Transcript ppt

Neutrino Physics
Pedro Ochoa
May 15th 2006
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James Chadwick
I. Historical Background
Radioactive beta decay as understood in the twenties:
n  p  e  like for example in 60Co60Ni  e 
Observed electron
(positron) spectrum
Do you see any problems with this picture?
Energy conservation !
YES !
(also) Recoil of proton not always opposite to electron
(also) Spin seemed non-conserved
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Dear Radioactive Ladies and Gentlemen,
As the bearer of these lines, to whom I graciously ask you to listen, will
Wolfgang Pauli
explain to you in more detail, how because of the "wrong" statistics of the N
and Li6 nuclei and the continuous beta spectrum, I have hit upon a
desperate remedy to save the "exchange theorem" of statistics and the
law of conservation of energy. Namely, the possibility that there could
exist in the nuclei electrically neutral particles, that I wish to call neutrons,
which have spin 1/2 and obey the exclusion principle and which further differ
from light quanta in that they do not travel with the velocity of light. The mass
of the neutrons should be of the same order of magnitude as the electron
mass and in any event not larger than 0.01 proton masses. The continuous
beta spectrum would then become understandable by the assumption that in
beta decay a neutron is emitted in addition to the electron such that the sum
of the energies of the neutron and the electron is constant...
I agree that my remedy could seem incredible because one should have seen those neutrons very
earlier if they really exist. But only the one who dare can win and the difficult situation, due to the
continuous structure of the beta spectrum, is lighted by a remark of my honored predecessor, Mr Debye,
who told me recently in Bruxelles: "Oh, It's well better not to think to this at all, like new taxes". From now
on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge.
Unfortunately, I cannot appear in Tubingen personally since I am indispensable here in Zurich because
of a ball on the night of 6/7 December. With my best regards to you, and also to Mr Back.
Your humble servant
. W. Pauli
Note: In 1933 Pauli recognized the possibility of neutrinos having zero mass.
Do you know why they were not named neutrons after all?
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In 1934, Hans Bethe and Rudolf Peierls showed that the cross-section
(related to the interaction probability) between neutrinos and matter should
be extremely small…. BILLIONS of time smaller than that of an electron.
Most people thought this “neutrino” was never to be observed…
Never say never !
In 1953-56, Frederick Reines and Clyde Cowan made the first observation
of electron antineutrinos.
 How?
Because of tiny cross-section, need very abundant flux of neutrinos
and/or large detector:
-Nuclear bomb
2 choices; go near a:
-Nuclear plant
They chose the nuclear plant of Hanford, Washington (and later on Savannah
river, SC)
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2 things happen after a neutrino interacts in the detector:
 e  p  n  e
n108Cd 109Cd *109Cd  
e  e    
The detection of a gamma after 5µs of the detection of the initial gamma pair
provided a unique signature for antineutrino events.
F. Reines got the Nobel Prize in 1995 for his contributions to neutrino physics.
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A question remained: Are the neutrinos associated with the electron (i.e. from
beta decay) different than the ones associated with the muon (i.e. pion decay)?
In modern terms:
 e   ?


Earlier failed attempts to observe the reaction   e   suggested that even
if the weak coupling appeared to be universal, the two neutrino species were
different.
L. Lederman, M. Schwartz and
J. Steinberger (Nobel Prize
1988), along with other
collaborators answered this
question, by showing that
   n     p goes, but
   n  e  p does not go!
muons leave nice tracks
Beam made mostly of
In 34/40 interactions, they got a muon !
Schematic of the experimental apparatus
used at the Alternating Gradient
Synchrotron at BNL
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It wasn’t until 2000 that the DONUT collaboration reported the observation
of the tau neutrino:
Observed
  d    u
in their detector
(5 interactions!)
Schematic of the DONUT
beam at Fermilab
This concept for making a neutrino beam is very similar to NuMI,
the beam aimed at MINOS.
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But not everything added up !
Since 1969 a physicist named Ray Davis tried to catch a few electron neutrinos
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 37
from the sun every year through the reaction  e  Cl  e  Ar (Argon is a
radioactive noble gas with half life ~35 days)
600 tons of chlorine
expectation based on solar model
Only ~1/2 of the expected neutrinos were found !!! Later, GALLEX, SAGE
and KAMIOKANDE reported similar results.
Either the solar model was wrong or…. (see next slide)
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II. Neutrino oscillations
Underlying principle: weak eigenstates
 mass eigenstates
The oscillation probability is given by:
where E[GeV], L[km], mij2[ eV 2 ], and
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Do you understand this “mixing” concept?
Let’s see what this gives for the 2 flavor model (see board & next slide).
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 e  cos   1  sin   2
We have:
    sin   1  cos   2
If
then
 (0)   e
 (t )  eiE t cos 1  eiE t sin   2
1
2
We obtain:
 L 
 1  sin 2 (2 ) sin 2  
 L0 
2
 L 
   (t )  sin 2 (2 ) sin 2  
 L0 
4p
L0 
m 2
 e  (t )
where
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Do these oscillations happen for real? We’ll try to answer this question…
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But before answering let’s have a word on cosmic rays…
Neutrinos produced by:
cosmic rays
(protons mostly)
strike earth from
all directions
Note that:

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Movie time !
1TeV proton shower on Chicago
http://astro.uchicago.edu/cosmus/projects/aires/
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The Super-Kamiokande Experiment
So cosmic rays give us a practically isotropic flux of muon neutrinos at the
earth’s surface ! The Super-K experiment uses those neutrinos to study
neutrino oscillations:
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Two examples of events at SK:
Muon like event
Electron like event
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What they observed (1998):
expected
observed
best fit
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The interpretation: Observation of     
oscillations!
   (t )
2
2


m
L
2
2

 sin (2 ) sin 
 4E 
Such that:
1.5 103 eV 2  m 2  3.4 103 eV 2
sin 2 (2 )  0.92
at 90% confidence level.
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The SNO Experiment
1kton of heavy water
Neutral current
interaction (through Z)
Charged current
interaction (through W)
Sensitive to  x
Sensitive to  e
In 2001 the SNO collaboration announced that they observed:
1) ~1/3 of the electron neutrinos expected according to the solar model
2) ~exact flux of all types of neutrinos expected according to the model.
 The electron neutrinos are also changing flavor !
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The MINOS Experiment
Fermilab, IL
Soudan, MN
735 km
NUMI beam &
Near detector
  beam
120 GeV protons from
the Main Injector
Near detector
Measures the unoscillated
energy spectrum
# of CC events
NUMI
Far detector
250
m 2  0.002eV 2
sin 2 (2 )  0.9
200
Far detector
150
Measures the oscillated
energy spectrum
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50
0
0
10
20
E (GeV )
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How do you make a beam of neutrinos?
Hadrons decay
into neutrinos (and
other stuff)
Focus positively
charged particles
non-neutrino stuff
gets absorbed
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The two detectors:
Far Detector
Near Detector
Veto Shield
Coil
5.4 kton mass, 8x8x30m
484 steel/scintillator planes
1 kton mass 3.8x4.8x15m
282 steel and 153 scintillator planes
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What MINOS has seen (2006):
completely consistent with:
   (t )
2
 m 2 L 

 1  sin (2 ) sin 
 4E 
2
2
E (GeV)
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MINOS confirmed the hypothesis of      oscillations and will make a
2
10% measurement of m :
2006 results
The future for MINOS
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