lecture 12 - neutrino properties - experimental nuclear physics

Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 1

Lecture 12 - Neutrino Properties -

Experimental Nuclear Physics PHYS 741

[email protected]

References and Figures from:- Basdevant et al., “Fundamentals in Nuclear Physics”- Henley et al., “Subatomic Physics”- Oser, “Lake Luise Lectures”

mailto:[email protected]

mailto:[email protected]

Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin

Experimental Indications for Neutrino Oscillations

LSND Experiment L = 30m E = ~40 MeV

Atmospheric Neutrinos L = 15 - 15,000 km E = 300 - 2000 MeV

Solar Neutrinos L = 108 km E = 0.3 to 10 MeV

Δm2 = ~ 2 to 8 × 10-5 eV2 ProbOSC = ~100%

Δm2 = 0.3 to 3 eV2 ProbOSC = 0.3 %

Δm2 = ~ 1 to 7 × 10-3 eV2

ProbOSC = ~100%2

Karsten Heeger, Univ. of Wisconsin ANL, May 23, 2008

Neutrino Oscillation

Neutrino States

Time Evolution

€

Pi→i = sin2 2θ sin2 1.27Δm2 LE

First Second First Second

Mass states

Time, t

Weak states

ν1 ν2 νe

νe cosθ sinθ2sinθ cosθ νµ

νµ

( ) ν2( )( )=ν1

ν1

ν2

νe

νµ

ν2

ν1

cosθ

sinθ

θ

θ

2

Pure νµ

0

Pure νµPure νµ

Mass States Weak States

Time, t

Pure νµ Pure νµ Pure νµ

First FirstSecond Second

νµ

νeνeνµ

=

cosθ sinθ2sinθ cosθ

ν1ν2

Pontecorvo, 1968

Neutrino Oscillations

6

Illustrate with only two generations

|νa〉 = cos θ|ν1〉 − sin θ|ν2〉|νb〉 = sin θ|ν1〉+ cos θ|ν2〉


|ν(t)〉 = e−iHt|ν(t = 0)〉


|ν(t)〉 = e−iHt|ν(t = 0)〉

H|ν1〉 = E1|ν1〉 E1 =(p2 + m2

1

)1/2

H|ν2〉 = E2|ν2〉 E2 =(p2 + m2

2

)1/2

oscillation → energy and baseline- dependent effect


Discovery of Massive Neutrinos through Oscillations

Solar (SNO)

νµ ⇒ ντ

νe ⇒ νµ,τ

Atmospheric (Super-K)

Reactor (KamLAND)

Accelerator (K2K)

• Neutrinos are not massless • Evidence for neutrino flavor conversion νe νµ ντ• Experimental results show that neutrinos oscillate

4


What if neutrinos have mass?

5

If neutrinos have mass and lepton-family number is not conserved, a muon neutrinoemitted at the first weak-interaction vertex could become an electron neutrinothrough interaction with the Higgs background and be transmuted into an electron e-

at the second vertex.

Reaction μ - -> e- + ϒ could proceed if mixing occurred across lepton families.

difficult to detect: ~ 10-40 of normal μ decay

Lepton Family Mixing


Neutrino Interactions

W exchange gives Charged-Current (CC) events and Z exchange gives Neutral-Current (NC) events

€

l− →ν

l+ →ν

In CC events the outgoing lepton determines if neutrino or antineutrino

6


Neutrino Cross Section is Very Small

MW ~ 80 GeVMZ ~ 91 GeV

σweak ∝ GF2 ∝ (1/MW or Z)4

Weak interactions are weak because of the massive W and Z boson exchange

For 100 GeV neutrinos: σ(νe)~10-40 σ(νp)~10-36 cm2 σ(pp)~10-26 cm2

Mean free path length in steel ~ 3×109 m → Need big detectors and lots of νʼs

σEM ∝ 1/Q4

At Hera see W and Z propagator effects - Also weak ~ EM strength

7


Spin 1-2 Particle

8

right helicity: p and s in same direction

left helicity: p and s in opposite direction


Standard Model (massless neutrino)

9

each handedness state can be written as a linear combination of helicity states;for massless neutrino helicity = handedness


Neutrinos Are Left-Handed - Helicity and Handedness

Helicity is projection of spin along the particles direction.Frame dependent (if massive)

Neutrinos only interact weaklywith a (V-A) interaction

All neutrinos are left-handed

All antineutrinos are right handed

right-helicity left-helicity

Handedness (or chirality) is Lorentzinvariant. Only same as helicity formassless particles.

