Transcript Slide 1

Neutrino Physics 2
Pedro Ochoa
May 22nd 2006
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What about solar neutrinos and the solar neutrino problem?
 Kamland is an experiment which studies the disappearance of reactor neutrinos
Kashiwazaki
KamLAND uses
the entire Japanese
nuclear power
industry as a
longbaseline source
Takahama
Ohi
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In a fission reactor, there is a flux of v e associated with
that you can predict to a good accuracy.
235U, 239Pu, 241Pu
and 238U
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You can detect these antineutrinos via inverse beta decay:
ve  p  n  e
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How the detector looks from the inside
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At Kamland’s average L of about 180 km, the disappearance probability in the
three neutrino model is, to a very good approximation:
2

2
2 m12 L 

P(ve  ve )  cos 13 1  sin 212 sin
4E 

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So if there are oscillations, this spectrum will be distorted:
Notice
similarity with 2
flavor approx.
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 Oscillations were observed indeed (2002)!! Kamland was actually the first
experiment to observe the disappearance of “earthly” electron antineutrinos
Other experiments hand’t seen
anything (they were too close)
Conclusive evidence of reactor
N obs  Nbkgd
N no osc
v e disappearance:
 0.658 0.044(stat)  0.047(syst )
( 99.998% C.L.)
The solar neutrino problem was finally solved !
Best fit values:
m122  7.9 105 eV 2
tan2 12 0.45
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III. Open Questions
What are some of the unsolved problems in Neutrino Physics?
We know that neutrinos have mass !!!!!
Things make sense in the light that
neutrino mass eigenstates mix with the
weak eigenstates creating the oscillation
phenomenon measured in many
experiments. We think that:
m122  8  10 5 eV 2
2
m23
 3  10 3 eV 2
tan 2  12 0.45
sin 2 ( 2 23 )  0.9
Everything fits the model extremely well !
Well, almost…. there is always a
black sheep !!
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Let’s first discuss “The LSND Anomaly” (1995)
The Liquid Scintillator Neutrino Detector Experiment:
• Beam of protons on water
produces π+ mainly
     
Stop at Cu
e e 
target
Oscillations?
e
• Search for  e through
Baseline ~30 m
 e p  e n
Neutrino Energy
20-55 MeV,
detect prompt e track,
20<Ee<60 MeV
(+ scintillation)
• neutron capture:
np  d
e

2.2 MeV scintillation
signal, 186µs later
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The interior of the LSND detector:
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What the LSND experiment saw:
• Through ve
•
 p  e  n they observed:
v e excess of:
87.9  22.4  6.0
• oscillation probability:
(0.264 0.067 0.045)%
Do you see a problem with this picture?
Yes !! Only two independent m2 if three neutrinos !!
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How to explain the LSND anomaly?
1) There are more neutrinos :
But LEP showed that there are three active (i.e. that interact with the Z)
neutrino flavors only… the extra neutrinos would have to be sterile !
2) CPT Violation (in other words, mv  mv ):
Before it could all nicely fit in a
spectrum like
But now it would have to be
something like
, which is unlikely.
3) Some weird combination (CPT + 1 sterile neutrino, sterile
neutrino decay…)
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Other experiments have ruled out parts of the LSND allowed region:
The MiniBoone experiment at Fermilab will
be able to put this issue to rest
800 tons of mineral oil
D=12m
green=unexplored
If MiniBoone finds a signal  new exciting physics !!
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2
5
2

