Y2 Neutrino Physics (spring term 2017)

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

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

Lecture 7

Atmospheric neutrino experiments

Previous lecture

1

The most precise neutrino “absolute” mass measurements

come from studies of the 3H beta-decay spectrum near the endpoint.

The absolute neutrino mass scale: unknown but below 2 eV/c2.

Neutrino mixing leads to the flavour oscillations phenomenon.

Oscillations have been observed. Therefore the neutrinos are

massive, and lepton flavour is not conserved at large distances.

Two-flavour oscillations are described by two fundamental

parameters: the mass splitting m2 and the mixing angle .

Principal concepts: oscillation length, oscillation maxima,

appearance and survival probabilities.

Three-flavour oscillations are described by the

neutrino mixing matrix.

This lecture

2

Atmospheric neutrinos and their flavour composition.

The Cherenkov effect.

Water Cherenkov neutrino detectors; Super-Kamiokande.

Observables sensitive to atmospheric neutrino oscillations.

Evidence of atmospheric neutrino oscillations.

Cherenkov neutrino telescopes.

Reading list:

K. Kleinknecht. Detectors for particle radiation. Chapter 5.3.

D. Perkins. Introduction to high energy physics. Chapter 9.7.2.

“Atmospheric neutrinos” in Soler et al.

Journal articles: see course webpage.

Cosmic rays

3

Primary cosmic rays:

mainly high energy protons and 4He

from astrophysical sources

(including supernovae).

Secondary cosmic rays (air showers):

light particles produced in

inelastic interactions with air

(N, O nuclei).

Main source of atmospheric neutrinos:

decays of secondary pions and kaons

p

Atmospheric neutrinos

4

Main source of atmospheric neutrinos:

At energies E<1 GeV, most and decay:

Highly energetic muons reach the ground

(and even penetrate deep underground):

(relativistic time dilation) Mean free path:

E=1 GeV muons:

E=1 GeV pions: (atmosphere thickness: ~10 km)

Atmospheric neutrino properties

5

Computed muon (anti)neutrino flux

1 10 102 103

E, GeV

0.1

Flavour composition

Neutrino flux: ~10 cm2 s1.

Directionality: roughly isotropic, up-down symmetric.

Typical energy: ~1 GeV, wide energy range.

Typical uncertainty on the calculated atmospheric neutrino flux: ±20%.

Uncertainty of the /e ratio: ±3%.

E, GeV

Insufficient time

for muon decay

Atmospheric neutrino detection

6

Charged current reactions (inverse beta decays)

Thresholds are not crucial for atmospheric neutrinos:

E(e)=1.8 MeV, E()=110 MeV, E()=3.5 GeV.

A possibility: multi-kilotonne water Cherenkov detectors

Comparison with reactor experiments (see lecture 4)

Larger cross-section than for reactor neutrinos:

(E=1GeV)~1038 cm2 vs (E=1MeV)~1043 cm2.

Lower neutrino flux than in reactor experiments:

ATM~10 cm2 s1 vs REACTOR~1013 cm2 s1.

Therefore neutrino interaction rates per nucleon (~) are ~107 lower:

FATM~0.1/year/tonne vs FREACTOR~100/hour/tonne.

W

Requirement to oscillation experiments:

flavour identification of the charged leptons produced.

Cherenkov effect

7

Cherenkov radiation is emitted when a charged particle

passes through a dielectric medium at a speed greater than

the speed of light in that medium (>1/n).

Emission angle wrt particle direction

is fixed (the “Cherenkov angle”):

Threshold velocity:

Example: water (n=1.33).

Threshold velocity: min = 1/n = 0.75; maximum angle: max = 41o.

Measurement of velocity . If momentum known, compute mass:

Cherenkov particle identification. Not sensitive to particle charge (+/–).

For highly relativistic particles (1),

C

C

(n: refractive index)

Cherenkov photon spectrum

8

Wavelength, nm

Typical Cherenkov photon spectrum

FrankTamm spectrum

with chromatic dispersion

Dispersion

neglected

“red” “violet”

(a gas Cherenkov detector co-developed by the Birmingham group

for the NA62 experiment at CERN)

FrankTamm formula:

(Dispersion: the refractive index n depends on wavelength)

Cherenkov light emission

near a nuclear reactor:

(www.spectrum.ieee.org/image/37182)

dx: path; d: wavelength interval; Dominated by low wavelength; Integrated intensity is determined by the Cherenkov angle C.

Arb

itra

ry s

cale

Water Cherenkov detectors

9

41°

Neutrino detection in WCD

Highly relativistic charged leptons (max)

produce ~40 photons/cm.

Ionization energy loss by a muon: ~2 MeV/cm.

Dimension of the largest detector: 40 m.

Charged lepton traverses it in ~100 ns,

emitting ~105 Cherenkov photons.

Photons detected by PMTs along the walls (time resolution ~1 ns). a series of “Cherenkov rings”

Measurement of speed & direction

of the charged lepton. e,

Neutrino and charged lepton directions

are correlated. Typical angular resolution:

25° at E = 1 GeV; 0 for higher E.

e,

(application for atmospheric neutrinos)

(GeV energy range)

Inverse beta decays:

Light sensitive detectors (PMTs)

Lig

ht

sensi

tive d

ete

cto

rs (

PM

Ts)

Lig

ht

sensi

tive d

ete

cto

rs (

PM

Ts)

Super-Kamiokande

10 Photos: © Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo

The largest water Cherenkov detector built so far

Cosmic ray shield (Mt. Ikeno,

Japanese Alps): 1000 m below surface.

50k tonnes of pure H2O.

