The Neutrino World

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Status and New Opportunities in Neutrino Physics
Boris Kayser PANIC05 October 26, 2005
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Neutrinos and photons are, by far, the most
abundant particles in the universe.
The study of neutrinos involves

• Astrophysics
• Nuclear physics
• Particle physics
• Accelerator and detector technology

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Solar Neutrinos
History
Nuclear reactions in the core of the sun produce
?e. Only ?e.
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Theorists, especially John Bahcall, calculated
the produced ?e flux vs. energy E.
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Ray Davis Homestake experiment measured the
higher-E part of the ?e flux ??e that arrives at
earth.
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The Homestake experiment could detect only ?e.
It found
The Possibilities
The theory was wrong.
The experiment was wrong.
Both were wrong.
Neither was wrong. Two thirds of the ?e flux
morphs into a flavor or flavors that the
Homestake experiment could not see.
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The Resolution

• Sudbury Neutrino Observatory (SNO) measures, for
  the high-energy part of the solar neutrino flux
• ?sol d ? e p p ? ??e
• ?sol d ? ? n p ? ??e ??? ???
• From the two reactions,

??e ??e ??? ???
0.340 0.023 (stat) 0.030 (syst)
Clearly, ??? ??? ? 0 . Neutrinos change
flavor.
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SNO ??e ??? ??? (4.94 ? 0.21 ? 0.36) ?
106/cm2sec Theory ?total
(5.69 ? 0.91) ? 106/cm2sec
Bahcall, Basu, Serenelli
John Bahcall and Ray Davis both stuck to their
guns for several decades, and both were right all
along.
Change of neutrino flavor implies neutrino mass.
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What We Have Learned
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The (Mass)2 Spectrum
or
(Mass)2
?m2atm
Inverted
Normal


?m2sol 8 x 105 eV2, ?m2atm 2.5
x 103 eV2
Are there more mass eigenstates, as LSND suggests?
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Leptonic Mixing
When W ? l? ?? , the produced neutrino
state ??gt is ?? gt ? U?i ?igt
. Neutrino of flavor ? Neutrino of
definite mass mi Unitary
Leptonic Mixing Matrix Inverting ?i gt ? U?i
??gt . Flavor-? fraction of ?i U?i2 .
e, ?, or ?
i
i
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The spectrum, showing its approximate flavor
content, is
Inverted
Normal
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The Mixing Matrix
Solar
Atmospheric
Cross-Mixing
cij ? cos ?ijsij ? sin ?ij
Majorana CP phases
?12 ?sol 34, ?23 ?atm 37-53, ?13 lt
10 ? would lead to P(??? ??) ? P(??? ??).
CP But note the crucial role of s13 ? sin ?13.

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The Open Questions Identified by the APS
Multi-Divisional Study
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Neutrinos and the New Paradigm

• What are the masses of the neutrinos?
• Is the spectrum like or ?
• What is the pattern of mixing among the different
  types of neutrinos?
• What is ?13? Is ?23 maximal?

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• Are neutrinos their own antiparticles?
• Do neutrinos violate the symmetry CP? Is P(?? ?
  ??) ? P(?? ? ??) ?

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Neutrinos and the Unexpected

• Are there sterile neutrinos?

• Do neutrinos have unexpected or exotic properties?

We must be alert to further surprises.
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• What can neutrinos tell us about the models of
  new physics beyond the Standard Model?
• The See-Saw Mechanism relates ? masses to physics
  at the high-mass scale where the forces become
  unified.
• A signature feature of the See-Saw is that ? ?.

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Neutrinos and the Cosmos

• What is the role of neutrinos in shaping the
  universe?
• Is CP violation by neutrinos the key to
  understanding the matter antimatter asymmetry
  of the universe?
• What can neutrinos reveal about the deep interior
  of the earth and sun, and about supernovae and
  other ultra high energy astrophysical phenomena?

