leptons. These assumptions, in particular the massless assumption, were built up from experiments.

5. Massive neutrinos and neutrino oscillations In 1998 there was a turning point in neutrino physics. Data from atmospheric neutrinos collected by the Superkamiokande detector showed that there were neutrino oscillations. As we will see neutrino oscillations requires that neutrinos have mass and that there is lepton mixing. The SK results were preceded by many experiments on solar neutrinos that showed a deficit, with respect to solar models, on the number of neutrinos coming from the Sun. New solar neutrino experiments, in particular SNO (Sudbury Neutrino Observatory) have now shown unequivocally that the deficit of solar neutrinos is also due to oscillations.

6. What do we mean by oscillations? (ref.: B. Kaiser, hep-ph/0506165). Let’s assume that a neutrino of flavor a, na, is produced at the source. When it interacts at the target it does so as a neutrino of a different flavor, nb.

7. Neutrino Oscillations: Oscillation requires both mixing between the leptons and massive neutrinos. Suppose that there are several neutrino mass statesni. Mixing means that the state produced together with charged lepton la is a superposition of differentni: U*ai= amplitude of W+ decay to la ni The set of all U*ai (for 3 ni) form a unitary matrix. Inverting it: Pontecorvo-Maki-Nakagawa-Sakata matrix

8. Neutrino Oscillations: The oscillation probability is given by the square of the amplitude:

9. Neutrino Oscillations: In the n rest frame: In terms of laboratory variables: To interfere coherently, the different ni have to have the same E. (if they had the same p the phase would be exp (-i(Ei-Ej)t) which, averaged over t, would be zero, unless Ei=Ej). The momentum is given by (for mi2<<E2): Therefore the phase is: (constant for all ni, thus not contributing to the probability)

10. Neutrino oscillations. Squaring the amplitude: The oscillation implies that lepton family number is no longer conserved.

11. Neutrino Oscillations: This is entirely similar to what happens in the case of the quarks, where favor is not conserved in weak decays, e.g L (uds) p (uud)+ p- (du) The reason for the non-conservation of “quark family number” or “flavor” is quark mixing, the fact that weak and mass eigenstates are different. u d’ c s’ t b’ The difference is that we produce weak-interaction quarks (in the weak decay of the L) but observe them as mass states (in the p or p).

12. Neutrino oscillations. Squaring the amplitude: From this expression we see that: 1) as required by CPT invariance. 2) In general (if U complex): CP violation. 3) The sin2[..L/E] gives “oscillatory” pattern. The above formula is very complicated but nature has been kind enough as to make it simple in certain cases of interest.

13. Neutrino oscillation: To gain some understanding of the above formula write: For a given term to be relevant the argument of sin2() should not be much smaller than p/2, otherwise sin2( ) is too small. It can also be that for a given experiment only one mixing angle is relevant. The bottom line is that in some cases the oscillation can be treated as a two-family mixing.

14. Neutrino oscillation: Oscillation probabilities (2 neutrino case; relevant for CNGS beam):

15. The first clear signature of oscillations came from the SuperKamiokande experiment in 1998

16. SuperKamiokande detector principle Detect Cherenkov light produced by charged lepton l  from reacction n+Nl  +X (l =e,m), or e- from n+e-n+e- . Detector operates in real time and has directional information.

17. SuperKamiokande events (fairly typical) muon electron

18. Evidence for neutrino oscillations SK atmospheric n

19. The evidence for oscillations: solar neutrinos The flux of solar neutrinos is very large but their detection is very difficult. The pioneer experiment of R.Davies took place at the Homestake Mine in S. Dakota (at 1350m depth). Large (600 tons) of Perchloroethylene (C2Cl4). The detection method is radiochemical. About 2 radioactive atoms/day!

20. The evidence for oscillations: solar neutrinos The measurement was repeated by other experiments using Galium instead of Clorine. All of them saw less neutrinos than expected. This was known as the “solar neutrino problem”. Kamiokande, and later SK, measured the “elastic-scattering” reaction nx + e-→ nx + e- where nxis mostlyne. ne e- ne ne W Z e- e- e- ne nm,t nm,t Z e- e-

21. The evidence for oscillations: solar neutrinos

22. The evidence for oscillations: solar neutrinos

23. Definive solution of the Solar neutrino puzzle The Sudbury Neutrino Observatory (SNO) SNO: 1 kT of D2O (heavy water) surrounded by 7.8kT of ultra pure H2O. Located at 2000m depth at the INCO mine in Sudbury, Ontario, Canada.

24. Definive solution of the Solar neutrino puzzle SNO A neutrino of En>2.2 MeV can disociate the Deuterium nucleous, into proton and neutron. This NC process takes place for any neutrino species. SNO detects 3 reactions: ne + D  p + p + e- (CC) nx + e-→ nx + e- (ES; like SK) nl + D  nl + p + n (NC; l = e,m,t) The neutron is captured producing 6.25 MeV. But detecting a single neutron is difficult...

