Transcript Neutrino and Nucleon Decay Physics with Water Detectors

Neutrino Oscillations,
Proton Decay
and Grand Unified Theories
D. Casper
University of California, Irvine
Outline
A brief history of neutrinos
How neutrinos fit into the “Standard Model”
Grand Unified Theories and proton decay
Recent neutrino oscillation discoveries
Future prospects for neutrino oscillation and
proton decay
A Desperate Remedy
Enrico Fermi
Wolfgang
Pauli
Operation Poltergeist
Clyde Cowan
Fred Reines
Two Kinds of Neutrinos
Reines and Cowan’s
neutrinos produced in

reaction: n   e p
Observed reaction was:   p  e  n
Muon decay was known to involve two


neutrinos:   e 
If only one kind of neutrino, the rate for the
unobserved process:    e  much too large
Proposal: Conserved “lepton” number and
two different types of neutrinos(e and )
Produce beam with neutrinos from      ( )
Neutrinos in beam should not produce electrons!
Last, but not least…
Three’s Company
Number of light
neutrinos can be
measured!
Lifetime (and width) of
Z0 vector boson
depends on number of
neutrino species
Measured with high
precision at LEP
N = 3.02 ± 0.04
Probably no more
families exist
Particles of
“The Standard Model”
Three “families” of particles
Families behave identically, but have
different masses
Keeping it “in the family”?
Quarks from different families have a
small mixing – do the neutrinos also
mix?
Each quark comes in three “colors”
The electron and each of its “copies”
has a neutrino associated with it
Neutrinos must be massless, or the
theory must have something new
added to it.
u  e 
  
d d  e 
c c c  

  
 s s s  
u

d
u
 t t t   

  
b b b  
Quarks
Leptons
Forces of The Standard Model

Z

Z
W
W
Gravity – the weakest, not included
in Standard Model
Electromagnetism – charged
particles exchange massless
photons
Strong force – holds quarks
together, holds protons and
neutrons together inside nucleus;
particles exchange massless
“gluons”
Weak force – responsible for
radioactivity; particles exchange W
and Z particles

Z
 u u u  e 

  
d d  e 
 dgluons
W
Four known forces hold
everything together:

Z
c c c  

  
 s s s  
 t t t   

  
b b b  
Weakly Interacting Neutrinos
Neutrinos interact only via the
two weakest forces:
u
d
Gravity
Weak nuclear force
W and Z particles extremely
massive
W+
W mass ~ Kr atom!
Force extremely short-ranged
This makes the weak force weak
Neutrinos pass through lightyears of lead as easily as light
passes through a pane of glass!
µ
µ
Mysteries of the Standard Model
Why three “families” of quarks and leptons?
Why are do particles have masses?
Why are the masses so different?
m < 10-11  mt
Are neutrinos the only type of matter without
mass?
Can quarks turn into leptons?
Are there really three subatomic forces, or
just one?
Grand Unified Theories
Maybe quarks and
leptons aren’t different
after all?
Maybe the three
subatomic forces aren’t
different either?
Maybe a more complete
theory can predict
particle masses?
X,Y
u  e 
  
d d  e 
c c c  

  
 s s s  
u

d
u
 t t t   

  
b b b  
Proton Decay
Generic prediction of most
Grand Unified Theories
Lifetime > 1033 yr!
Requires comparable number of
protons
Colossal Detectors
Proton decay detectors are
also excellent neutrino
detectors (big!)
Neutrino interactions are a
contamination which proved
more interesting than the (as
yet unobserved) signal
0
Proton
Proton Decay
e+
e
0
Neutron
Neutrino
e–
Proton
IMB
World’s first large, ringimaging water detector
Total mass 8000 tons
Fiducial mass 3300 tons
2048 Photomultipliers
Built to search for
proton decay
Operated 1983-1990
Water Cerenkov Technique
Muon
Cheap target material
Surface
instrumentation
Vertex from timing
Direction from ring
edge
Energy from pulse
height, range and
opening angle
Particle ID from hit
pattern and muon
decay
Electron
The Rise and Fall of SU(5)
SU(5) grand-unified
theory predicted proton
decay to e+0 with
lifetime 4.51029±1.7
years
With only 80 days of
data, IMB was able to
set a limit > 6.51031
years (90%CL)
SU(5) was ruled out!
Nova
February 1987: Neutrino
pulse from Large Magellanic
Cloud observed in two
detectors
Confirmed astrophysical
models
Neutrino mass limits
comparable to the best
laboratory measurements of
that time (from 19 events!)
Atmospheric Neutrinos
Products of hadronic showers in
atmosphere
2:1 µ:e ratio from naive flavor
counting
Flavor ratio (/e) uncertainty
± 5%
Neutrinos produced above
detector travel ~15 km
Neutrinos produced below
detector travel all the way
through the Earth (13000 km)
Primary cosmic ray
/K