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How the Questions May Be Answered
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Are Neutrinos Their Own Antiparticles?
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Why Many Theorists Think L Is Not Conserved
The Standard Model (SM) is defined by the fields
it contains, its symmetries (notably Weak Isospin
Invariance), and its renormalizability. Anything
allowed by the symmetries occurs in nature. The
SM contains no ? mass, and no ?R field, only
?L. This SM conserves the lepton number L. But
now we know the neutrino has mass. If we try to
preserve L, we accommodate this mass by adding a
Dirac, L - conserving, mass term mD?L?R.
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To add a Dirac mass term, we had to add ?R to the
SM. Unlike ?L , ?R carries no Weak Isospin. Thus,
no SM symmetry prevents the occurrence of the
Majorana mass term mM?Rc ?R. This mass term
causes ? ? ?. It does not conserve L. If anything
allowed by the extended SM occurs in nature, then
L is not conserved.
If the nonconservation of L comes from Majorana
masses, any attempt to find it must overcome the
smallness of neutrino masses.
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To Demonstrate That ?i ?i Neutrinoless Double
Beta Decay 0???
By avoiding competition, this process can cope
with the small neutrino masses.
Observation would imply L and ?i ?i .
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In Pursuit of ?13

• Both CP violation and our ability to tell
  whether the spectrum is normal or inverted depend
  on ?13.

If sin22?13 lt 0.01, a neutrino factory will be
needed to study both of these issues.
How may ?13 be measured?
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sin2?13
?3
?m2atm
(Mass)2
?2

?m2sol
?1

• sin2?13 ?Ue3?2 is the small ?e piece of ?3.
• ?3 is at one end of ?m2atm.
• ?We need an experiment with L/E sensitive to
  ?m2atm (L/E 500 km/GeV) , and involving ?e.

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Complementary Approaches
Reactor Experiments
Reactor ?e disappearance while traveling L
1.5 km. This process depends on ?13 alone
P(?e Disappearance)
sin22?13 sin21.27?m2atm(eV2)L(km)/E(GeV)
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Accelerator Experiments
Accelerator ?? ? ?e while traveling L gt Several
hundred km. This process depends on ?13, ?23, on
whether the spectrum is normal or inverted, and
on whether CP is violated through the phase ?.
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(Lindner)
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The Mass Spectrum or ?
Generically, grand unified models (GUTS) favor
GUTS relate the Leptons to the Quarks.

is un-quark-like, and would probably
involve a lepton symmetry with no quark analogue.
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How To Determine If The Spectrum Is Normal Or
Inverted
This changes both the spectrum and the mixing
angles.
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Matter effects grow with energy E. At E 1 GeV,
matter effects ? sin2 2?M sin2 2?13 1 S
. Signm2( ) - m2( ) At
oscillation maximum, P(??? ?e) gt1 P(???
?e) lt1 30 E 2 GeV (NuMI)
10 E 0.7 GeV (T2K)

()
()

Note fake CP violation.

The effect is
T2K cannot address the mass hierarchy. (Feldman)
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Larger E is better. But want L/E to correspond
roughly to the peak of the oscillation. Therefore,
larger E should be matched by larger L. Using
larger L to determine whether the spectrum is
normal or inverted could be a special
contribution of the U.S. to the global program.
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The most popular theory of why neutrinos are so
light is the See-Saw Mechanism
Why would CP in ? oscillation be interesting?

Familiar light neutrino
?

