Understanding the Particle Nature of Neutrinos

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1 Understanding the Particle Nature of Neutrinos Karsten M. Heeger Lawrence Berkeley National Laboratory U e2 U µ2 * S = 2J U e3 U µ3 * U e1 U µ1 *

2 Particle Properties Interactions Flavors & States Charge Spin Particle Type Mass

3 N N + e - M parent c 2 E daughter + E electron some nuclei emit electrons! 1914 KE electron = M parent c 2 - M daughter c 2 - m electron c 2

4 Postulate of the Neutrino 1930 Wolfgang Pauli Princeton University Pauli proposed that an undetectable particle shared the energy of beta decay with the emitted electron.

5 Neutrinos Don t Matter to Chemists

6 Fermi s Theory of Beta Decay Enrico Fermi Univ. of Chicago Fermi s Theory of Beta Decay based on Pauli s Letter of Regrets Experiment: Conjecture: M n c 2 E p + E e M n c 2 = E p + E e + E ν Consistency requires that E ν is not observable! Fermi s theory still stands (parity violation added in the 50s).

7 Weak Interactions in the Standard Model The weak gauge bosons W ± act on left-handed doublets (charged-current interaction) β-decay Since m w =80.4 GeV >> m p decay is governed by Fermi coupling G F Fermi coupling G F 2 = g m W Weinberg angle g 2 = W gauge coupling e g 2 = sinθ W = 0.48

8 Fermi s Idea for Measuring m ν

9 First Direct Detection of the (Anti)Neutrino Reines and Cowan 1956 ν e + p e + + n ν ν µ

10 Discovery of Muon Neutrino 1962 Lederman, Schwarts, Steinberger spark chamber with Al plates 20 m µ produce nice tracks as they go through the chamber (29 events) e produce showers as they cross Al (0 events) 10 4 ν µ and ν µ

11 Muon Neutrino Mass Studies Current best limit from studies of the kinematics of π µ ν decay p µ 2 + m µ 2 = (m π 2 + m µ 2 m ν 2 ) 2 /4m π 2 Can use π-decay: At Rest: Mass of π is dominate uncertainty In Flight: Resolution on p π p µ limited experimentally Best mass limit is from π-decay at rest < 170 kev at 95% CL (Assamagan et al., PRD 1996) New BNL Experiment using g-2 setup (sensitivity for > 8 kev)

12 Proposed BNL NuMass Experiment BNL g-2 Neutrino Mass Experiment m(ν µ ) < 8 kev/c 2 Forward-going decay muons orbit a larger diameter by ΔD CM ν µ π µ q = 29.7 MeV/c undecayed pions D ΔD p µ - p π 0.7 MeV/c 3.26 mm D p π 3 GeV/c 14 m non-zero m ν shrinks ΔD decay µ s ΔD δd depends on m(ν) δd -m ν 2 D 2 q m π 0.04 mm for current limit

13 Number of Active Neutrinos Precision studies of Z-line shape, determine number of active light neutrinos Each separate (νli)l adds to total Z-width. From LEP, one finds: tly! c e r i N ν= ±0.008 dd e t c e t which argues strongly for only having 3 generations as ν τ de re w Befo Big bang nucleosynthesis gives a constraint on the effective number of light neutrinos at T~ 1 MeV: 1.2 < N νeff < 3.3 [99% CL] Because mixing effects are likely to bring sterile neutrinos into equilibrium, above suggests that the number of (νli)r is also limited to 3. Karsten Heeger, LBNL CWRU - April 30, 2004

14 Search for tau Neutrino Discovery of τ lepton at SLAC there should be a corresponding neutrino. In 1989, indirect evidence for the existence of ν τ in measurement of Z-width no one had directly observed the tau neutrino. The tau neutrino interact and form a tau that has an 18% probability of decaying to - a muon and two neutrinos (long event) - an electron and two neutrinos (short event) 86% of all tau decays involve only 1 charged particle (a kink) which is the particle physicists are looking for in DONUT experiment

Discovery of tau Neutrino 2000 An 800 GeV beam of protons from the TeVatron collides with a block of tungsten Experimental Challenges: - Very short lifetime of the τ.

