Finding the Invisible

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Finding the Invisible

• Stephen Miller
• Saturday Morning Physics
• October 25, 2003

Detailed drawing of invisible particle
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Atoms
Atoms built from Electrons, Protons,
Neutrons
Particle Charge Proton
1 Neutron 0 Electron
- 1
Not to Scale!
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Atoms to Scale
Helium Nucleus
P
N
P
N
Nearest Electron Outside Building
Matter is mostly empty space!
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Electromagnetic Force

• Carried by Photon
• Force due to exchange of photons
• Photon is massless
• Long Range Force
• Keeps electrons around nucleus
• Opposite charges attract
• Keeps electrons apart
• Like charges repel
• Keeps us from falling through our chairs
• And from sliding off

Charged particles interact by photon exchange
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Protons, Neutrons and Quarks
Protons and neutrons made out of quarks
Quark Charge up
2/3 down -1/3
Quarks held together by strong force - carried by
gluons Only acts over a short range
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Radioactive Beta Decay

• Electron (beta particle) emitted in radioactive
  decay of nucleus
• Energy of electron and left over nucleus didnt
  add up to total
• Energy spectrum of electron was continuous
• Apparent violation of law of conservation of
  energy and momentum
• Solution An undetected particle
• (Pauli 1930)
• Neutrino little neutral one
• (Fermi 1933)

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Neutron Decay

• Decay caused by the Weak Force
• Weak Force particle is W boson
• Weak decay
• 1 particle turns into a lighter particle
• W splits into electronneutrino
• Neutron decay
• down ? up electron neutrino
• Neutrino
• Electrically neutral
• No color charge

ne
e
u
W-
d
Feynman diagram
time
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Neutrino Facts
About 10 trillion neutrinos pass through your
body each second
Most come from the sun Some come from cosmic
rays Some are leftover from the big bang
A neutrino could go through a lead block the
length of a light year without interacting
How can we detect neutrinos?
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Neutrino Literature
Cosmic Gall John Updike 1960 NEUTRINOS, they
are very small. They have no charge and have no
mass And do not interact at all. The earth is
just a silly ball To them, through which they
simply pass, Like dustmaids down a drafty hall
Or photons through a sheet of glass. They snub
the most exquisite gas, Ignore the most
substantial wall, Cold shoulder steel and
sounding brass, Insult the stallion in his
stall, And scorning barriers of class,
Infiltrate you and me! Like tall and painless
guillotines, they fall Down through our heads
into the grass. At night, they enter at Nepal
and pierce the lover and his lass From
underneath the bed-you call It wonderful I call
it crass.
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Discovery of Neutrinos
Neutrinos detected from fission in nuclear
reactor
Reines and Cowan- 1956
Reines gets Nobel prize - 1995
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Neutrino Detection
Neutrinos only interact via the Weak force Weak
force particles have mass Force is weak and has
very short range
Inverse beta decay - incoming neutrino turns
into electron
Key is an intense source of neutrinos and/or a
large detector
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Muon Neutrino
2nd kind of neutrino discovered - muon neutrino
(1962) produces a muon instead of electron
Neutrino beam produced by decay of particles
produced by Particle accelerator
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Leptons
Electron, Muon, Tau have negative charge
Neutrinos 3 types have no charge
Tau neutrino directly observed in 2000
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Solar Neutrinos
Neutrinos produced in fusion reaction inside
sun Light produced in sun takes million years
to escape the sun due to scattering inside
sun Neutrinos get out of sun in a few seconds
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Homestake Solar Neutrino Detector
Large tank of C2Cl4 (cleaning fluid) in
Homestake mine
Uses reaction ? Cl ? Ar e-
Count Argon atoms produced Expected about 1 a day
Started experiment 1968 Ray Davis- Nobel prize
2002
Found only 1/3 as many Argon atoms as expected
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Super Kamiokande Detector
40 meters in diameter, 40 meters tall 12.5
million gallons of water Inside a mountain
Inside walls lined with phototubes to detect
light
Cleaning detector during filling - no
radioactive dust allowed
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Water Cerenkov Detectors

