1. Neutrinos: Detecting the Invisible!
The MINOS Neutrino Oscillation Experiment
Andy Blake, Cambridge University

2. Current “Standard Model” of elementary particles
and their interactions:
All matter is composed
of quarks and leptons.
Four fundamental forces.
– Electromagnetic force.
– Weak nuclear force.
– Strong nuclear force.
– (Gravity).
Quarks interact through
electromagnetic, weak
and strong forces.
Charged leptons interact
through electromagnetic
and weak force.
Neutrinos interact through
weak force only.
Particle masses generated
by Higgs mechanism (?)

3. Properties of Neutrinos
Three “flavours” of neutrino:
– electron, muon, tau.
Known properties:
– Spin = ½ (i.e. fermions).
– Charge = 0 (i.e. no electromagnetic interactions).
– Colour = 0 (i.e. no strong interactions).
Neutrino mass is known to be very small!
– direct limit on e mass is <1eV/c2 (from studies of tritium beta decay)
– 1 eV/c2 ≈ 500,000 times lighter than mass of electron.
– 1 eV/c2 ≈ g !
Challenge to physicists is to measure this mass.
– Need the most sensitive set of weighing scales ever created!

4. The universe is filled with a dense flux of “relic neutrinos”
created in the big bang – this makes neutrinos the most
abundant known form of matter!

5. atmospheric neutrinos
solar neutrinos
supernova neutrinos
geo-neutrinos
atmospheric neutrinos
reactor neutrinos

6. Each of you emits 300 million neutrinos per day!

7. 
But neutrinos are weakly interacting – the probability that a neutrino
will interact anywhere in the entire earth is 1 in 1,000,000 !

8. Detecting Neutrinos Neutrino Type Typical Energy
Neutrino energies and interaction rates
Note units!
Neutrino Type
Typical Energy
Interactions per kT-Yr
Solar
(electron neutrinos)
MeV
~10,000
Reactor
1 - 5 MeV
~1,000
Atmospheric
(electron/muon neutrinos)
GeV
~100
Compare with surface cosmic ray flux of ~10,000 m-2 s-1
“This makes looking for a needle in a haystack seem easy!”
(John Bahcall, neutrino physicist)

9. Detecting Neutrinos Need a very intense flux of neutrinos !
Need a very massive
neutrino detector !
Build the detector
deep underground
to shield it from
surface radiation.
Cannot observe
neutrinos directly
but can detect the
particles produced
by their interactions.

10. Borexino
IMB
Homestake
MINOS
SNO
KamLAND
Super-Kamiokande

11. The Super Kamiokande Neutrino Detector
Located at depth of 1,000m beneath
Mt. Ikenoyama in Japan.
Cylindrical tank of ultra-pure water.
– 40m high x 40m diameter.
– total detection mass of 50 kT.
Neutrinos interact with nuclei in
water to produce charged leptons.
– Electron neutrinos produce electrons.
– Muon neutrinos produce muons.
The charged leptons emit cones of
“Cherenkov” light along their path.
– Equivalent to sonic boom (the charged
particles are travelling faster than the
speed of light in water).
Detector is instrumented by array
of 13,000 photo-multiplier tubes.
– Convert light into electrical signals.
– Use PMT hits to map out progression
of Cherenkov cone through detector.
– Measure energy/direction of lepton,
infer energy/direction of neutrino.

12. Neutrino Interaction in Super-K Detector.

13. Atmospheric Neutrinos
The earth is continually bombarded
by a stream of cosmic particles.
– mostly protons and helium nuclei.
Cosmic rays interact in atmosphere
to produce secondary particles.
– mostly pions () and kaons (κ).
Decay chain of /κ produces e/.
primary cosmic rays
/κ  e  e
~20 km
p/He
p / He + N → X + ± / κ±
± / κ± → ± +  ()
± → e± + e (e) +  ()
Neutrinos travel right through the earth,
and a tiny fraction interact in Super-K.
– down-going neutrinos travel ~20 km.
– up-going neutrinos travel ~13,000 km.
Expect following flux ratios:
R / Re ~ 2 and Rup / Rdown ~ 1
Atmospheric neutrino production

14. Atmospheric Neutrino Oscillations
electrons
muons
Super-K measured the angular
distribution of the atmospheric
electron and muon neutrinos.
– Most atmospheric neutrinos travel
straight through the earth but a
tiny fraction interact in Super-K.
– since cosmic rays are isotropic,
expect atmospheric neutrinos
to be isotropic as well.
Electron neutrinos consistent
with isotropic expectation.
Muon neutrinos exhibit clear
up-down asymmetry.
– deficit of up-going neutrinos,
which travel ~100 times further
than down-going neutrinos.
Up-going muon neutrinos have
“oscillated” into tau neutrinos.
– evidence for neutrino mass!
null oscillations
cos Θ = -1 up-going
cos Θ = +1 down-going
nm  nt oscillations

15. Masatoshi Koshiba receives Nobel Prize
Nobel Prize for Physics 2002
Masatoshi Koshiba receives Nobel Prize

16. Neutrino Oscillations
Neutrinos undergo spontaneous transitions between flavours.
– These “oscillations” have been observed in atmospheric neutrinos,
solar neutrinos, and reactor neutrinos. Compelling evidence!
Neutrino oscillations imply that neutrinos have mass!
– The neutrino flavour states are actually just quantum mechanical
mixtures of a set of neutrino mass states.
– Can determine neutrino mass by measuring neutrino oscillations.
Neutrino oscillations are a purely quantum mechanical effect.
– The effect is so weak that it is observed over macroscopic distances.
“Flavour States”
“Mixing Matrix”
“Mass States”

17. Neutrino Oscillations
A beam of neutrinos, initially produced as muon neutrinos,
will oscillate into tau neutrinos and then back again!
– The rate of oscillations is dependent on the neutrino mass.   
probability
propagation distance
Wavelength of oscillations:
 = 2.47 E / m2
(  = wavelength (km) ; E = energy (GeV) ; m2 = mass splitting (eV2) ).

