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“The Story of the neutrino”

VIEWS: 16 PAGES: 20

									  “The Story of the
     neutrino”
               or


Does the missing matter matter?
                        An Introduction



 2km underground

To observe the Sun
                                                    Sudbury Neutrino
!
                                                    Observatory (SNO)
 21st/22nd March 2002       Particle Physics Masterclass                2
                                     Oxford University
        Discovery of Radioactivity
Henri Becquerel, 1896
                 Accidentally left photographic
                 plates in a dark drawer with some
                 uranium salts...
                                The plates became “fogged”




The Uranium was emitting
something which was
interacting with the film...
 21st/22nd March 2002   Particle Physics Masterclass     3
                                 Oxford University
  Radioactivity and momentum
         conservation
Recoiling (more)                            • A* is at rest (PTOTAL = 0)
stable nucleus                                             Conservation of momentum

                                            •A and B must have equal
              Unstable nucleus              and opposite momenta:
p                                                    p + (-p) = 0
        A
                                            • But, E2 = p2 + m2 and the
       MA        A*                         total energy loss is fixed...
                        B
                                                           Einstein: Relativity
                       mB
                                            • So, there can be only one
                               -p           value of the energy of the
                                            escaping particle...
21st/22nd March 2002        Particle Physics Masterclass                          4
                                     Oxford University
            The Beta-decay puzzle
                                              N
            The electron has a
            range of energies


A nucleus decays giving
out an electron...
                                                           Beta-particle energy
        A                                Energy and momentum
                 A*                      aren’t conserved!?
                       e-                 Something must be wrong
                                          with the picture...?
21st/22nd March 2002        Particle Physics Masterclass                  5
                                     Oxford University
       The “neutrino” hypothesis
What Pauli “saw”:

“What if the missing
energy is carried off by
an otherwise “invisible”
particle...?”

        A
                 A*            C

                       e-
                                                           Wolfgang Pauli, 1930
21st/22nd March 2002        Particle Physics Masterclass                   6
                                     Oxford University
          The birth of the neutrino
    • It must be very light, possibly massless:
                      light
          (sometimes, the electron takes all the energy in the decay)

    • It must be electrically neutral:
                              neutral
          (charge conservation in beta decay)

    • It is produced along with an electron:
          (they can’t be made on their own...)

    • It must interact very rarely:
                            rarely
          (it always escapes the detector without being seen)

Pauli: “I have done a terrible thing. I have invented
a particle which cannot be detected...”
21st/22nd March 2002       Particle Physics Masterclass                 7
                                    Oxford University
        25 years later: Hard but not
                impossible
Beta decay in a                                 It would take 1019 m (300 light years)
nuclear reactor               e-                of water to absorb a single neutrino.

                                                Therefore we need many neutrinos...
    n                     p
                                                    …and a very large detector
                               ne

         ...and it’s the same                          “Inverse” Beta decay
         type of reaction in                                             e+
         “reverse”                                      ne

                                                                   p      n
   21st/22nd March 2002             Particle Physics Masterclass                    8
                                             Oxford University
            Neutrinos from the Sun
  Two protons fuse to                   The biggest nuclear reactor
  give a “deuteron”, a                         of them all...
  positron and an
  electron neutrino
  p                     e+
                  p
                   n
  p                     ne

 10 billion neutrinos from
the sun pass through your
 finger tip every second!
 21st/22nd March 2002    Particle Physics Masterclass          9
                                  Oxford University
     380,000 bottles of cleaning
               fluid
Neutrinos can change
chlorine atoms to argon
atoms by Inverse Beta
Decay
        Cl + ne Þ Ar + e-
          n + n e Þ p + e-
Measure the amount of
argon produced in one
month...
...find 10 atoms in 1030!
21st/22nd March 2002     Particle Physics Masterclass   10
                                  Oxford University
       Some of our neutrinos are
              missing...
 p + p Þ p + n + e + ne + (Energy)                • We “understand” how each
                                                  nuclear reaction works...

                “Light detector”
                                     Sun          • We can measure how much
Total energy Ü
                                                  energy the sun gives out...


 Nneutrinos =    Total energy                     • We can accurately predict the
                Reaction energy                   number of neutrinos we expect!


    Something must be wrong with the neutrinos...
21st/22nd March 2002               Particle Physics Masterclass                11
                                            Oxford University
             Neutrinos as waves...
                 Oscillations!
• What if neutrinos were made
up of two quantum waves with
different wavelengths...?


• These waves would interfere


• Electron neutrinos disappear
depending on the distance and
mass difference of the neutrinos

 Quantum waves interfering
   over 100 million miles!
21st/22nd March 2002   Particle Physics Masterclass   12
                                Oxford University
    SNO - two measurements at
              once
                                             100 million km to the
                                             sun

        2039m to
        surface
1000 tonnes of heavy
    water (D 20)


10,000 light detectors



 21st/22nd March 2002   Particle Physics Masterclass                 13
                                 Oxford University
Sudbury Neutrino Observatory




21st/22nd March 2002   Particle Physics Masterclass   14
                                Oxford University
        SNO - two measurements at
                  once
•Reaction like beta decay to
detect electron neutrinos

          ne + d ® p + p +   e
•All neutrinos can break up
deuterons ( electron , muon or tau)

         nany + d ® p + nany + n
•See   e or n from flash of light
•If oscillation occurs:

         Number of ne
         Number of nany       <1
    21st/22nd March 2002         Particle Physics Masterclass   15
                                          Oxford University
                 Cosmic encounters...
 • High energy protons from
 space in all directions                                           proton

 • Hit the atmosphere and
 produce a “shower” of
 particles

    This is why all
 • This shower includes muons
neutrino detectors
 • These particles decay like
 beta underground
 aredecay to give...
                                                               100 particles m-2s-1
                          ...neutrinos!

   21st/22nd March 2002         Particle Physics Masterclass              16
                                         Oxford University
               Atmospheric neutrinos
• Muons and electrons can’t just be
made or decay on their own...                                              “Shower
                                                                           particle”
•Particle “type” must be conserved
                                                        muon
                                                      neutrino     muon
• A shower particle produces a muon and
a muon neutrino.

• The muon decays to a muon neutrino...                                muon
 ...and an electron along with an electron                    e        neutrino
neutrino
                                                            electron
                                                            neutrino

   21st/22nd March 2002      Particle Physics Masterclass    2 nm : 1 ne          17
                                      Oxford University
                  Detecting neutrino
                    oscillations...
         Soudan 2
Knowing what ratio of
neutrino types to expect...
 Atmospheric       neutrinos

...count neutrinos after
different distances
Compare “up”           and “down”

See how many neutrinos
of each type have
disappeared (oscillated)

21st/22nd March 2002                Particle Physics Masterclass   18
                                             Oxford University
            Soudan 2: Disappearing
                 neutrinos...
  What do electron and muon neutrino interactions look like?
Interacting electron neutrinos
produce electrons                ne           e
Electrons create e+e- pairs
“Electromagnetic Showers”

                                              1000 tonnes of iron!
Interacting muon neutrinos
produce muons
                              nm               muon
Muons are heavier than
electrons Þ Straight track
until it runs out of energy
   21st/22nd March 2002   Particle Physics Masterclass               19
                                   Oxford University
           What we (don’t) know...
• There are three types of                     e        mu      tau
neutrino, each associated
                                            nu-e       nu-mu   nu-tau
with a “lepton”
• They appear to change
(oscillate) between neutrino
types


• Real quantum waves in
action over millions of miles!

 21st/22nd March 2002   Particle Physics Masterclass              20
                                 Oxford University

								
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