1. HCP: Particle Physics Module, Lecture 4
Plan for today:
Neutrinos
mass (?)
dark matter problem in the universe
solar neutrino problem
oscillations!
Possible future of particle physics
 superstring theory

3. New chart:
Old chart:
Why the change?

4. Neutrinos
Neutrinos are zero (or nearly zero) mass particles that only interact
through the weak interaction.
Originally proposed by Pauli (1933) to explain the apparent “missing”
energy and momentum in the beta decay process:
n  p e e
 First directly detected in late 1950’s using neutrinos from a nuclear
reactor with the process:
e p  n e+
Very difficult experiment because neutrinos hardly interact at all:
Example: Probability of a neutrino interacting as it passes through the
Earth is 1 part in 1015 = !

5. Neutrino Mass Why do we say that the neutrino mass is “nearly zero”?
From experiments we have “upper limits” on the mass of the neutrinos:
e : mass < 3 eV/c2  : mass < 0.2 MeV/c  : mass < 20 MeV/c2
Ways to directly measure neutrino mass:
Search for “missing energy” in nuclear beta decays
Look at neutrinos from a supernova!
“Direct search” for neutrino mass using tritium beta decay:
Search for “missing energy” near end point; the highest energy decay
electrons would have less energy if the neutrinos have mass.

6. Or Look at Electron Neutrinos from a Supernova (exploding star)
Supernova 1987A: observed in optical telescopes and in two large
underground neutrino detectors.
Huge burst of neutrinos (of varying energies) emitted during the
explosion
Measure: 1. neutrino energy in the detector
2. neutrino arrival time: if neutrinos have mass, the lower
energy ones would arrive later
12 neutrinos observed
This analysis led to an
upper limit on the electron
neutrino mass of 20 eV/c2
Neutrino arrival time (seconds)

7. Why is Neutrino Mass of Interest?
Possible explanation of the “dark matter problem” in the universe
Solar neutrino problem
Neutrino oscillations (if neutrino oscillations are observed, then
neutrinos definitely have mass)
4. The indirect evidence of neutrino mass from the recent observations
of neutrino oscillations is the first evidence of physics “beyond” the
Standard Model (the neutrinos are assumed to be massless in the
Standard Model)

8. Dark Matter Problem
Observation of the motion of stars in our galaxy indicates that there
is not enough visible matter to account for how fast the stars are moving.
Conclusion: 90% of the mass of the galaxy is in some “invisible” (not giving
off light) form
Could it be neutrinos?
There are ~ neutrinos in every cubic inch of space (the
“relic” neutrinos from the Big Bang).
If they had a mass of around 2 – 20 eV/c2 they could explain the
problem.
Observed
Velocity of stars
in our galaxy 
Expected if all
mass is “visible”
of the galaxy

9. How Does the Sun Shine?
The Sun’s energy comes from nuclear fusion reactions:
proton – proton “chain” of nuclear fusion reactions
Basically, p + p + p + p  He “stuff” MeV
4 protons fuse released energy
Part of the released energy comes off as visible sunlight, BUT some of
it is carried off by neutrinos that reach us here at Earth.
 Observation of these neutrinos here on Earth is some of the
best evidence that nuclear fusion reactions power the Sun.

10. Solar Neutrino Detection
The neutrinos coming from the sun are all electron type neutrinos (e):
They come in many different energies (from 0.1 – 16 MeV).
We can detect them here on Earth with:
LARGE (due to low neutrino interaction rate)
UNDERGROUND (to reduce cosmic ray background (mostly +,-))
DETECTORS

11. The Chlorine Experiment (The Original Solar Neutrino Experiment)
Large tank full of 100,000 gallons of cleaning fluid located 1 mile
underground in the Homestake Gold Mine in South Dakota.

12. The Chlorine Experiment (The Original Solar Neutrino Experiment)
Large tank filled with 615 tons of cleaning fluid (C2Cl4). Located 1 mile
underground in the Homestake Gold Mine in South Dakota.
Search for products of the reaction:
e Cl  e Ar
 
from the Sun in the cleaning fluid
Only one 37Ar atom is created every 2 days!
They chemically “sweep” the tank every two months to collect about
30 37Ar atoms.

13. The Chlorine Experiment
Kamiokande and Super-Kamiokande

14. Solar Neutrino Problem
As of 2001, four different solar neutrino experiments lead to the
same conclusion 
The amount of neutrinos coming from the Sun is much less than the
predictions of the Standard Solar Model
Theory 
Experiment 
Results from four different experiments on the number of electron
neutrinos coming from the Sun.

15. Solar Neutrino Problem
As of the year 2001, the question was:
What is causing the apparent deficit of e coming from the Sun?
Error in the Standard Solar Model of the Sun OR
b) New neutrino physics?
 The e neutrinos are oscillating to other flavors ( or ) that we
do not detect.
As we will see now, experiments since 2001 have confirmed that the
answer is b).

16. Neutrino Oscillations
If neutrinos have non-zero mass, they can oscillate.
Oscillation: The neutrino can change from one type to another as it
travels along:
e    e
This type of effect turns out to be responsible for the solar neutrino
problem  new neutrino physics!