18. The MINOS Experiment (“Main Injector Neutrino Oscillation Search”)
The first of a new generation of neutrino experiments.
Objective is to determine the neutrino mass through a
precision study of neutrino oscillations.
Manufacture accelerator beam of muon neutrinos.
Measure muon neutrino spectrum before/after oscillations.
– near detector constructed adjacent to neutrino source.
– far detector constructed hundreds of miles away.
– search for muon neutrino disappearance in far detector.
Near
Detector
Far
Detector
Neutrino
Source  
730 km

19. 735 km The MINOS Experiment Accelerator beam of muon neutrinos
produced by NuMI facility at Fermilab.
Near Detector at Fermilab to measure
spectrum and composition of beam.
Far Detector at Soudan mine to study
neutrino disappearance in beam.
Soudan Mine,
Minnesota
735 km
Fermi Laboratory,
Chicago

20. THE MINOS COLLABORATION
Argonne • Athens • Benedictine • Brookhaven • Caltech • Cambridge • Campinas
Fermilab • Harvard • IIT • Indiana • Minnesota Duluth • Minnesota Twin Cities • Oxford
Pittsburgh • Rutherford • Sao Paulo • South Carolina • Stanford • Sussex • Texas A&M
Texas Austin • Tufts • UCL • Warsaw • William & Mary • Wisconsin

21. Fermi Laboratory

22. CDF D0
The Tevatron
The CDF Detector
The D0 Detector

23. Main Injector p p p p Booster NuMI Beam Line
Proton beam generated from H- Source.
– Ramped to 400 MeV in linear accelerator.
– Ramped to 8 GeV in “Booster” ring.
– Ramped to 120 GeV in “Main Injector” ring.
Protons kicked into neutrino beam line.
Booster p p
NuMI
Beam Line
Main Injector p p

24. The NuMI Beam
protons p+ n
The NuMI beam
(“Neutrinos from the Main Injector”)
1.5 km
Direct protons onto 50g segmented graphite target.
– produces an intense flux of secondary pions and kaons.
Focus +/κ+ into tight beam using magnetic focusing.
– requires two 200kA parabolic electromagnets (act as lenses).
Direct +/κ+ into 675m evacuated decay pipe.
– need to point the beam 3 degrees into earth to reach Soudan!
– +/κ+ decay in pipe to produce +/ (and 1% e+/e).
Absorb  in 200m rock to leave pure neutrino beam.
– produce ~1 neutrino for each proton on target. #
– proton beam intensity is 1013 s-1.

25. Guinness Book of World Records 2007.
(page 150)

26. The MINOS Detectors Far Detector Near Detector 1 kT mass
coil
coil
Near Detector
Far Detector
Functionally Identical Detectors
1 kT mass
1 km from target
282 steel planes
153 scintillator planes
100m underground
5.4 kT mass
735 km from target
486 steel planes
484 scintillator planes
700m underground
steel and scintillator
sampling calorimeters.
Magnetized steel (B ~1.3T).
GPS time-stamping for
synchronization.

27.  MINOS Near Detector The MINOS near detector measures the spectrum of
neutrinos before oscillations.
Neutrino beam is pulsed.
– Several interactions per pulse!
Detector composed of many
layers of magnetized steel
and plastic scintillator.
– Neutrinos interact in steel to
produce charged particles.
– Charged particles induce light
emissions in scintillator.
Scintillator planes divided
into many thin strips.
– Light in each strip measured
using photo multiplier tubes.
– Use scintillator hits to piece
together particle tracks
from neutrino interactions. 

28. MINOS Neutrinos
steel
scintillator  
Nuclear
Scattering

29. MINOS Far Detector shaft MINOS Soudan mine
Expected interaction rate is ~1/day.
Far detector is constructed 700m
under the ground to shield against
surface cosmic radiation.
MINOS
Soudan 2/CDMS II
shaft
Photo by Jerry Meier

30. Far Detector Neutrinos
Online event display:

31. Neutrino Oscillations
Use observed near detector energy spectrum to predict far detector
energy spectrum in the absence of neutrino oscillations.
If oscillations have occurred, expect to see an energy-dependent
deficit at the far detector relative to this prediction.
nm spectrum
spectrum ratio
Unoscillated
Oscillated
Simulation
Simulation

32. Neutrino Oscillations observed spectrum ratio
Results after 2½ years of MINOS running:
Phys. Rev. Lett. 101, (2008)
observed nm spectrum
observed spectrum ratio
Difference between masses of muon and tau neutrinos is:
m2 = ( 2.43 ± 0.13 ) x 10-3 eV2

33. Neutrinos in the Early Universe
One of the great unsolved problems
in physics is the imbalance between
matter and anti-matter in the universe.
– Physicists believe that the universe was
created with equal amounts of matter
and anti-matter.
– The matter and anti-matter annihilated
(this is the origin of the cosmic microwave
background that we observe today).
– However, some matter was left over!
Neutrinos provide a possible mechanism for this imbalance.
– The Standard Model allows an asymmetry between neutrinos and anti-neutrinos.
– This asymmetry would have been at work in the early seconds of the universe.
– The extent of this asymmetry can be measured by studying oscillations.
The MINOS experiment is a step along the road to solving this puzzle.

34. Andy’s A-Level Dissertation (1996) “Neutrino oscillations!”
Andy’s PhD Thesis (2005)
“Solar deficit”
“Atmospheric anomaly”
“Neutrino oscillations!”