15 Discovery of tau Neutrino 2000 An 800 GeV beam of protons from the TeVatron collides with a block of tungsten Experimental Challenges: - Very short lifetime of the τ. - ν τ is extremely non-interacting (detector must have a very fine resolution). Detecting a τ Neutrino τ decay D s decay into τ and ν τ neutrino D s ν τ + τ τ ν τ + X 6,000,000 candidate events on tape 4 clean tau events

16 A ν τ interacted with a nucleon in a steel layer, producing a τ. Long tau decay because it decays to one charged particle, the electron, and the decay vertex occurs several sheets downstream from the neutrino interaction vertex. Short tau decay to an electron in less than the distance it takes to traverse an emulsion layer.

Direct ν τ Mass Limits Look at tau decays near the edge of the allowed kinematic range τ 2π π + ν τ and τ 3π 2π + (π 0 ) ν τ Massive ν τ shifts edge of the kinematic distribution Outer

17 Direct ν τ Mass Limits Look at tau decays near the edge of the allowed kinematic range τ 2π π + ν τ and τ 3π 2π + (π 0 ) ν τ Massive ν τ shifts edge of the kinematic distribution Outer lines: m ντ = 0 Inner lines: m ντ = 30 MeV Fit to scaled visible energy vs. scaled invariant mass (e.g. hep-ex/ , CLEO) Best limit is m(ν τ ) < 18.2 MeV at 95% CL (Aleph, EPJ C )

Standard Model Neutrino Physics 1914 Electron Spectrum in β decay is continuous 1930 Pauli postulates that a new particle is emitted 1933 Fermi names the new particle neutrino and introduces

18 Standard Model Neutrino Physics 1914 Electron Spectrum in β decay is continuous 1930 Pauli postulates that a new particle is emitted 1933 Fermi names the new particle neutrino and introduces four-fermion interaction 1956 Reines and Cowan discover the neutrino 1962 At least two neutrinos: ν e ν µ 1973 Discovery of neutral currents at CERN 1983 Discovery of the W and Z 1989 Measurement of Z width at CERN N ν = tau neutrino discovered.

20 Elementary Particles 3 ν flavors Only upper limits on m ν from kinematic studies. Are ν massless? (ad hoc assumption in SM)

21 Neutrino Interactions W exchange gives Charged-Current (CC) events and Z exchange gives Neutral-Current (NC) eventss In CC events the outgoing lepton determines if neutrino or antineutrino l ν l + ν

22 Neutrino Cross Section is Very Small Weak interactions are weak because of the massive W and Z boson exchange σ weak G 2 M W ~ 80 GeV F (1/M W or Z ) 4 M Z ~ 91 GeV 2 2 gw 5 G = = / 2 ( 0.7) 8 F GeV gw M W For 100 GeV neutrinos: σ(νe)~10-40 σ(νp)~10-36 cm 2 Mean free path length in steel ~ m Need big detectors and lots of ν s σ(pp)~10-26 cm 2 At Hera see W and Z propagator effects - Also weak ~ EM strength σ EM 1/Q 4

Neutrinos Are Left-Handed - Helicity and Handedness Helicity is projection of spin along the particles direction. Frame dependent (if massive) Handedness (or chirality) is Lorentz invariant.

23 Neutrinos Are Left-Handed - Helicity and Handedness Helicity is projection of spin along the particles direction. Frame dependent (if massive) Handedness (or chirality) is Lorentz invariant. Only same as helicity for massless particles. right-helicity left-helicity Neutrinos only interact weakly with a (V-A) interaction All neutrinos are left-handed If neutrinos have mass then lefthanded neutrino is: mainly left-helicity But also small right-helicity component m/e Only left-handed charged-leptons (e,µ,τ ) interact weakly but mass brings in right-helicity: All antineutrinos are right handed

27 Dirac and Majorana Neutrinos Dirac Neutrinos Majorana Neutrinos ν ν ν ν only difference is handedness ν are left-handed ν µ ν are right-handed ν µ + Lepton number conserved Neutrino µ Antineutrino µ + Lepton number not conserved Neutrino Antineutrino with spin flip Dirac Mass Term Majorana Mass Term

28 Neutrino Mass: Theoretical Ideas No fundamental reason why neutrinos must be massless. But why are they much lighter than other particles? Fermion Masses PDG 2000 ν e ν µ ν τ PDG SNO + SK (ν 3 ) < ν 1 < ν 2 < (ν 3 )

