# Neutrino detection

Neutrino detection

Neutrino detection

Neutrinos are very special:

there is a lot of them around but they are very difficult

to detect

Neutrino interactions (cross sections)

Going underground with detectors

Techniques of very large detectors

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Cross sections

A cross section σ is a measure of interaction probability.

A number of interactions is:

N int = N target * σ *

where is a flux ( cm -2 ) of the projectile particles.

Hence σ is measured in cm 2 . It can be seen as an effective surface of

a target.

For strong interactions is ~1barn=10 24 cm 2

Differential cross sections:

distributions of energies and angles of secondary particles

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Cross section – Fermi theory

Propagator:

For small 4-momentum

transfer q:

dq 2 g 1 f (q)g 2

2 ⎛ g ≈ 1

g 2

⎜ 2

⎝ M W

2

≡ G F

2

lepton universality:

g 1

= g 2

≡ g

Fermi coupling constant:

contact

interaction

c = 1 = 200 MeV ⋅fm ⇒ G F 2 = 5.5⋅10 −38 cm 2

GeV 2

Neutrino cross sections CC

For the CC interaction: ν e

+ e − → ν e

+ e −

2

dq 2 G F

when integrating from

q 2 2

= 0 to q max

= 4 p 2 = s

σ = G 2 s

F

π

E ν

M ⇒ s = 2 ME ν

σ = 2G 2

F

π

M ⋅ E ν

G F 2 = 5.5⋅10 −38 cm 2

GeV 2

Similarily one can get for:

σ = 1.7 ⋅10 −41 cm 2

E ν

GeV 2

but here a proton formfactor has to be included

σ 1⋅10 −38 cm 2

E ν

GeV 2

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Neutrino cross sections CC

For reaction: ν e

+ p → e + + n E ν

M (target mass)

s = 2E ν

cms

σ G 2

π s

E ν cms = γ E ν

(1− β cosϑ) where β= E ν

M → 0,

γ = 1 and E cms ν

E ν

For a threshold Q: E ν

⇒ E ν

− Q

σ = 6 × 10 −44 (E ν

-Q) 2 cm 2

(E ν

in MeV)

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Neutrino cross sections NC

For reaction:

ν µ

+ e − → ν µ

+ e −

one has only NC diagram:

Z 0 ν µ

then one needs to change propagator:

f (q) ≈ 1 M W

2

⇒ f (q) ≈ 1 M Z

2

e −

σ ⇒ M 4

W

4

M Z

G 2

π s

σ ( ν µ

e − → ν µ

e −

) ≈ 0.6 ⋅σ ( ν µ

e − → µ − ν ) e

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Neutrino interaction length

or mean free path between collisions:

For neutrinos of 1 GeV pathing Earth:

let’s take:

Energy needed for λ to become of

the size of Earth:

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Neutrino interactions with

nucleons

At low energies:

only anti neutrinos on free

protons

neutrinos only on neutrons,

(but those are always

bound in nuclei)

extra energy needed to compensate

for nuclear binding

Neutrino scattering on electrons

CC

NC

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Interactions in water

for ν e at low energies

ν e

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Interactions in water

for ν µ at low energies

µ mass =106 MeV

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Neutrino interactions at different

energies

A neutrino of energy E and wavelength λ=h/E can

interact with:

for = c = 1

1GeV −1 = 0.2 fm

an electron on the atomic orbit R proton

~ 0.8 fm

a nucleus as a whole (when λ is comparable to the nucleus size)

a free proton or a nucleon bound in a nucleus (when λ is

comparable to the nucleon size)

a quark (when λ is much smaller than the proton size)

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Interactions of ν µ with nucleons

– high energies

QE:

(quasi-elastic)

Single Pion

production:

(resonances)

DIS:

(deep inelastic

scattering)

at high energies:

σ tot ≈ const ⋅ E ν

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Angular distributions in ν e -

scattering

Let’s look at Lorentz transformation from CMS to LAB:

E = γ E cms

p z

= γ E cms

( 1+ β cosϑ ) cms

( cosϑ cms

+ β)

For E m e

β =

E ν

E ν

+ m e

→ 1:

cosϑ lab

p z

E = β + cosϑ cms

⎯⎯⎯→

1

β→1

1+ β cosϑ cms

ϑ cms

e −

independently of ϑ cms

both neutrino and electron mostly forward

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

ν

e −

ν

Angular distributions of electrons

from ν e - scattering

ν e

ν e

ν µ

ν µ

ν e

ν e

ν µ

ν µ

E ν

= 5 MeV

E ν

= 10 MeV

e

( ) cos( ϑ lab )

e

cos ϑ lab

e

ϑ lab

( )

= p e

, p νi

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Angular distributions in ν N scattering

at low energies

e

( ) e

cos( ϑ lab )

e

cos ϑ lab

ϑ lab

( )

= p e

, p νi

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

How to detect neutrinos – i.e. products

of their interactions?

Go underground to shield the detector from other particles

use large volumes of cheap materials

Typical detection techniques:

water (light or heavy) – record Cherenkov light

scintillators – record scintillation light

liquid argon – record drifting electrons from ionization

iron slabs as targets and various detectors to record

exiting particles

We have to go underground...