17. Super-Kamiokande in Japan: 50,000 tons of water + 11,000 phototubes

24. Example of
Cerenkov
light
cone
in the
Super-K
detector

25. but sometimes accidents happen …

26. Sudbury Neutrino Detector in Ontario,Canada

29. Acrylic vessel containing
“heavy” water surrounded
by 10,000 phototubes on
a geodesic dome structure

30. Excavation of the cavity

31. Acrylic sphere with phototube dome partially done

32. Geodesic dome
containing 10,000
phototubes

33. First Observation of Neutrino Oscillations
Neutrino oscillations have in fact been observed. The first clear
evidence for neutrino oscillations came in 1998
Super-Kamiokande in Japan: underground detector with 50,000 tons of
water and 11,200 photomultiplier tubes
Process: Pions are generated from cosmic ray events
Pions decay to neutrinos in the upper atmosphere
+   
e  + e
A detector on Earth should detect two muon neutrinos for every
one electron neutrino.
Super-Kamiokande observes a ratio of 1:1 NOT 2:1 as expected.
Explanation: The muon neutrinos () are oscillating to another neutrino
type (probably tau neutrinos ()) which is not detected by Super-K

34. Solution of Solar Neutrino Problem – SNO 2001
SNO: (Sudbury Neutrino Observatory, Canda) : large, underground tank
of “heavy” water (D2O) viewed by 10,000 photomultiplier tubes
Previous solar neutrino experiments could only detect electron type
neutrinos (e ), while the SNO detector can detect all three types
(e   )
They agree with all the other experiments that they are seeing only
about 1/3 of the expected number of electron type neutrinos (e)
BUT
When they sum all three types they observe as many solar neutrinos
as expected!
So, the “missing” electron type neutrinos are oscillating to another
type (probably muon type neutrinos, but SNO can’t distinguish between
those and the tau type neutrinos)

35. Solution of Solar Neutrino Problem – SNO 2001
They really are coming from
the Sun’s direction!

36. Further Evidence that Electron Neutrinos Oscillate - 2002
KAMLAND: Large, underground tank in Japan with 1000 tons of
scintillator and 2000 photomultiplier tubes
One of the radioactive decay products from nuclear reactors is
electron type anti-neutrinos (e); KAMLAND detects these from
nuclear reactors all over Japan and Korea
They also observe less electron-type neutrinos than expected!
Interpretation: The electron type neutrinos are oscillating to another
type – probably muon neutrinos

37. Summary of Three Pieces of Experimental Evidence for Neutrino Oscillations
1. Super-Kamiokande (1998) in Japan
50 kton underground tank of water viewed by 11,200 phototubes
sensitive to "atmospheric" neutrinos from
Observe less muon neutrinos than expected
Interpretation: The muon neutrinos are OSCILLATING to tau neutrinos
2. SNO (Sudbury Neutrino Observatory) (2001) in Canada
Large, underground tank of "heavy" water (D2O) viewed by 10,000 phototubes
Observing solar neutrinos; sensitive to all three types
They observe as many solar neutrinos as expected
Solar neutrino problem solved! "Missing" electron type neutrinos are oscillating to another type.
3. KAMLAND (2002) in Japan
Large, underground tank with 1 kton of scintillator and 2000 phototubes
Observing electron type neutrinos from nuclear reactors throughout Japan
They observe less electron type neutrinos than they expect
Interpretation: the electron type neutrinos are oscillating to muon type neutrinos

38. Neutrino Mass Spectrum
The observation of neutrino oscillations implies that at least two of the
known neutrinos have mass:

39. Where Are We Headed?  “Theory of Everything”
What does that theory need to be able to do?
Unify gravity with the three other forces
Until recently this seemed very difficult because there was no
successful way to make a quantum theory of gravity.
Possible solution:
“Superstring theory” = Supersymmetry + string theory
Allows for gravity to be quantized
2. Gives unification of all 4 forces into a single unified force

40. Why Is It Difficult to Make a Quantum Theory of Gravity?
At distances as short as the Planck length (10-35 m) the energy fluctuations
implied by the Heisenberg uncertainty principle become very large 
In fact, the energy fluctuations are so large that virtual black holes
form.
Space-time looks highly curved and “foamy” on these length scales.
It is not the smooth collection of spacetime points required by
quantum field theories like QED = quantum electrodynamics
QCD = quantum chromodynamics

41. String Theory
String theory: Instead of being pointlike, the fundamental particles of
nature (quarks and leptons) are described by strings.
R ~ m (Planck length)
The fundamental particles correspond to the different vibrational
modes of the string.

42. Supersymmetry
A symmetry where every known particle has a supersymmetric partner
If this symmetry exists, it is clearly badly broken in our low-energy
world since we don’t seem to see the supersymmetric partners.
The supersymmetric particles will be searched for at the new high
energy accelerators (like LHC)

43. Superstring Theory
Superstring theory is the “marriage” of these two concepts:
Superstring theory
= string theory
+ supersymmetry
Probem with superstring theory:
It is currently only understood how it works at the Planck energy (1019 GeV).
There are no predictions that can be checked with current accelerators
which have the highest energies ~ 104 GeV.
 Something to look forward to!