29 Neutrino Mass: Theoretical Ideas No fundamental reason why neutrinos must be massless. But why are they much lighter than other particles? PDG SNO + SK (ν 3 ) < ν 1 < ν 2 < (ν 3 ) Grand Unified Theories Dirac and Majorana Mass See-saw Mechanism Many of these models have at least one Electroweak isosinglet ν Modified Higgs sector to accommodate neutrino mass Extra Dimensions Neutrinos live outside of space Right-handed partner of the left-handed ν Mass uncertain from light (< 1 ev) to heavy (>10 16 ev) Would be sterile Doesn t couple to standard W and Z bosons

30 See-Saw Mechanism Dirac Neutrinos Majorana Neutrinos See-Saw Mechanism with Both Majorana and Dirac Terms:

31 Direct Neutrino Mass Experiments Techniques Electron neutrino Study E e end point for 3 H 3 He + ν e + e - µ (kev) Muon neutrino Measure P m in π µν µ decays Tau neutrino Study nπ mass in t (nπ) ν τ decays e (ev) τ (MeV) (Also, information from supernova time-of flight)

32 Direct Neutrino Mass Searches Model-Independent Neutrino Masses from ß-decay Kinematics N(E e ) p e E e (E 0 E e ) (E 0 E e ) 2 m ν 2 c 4 E ν p ν Current best limit m ν < 2.2 ev Search for a distortion in the shape of the β-decay spectrum in the end-point region entire spectrum close to β endpoint

33 Mainz Neutrino Mass Experiment T 2 source electrodes solenoid detector

34 Past Tritium Beta Decay Experiments

35 Motivation for a Next-Generation T 2 Experiment Validate/rule out models with quasi-degenerate masses Role of ν s as hot dark matter, constrain Ων

36 Next Generation T 2 β-decay Experiment (m e < 0.35 ev) Main challenge: XHV conditions p < mbar

37 Neutrinoless Double Beta Decay (0νββ) The Next Frontier in Neutrino Physics 2ν mode: conventional 2 nd order process in nuclear physics 0ν mode: hypothetical process only if M ν 0 AND ν = ν Γ 2ν = G 2ν M 2ν 2 Γ 0ν = G 0ν M 0ν 2 m ββ 2 G are phase space factors G 0ν ~ Q 5 important physics

38 Neutrinoless Double Beta Decay (0νββ) The Next Frontier in Neutrino Physics 2ν mode: conventional 2 nd order process in nuclear physics 0ν mode: hypothetical process only if M ν 0 AND ν = ν The only known practical approach to discriminate Majorana vs Dirac ν Two Neutrino Spectrum Zero Neutrino Spectrum 1% resolution Γ(2ν) = 100 * Γ(0ν) Sum Energy for the Two Electrons (MeV) 2.0

39 Neutrino Masses: What do we know? The results of oscillation experiments indicate ν do have mass, set the relative mass scale, and a minimum for the absolute scale. m i > 2 Δm atm 50meV βdecay experiments set a maximum for the absolute mass scale. 50 mev < m ν < 2200 mev : For the next experiments <m β > in the range of mev is very interesting.

40 Distinguishing the Mass Hierarchy in 0νββ Δm 2 Δm atm 2

41 Several Proposed Experiments COBRA DCBA NEMO CAMEO CANDLES CUORE EXO GEM GENIUS GSO Majorana MOON Xe XMASS Te-130 Nd-150 Mo-100, Various Cd-114 Ca-48 Te-130 Xe-136 Ge-76 Ge-76 Gd-160 Ge-76 Mo-100 Xe-136 Xe kg CdTe semiconductors 20 kg Nd layers between tracking chambers 10 kg of ββ isotopes (7 kg of Mo) 1 t CdWO 4 crystals Several tons CaF 2 crystals in liquid scint. 750 kg TeO 2 bolometers 1 ton Xe TPC (gas or liquid) 1 ton Ge diodes in liquid nitrogen 1 ton Ge diodes in liquid nitrogen 2 t Gd 2 SiO 5 :Ce crystal scint. in liquid scint. 500 kg Ge diodes Mo sheets between plastic scint., or liq. scint t of Xe in liq. Scint. 10 t of liquid Xe

42 Selected Proposals for 0νββ Experiments Proposed ton-year = M * T * ε Anticipated <m ee >, (QRPA) CUORE 0.21*5*1 = 1 60 mev EXO 6.5*10*0.7 = mev GENIUS 1*2*1 = 2 20 mev MAJORANA 0.5*10*1 = 5 25 mev MOON 3.3*3*0.14 = mev The <m ββ > limits depend on background assumptions andmatrix elements which vary from proposal to proposal.