25. Definive solution of the Solar neutrino puzzle SNO Results

26. Oscillation signatures Atmospheric neutrino disappear, but, is it due to oscillations? A controlled accelerator experiment: K2K (KEK to Kamioka).

27. K2K (KEK to Kamioka) ~1 event/2days ~1011nm/2.2sec (/10m10m) ~106nm/2.2sec (/40m40m) 1º tilt downwards nm 12GeV protons nt SK p+ m+ Target+Horn 100m ~250km 200m decay pipe p monitor Near n detectors (ND) m monitor (monitor the beam center) • Signal of n oscillation at K2K • Reduction of nm events • Distortion of nm energy spectrum

28. GPS SK Tspill TSK TOF=0.83msec SK Events Decay electron cut. 500msec 20MeV Deposited Energy No Activity in Outer Detector Event Vertex in Fiducial Volume More than 30MeV Deposited Energy 107 events Analysis Time Window 5msec for 0.89x1020 p.o.t. -0.2<TSK-Tspill-TOF<1.3msec (BG: 1.6 events within 500ms 2.4×10-3events in 1.5ms) TDIFF. (ms)

29. K2K near-detector complex • 1KT Water Cherenkov Detector (1KT) • Scintillating-fiber/Water sandwich Detector (SciFi) • Lead Glass calorimeter (LG) before 2002 • Scintillator Bar Detector (SciBar) after 2003 • Muon Range Detector (MRD)

30. Oscillation signatures E. Aliu et al., Phys. Rev. Letters 94:081802, 2005.

31. KAMLAND Reactor Experiment “Solar” neutrino oscillations in a controlled reactor-experiment

32. Evidence for neutrino oscillations SK atmospheric n Solar experiments KAMLAND reactor exp. K2K

33. Neutrino oscillations: In addition to the Solar (+Kamland) and Atmospheric (+K2K), there are two other very relevant experiments:  Chooz reactor experiment. Sees no oscillation of reactor ne over a baseline of 1 Km.  LSND accelerator experiment. Sees positive signal of oscillations of nm→ ne over 30 m baseline conveniently ignored Excess of 87.9±22.4±6.0 events!

34. Results of the analysis of the oscillation data

35. Results of the analysis of the oscillation data

36. Oscillation parameters Dm2atm≡ Dm232=m23-m22= [(2.40.3)x10-3 eV2] dm2sol≡ Dm221=m22-m21= (0.80.3)x10-5 eV2 Mixing angles: q12  qsol  34º2º q23 qatm 45º3º q13 < 12º (at 3s) sin2q13≡|Ue3|2 < 0.04 n12/3 ne n21/3 ne n30% ne

37. CP violation phase atmospheric solar Links atmospheric & solar sectors Parameterization of the PMNS matrix In view of the results it is convenient to parameterize the PMNS matrix as: cij≡cosqij sij≡sinqij

38. Solar Oscillations and the MSW effect The solar neutrinos pass through the very dense Sun core. Electron neutrinos can interact forward with the solar matter in two ways, while mu or tau neutrinos only do so through NC. ne,m,t ne,m,t ne e Z W e e e ne Forward interactions cannot be distinguished from no-interaction at all coherent scattering, which affects propagation through matter. The extra term for ne gives an extra phase to mass eigenstates which interplays with that which gives rise to oscillations. The effect has the opposite sign for neutrinos and antineutrinos (has nothing to do with CP violation).

39. Solar Oscillations and the MSW effect Formulae are similar to those in vacuum with the replacement: + for neutrinos - for antineutrinos x < cos2q12 sign of x depends on sign of Dm2 x > 1 Pee E 1 MeV

40. Accelerator experiments and their primary goals: MiniBoone (FNAL):prove or disprove LSND Near Term K2K (KEK-Kamioka):check SK, improve Dm MINOS (FNAL-Soudan)check SK, improve Dm, q13? OPERA (CERN-LNGS)see nt appearance in nm beam T2K (KEK-Kamioka)try to measure q13 . . . Longer Term Nona (FNAL-Nth Minn.)try to measure q13 . . . Many ideas for futureq13, CP-violation, ...

41. Conclusions • Neutrinos played a crucial role in establishing the Standard Model. • Neutrinos have mass and mix. This is physics beyond the Standard Model. • The masses and pattern of mixing is quite different from that of quarks. This may be a hint to the physics beyond the SM. • Accelerator experiments permit the control of E, L and the initial neutrino state. They will have a role in elucidating fully the pattern of masses and mixings. • Progress will require a variety of experiments at different energies and baselines.