• Neutrino scattering
• Neutrino interacts with electron via the weak
  force
• Electron goes in neutrino direction
• Can measure time and direction of neutrino

• Cerenkov radiation
• Charged particles going faster than light in
  matter
• Creates shockwave of visible light similar to
  sonic boom
• Visible ring of light can be detected

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Neutrino Detection at Super-K
Ring shows direction of electron Can point back
to the sun
The sun as seen with neutrinos
Super-K also sees deficit of neutrinos from the
sun
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The solar neutrino problem

• Multiple experiments saw 2-3x fewer neutrinos
  than expected from the sun
• Was it experimental error?
• Mistake in theoretical calculation?
• Or some new physics at work?
• Answer New Physics!
• Neutrino oscillations - ?e ? ?µ
• Neutrinos can change from one type to another
• Implies that neutrinos have mass

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The SNO Detector
Underground Water Cerenkov Detector
Heavy water sensitive to all 3 kinds of neutrinos
Total number of neutrinos detected equal to
number expected from the sun
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Neutrino Oscillation Experiments
Study neutrino oscillations in a controlled way
Use neutrino beam produced by accelerator
Beam from Fermilab to Minos
detector
Mini-Boone experiment at Fermilab with U of M
physicists Uses large tank of liquid
scintillator detects both Cerenkov and
scintillation light
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Supernovas
Massive stars die by exploding as a Supernova
Supernova can be as bright as galaxy
98 of energy released as neutrinos
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Supernova SN1987a

• 1987 Supernova explodes in nearby galaxy
• IMB and Kamiokande detectors observe a burst of
  neutrinos at the same time
• Confirms theory of stellar collapse
• Current neutrino detectors are networked to
  report coincidence of neutrino bursts SNEWS
• Provides early warning for supernovae
• But only expect about 1/100 years in the local
  part of the universe

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Cosmic Neutrinos

• High energy neutrinos also arrive from distant
  cosmic sources
• High energy charged particles also arrive from
  cosmic sources
• Can be absorbed by dust
• Trajectory bent by magnetic fields in galaxy,
  near earth
• A new way of doing astronomy
• Low rate of cosmic neutrinos
• Requires very large detector

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Amanda/ICE Cube Detector

• Need very large detector to detect low rate of
  cosmic neutrinos
• Cerenkov light in frozen water
• Large supply at south pole
• Ice under pressure has no bubbles
• Light travels a long way
• Phototubes deep under ice cap
• Muon emits Cerenkov light
• See upward going muon
• Created by neutrino interaction in the ice
• Experiments also exist using the ocean and deep
  lakes

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Dark Matter
Measure rotation speed of stars in galaxies Stars
close to center should move faster than outer
stars Depends on the amount of mass (gravity) in
the galaxy
Rotation speed inconsistent with mass in
stars Implies mass contained in Dark Matter
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WIMP Detector
Weakly Interacting Massive Particles
Only interact via weak force Small amount of
energy in collision Need very sensitive detectors
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DAMA Experiment
Earth moves through WIMP wind Rate of events
depends on earth moving with or against the wind
Observed rate depends on time of year evidence
for WIMPS! Result needs to be confirmed by
other experiments
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Missing Energy in Collisions
We detect neutrinos in collider experiments
from missing energy Searching for WIMPs produced
and detected in the same way
Event Display of Collision
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What to Remember

• Billions of neutrinos pass through you each
  second
• Most produced in the sun
• Neutrinos only interact via weak interaction
• Makes them difficult to detect
• With large enough detector can detect neutrinos
  
• From sun, cosmic rays, accelerators
• Deficit of neutrinos observed from the sun
• Really neutrinos oscillating into other type of
  neutrinos
• New kind of particle WIMP responsible for
  dark matter
• Still trying to discover and understand this
  particle

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Some Particle Physics Websites

• particleadventure.org a great intro to particle
  physics
• www.fnal.gov Fermilabs website
• www-cdf.fnal.gov The CDF experiment I work on

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