3 ev Combining WMAP and large scale structure Ω ν h 2 <0.

43 Cosmological Information on Neutrino Mass WMAP large scale structure formation Neutrinos contribution to the Universe s energy density Ω ν h 2 =Σ i m i /95.3 ev Combining WMAP and large scale structure Ω ν h 2 < ev (95% CL) If m νe ~ m ντ (degenerate neutrino species) m ν < 0.23 ev Cosmological neutrino mass limits probe Dirac and Majorana ν masses! Mass limits comparable to 0νββ experiments.

44 Supernova Neutrinos In a supernova explosion Neutrinos escape before the photons Neutrinos carry away ~99% of the energy The rate of escape for ν e is different from ν µ and ν τ (Due extra ν e CC interactions with electrons) Neutrino mass limit can be obtained by the spread in the propagation time t obs -t emit = t 0 (1 + m 2 /2E 2 ) Spread in arrival times if m 0 due to ΔE For SN1987a assuming emission time is over 4 sec m ν < ~30 ev All arrived within about ~13 s after traveling 180,000 light years with energies that differed by up to a factor of three. Neutrinos Karsten Heeger, arrived LBNL about 18 hours CWRU before - April 30, the 2004light was seen.

45 What about a Neutrino Magnetic Moment? dσ = weak int+ πα 2 µ ν dt e m e T e E ν ν e - e - from U 235 at a reactor Electron Recoil T (MeV)

46 Low Electron Recoil Energy Experiment Experiment at Nuclear Reactors (low energy source of ν e ) Time Projection Chamber High density, relatively low Z, good drifting properties V=1 m 3 L=1.6 m D=0.9 cm

47 Neutrino Magnetic Moment Experimental Results Reactor Experiments UC Irvine µ reac ν < 2-4 x10-10 µ B Kurchatov µ reac ν < 2.4 x10-10 µ B Rovno µ reac ν < 1.9 x10-10 µ B MUNU µ reac ν < 1.0 x10-10 µ B (90% CL) Solar (Anti)Neutrino Experiments Super-Kamiokande KamLAND µ ν sol < 1.5 x10-10 µ B

Experimental Indications for Neutrino Oscillations LSND

Neutrinos L = 15-15,000 km E = 300-2000 MeV ν µ ν x

3 to 3 MeV ν e ν x Δm 2 = ~ 2 to 8 10-5 ev 2 Prob OSC =

48 Experimental Indications for Neutrino Oscillations LSND Experiment L = 30m E = ~40 MeV ν µ ν e Atmospheric Neutrinos L = 15-15,000 km E = MeV ν µ ν x Solar Neutrinos L = 10 8 km E = 0.3 to 3 MeV ν e ν x Δm 2 = ~ 2 to ev 2 Prob OSC = ~100% Δm 2 = 0.3 to 3 ev 2 Prob OSC = 0.3 % Δm 2 = ~ 1 to ev 2 Prob OSC = ~100%

Reactor (KamLAND) Neutrinos are not massless Evidence for neutrino

49 Discovery of Massive Neutrinos through Oscillations ν µ ν τ Atmospheric (Super-K) Accelerator (K2K) ν e ν µ,τ Solar (SNO) Reactor (KamLAND) Neutrinos are not massless Evidence for neutrino flavor conversion ν e ν µ ν τ Experimental results show that neutrinos oscillate

50 Neutrino Mixing Matrix/Leptonic Unitarity Triangle Global Fit of Oscillation Data (3σ) U PMNS = U e1 U µ1 U e2 U µ2 U e3 U µ3 U τ1 U τ2 U τ3 = Can we reconstruct the triangle? Can we use it to determine the CP-violating phase? Ref: Gonzalez-Garcia U e2 U µ2 * S = 2J U e1 U µ1 * U e3 U µ3 * U e3 = 0.16 nearly best fit values of other angles J = s 12 c 12 s 13 c 132 s 23 c 23 sinδ Problem: coherence (we deal with coherent states and not mass eigenstates of neutrinos) Ref: Farsan, Smirnov

