Y2 Neutrino Physics (spring term 2016)

Dr E Goudzovski eg@hep.ph.bham.ac.uk

http://epweb2.ph.bham.ac.uk/user/goudzovski/Y2neutrino

Lecture 11

Reactor neutrino experiments. Lepton flavour and number violation.

Previous lecture

1

Modern accelerator-based experiments typically use

GeV energy beams, ~1000 km baselines, near and far detectors.

Observables: survival probability; e, appearance probabilities.

Neutrino detection techniques used by accelerator experiments:

sampling calorimeters (steel/scint, Pb/emulsion); water Cherenkov.

[Liquid Argon detectors are not discussed]

Accelerator experiments provided precision measurements

of the atmospheric mass splitting m232 and the mixing angles

23 (via disappearance) and 13 (via e disappearance).

This lecture

2

Physics programme of reactor experiments.

Results from the Daya Bay and KamLAND experiments.

Oscillation-induced charged lepton flavour violation.

Dirac and Majorana neutrinos.

Neutrinoless double beta decay.

Open problems in neutrino physics.

Reading list:

“Neutrino experiments with reactors” and

“Neutrinoless double beta decay” in Soler et al.

B.R. Martin and G. Shaw. Particle physics. Chapters 2.3, 11.6.

D. Perkins. Introduction to HEP. Chapters 7.6, 9.6, 9.7.

C. Sutton. Spaceship neutrino. Chapter 3.

N. Solomey. Elusive neutrino. Chapters 3, 6.

Reactor experiments

3

Reactors emit anti-e with energies of a few MeV. [Lecture 4]

Below thresholds for +, + production: appearance experiments not possible.

Disappearance reactor experiments:

Slow “solar” oscillations: note the 3-flavour formalism.

12=34°; m212=8×105 eV2.

Fast “atmospheric” oscillations:

13=9° (small amplitude); m213=2×103 eV2.

Oscillation lengths for the mean reactor energy (E=3.6 MeV):

Both distance

scales are

accessible

The two distance scales

4

Slow “solar” oscillations (12; m212).

Need a very powerful source and

a large detector, because Flux~1/L2.

Electron antineutrino survival probability vs distance (E=3 MeV)

Distance L, km

Near and far detectors for

measuring the fast “atmospheric”

oscillations: (13, m213).

The effect is small due to the

“wrong” flavour (e) at the source.

~1/m213

sin2(213)

~1/m212

sin2(212)

5

Antineutrino energy measurement IBD signature:

prompt signal from the positron annihilation +

delayed signal from the neutron capture

Positron detection: via annihilation

Neutron detection:

via thermalization & capture, e.g.

p

e

p

Energy conservation (neglecting n recoil ~10 keV):

“Prompt energy” deposited by positron:

Therefore,

Direct measurement of the antineutrino energy

6

Daya Bay (China)

Phys.Rev.Lett. 112 (2014) 061801

Discovery of non-zero 13 in 2012.

Recent results: measurements

Disappearance measurements:

fast atmospheric oscillations

KamLAND experiment (Japan)

Power plants in Japan

7

Need a very powerful source

and a large detector.

Ideal place: Japan

(~50 reactors within ~200 km).

“Solar” oscillations with reactors:

oscillation length of ~35 km;

flux falls as 1/L2.

Detector: 1 kilotonne of liquid scintillator

watched by ~2000 PMTs.

Located underground in the Kamioka lab.

Surrounded by a water Cherenkov detector

to absorb neutrons and tag cosmic muons.

Data taking: 20022007.

Nylon balloon with

liquid scintillator (13m diameter)

KamLAND results

8

Prompt energy (=E0.8 MeV), MeV

Survival probability Pee

Phys.Rev. D100 (2008) 221803

Prompt energy

tan212

m

212,

eV

2

First hint for geo-neutrinos.

Clear signal for e disappearance

at the large (“solar”) distance scale.

Two oscillation maxima observed.

The first accurate measurement

of the solar mass splitting m212.

Effective baseline: L0=180km

Oscillation-induced LFV

9

W W

(range of the weak force

by the uncertainty principle)

Forbidden in the Standard Model by

lepton number conservation.

Allowed in the extended SM (m0)

via oscillation of a virtual neutrino.

(oscillation length

in natural units)

(=1/137: electromagnetic coupling constant)

(maximum mixing assumed)

2.4×103 eV2 80 GeV 1.8 GeV Totally negligible;

out of experimental reach

A “loop diagram”

(atmospheric

mass splitting)

Neutrino & antineutrino (1)

10

Charged particles: antiparticle is different from particle

(opposite electric charge quantum number).