Flux of atmospheric muons below the surface of Earth::

One order of magnitude per 650 m

Water

Cherenkov detectors

Super-Kamiokande

SNO

cheap material

directionality

time of every event

threshold 4-5 MeV

Cherenkov Light Emission

Charged particles with velocities

(where n is the refractive

index of the medium)

produce the electromagnetic shock-wave

along the conical wavefront at an angle

there is a thereshold effect:

we get light if

e.g. in water total energy above ~1.5 mass

It is used to measure particle velocity (angle gives ) for slow particles,

for relativistic particles the angle is always very similar (in water about 42 deg)

can be used to measure particle direction

and vertex reconstruction (the point from which light is emitted

at the earliest)

Super-Kamiokande detector

50,000 tons of ultra-pure water

1000 m underground

11,146 photomultipliers (PMT)

20” dimension

1,885 PMTs in outer layer

42m

Super-

Kamiokande

Kamioka mine

Experiments:

• Kamiokande

• Super-Kamiokande

• KamLand:

Entrance to

Kamioka mine

Photomultipliers (PMTs)

Dimension 20”

Time uncertainty 1nsec

Stopping Muon in Super-

Kamiokande

Colors – time of

every hit PMT

corrected for

the photon flight

time

Muon like –

from the sharp

ring edge

Energy - from

the sum of all

PMT signals

Stopped –

from the ring

empty inside

Contained eventno

signal in outer

detector

Second ring –

decay electron

muon stopping in the

detector

Colors:

charge

at each

PMT

which

measures

the number

of photons

provide

event

energy

two tracks

Two tracks passing

rings filled – tracks

leaving the detector

What can be seen in a Cherenkov

detector : looking at gammas

this is only schematic drawing

in reality angle between e + and e -

is ZERO degree

γs are not directly seen by PMTs while

electrons give light if above threshold

BUT

γs produce electromagnetic

• they convert to e + , e - pairs

• electrons emit bremstrahlung γs

• electron energy is degraded, they

undergo multiple Coulomb scattering

• light is emitted in directions

different from original

In effect

electrons or γs produce

Cherenkov rings more smeared

than muons or pions

• low energy particles (below threshold) are not visible

• neutrons are only visible by products of their interactions

Particle

identification

electrons, gammas:

most likely:

ν e

+ n → e − + p

(protons under

Cher. threshold)

ν µ

+ n → µ − + p

muons, pions, protons:

Used in atmospheric ν

detection

µ − → e − + ν µ

+ ν e

Solar neutrinos in SuperKamiokande

initial neutrino directions.

( Sun dir., electron dir. )

Cherenkov light directionality allows reconstruction of

neutrino direction (approximatly)

neutrinogram of the Sun made in the mine

PMT photo

Super-Kamiokande accident

Super-Kamiokande – being built......

In 1995

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Super-Kamiokande

In Sep 2001 the upgrade finished

In Oct 2001 the detector filling started

In Nov 2001:

one bottom PMT imploded

a nearby seismograph records

above 3 in Richter scale

\$20 M worth of PMTs gone

in 40 msec

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Super-Kamiokande after implosion

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Super-Kamiokande

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Super-Kamiokande

„From neutrinos to cosmic sources”, D. Kiełczewska and E. Rondio

Rebuilding Super-Kamiokande

Decisions in early 2002:

Phase-1, SK-II quick restart of K2K

rebuild SK with 47% inner PMTs

by autumn of 2002

PMT vessel to avoid chain reaction

of explosion

Phase-2, SK-III full detector before

the time of commissioning of T2K

- reconstruction starts in Oct 2005

Acrylic + FRP vessel

Super-Kamiokande – phase II

2002 – 2005

Super-Kamiokande

reconstruction

in 2005/2006

P. Mijakowski & P. Przewłocki

Super-

Kamiokande III

- after

reconstruction

in 2005/2006

Super-K III filling

SNO

(Sudbury Neutrino Observatory)

Water detector with

a difference:

2 km underground

1000 tonnes D 2 O

10 4 - 8” PMTs

6500 tons H 2 O

SNO under construction

SNO

Results from D2O

Neutrino reactions in heavy water

(SNO)

Charged Current Reaction:

6-9 events per day

CC

ν e

+ d → e − + p + p

ν e flux and energy spectrum

Some directional sensitivity (1 - 1/3cos(ϑ e )

ν e

n

E thresh

= 1.4 MeV

W

e -

p

Neutral Current Reaction:

NC

1-2 or 6-8 events per day

(different detection mechanisms)

Total solar 8 B active neutrino flux

ν x

+ d → ν x

+ p + n

ν

Z

n/p

ν

n/p

E thresh

= 2.2 MeV

Elastic Scattering Reaction:

1-2.5 events per day

ES

Directional sensitivity (very forward peaked)

ν

x

+ e − → ν

x

+ e − E thresh

= 0 MeV

ν e -

e ν x

ν x

ν e

e - W Z

e - e -

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