Very heavy neutrino
N
The heavy neutrinos N would have been made in the
hot Big Bang.
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The heavy neutrinos N, like the light ones ?, are
Majorana particles. Thus, an N can decay into l
or l.
If neutrino oscillation violates CP, then quite
likely so does N decay. Then, in the early
universe, we would have had different rates for
the CP-mirror-image decays N ? l
and N ? l This would have led to
unequal numbers of leptons and antileptons
(Leptogenesis). Perhaps this was the original
source of the present preponderance of Matter
over Antimatter in the universe.
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How To Search for CP
?? ? ?? is a different process from ?? ? ??
even when ?i ?i
?e ? ??
e-
?-
?
Source
Detector
?e ? ??
?
e
?
Source
Detector
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The NuMI T2K Relationship
Eventually, the CP and matter-effect sources of
any ? ? asymmetry must be disentangled.
Distance L
Energy E
Owing to its higher E, the NuMI experiments will
have a three-fold bigger matter
effect. Combining the NuMI and T2K results will
greatly facilitate the separation of CP from
matter effects.
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The Future The Proton Driver and Large Detector
These facilities, or their equivalents, are
needed if we are to be able to determine whether
the spectrum is normal or inverted, and to
observe CP violation, for any sin22?13 gt (0.01
0.02).
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Neutrino Factories and ? Beams
The ultimate in sensitivity, with intense,
flavor-pure beams.
Neutrino Factory A muon storage ring, producing
neutrinos via ? ? e ?e ??

• ? Beam A boosted-radioactive-ion storage ring,
  producing neutrinos via
• 18Ne ? 18F e ?e
• Then look for ?e ? ??

Monoenergetic ?e from e- capture
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Conclusion
We have a very rich opportunity to do exciting
physics.
Neutrino physics has connections to astrophysics
and cosmology, and to both nuclear and particle
physics.
Answering the questions raised by the discovery
of neutrino mass should prove very interesting!
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BACKUP SLIDES
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From Y. Suzuki at Lepton -Photon 2005
From L/E
Atmospheric ?m2 and mixing angle from
SuperKamiokande L/E analysis and full data set
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From nucl-ex/ 0502021
Solar ?m2 and mixing angle from SNO analysis of
solar neutrino and KamLAND data
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The APS Study
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A year-long study of the future of neutrino
physics, sponsored by the American Physical
Society Divisions of
Nuclear Physics Particles and Fields Astrophysics
Physics of Beams
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The APS Multi-Divisional Neutrino Study

• Over 200 Participants
• Seven Working Groups
• Organizing Committee Janet Conrad, Guido
  Drexlin, Belen Gavela, Takaaki Kajita, Paul
  Langacker, Keith Olive, Bob Palmer, Georg
  Raffelt, Hamish Robertson, Stan Wojcicki, Lincoln
  Wolfenstein
• Co-Chairpersons Stuart Freedman, Boris Kayser

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• The aim To develop a strategy for the U.S. role
  in a global neutrino program.
• The U.S. effort should complement, and cooperate
  with, the efforts in Europe and Asia.

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Our Main Report, The Neutrino Matrix, and the
reports of the Working Groups, may be found at
www.aps.org/neutrino
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How Can We Demonstrate That ?i ?i?
We assume neutrino interactions are correctly
described by the SM. Then the interactions
conserve L (? ? l ? ? l). An Idea that Does
Not Work and illustrates why most ideas do not
work Produce a ?i via
Spin
Pion Rest Frame
?
?i
?
?
Give the neutrino a Boost
??(Lab) gt ??(? Rest Frame)
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• The SM weak interaction causes

?
?

?i
Target at rest
Recoil
?i ?i means that ?i(h) ?i(h).
helicity
?
?

??????i
,
????i
?
??????i
will make ? too.
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Minor Technical Difficulties

• ??(Lab) gt ??(? Rest Frame)
• E?(Lab) E?(? Rest Frame) m? m?
• ? E?(Lab) gt 105 TeV if m? 0.05 eV
• Fraction of all ? decay ?i that get helicity
  flipped
• ? ( )2 10-18 if m??? 0.05 eV
• Since L-violation comes only from Majorana
  neutrino masses, any attempt to observe it will
  be at the mercy of the neutrino masses.
• (BK Stodolsky)

? gt
i

i
m? E?(? Rest Frame)
i
i
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