m

8

10
eV
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Let’s change topics now. Earlier I said that:
tan 2  12 0.45
 What parameter am I leaving out?
13 !!!
2
m23
 3  10 3 eV 2
sin 2 ( 2 23 )  0.9
Is it zero or just
very small?
Nobody knows…
U
 13 is directly related to whether or not there is CP violation in the
neutrino sector!
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Best limit on θ13 comes from a reactor experiment called CHOOZ:
MINOS will actually expand that limit (or
discover θ13):
m2 = 0.0025 eV2
At their baseline (~1km):
P(ve  ve )  1  sin 2 213 sin 2
m L
4E
2
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At MINOS baseline (~735 km):
2
2
2
2 m23 L
P(v  ve )  sin 213 sin  23 sin
4E
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NOVA (NuMI Off-Axis Experiment) will be able to assess this much better:
Use the same NuMI beam that used for
MINOS !!
But remember:
2
m23
L
P(v  ve )  sin 213 sin  23 sin
4E
By going off-axis we can get more
neutrinos in the energy region where
we’re more interested.
2
2
2
At 14mrad the spectrum peaks just
above the first oscillation maximum.
Given the current limit of θ13 set by
CHOOZ, the oscillation probability
cannot be larger than 5%. This is why
to study these oscillations we need a
monster detector.
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What will be achieved:
NOVA may also address one of the biggest puzzles in neutrino physics:
What is the
right hierarchy?
(note: we know
m221 > 0 from
solar neutrinos)
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This would be achieved through something called Matter Effect:
The basic concept is that electron
neutrinos, besides oscillating in
the usual way, can interact with
the electrons in rock while they
propagate:
ve  e  ve  e
e
ve
W
ve
t
e
But this will not happen for
ve , v ,v ,
This creates a small difference in
the probability of seeing v e vs. v e
The direction of this effect
depends on the mass hierarchy.
This effect must be disentangled from possible CP violation, which also implies
that P(v  ve )  P(v  ve )
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Besides the hierarchy, there’s something about neutrino masses we still don’t
know:
??
 We don’t know the absolute scale of the neutrino masses !!
There is actually a way of searching for this directly…
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Neutrino mass should affect the spectrum of tritium decay:
Endpoint energy
E=18.57keV
An experiment called KATRIN (Karlsruhe Tritium Neutrino Experiment) in Germany
will look for this effect.
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Observing this effect is a major technological challenge. The way they’ll do it is
with a “MAC-E-Filter”:
The beta electrons are
transformed into a broad
beam of electrons flying
almost parallel to the
magnetic field lines.
Because of the electrostatic
potential, all electrons with
enough energy to pass the
barrier will make it to the
detector.
Varying the E-field allows to
measure the beta spectrum
in a integrating mode.
KATRIN will be able to measure the neutrino mass down to 0.2eV (90% CL).
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The other way is through Neutrinoless Double Beta Decay (denoted 0vββ):
2 neutrino Double Beta Decay is actually known to exist and allowed by the
Standard Model:
No piece of cake. First
calculated in 1935 by M.
Goeppert-Mayer, and
first observed in 1987
(any ideas why so hard
to observe?)
But 0vββ has not been seen (convincingly at least):
What is the condition for
this to happen? That
neutrinos are their own
antiparticle !! (i.e. they
are Majorana Particles)
If 0vββ is observed then we’d know for sure that v
v
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The 2vββ decay is a major background for the 0vββ search.
It is a very hard measurement !!
T1/2 (2vββ) ~ 1020 years
T1/2 (0vββ) ~ 1025-27 years.
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If 0vββ is observed, would that tell us something about the neutrino mass?
Yes !! The effective neutrino mass (due to mixing) can be disentangled:
1
 GF M 0v
T1/ 2
Called the
Matrix
Element
2
m
2
Remember:
electron neutrino
has no definite
mass
Calculating the matrix elements is no picnic, and many authors disagree
among themselves:
Rodin et al,
nucl-th/0503063
Several 0vββ candidates. Each experiment
uses a different one (and different technique)
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We cannot go through all the proposed experiments that are going to try to measure
0vββ.
However, you should know that somebody claims to have observed it already.
Most sensitive experiments to date are based on germanium 76. This is
one of them.
Note that only a subset of the Heidelberg-Moscow collaboration claims
the observation of 0vββ. The collaboration actually split over this.
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Would you buy this?
Other experiments will
look in the same region
to confirm/disprove it.
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SUMMARY & CONCLUSIONS
Neutrino physics is a field full of surprises, and with
plenty of room for discovery !!
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