Experimental Neutrino Physics - PowerPoint Presentation

Slide1

Experimental Neutrino Physics

Susan CartwrightUniversity of SheffieldSlide2

Introduction:

Massive neutrinos in the Standard Model

Dirac and

Majorana massesThe mixing matrix and neutrino oscillations Slide3

Massive neutrinos in the

Standard Model

In the original Standard Model, neutrinos are two-component spinors with mass exactly zerodisproved by existence of neutrino oscillations—see later

There are two ways to add a neutrino mass term to the SM LagrangianDirac:

exactly like other

fermion

masses

Majorana

:where νc = Cν̄T = Cγ0ν*different chiral states need not have same mass in this case neutrino and “antineutrino” same particle, different chiralitySlide4

Seesaw mechanism

General mass term has both Dirac and Majorana

components:

If , we can diagonalise matrix to get eigenstates

with masses

naturally small mass for LH state Slide5

Neutrino oscillations

If neutrinos have mass, then they can be described in terms of mass

eigenstates as well as weak (flavour) eigenstates

no reason why these should align (and they don’t), so we have a 3×3 unitary mixing matrix (the PMNS matrix) U:

mass

eigenstates

propagate according to

if

c = 1 and v ≈ c (and hence L ≈ t)therefore even if |ν(0)⟩ is a pure flavour state, |ν(L)

⟩ is notSlide6

Neutrino oscillations

Probability of observing neutrinos of flavour ℓ' at distance

L (in vacuo) from a beam of initial flavour ℓ

:For two-flavour case we have

giving

therefore key variable for experiments is

L

/

E

can’t measure absolute massesSlide7

Matter effects

ν

e

interact with electrons via W exchange; νμ

,

ν

τ

do not

This leads to an increased effective mass for a νe-dominated state in dense matteras ν propagates out through decreasing density, effective mass drops, eventually crossing another eigenvalueresulting resonant conversion can greatly enhance oscillationcritical for solar neutrinos, significant for long baseline terrestrial too

MSW effect—sees sign of ΔM

2Slide8

Theory Summary

Non-zero neutrino masses imply

neutrino oscillation (change of effective flavour)hence, non-conservation of lepton family

numberthis is experimentally establishedalso, if 3×3 mixing, non-conservation of CPimaginary phase δ

in PMNS matrix

not

established yet

if

non-zero Majorana mass, ν = ν̄hence, non-conservation of global lepton numbernot establishedExperimental tasksdetermine oscillation parameters (Δm2, θ, δ

)measure neutrino massesSlide9

Neutrino oscillations

Principles of oscillation measurements

Solar neutrinos,

θ12

Atmospheric neutrinos,

θ

23

New measurements of

θ13Slide10

Principles of oscillation measurements

Relevant physical properties are

Δm2ij

and θijExperiment parameters are

L

,

E

and initial flavour

e, μbut physical parameter is L/E, so result is conversion probability P(L/E), giving contour on Δm2 – sin2 2θ plane

Two distinct experimental techniquesdisappearance experiments look for reduction in flux of original flavouronly possibility for very low-energy neutrinos (reactor

ν̄

e

, solar

ν

e

)

appearance experiments

look for converted flavour

e.g.

ν

e

events from a

ν

μ

beamSlide11

The PMNS matrix

atmosphericneutrinos

ν

μ

↔

ν

τ

solar neutrinos

ν

e

↔

ν

X

reactor &

accel

. neutrinos

ν

μ

↔

ν

e

need all three mixing angles to be non-zero

for CP violation to be possibleSlide12

Solar neutrinos

Produced as by-product of hydrogen fusion

4 1H →

4He + 2e

+

+ 2

ν

e

reaction goes by many paths which produceneutrinos of differentenergiesinitial flavour state νe

as too little energy toproduce

μ

,

τ

Detected by inverse

β

decay, elastic scattering,

or dissociation of

2

H

http://www.kip.uni-heidelberg.de/tt_detektoren/neutrinos.php?lang=enSlide13

Solar neutrinos:

θ12, Δm

212

requires L/E ~ 30 km/

MeV

in vacuum

experimental approaches:

solar neutrinos:

νe → νX disappearanceresonant conversion via MSW effect in solar interiorexpected flux calculated from models of solar luminosity (John Bahcall

et al.)experimental normalisation via NC reactions (SNO)reactor neutrinos:

ν̄

e

→

ν̅

X

disappearance

requires long-baseline experiment owing to small

Δ

m

2

expected flux from known reactor power outputSlide14

Solar neutrinos:

θ12, Δm

212

NC: d +

ν

X

→

p + n +

ν

X

CC: d +

ν

e

→

p + p + e

−

ES: e

−

+

ν

X

→

e

−

+

ν

X

Slide15

Solar neutrinos:

