1. Discovery of the Neutrino Mass
P1X* Frontiers of Physics Lectures
21-22 October 2003
Dr Paul Soler
University of Glasgow

2. Discovery of the Neutrino Mass
Outline
1. Introduction: the structure of matter
2. Neutrinos:
2.1 Neutrino interactions
2.2 Neutrino discovery and questions
2.3 Neutrino oscillations
3. Atmospheric neutrinos
3.1 Superkamiokande experiment
3.2 Discovery of neutrino mass
3.3 Long-baaseline neutrino experiments
4. The Solar Neutrino Puzzle:
4.1 Solar model and the Homestake experiment
4.2 Kamiokande and Superkamiokande experiments
4.3 Gallium experiments
4.4 Sudbury experiment: the solution of the puzzle
5. The future: a neutrino factory?
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Motivation
Motivation for the Frontiers of Physics lectures:
Bring to your attention some of the most exciting fields of physics research at a level that can be easily understood
Help you to understand the link between undergraduate physics and front-line research.
Use some of the concepts learned in these lectures to improve understanding of undergraduate physics
Neutrino physics: exciting recent discoveries have shown that neutrinos have mass, Nobel prizes for R. Davis and M. Koshiba.
Motivation for the Discovery of the Neutrino Mass
lectures:
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4. Discovery of the Neutrino Mass
References
Reading for the “Discovery of Neutrino Mass” lectures
“Detecting Massive Neutrinos”, E. Kearns, T. Kajita, Y. Totsuka, Scientific American, August 1999.
“Solving the Solar Neutrino Problem”, A.B. McDonald, J.R. Klein, D.L. Wark, Scientific American, April 2003.
Web references:
2002 Nobel Prize in Physics:
Super-Kamiokande and K2K web-sites:
Sudbury web-site:
More on neutrinos:
Discovery of the Neutrino Mass

5. 1. The structure of matter
How do we find out about the smallest constituents of matter?
Build more powerful “microscopes”:
Wavelength of probe
becomes smaller as
energy (momentum)
of probe becomes
larger.
For sub-atomic
particles, we use
powerful accelerators
(e.g. CERN, Fermilab).
Discovery of the Neutrino Mass

6. 1. The structure of matter (cont.)
Two types of particles:
Fermions (half-integer spin particles): make up the known matter and occupy “space” because of Pauli exclusion principle.
Examples: quarks, protons, neutrons, electrons, muons, neutrinos, ...
Bosons (integer spin particles): carriers of the forces between fermions
Examples: photons for electromagnetic interactions, W and Z bosons for weak interactions, gluons for strong interactions
Fermions come in three families (why?, we don’t know) and have antiparticles as well. One neutrino for every electron, muon and tau.
Quarks give
most of mass
Electrons take
up space
Protons: uud
Neutrons: ddu
Muons unstable
(cosmic rays)
Exotic quarks:
rare and unstable
Discovery of the Neutrino Mass
Taus very unstable

