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 5

Discoveries of the leptons

Previous lecture

1

In 1940s, nuclear reactors became the first powerful

continuous artificial anti-neutrino sources

(production rate: ~1020 s1GW1; typical energy: few MeV).

Anti-neutrinos are detectable via the inverse beta decay (IBD)

reaction , with a threshold of Eth=1.8MeV.

We have defined the reaction cross-section and mean free

path , and found how they are related:

For MeV-energy neutrinos, the interaction cross-section is tiny

(~1043 cm2), free path in matter is astronomical (light years).

Design of the reactor anti-neutrino experiment:

the expected detection rate is ~0.01/hour/kg.

This lecture

2

Discovery of the electron antineutrino:

the CowanReines experiment (1956).

Discoveries of the second generation leptons.

Discoveries of the third generation leptons.

Reading list:

C. Sutton. Spaceship neutrino. Chapters 3, 4.

N. Solomey. The elusive neutrino. Chapters 4, 5, 9.

3

IBD detection principle IBD signature:

prompt signal from the positron annihilation +

delayed signal from the neutron capture

(typical capture time ~200 s)

(~10s for Cd, Gd-doped targets)

Positron detection: via annihilation

Neutron detection:

via thermalization & capture, e.g.

A possible detector type: scintillation detector

Mean free path of a MeV photon in water: 10 cm.

Interaction of a MeV photon: mainly Compton scattering (ee).

Scintillation: fast (~1 ns) isotropic luminescence produced

by absorption of ionising radiation a real-time experiment

p

e

p

CowanReines experiment

4

(Savannah River nuclear power plant, South Carolina, US, 195556)

Reines et al., Phys. Rev. 117 (1960) 159

Experimental setup

Thin H2O+CdCl2 target

tanks (0.2m3 each).

Cd/H atomic ratio = 1%.

Liquid scintillator

detectors

(each equipped with

110 photomutipliers)

Pb shielding Antineutrino interaction event

0.511 MeV

0.511 MeV

Prompt signal: 2×0.511 MeV photons.

Delayed signal: n capture on Cd, ~8 MeV.

Both signals: coincidence in two detectors.

Top

triad

Bottom

triad

Photomultipliers

5

Typical quantum efficiency:

two modern Hamamatsu PMTs

R9880U-110

R7400U-03

Photocathode:

photoelectric effect Dynodes:

secondary emission

Typical operating voltage: 1000 V.

Typical number of amplification stages: 10.

Typical gain: ~106.

Typical time resolution: <1 ns. Light absorption in

(quartz) input window

Insufficient photon energy

for photoelectric effect

Wavelength, nm

Detectors of visible/UV light

First neutrino oscillograms

6

Signal in the

top triad:

t=2.5s

Signal in the

bottom triad:

t=13.5s

PROMPT DELAYED

Reines et al., Phys. Rev. 117 (1960) 159

The discovery of e

7

Reines et al., Phys. Rev. 117 (1960) 159 Top triad

Bottom triad

Counting the prompt+delayed coincidences:

• (S) Signal region: 0.75s<t<7s;

• (C) Control region: 11s<t<25s; subtraction of accidental counts • Cross-check: run with the reactor switched off.

(S)

(S) (C)

(C)

Accidental background:

• Background/Signal 25%;

• Mostly non reactor associated.

accidentals

Counting rates after background subtraction:

• Top triad: F = (1.690.17) hr 1;

• Bottom triad: F = (1.240.12) hr 1. compatible after correcting for the distance to reactor

Further cross-checks:

• double Cd concentration in target or remove Cd;

• dissolve 64Cu (+ emitter) in target; etc.

Cross-section measurement

8

CowanReines IBD cross-section measurement:

Reines et al., Phys. Rev. 117 (1960) 159

A remarkable agreement !

The neutrino was discovered in 1956. Nobel Prize awarded in 1995.

Our expectation for MeV neutrinos,

assuming weak interaction in low-energy regime (see lecture 4):

9

Discoveries in the cosmic rays Particles known by 1937: proton, neutron, electron, positron

(the neutrino was proposed in 1930 to explain the beta decay spectrum)

Particles not present in “ordinary” matter

and decaying by the weak interaction

discovered in cosmic rays:

1937: muon (the “heavy electron”)

decays into electron (or positron);

= 2.2×106 s; c = 660 m.

1947: pion (the second lightest hadron) •

quark content: + = ud; = ud;

decays into muon;

= 2.6×108 s; c = 7.8 m;

short lifetime: discovered at high altitudes.