In operation since 1996.

inner detector,

“fiducial volume”

outer detector to

veto external activity

Super-K (1)

11 World’s largest 20’’ diameter PMTs

Super-K (2)

12

© Kamioka Observatory,

ICRR (Institute for Cosmic Ray Research),

The University of Tokyo

Super-K Cherenkov images

13

603 MeV muon:

sharply defined

ring edge

Super-K Cherenkov images

14

492 MeV electron:

diffused ring edge

due to multiple

scattering

NB: leptons

are not observable

due to the short lifetime

Lepton flavour

identification

(e vs )

Atmospheric : observables

15

(1) Oscillation is the change of neutrino flavour.

The /e flux ratio in the absence of oscillations is known.

Therefore, the simplest observable is the measured /e flux ratio.

(2) Oscillations depend on the travelled path L.

Downward-going : ~10 km path.

Upward-going : ~ 2REarth ~ 13000 km path.

Observables: up-down asymmetries of and e fluxes.

Generalization: zenith angle (z) distribution of and e.

(3) Oscillations depend on the path-to-energy ratio.

Observable: path-to-energy ratio (L/E) distribution of and e.

Appearance probability

(two-favour case):

The goal: search for neutrino oscillations

(1) Muon/electron flux ratio

16

Measured/expected ratio

Muon/electron neutrino flux ratio

Experi

ment

The “atmospheric neutrino anomaly”: deficit of atmospheric

muon neutrinos (), an early hint for oscillations (1980s).

Inconclusive: could be e.g. proton decays in the detector ( )

(lepton number violation; emission of a ~300 MeV positron)

(2a) Up-down asymmetry

17

Oscillation length for a 1 GeV atmospheric neutrino:

L0 ≫ atmosphere thickness (~10 km).

L0 ≪ Earth diameter (~13000 km).

Therefore the Earth is an ideally sized lab!

Downward-going neutrinos do not oscillate: survival prob.

Upward-going neutrinos undergo many oscillation cycles.

(except for the minority at extremely high energies above ~100 GeV)

Survival probability for the upward-going neutrinos:

Averaged over L/E

(the effect of “solar” oscillations, L0~104 km, cancels accidentally)

(m2 known from

experiment)

Upward-going neutrinos

18

(Half of the original flavour neutrinos survive even at maximal mixing)

Indefinite integral of f(x) = sin2x:

Mean value of a function:

Mean value of f(x) = sin2x over a period (0<x<):

Survival probability for upward-going neutrinos:

(2b) Zenith angle z

19

Distance between neutrino

production point and detector L

depends on zenith angle z

Uncertainty on the production point

(~atmosphere thickness): L=5km.

A non-assessed problem: obtain the L = f(cosz) dependence

Earth

Detector

Atmosphere

local vertical axis

Downward-going :

z = 0

Upward-going:

z = 180°

Horizontal :

z = 90°

Generalization of the “up/down” approach

–1 –0.5 0 0.5 1

104

103

102

10

Distance vs zenith angle

cosz

L ,k

m

L = f(cosz)

downward-

going

upward-

going

0º z 180º

+1 ≥ cos z ≥ 1

~10 km L 13000 km

The expected oscillation pattern

20

Horizontal neutrinos Upward-going neutrinos

z measurements at Super-K

21

Data and best fit

to oscillations

Prediction:

no oscillations

Up-down flux asymmetry.

Deficit of upward-going .

No excess or deficit of e.

The data are compatible

to oscillations

“Sub-GeV” = low energy and worse directional correlation

downward-

going upward-

going

downward-

going upward-

going

(3) L/E analysis at Super-K

22

L/E (km/GeV)

Muon (anti)neutrino counts:

data/(expectation for no oscillations)

The dip = first oscillation maximum.

Higher maxima not visible due to insufficient resolution.

Fits to decay and decoherence

models fail to explain the data

Fit to oscillations

No oscillations 400 km/GeV

Oscillation parameters

23

Atmospheric neutrinos: disappearance experiments

(MINOS and T2K accelerator experiments will be discussed later)

Maximum mixing:

sin2(223) 1

(almost complete

disappearance

at oscillation maxima)

Experimentally, m2atm = (2.40.1)×103 eV2 and atm = (457)°

R. Nichol @ Neutrino 2012, Kyoto, Japan

Megatonne & gigatonne detectors

24

Megatonne water Cherenkov experiments:

Hyper-Kamiokande in Japan might be operational by 2023

(~25 times Super-Kamiokande mass; several identical water tanks).

Gigatonne neutrino telescopes using natural Cherenkov radiators:

WATER ICE

Large PMT arrays operate in the

Mediterranean Sea and at Lake Baikal

IceCube neutrino telescope

25

World’s largest neutrino detector:

1 km3 (0.9 Gigatonne) of

Antarctic ice instrumented with PMTs.

IceCube location at the South Pole

Photodetector module

~100 m spacing between PMTs: sensitive mainly to

high-energy astrophysical neutrinos (100 GeV<E<1 PeV).

DeepCore: atmospheric neutrinos (1 GeV<E<100 GeV).

26

Neutrino detection via

Cherenkov light emitted by a muon

A PeV (1015 eV) energy neutrino event

Excellent angular resolution:

moon shadow in the cosmic rays

seen with TeV (=1012 eV) muons

IceCube neutrino telescope

Summary

27

Atmospheric energy spectrum: wide, GeV range; flavour

composition: /e2 at GeV energy, /e>2 at higher energy.

The leading atmospheric (and astrophysical) neutrino detection

technology: water and ice Cherenkov detectors.

Observables sensitive to oscillations: muon/electron ratio,

up-down asymmetries, zenith angle (z) and L/E distributions.

Atmospheric neutrino observations are consistent with

oscillations with m2=2.4×103 eV2 and

near-maximal mixing (atm 45o). [Oscillations established in 1998]