What about the neutrino ?

Neutral baryons: antiparticle is different from particle

(opposite baryon number).

Some particles are fully neutral, i.e. identical to their particles.

Examples:

?

Lepton helicity

11

H=+1: “right-handed”

H=1: “left-handed”

Spin structure of the weak interaction:

Ultra-relativistic leptons (antileptons) are mostly left- (right-) handed.

The “forbidden” helicity state is suppressed by a factor

In the SM, right-handed neutrinos (and left-handed antineutrinos)

do not take part in weak interactions

Helicity: p s

s

Spin of a particle is quantised along the direction of motion:

+½ and ½ for a spin-½ particle.

For a Standard Model neutrino,

Leptonic pion and kaon decays

12

+(K+) e+,+ e,

s se,

S = 0; H = 1 therefore He, = 1

Muon is non-relativistic:

At first order,

Excellent agreement with experiment (~0.1% precision) This determines atmospheric composition, accelerator production.

( )

No strong suppression of

H = 1 helicity state

~ 104

Positron is highly

relativistic: : He = 1 is suppressed

~ 2 ~ (m/me)2 ~ 104

Helicity factor ~105 Kinematic factor: not discussed here

Neutrino & antineutrino (2)

13

SM neutrino is left-handed and SM antineutrino is right-handed:

irrespectively of whether they are distinct particles.

For massless neutrinos (m=0), the two options are indistinguishable.

For m>0, the “forbidden” helicity states exist: ~ (m/E)2.

In the Majorana case with m0, to transition is possible.

Physics beyond the Standard Model: lepton number violation.

Two possibilities

1) Dirac neutrino:

and are different particles;

2) Majorana neutrino:

and are different helicity states of the same particle.

Double beta-decay

14

Double beta-decay:

can be observable if the beta-decay

is forbidden by energy conservation,

Second-order process in the weak interaction (rate ~ ).

Experimentally observable in even-Z, even-N nuclei

(stable against beta-decay due to spin-coupling).

Typical lifetimes: T1/2~1020 years (cf. age of Universe ~1010 years)

Observed for over 10 isotopes.

The first directly observed DBD (1987):

Neutrinoless double beta-decay

15

Neutrinoless double beta-decay:

Kinematics: 2 and 0

Decay rate:

“Effective Majorana neutrino mass”:

Physics beyond the Standard Model:

violation of lepton number by two units.

For ,

expected lifetimes are 1/2~1026 years.

Extremely rare: ~0.01 decay/year/kg.

Not observed (experimental upper limits

on 1/2 are ~1025 years): Majorana mass upper limit ~1 eV/c2.

If observed, strong evidence for Majorana nature of the neutrino.

e

e

e

W

W

e

Virtual massive

Majorana

neutrino

Total energy of two electrons

The 6 fundamental parameters

16

Solar & reactor

Atmospheric & accelerator

Reactor & accelerator

Future experiments

Types of experiments Parameters

Open problems:

What is the value of ? Do properties of neutrinos and antineutrinos differ?

Is that related to matter-antimatter asymmetry of the universe?

What is the absolute mass scale of the neutrino?

Why are neutrino masses so small (<1 eV)?

Are there additional neutrino flavour and mass states?

Are neutrinos Dirac or Majorana particles?

The unknown CP-violating phase

Neutrino mass hierarchy

17

Two distinct oscillation mass scales occur in nature:

• Solar oscillations: |m2|sol 8×105 eV2;

• Atmospheric oscillations: |m2|atm 2×103 eV2.

Two possible assignments of the mass hierarchy:

m2atm 30m2

sol

Absolute mass scale and hierarchy are still unknown.

A possibility: degenerate hierarchy (m21,2,3≫m2

atm).

Normal Inverted

m2sol

m2atm OR

m2atm

m2sol

e

|m231| |m2

32|

Three mass states: therefore 3 mass differences (mij).

Summary

18

Reactor experiments have confirmed both “atmospheric”

(Daya Bay) and “solar” (KamLAND) oscillations by observing

electron anti-neutrino disappearance.

Non-zero neutrino masses induce lepton flavour violation

in the charged lepton sector (but it is tiny).

Massive Majorana neutrinos lead to physics beyond the SM:

the lepton number violating neutrinoless double beta decay.

Many fundamental questions in neutrino physics

remain unanswered.

The revision lecture: Wed 27 April at 10:00, LAW LT2

The default format: 5-minute summaries of each lecture.

Alternatively, could work through particular problems (please let me know your preferences by 22 April).