θ

12, Δ

m212

3-flavour analysis of

SNO+KamLAND

data gives

(

arXiv 1109.0763)Slide16

Solar neutrinos: new results

Measurement of 7Be and

pep flux by Borexinoline fluxes, therefore potentially more informative about energy dependence

Also no day-night asymmetryexcludes low Δ

m

2

region of plane

this exclusion previouslydepended on reactordata (ν̄)Slide17

Atmospheric neutrinos:

θ23, Δ

m223

Initially studied using neutrinos produced in cosmic-ray showersincident proton or heavier nucleus produces

pions

which decay to

μ

+

νμsome of the muons then also decay (to e + νe + νμ)if they all do so then νμ:νe ratio ~ 2Also addressed by accelerator-generated neutrino beamsessentially identical process: collide proton beam from accelerator with target, collimate produced

pions with magnetic horns, allow to decay in flightmagnets select charge of pion, hence either

ν

μ

or

ν̄

μ

beamSlide18

Atmospheric and accelerator ν

s

−1 0 +1 −1 0 +1

cos

(zenith angle)Slide19

Atmospheric neutrinos:

θ

23

, Δm223

MINOS combined fit

3-flavour global fit

first time that best fit

θ

23 ≠ 45°!Slide20

Third mixing angle

θ13

Absence of signal in

ν

e

shows that

atmospheric mixing is

ν

μ → ντmeasurements of ντ appearance in OPERA and Super-K are statistics-limited

but in qualitative agreement with this Therefore 3

rd

mixing angle

θ

13

involves

ν

e

can be seen in

ν

e

appearance in

ν

μ

beam or

ν̅

e

disappearance from reactors

because

Δ

m

2

13

=

Δ

m

2

23

±

Δ

m

2

12

and

Δ

m

2

12

≪

Δm223,

ν

μ

disappearance always dominated by

θ

23Slide21

νe

appearance:

Off-axis geometry produces lower-energy,

much more monochromatic

beam

T2K beam is 2.5° off-axis—optimised for oscillation measurementSlide22

T2K analysis

Super-Kamiokande measures Cherenkov radiation from e/

μ

produced in interaction

can distinguish the two based on ring morphology

sharp

muon

ring

fuzzy electron ring

11 events observed

3.22±0.43 expected

3.2

σ

effectSlide23

ν

̅

e disappearance:

Multiple detectors associated with extended reactor complexν̅

e

detected via inverse

β

decay

in Gd-loaded liquid scintillatorSlide24

Daya Bay analysis

Large difference between

Δm213 and

Δm212 means that L

/

E

can be “tuned” for

θ

13no ambiguity—simple 2-flavour system“near” and “far” detectorsidentical to minimise systematicsfar/near ratio R = 0.940 ± 0.011 ± 0.0045.2σ effectSlide25

ν̅

e disappearance:

Detector design and analysis very similar to Daya BayResults very similar too:

R = 0.920 ± 0.009 ± 0.0144.9σ effectSlide26

Results for

θ13

T2K :Daya Bay:

sin2 2θ

13

= 0.092 ± 0.016 ± 0.005

Reno:

sin

2 2θ13 = 0.113 ± 0.013 ± 0.019Global fit (Fogli et al. arXiv 1205.5254):

measurement of Δm2

still best done by

combining solar & atmospheric

Δ

m

2Slide27

Open questions

We know the ν

e-dominated state m1 is lighter than

m2 (from MSW effect), but we still don’t know if m

3

>

m

2

(normal hierarchy) or vice versa (inverted hierarchy)longer baseline experiments, e.g. NOνA, should be able to sort this out via matter effects in EarthConstraints on phase δ are very weakcan be constrained by antineutrino running and/or matter effects (NOν

A again)Slide28

Effect on models

Tri-bimaximal

mixing predicts

and hence θ

13

= 0, which it clearly isn’t.

Theorists are of course trying to rescue this with perturbations of various kinds

The current hint that

θ23 ≠ 45° is also inconsistent with tri-bimaximal predictionsSlide29

Neutrino masses

Tritium beta-decay

Neutrinoless

double beta decayAstrophysical constraintsSlide30

Neutrino mass: β

decay

Basic principle: observe electron spectrum of β-decay very close to endpoint

presence of mc2 term for neutrino will affect thisunfortunately not by much!!

tritium (

3

H) favoured because of combination of low

Q

-value (18.57 keV) and shortish half-life (12.3 y)Slide31

β decay status and prospects

Best efforts so far by Mainz and

Troitsk experiments of late 90s:

Two experiments in pipeline should do much betterKATRIN—tritium decay experiment with planned sensitivity ~0.2 eV