7. 1. The structure of matter (cont.)
Forces:
Gravity: very weak, long interaction, mediated by graviton (never observed!).
Electromagnetic: keeps atoms together, mediated by photon
Strong: keeps nuclei and nucleons (ie. protons, neutrons) together, mediated by gluons. Very short range interaction
Weak: responsible for some radioactive decays (ie. beta decay), mediated by W+, W- and Z0 massive gauge bosons. Relatively short range and weak due to mass of the bosons.
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2. Neutrinos:
Neutrinos:
Originally suggested by Pauli in 1930 as a “desperate remedy to overcome law of conservation of energy” in beta decay:
1.05
KE (MeV)
Radium-E spectrum
(Bi-210)
Number
beta rays
Why is the electron spectrum continuous?
A third particle (neutrino) is taking away part
of the energy
The neutrino was originally postulated as a massless, chargeless and very weakly interacting particle: practically indetectable!
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9. 2.1 Neutrino interactions
One of the ways neutrinos interact is through inverse beta decay: or
Cross-section s (average area of neutrino in collision) is very small: on average a neutrino would travel 1600 light-years of water before interacting!
Mean free path:
light-years
In water:
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2.2 Neutrino discovery
Reines and Cowan observed neutrinos for the first time in 1953 (Nobel prize for Reines in 1995)
They used 400 l of a mixture of water and cadmium chloride (Cd)
An antineutrino from a nuclear reactor (6 x1020 s-1) very rarely interacted with the protons in the target (2.8 hr -1):
The positron (e+) produces two photons, followed about 20 ms later by the neutron interacting with a Cd nucleus that produced another spray of photons
Scintillator
H2O + CdCl2
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2.2 Neutrino questions
Neutrinos are all around us:
Produced by nuclear reactions in radioactive rocks (trace uranium thorium in granite, etc.), in the sun (solar neutrinos) and from cosmic rays hitting the atmosphere (atmospheric neutrinos).
Very difficult to detect because they are so weakly interacting.
Produced in copious quantities inside nuclear reactors.
Generated by high energy accelerators
Two main problems:
Solar neutrino problem: nuclear reactions in the sun produce electron neutrinos ne (energies up to 14 MeV). The number detected on earth by experiments is between 30%-50% of what is expected.
Atmospheric neutrino problem: high energy particles (cosmic rays) hitting the upper part of the atmosphere. There should be twice as many muon neutrinos nm as electron neutrinos ne (energies up to 10 GeV). Experiments detect approximately equal numbers.
Both can be resolved through neutrino oscillations
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12. 2.3 Neutrino oscillations
If neutrinos have mass, theoretically, a neutrino of one species could change into another species:
For example: a muon neutrino changes into a tau neutrino
Probability that a nm of energy E converts to a nt after travelling a distance L is:
Notice that the probability of oscillations is zero if the mass of the neutrinos are zero!
L=length of neutrino path (in m) E=energy neutrino (in MeV)
mnm= mass of nm (in eV) mnt= mass of nt (in eV)
qmt=mixing angle between two neutrinos
(eV= electronVolt=energy of one electron accelerated by 1 Volt=1.6x10-19 J)
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3. Atmospheric Neutrinos
Cosmic rays provide an abundant source of neutrinos.
Protons hit upper part of atmosphere producing cascade of particles including pions that decay (on average) into 2 muon neutrinos for each electron neutrino produced in an interaction
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3.1 Super-Kamiokande experiment
Kamiokande experiment: started 1987, 5000 tons water, 1000 photomultipliers
Super-kamiokande experiment: started 1997 (M. Koshiba leader experiment)
50,000 tons of water, surrounded by 11,000 phototubes to detect flashes of light in the water.
Super-Kamiokande
experiment is underground
Inside a mine in Japan to
shield it from the very
large number of cosmic
rays.
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3.1 Super-Kamiokande experiment
Super-Kamiokande detects faint flashes of Cherenkov light inside huge tank of 50,000 tons of water.
Electron neutrinos make a recoil electron and muon neutrinos make a recoil muon.
Rings of Cherenkov light are formed from the electron or the muon. The detector can distinguish between electrons (fuzzy rings) and muons (clean edge on ring).
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3.2 Discovery of neutrino mass
Results from Super-Kamiokande:
There are less muon neutrinos than expected. The number of muon neutrinos disappearing depends on the angle of the neutrino (ie. It depends on whether the neutrino was produced in the atmosphere above or on the other side of the earth). First evidence for neutrino oscillations in 1998 !!!!
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3.2 Discovery of neutrino mass
As the distance from production increases then more muon neutrinos disappear.
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Therefore 84% of nm survive journey!