Relativistic time dilation:

mean free path in lab frame

is enhanced by the Lorentz-factor

The pion decay chain

10

e

“kink”:

an undetected neutrino

“kink”

The decay chain observed in

photographic emulsions

exposed at Pic du Midi (2,877 m)

in the French Pyrenees:

(Powell et al., Bristol University, 1947;

Nobel Prize 1950)

3-body decay:

2-body hypothesis ruled out by

the continuous positron spectrum

A possibility of producing

neutrino beams at accelerators

First accelerator neutrinos (1950s)

11

Accelerator-produced (GeV) neutrinos are ~105 times more likely to interact than the reactor ones

Interaction probability in L=2.25m thick Al block (the first detector): P = L/.

Production rates required for an experiment:

Neutrino interaction (IBD) cross-section:

The first accelerator proton beam of the required intensity became available at the Brookhaven lab (US) in the early 1960s

Are the produced together with

muons identical to the produced together

with electrons (e.g. in a reactor)?

density of relevant nucleons

(high intensity)

Protons

(~10 GeV) target

(diverging beam)

The discovery of

12

LedermanSchwartzSteinberger experiment, Brookhaven, 1962

Target (Be)

Trigger counter

(trigger synchronized

with proton delivery) shielding

Veto counters

Results:

29 muon tracks identified:

No electron tracks identified:

the reaction

WAS NOT OBSERVED

First large scale particle experiment

• Photographic detection.

• Exposure: 8 months 25 “good” days.

• Detector “ON” for a total of 5.5 s.

• ~1014 neutrinos through the detector.

• ~5000 spark chamber photographs taken.

/=0.012 Proton beam

(15 GeV)

Spark chamber:

~10 tons of Al.

Method:

• Detect inverse beta decay in the

spark chamber: e.g.

• Identify the lepton type (e or ).

e and demonstrated to be different particles: Nobel Prize 1988

e,?

Spark chambers

13

The Brookhaven spark chamber

Proton-antiproton collision

seen by a spark chamber

in a different experiment (at CERN)

Muon tracks are visible

Q: How to identify e/ in a spark chamber?

A: Muons are ~200 times heavier: smaller energy loss due to bremsstrahlung.

Muons travel large distances and leave straight tracks.

Stack of metal plates, HV between pairs of plates. 10 tons of aluminium.

images

14

Photographs of the muon tracks produced in interactions

taken by the Brookhaven experiment in 1962

The discovery of the lepton

15

Tau-lepton production was discovered at the

SPEAR e+e collider at SLAC (California) in 1975

(threshold CM energy: 2m=3.55 GeV):

is the only lepton massive enough to decay into hadrons

(m=1.777 GeV)

(by lepton universality, almost independent of daughter lepton type)

Experimental statement: opposite sign

e-pair and at least 2 missing particles.

NB: e+e and + pairs

can be produced by e+e scattering.

undetected:

c = 87 m

undetected

Nobel Prize 1995

The observation of

16

postulated following discovery in 1975;

directly observed by the FNAL E872

(DONUT) experiment in 2000.

Primary tau-neutrino source:

Secondary beam production:

(~5% of all ’s are expected to be )

Mean free path: c 2mm; decay into

a single charged track: “track with a kink”

(tungsten)

Detector type: Pb/emulsion sandwich + spectrometer

[BR=5.6%]

;

Are there further neutrino flavours?

17

OPAL electromagnetic calorimeter “endcap”:

1132 Pb glass blocks (25 kg each)

The LEP e+e collider at CERN:

Operation: 19892000.

Circumference: 27 km.

Four large detectors.

ECM up to 209 GeV.

A “Z0 factory”.

~17M Z0

~36k W+W-

1989-1995

1995-2000

~1/E2

e

Z0

e+

Z0 decay rate measurement

18

Measured

Measured number of generations:

N = (2.9840.008).

Standard Model expectation:

0.166 GeV

However there is still room for

heavy (m>MZ/2) or sterile neutrinos;

(sterile = no weak interactions).

“invisible part”

Total Z0 decay rate:

x

Z0

x

e+,+,+

Z0

e,,

q

Z0

q

Summary

19

The six known leptons were discovered in 18972000.

Electron (e): in cathode rays.

J.J. Thompson, Cambridge, 1897.

Positron (e+) and muon (): in cosmic rays.

C. Anderson, Caltech (US), 1932, 1936.

Electron antineutrino (e): produced at a nuclear reactor.

C. Cowan & F. Reines, South Carolina (US), 1956.

Muon neutrino (): produced at a proton accelerator.

L. Lederman, M. Schwartz, J. Steinberger,

Brookhaven laboratory, New York (US), 1962.

Tau lepton (): produced at an e+e collider.

M.L. Perl et al., SLAC, California (US), 1975.

Tau neutrino (): produced at a proton accelerator.

DONUT experiment, FNAL, Illinois (US), 2000.

There is no conclusive experimental evidence for further

generations of leptons or quarks.