MARE—rhenium-187 experiment, similar reach

187

Re has very low

Q

-value but extremely long half-lifeMARE uses single-crystal bolometers to get good energy resolution and measure differential spectrumVery hard experiments: don’t expect results for ~5 yearsSlide32

Neutrinoless

double-

β decay

Even-even isobars are lighter than odd-odd (pairing term)can be energetically permitted for nucleus (

Z

,

A

) to decay to (

Z±2, A) but not (Z±1, A)these decays do happen through conventional ββ2ν mode, albeit at very low ratelifetime ≫ age of universeif neutrino is

Majorana particle, can also happen with no neutrino emission, ββ0νSlide33

Key features

Violates lepton number by 2 units

possible relevance to

baryogenesisSensitive to

PMNS matrix multiplied by

diag

(1,

e

iα, eiβ) introducing two additional phases Rate for SM, amplitude ∝ mi/q2 where m ~ 0.5

eV and q ~ 108

eV

small!

nuclear matrix element

M

is a major systematic error

theoretical calculations disagree by factors of 2 or moreSlide34

Relation of

⟨mee⟩ to lightest m

ν

hep-ph/1206.2560Slide35

Experimental issues

Signature: (A, Z

) → (

A, Z

+2) + 2e

−

, so

E

(e−) = Q/2 —spike at energy endpointp(e1

) = −p(e

2

) —electrons are back to back

Two experimental approaches

source = detector

calorimetric; energy signature

target isotope fixed

source ≠ detector

tracker; topological signature

target isotope variable

Most

ββ

isotopes are only ~10% of natural element.

Enrichment is often needed.Slide36

232

Th

60

CoSlide37

Experimental results

Best results probe down to

⟨mee⟩

~ 0.2-0.6 eV not yet quite in rangeof interesting limits

Next few years:

improvement of ~ ×10

EXO-1000, CUORE,

KamLAND

-ZEN,GERDA/MAJORANAall hoping for ~0.02-0.06 eVSlide38

Effect on models

Sensitivity to hierarchy: IH

implies accessible minimummass (∝

Δm2

13

)

within reach of next generation

Non

-observation possible even with Majorana neutrinosin NH masses and phases can conspire to cancel effect Conversely, ββ0ν decays can be driven by mechanisms other

than Majorana mass (e.g. LR symmetry)

such mechanisms do imply that the neutrino

has

a

Majorana

mass, but it can be very smallSlide39

Astrophysical constraints on mν

Number density of relic neutrinos from early universe (C

νB) is 112 per species per cm3

these are hot dark matter and will affect structure formation—hence leave astrophysical signaturesSensitive to ∑mν,

which is bounded below by oscillations

Δ

m

223 ~ 0.0024 eV2 means ∑mν ≥ 0.05 eVbounds are within factorof 10 and will improve soon(e.g. Planck)Slide40

Model dependence

Quoted constraints assume

flat geometryexactly 3 neutrino species with Tν

= (4/11)1/3 TCMB

dark energy is a cosmological constant

There are correlations between ∑

m

ν

and other parametersWMAP5Slide41

Comparing different measures

ββ

0

ν

Cosmology

yes

no

yes

noKATRINyesQD+M

QD+D

QD

N-S C

no

N-S I

low IH/

NH/D

m

< 0.1

eV

/N-S C

NH

ββ

0

ν

yes

(IH/QD)+M

N-S

C/I

no

low IH/(QD+D)

NH

D = Dirac; M =

Majorana

; QD = quasi-degenerate; NH/IH = normal/inverted;

N-S C = non-standard cosmology; N-S I = non-standard interpretation of

ββ

0

ν

Assumes sensitivities of

m

β

= 0.2

eV

,

⟨

m

ee

⟩

= 0.02

eV

,

∑

m

ν

=

0.1

eV

W.

Rodejohann

, hep-ph/1206.2560

Conclusion: it does help to have multiple approaches.Slide42

Important things I don’t have time to discuss!

Non-standard interpretations of

ββ

0ν

Light sterile neutrinos

Neutrino astrophysics and cosmologySlide43

Summary and Conclusion

All 3 neutrino mixing angles are definitely non-zero—

naïve tri-

bimaximal mixing ruled outConstraints on

δ

and determination of hierarchy should be possible with next generation of oscillation experiments

Experimental limits on neutrino mass do not currently compete with cosmological constraints, but next decade should see

complementarity

developingPhysics of massive neutrinos is a rich and interesting field!Slide44

BackupsSlide45

Solar neutrinosSlide46

Double beta decaySlide47

Hot dark matter

Hannestad

et al.,

astro-ph/1004.0695

WMAP7 + halo power spectrum

WMAP7 + HPS + H

0

Giusarma

et al., astro-ph/1102.4774

Sterile neutrinos and axions can also contribute to hot dark matter