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3.2 Discovery of neutrino mass
Consequences of discovery:
Neutrino oscillations responsible for atmospheric muon neutrino deficit.
Since electron neutrino spectrum well predicted, it must be muon neutrinos nm changing into tau neutrinos nt .
Since then neutrinos have mass!!
Mass of the neutrinos have to be greater than eV.
If either the nm or nt is much smaller than the other, then mn~0.05 eV.
Both nm or nt could have a mass much larger than mn~0.05 eV as long as the difference of the mass squared is 3.2x10-3 eV2.
Since there were so many neutrinos produced soon after the big-bang, if they have a mass, it could provide a large portion of the missing mass of the universe (up to 20%).
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3.3 Long-baseline experiments
Long-baseline experiments with accelerators will verify that oscillations are really taking place in Super-Kamiokande.
K2K (from the KEK accelerator in Japan to Super-Kamiokande): 250 km baseline of neutrinos. So far they observe 56 nm events when they expected 80 events, consistent with 3x10-3 eV2 mass-squared difference.
MINOS: neutrino beam from Fermilab in Chicago to a mine in Minnesotta (750 km), will start taking data in Another beam from CERN to Gran Sasso (CNGS) laboratory in Italy (also 750 km) to start in 2006.
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4. Solar Neutrinos
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21. 4. The Solar Neutrino Puzzle
Ray Davis (Brookhaven National Laboratory) proposed an experiment in the 1960s to measure neutrinos from the sun.
Why are neutrinos emitted from the sun?
Nuclear fusion powers sun:
Energy of sun is due to burning hydrogen into helium. The measured photon luminosity is: 3.9x1026 J s-1.
Energy per neutrino = 26.7x106x1.6x10-19= 4.3x10-12 J/neutrino
Number of neutrinos = 3.9x1026/4.3x10-12 = 9.1x1037 neutrino s-1
Distance from sun to earth =R= 1.5x1013 cm.
Therefore:
(64 billion neutrinos per second through your finger nail of 1 cm2 !!!!)
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4. The Solar Model
In reality, chain of reactions needed to burn 4 hydrogen nuclei into helium nucleus.
There are two main cycles: the pp cycle (98.5% of the total sun’s power comes from these reactions) and the CNO cycle catalysed by carbon, nitrogen and oxygen (not very important in the sun with only 1.5% of power output).
Most abundant neutrinos are low energy (<0.42 MeV) pp reaction with flux 6.0x1010 cm-2 s-1. Most important for detection are 8B neutrinos because they have high energy (<14 MeV) but only consist of 10-4 of all solar neutrinos.
PP cycle:
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4.1 Homestake experiment
Ray Davis’ Chlorine experiment inside Homestake mine in Lead, South Dakota:
100,000 gallons (615 tons) of
cleaning fluid (C2Cl4)
Expect about 1.5 Ar atoms/day
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24. 4.1 Homestake and Solar Model
Results from the Ray Davis chlorine experiment: sensitive to 8B and 7Be neutrinos (0.814 MeV threshold).
Measured SNU (0.48 atoms/day),
Solar Model Expectation = SNU (1.5 atoms/day)
Observation about 1/3 the expected number of solar neutrinos
1 SNU = 1 interaction
per 1036 target atoms
per second
Is there something wrong
with experiment, something
wrong with solar model or
something wrong with the
neutrinos?
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25. 4.2 Super-Kamiokande experiment
Results Super-Kamiokande experiment: can also measure solar neutrinos
Proof that neutrinos come from sun: angular correlation
Neutrino flux is 46.5% that expected from the solar model
Confirmation
Solar Neutrino
Puzzle!
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4.3 Gallium experiments
Similar experiments to chlorine but with gallium:
Lower threshold (0.233 MeV) so sensitive to the lower end of the pp chain
Further evidence of missing solar neutrinos (55% of expectation)
Expectation: SNU
Observed: SNU
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27. 4.4 Sudbury Neutrino Observatory
Heavy water (D2O) experiment in Canada:
Surface: 2 km
(D=deuterium=
proton+neutron)
Phototube Support
Structure (PSUP)
1000 tonnes D2O
Acrylic Vessel
104 8” PMTs
6500 tonnes H2O
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28. 4.4 Sudbury Neutrino Observatory
Acryllic vessel with photomultiplier tubes:
All components
Made out of very
low radioactivity
materials
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29. 4.4 Sudbury Neutrino Observatory
Faint flashes of Cherenkov light recorded by photomultipliers:
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30. 4.4 Sudbury Neutrino Observatory
Results:
Charged current (CC):
Elastic scattering (ES):
Neutral current (NC):
(35% SSM)
(100% SSM) CC
1967.7
+61.9
+60.9
+26.4
+25.6 ES
263.6
+49.5
+48.9 NC
576.5
#EVENTS
About 35% electron neutrinos
make it to earth (from CC) but
flux of all neutrino species (from
NC and ES) as expected:
Neutrinos change species in flight:
Neutrino Oscillations!
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31. 4.4 Solar neutrino puzzle solution
Sudbury Neutrino Observatory has confirmed neutrino oscillations from solar neutrinos and has confirmed the solar model of fusion in the sun.
Experiments only sensitive to electron neutrinos (ne) see a deficit but Sudbury experiment that is sensitive to all neutrino flavours sees the expected total number of neutrinos.
Electron neutrinos (ne) oscillated into muon neutrinos (nm) in trajectory from the sun to the earth. However, the evidence shows that the transition happened inside the sun, due to an enhancement of the oscillations because of the high matter density of the sun.
Parameters:
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32. 4.4 Solar neutrino puzzle solution
More confirmation: KamLAND experiment in Kamioka mine in Japan shows that reactor (~2 MeV) disappearing in flight (~180 km).
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33. 4.5 The future: a neutrino factory?
Future directions for neutrino physics: build a neutrino factory to fire very intense beams of neutrinos at 700 km to one experiment and around 7000 km to other side of the world.
Main aim: why is the universe made of matter rather than antimatter? CP violation of neutrino oscillations might be explanation and a neutrino factory could measure this effect.
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Conclusions
Neutrinos are very misterious particles and we are only starting to undertand their nature.
Super-Kamiokande experiment discovered neutrino oscillations (nm to nt) from the deficit of muon neutrinos from cosmic rays hitting the upper layers of the atmosphere. This implies that neutrinos have mass with mass-squared difference of 3x10-3 eV2 (largest mass greater than 0.05 eV).
A number of experiments looking at solar neutrinos have also seen a deficit in the number of electron neutrinos from the sun. Confirmation of neutrino oscillations in solar neutrinos came from the Sudbury experiment that showed that it is only the electron neutrinos that are missing while the total flux of neutrinos is as expected. Hence electron neutrinos ne are changing to another species (probably nm) with a mass-squared difference of 7.1x10-5 eV2.
The field of neutrino physics still has a bright future, with open questions:
What is the absolute mass of neutrinos?
Are neutrinos their own antiparticle?
And the most ambitious question of all: are neutrinos responsible for the matter-antimatter asymmetry of the universe?
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