If neutrinos have mass then left-handed neutrino is: mainly left-helicityBut also small right-helicity component ∝ m/E

Only left-handed charged-leptons (e−,µ−,τ−) interact weakly but massbrings in right-helicity:

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Neutrino with Mass

13


Left-handed and Right-handed Neutrinos

14

sterile neutrinos: would not interact through weak force, only included to give Dirac neutrino a mass


Lepton-Number Non-Conservation in 0νββ

15


Pion Decay and Helicity

16


Neutrino Helicity

17

Goldhaber experiment to determine neutrino helicity


Dirac and Majorana Neutrinos

19


Dirac and Majorana Neutrinos

Dirac Neutrinos

ν ≠ ν

Majorana Neutrinos

ν ≠ ν

only difference is “handedness” ν are left-handed ν → µ−

ν are right-handed ν → µ+

Dirac Mass Term Majorana Mass Term

Lepton number conservedNeutrino → µ− Antineutrino → µ+

Lepton number not conservedNeutrino ⇔ Antineutrino with spin flip

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Neutrino Mass: Theoretical Ideas

No fundamental reason why neutrinos must be massless. But why are they much lighter than other particles?

PDG 2000 + SNO + SK

(ν3) < ν1< ν2 < (ν3)

Fermion Masses

νeνµ

ντ

PDG 2000

21


Neutrino Mass: Theoretical Ideas

PDG 2000 + SNO + SK

(ν3) < ν1< ν2 < (ν3)

Grand Unified Theories Dirac and Majorana Mass See-saw Mechanism

Modified Higgs sector to accommodate neutrino mass

Extra DimensionsNeutrinos live outside of 3 + 1 space

Many of these models have at least one Electroweak isosinglet ν

Right-handed partner of the left-handed ν Mass uncertain from light (< 1 eV) to heavy (>1016 eV)

Would be “sterile” – Doesnʼt couple to standard W and Z bosons

No fundamental reason why neutrinos must be massless. But why are they much lighter than other particles?

22


How Particles Get Mass

23


Majorana Neutrinos

24


Majorana Mass Terms

25


SeeSaw Mechanism

26


τ (MeV)

Direct Neutrino Mass Experiments

µ (keV)

e (eV)

Techniques

Electron neutrinoStudy Ee end point for 3H→3He + νe + e-

Muon neutrinoMeasure Pm in π→µνµ decays

Tau neutrinoStudy nπ mass in

t→ (nπ) ντ decays(Also, information from supernova time-of flight)

28


Direct Neutrino Mass Searches

entire spectrumclose to β endpoint

Search for a distortion in the shape of the β-decay spectrum in the end-point region

Model-Independent Neutrino Masses from ß-decay Kinematics

€

N(Ee )∝ peEe (E0 − Ee ) (E0 − Ee )2 −mν

2c 4

Eνpν

Current best limit mν < 2.2 eV

29


Mainz Neutrino Mass Experiment

T2 source electrodes solenoid detector 30


Past Tritium Beta Decay Experiments

31


Motivation for a Next-Generation T2 Experiment

Validate/rule out models with quasi-degenerate masses

Role of νʼs as hot dark matter, constrain Ων

32


Next Generation T2 β-decay Experiment (me < 0.35 eV)

Main challenge: XHV conditions p < 10-11 mbar33


KATRIN

34


The Next Frontier in Neutrino Physics

2ν mode: conventional 2nd order process in nuclear physics

0ν mode: hypothetical process only if Mν ≠ 0 AND ν = ν

Neutrinoless Double Beta Decay (0νββ)

€

Γ2ν =G2ν |M2ν |2

€

Γ0ν =G0ν |M0ν |2 mββ

2

G are phase space factors G 0ν ~ Q5 important physics

35


The Next Frontier in Neutrino Physics

2ν mode: conventional 2nd order process in nuclear physics

0ν mode: hypothetical process only if Mν ≠ 0 AND ν = ν

Neutrinoless Double Beta Decay (0νββ)

The only known practical approach to discriminate Majorana vs Dirac ν

2.01.51.00.50.0Sum Energy for the Two Electrons (MeV)

Two Neutrino Spectrum Zero Neutrino Spectrum

1% resolutionΓ(2ν) = 100 * Γ(0ν)

36


Neutrino Masses: What do we know?

The results of oscillation experiments indicate ν do have mass, setthe relative mass scale, and a minimum for the absolute scale.

For the next experiments <mβ > in the range of 10 - 50 meV is very interesting.