51 Quarks and Leptons Mixing Quarks Leptons 1-2, θ , θ , θ o 32 o 2.3 o 45 o ~ 0.5 o <13 o θ 12 + θ C = θ ο Hierarchy of Mass Neutrinos Charged leptons Down quarks Upquarks m 2 /m 3 > 0.18 m µ /m τ = 0.06 m s /m b ~ m c /m t ~ σ sin θ Ref: Smirnov

52 Other oscillations? Sterile Neutrinos? LSND Cannot be explained by 3 active neutrinos! ν µ ν e? ν µ ν τ Will be checked by MiniBoone at FNAL (2005) ν e ν µ,τ

53 The LSND Experiment π + µ + ν µ e + ν e ν µ Oscillations? ν e LSND took data from ,000 Coulombs of protons - L = 30m and 20 < E ν < 53 MeV Saw an excess of: 87.9 ± 22.4 ± 6.0 events. With an oscillation probability of (0.264 ± ± 0.045)%. 3.8 σ evidence for oscillation.

54 Too Many ν-oscillation Signals Three known neutrinos ν e ν µ ν τ cannot explain three different Δm 2 values. 2 Δm solar 2 Δm atmos then 2 Δm LSND = m 2 2 m 1 2 = ev 2 = m 3 2 m 2 2 = ev 2 = m 3 2 m 1 2 = ~ ev 2 But LSND sees ~ 1eV Experimental ideas - Not all 3 signals are neutrino oscillations - Unknown uncertainties give false signals Theoretical ideas - Neutrinos and antineutrinos have different masses - More than three types of neutrinos extra sterile neutrino types

55 LSND Oscillation Parameters ν µ ν τ ν e ν µ,τ

56 MiniBoone Experiment at Fermilab

57 Estimates of the MiniBoone Signal goal: p on target to cover LSND at 5σ

58 New Surprises Beyond Mixing of 3 Massive States? Sterile Neutrinos Are they light enough that we can see them? They would give a whole new spectrum of mass states and mixings MiniBooNE and follow-ups are key 3+2 models m 5 Probing for CP violation CP violation comes about when a process has a different rate for particles and anti-particles Need to measure last mixing angle θ 13 Then look at ν versus ν oscillations to measure δ New long baseline and reactor experiments are key ν e ν µ ν τ ~ 0.7 ~ 0.7 sinθ 13 e iδ? = ~ 0.5 ~ 0.5 ~ 0.7 ~ 0.5 ~ 0.5 ~ 0.7 ν 1 ν 2 ν 3

59 Experimental Program with Sterile Neutrinos If sterile neutrinos then many mixing angles, CP phases, and Δm 2 to include Measure number of extra masses Δm 142, Δm 15 2 Measure mixings: could be many small angles Map out mixings associated with ν µ ν e Compare ν µ and ν µ oscillations CP and CPT violations Map out mixings associated with ν µ ν τ

60 Theoretical Prejudices before 1995 Natural scale for Δm 2 ~ ev 2 since needed to explain dark matter In 2004 we know. Wrong Oscillation mixing angles must be small like the quark mixing angles Wrong Atmospheric neutrino anomaly must be other physics or experimental problem because it needs such a large mixing angle Wrong LSND result doesn t fit in so must not be an oscillation signal???

61 Beyond Neutrino Mass & Mixing with Oscilllation Experiments Neutrino oscillation Neutrino decay Neutrino decoherence Searching for Direct Evidence of Oscillations

62 Interdependencies/Redundancies of Experiments reactor + accelerator Need all types of experiments

63 Particle Properties of the Neutrino Interactions Flavors Charge Spin weak (and gravitational) only 3 active flavors sterile flavors? s=1/2? Type Mass Dirac ν ν Majorana ν = ν? m νe < 2 ev from tritium β decay m νµ < 170 kev from π decay m ντ < 18 MeV from τ decay

64 Open Questions in Neutrino Physics What are the values of Δm 2, U ij in (2,3)sector? Is U 13 = 0? What is the level ordering of 2,3 (or 1,3)? Is there CP violation for neutrinos? Principal goals of oscillation physics Is U 3-dimensional? 4? 6?? or, is the 3-D version unitary? or, are there sterile ν? MiniBoone What are the masses? Are ν Dirac or Majorana particles? Direct mass measurements -KATRIN 0νββ Majorana Exo Genius etc.

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