e


Neutrino Interactions
“Contained” (e , )
Fully-Contained (FC)
Partially-Contained (PC)
“Upward-Muon” ()
Stopping
Through-going
Difficult to detect 
Not enough energy in most
atmospheric neutrinos to
produce a heavy  particle
The Atmospheric Neutrino Problem
Early large water
detectors measured
significant deficit of 
interactions
What happened to
these neutrinos?
Smaller detectors did
not see the effect
Needed larger and
more sensitive
experiments, improved
checks
Neutrino Oscillation
Quantum mechanical interference effect:
Start with one type of neutrino and end up with
another!
Requires:
Neutrinos have different masses (m20)
Neutrino states of definite flavor are mixtures of
several masses (and vice-versa) (mixing 0, like
quarks mix)
Simplest expression (2-flavor):
Oscillation probability = sin2(2) sin2(m2L/E)
Checking the Result
A number of incorrect “discoveries” of neutrino
oscillation made over the years
Atmospheric neutrino problem was treated with
(appropriate) skepticism
Less exotic explanations were explored:
Incorrect calculation of expected flux?
Many comparisons of calculations failed to find any mistake
Systematic problem with particle ID?
Beam tests of water detector particle ID performed at KEK lab in
Japan – proved that water detectors can discriminate e and 
Conclusive confirmation required with higher
statistics, improved sensitivity
Super-Kamiokande
Total Mass: 50 kt
Fiducial Mass: 22.5 kt
Active Volume:
33.8 m diameter
36.2 m height
Veto Region: > 2.5m
11,146  50 cm PMTs
1,885  20 cm PMTs
Evidence for Oscillation
SuperK also sees deficit of
 interactions
Also clear angular (L) and
energy (E) effects
Finally a smoking gun!
All data fits  
oscillation perfectly
Surprise:
Maximal mixing between
neutrino flavors
SuperK Preliminary
1289 days
best fit:
sin22=1.0
m2 = 2.5  10-3 eV2
2 = 142/152 DoF
no oscillation:
2 = 344/154 DoF
Checking the Result (Again)
Look for expected
East/West modulation
of atmospheric flux
Due to earth’s B field
Independent of
oscillation
Fit the data to a
function of sin2(LEn)
Best fit at ~-1 (L/E)
The Solar Neutrino Problem
Homestake
experiment first to
measure neutrinos
from Sun, finds huge
deficit (factor of 3!)
Anomaly confirmed
by SAGE, GALLEX,
Kamiokande
experiments
Ray Davis
SuperK Solar Neutrinos
Real-time measurement
allows many tests for
signs of oscillation:
Day/Night variation
Spectral distortions
Seasonal variation
Allowed oscillation
parameter space is
shrinking
SMA is disfavored by SK
data
SNO
Water detector with
a difference:
Heavy water
Able to measure
charged current (e)
and neutral current
(x)
Can determine
(finally!) whether
solar neutrinos are
oscillating or not
Resolving the Solar Neutrino Problem
In July, 2000 SNO
published their first results
Measured the rate of D
charged-current scattering
(only e)
Compare with SuperK
precision measurement of e
scattering (x)
Significant difference
between flux of e and x
implies non-zero  +  flux
from the Sun: oscillation!
Combined flux of all
neutrinos agrees well with
solar model
SuperK pe+0
Require 2-3 showering rings, 0 e
0 mass cut if 3 rings
Overall Detection Efficiency: 43%
No candidates (0.2 background expected)
/ > 5.7 × 1033 yrs (90% CL)
16O15N* +
Prompt
6.3 MeV
K+, K+ +
236 MeV/c
+

16O
p


Present limit for K+:
/>21033 years
No candidates
Status of Proton Decay
The K2K Experiment
K2K Results
56 events observed at
Super-K, vs. 80±6
expected
Energy spectrum of
observed events
consistent with
oscillation
Appears completely
consistent with SuperK
More data next year
nd
2
Generation LongBaseline
(MINOS,CNGS)
730 km baselines
MINOS:
Factor ~500 more events than
K2K (at 3 distance)
Disappearance and appearance
(e, ) experiments
CNGS
Higher-energy beam from
CERN to look for  appearance
at Gran Sasso
Only a handful of signal events
expected
JHF/SuperK Experiment
Approved:
50 GeV PS
0.77 MW
(K2K is 0.005 MW)
Proposed:
Neutrino beamline to
Kamioka
Upgrade to 4 MW
Outlook:
Completion of PS in
2006
Neutrino Factory
The Ultimate Neutrino
Beam:
Produce an intense beam
of high-energy muons
Allow to decay in a
storage ring pointed at a
distant detector
Perfectly known beam
Technically very
challenging!
UNO (and Hyper-Kamiokande)
Fiducial Mass: 450 kton
20  Super-Kamiokande
Sensitive to proton
decay up to 1035 yr
lifetime
Able to study leptonic
CP violation
(with neutrino beam)
Hyper-Kamiokande
1 Mton Japanese version
A World-Wide Neutrino Web?
Enormous
interest in future
long-baseline
oscillation
experiments
world-wide!
Some theoretical
indications that
proton decay
may be within
reach
Solving the Mysteries
Why three “families” of quarks and leptons?
Quark and lepton family mixing seems very different
Only beginning to measure lepton mixings in detail
Why are do particles have masses?
Why are the masses so different?
m < 10-11  mt
Are neutrinos the only type of matter without mass?
It now seems clear that neutrinos have (very tiny) masses
Can quarks turn into leptons?
Are there really three subatomic forces, or just one?
Mixing between families, and the small neutrino masses may
tell us a lot about a Grand Unified Theory
Observation of proton decay would be direct evidence for it!