:€

mi > Δmatm2 ≈ 50meV

βdecay experiments set a maximum for the absolutemass scale. 50 meV < m ν < 2200 meV

37


Distinguishing the Mass Hierarchy in 0νββ

Δmatm2

Δm2

38


Several Proposed Experiments

COBRA Te-130 10 kg CdTe semiconductorsDCBA Nd-150 20 kg Nd layers between tracking chambersNEMO Mo-100, Various 10 kg of ββ isotopes (7 kg of Mo)CAMEO Cd-114 1 t CdWO4 crystals

CANDLES Ca-48 Several tons CaF2 crystals in liquid scint.

CUORE Te-130 750 kg TeO2 bolometers

EXO Xe-136 1 ton Xe TPC (gas or liquid)GEM Ge-76 1 ton Ge diodes in liquid nitrogenGENIUS Ge-76 1 ton Ge diodes in liquid nitrogenGSO Gd-160 2 t Gd2SiO5:Ce crystal scint. in liquid scint.

Majorana Ge-76 500 kg Ge diodesMOON Mo-100 Mo sheets between plastic scint., or liq. scint.Xe Xe-136 1.56 t of Xe in liq. Scint.XMASS Xe-136 10 t of liquid Xe

39


Selected Proposals for 0νββ Experiments

Proposed ton-year= M * T * ε

Anticipated<mee>, (QRPA)

CUORE 0.21*5*1 = 1 60 meV

EXO 6.5*10*0.7 = 45 13 meV

GENIUS 1*2*1 = 2 20 meV

MAJORANA 0.5*10*1 = 5 25 meV

MOON 3.3*3*0.14 = 1.4 30 meV

The <mββ> limits depend on background assumptions andmatrix elements which vary from proposal to proposal.

40


Cosmological Information on Neutrino Mass

large scale structure formation

WMAP

Cosmological neutrino mass limits probe Dirac and Majorana ν masses!

Mass limits comparable to 0νββ experiments.

Neutrinosʼ contribution to the Universeʼs energy density Ωνh2=Σimi/95.3 eV

Combining WMAP and large scale structure Ωνh2<0.0076 eV (95% CL)

If mνe ~ mντ (degenerate neutrino species) m ν < 0.23 eV

41


Counting Neutrinos in the Big Bang

42


Number of Neutrinos from LEP

43


Supernova Neutrinos

All arrived within about ~13 s after traveling 180,000 light years with energies that differed by up to a factor of three. Neutrinos arrived about 18 hours before the light was seen.

In a supernova explosion Neutrinos escape before the photonsNeutrinos carry away ~99% of the energyThe rate of escape for νe is different from νµ and ντ (Due extra νe CC interactions with electrons)

Neutrino mass limit can be obtained by the spread in the propagation time–tobs-temit = t0 (1 + m2/2E2 )–Spread in arrival timesif m≠0 due to ΔE

–For SN1987a assuming emission time is over 4 sec mν < ~30 eV

44


What about a Neutrino Magnetic Moment?

€

dσdTe

= weak int+ πα 2µν2

me2

1Te−1Eν

Electron Recoil T (MeV)

νe- e- from U235 at a reactor

45


V=1 m3

L=1.6 mD=0.9 cm

Low Electron Recoil Energy Experiment

Time Projection Chamber

Experiment at Nuclear Reactors (low energy source of νe)

High density, relatively low Z, good drifting properties

46


Neutrino Magnetic Moment

Experimental Results

Reactor Experiments

UC Irvine µνreac < 2-4 x10-10 µB

Kurchatov µνreac < 2.4 x10-10 µB

Rovno µνreac < 1.9 x10-10 µB

MUNU µνreac < 1.0 x10-10 µB (90% CL)

Solar (Anti)Neutrino Experiments Super-Kamiokande µν

sol < 1.5 x10-10 µB KamLAND

47


Theoretical Prejudices before 1995

Natural scale for Δm2 ~ 10 – 100 eV2 since needed to explain dark matter

Oscillation mixing angles must be small like the quark mixing angles

Atmospheric neutrino anomaly must be other physics or experimental problembecause it needs such a large mixing angle

LSND result doesnʼt fit in so must not be an oscillation signal

In 2008 we know ….

Wrong

Wrong

Wrong

(Wrong)

48


Interdependencies/Redundancies of Experiments

reactor + accelerator

Need all types of experiments

49


Particle Properties of the Neutrino

Interactions weak (and gravitational) onlyFlavors 3 active flavors sterile flavors?Charge

Spin s=1/2

Type Dirac ν ≠ ν Majorana ν = νMass m νe < 2 eV from tritium β decay m νµ < 170 keV from π decay m ντ < 18 MeV from τ decay

?

?

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lecture 12 - neutrino properties - experimental nuclear physics

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