1. Neutrino Oscillations: Experimental Results + Future Measurements
Introduction
“Solar” Neutrino Oscillations
D(m12)2, q12
“Atmospheric” Neutrino Oscillations
D(m23)2, q23
LSND-type Oscillations?
q13, dCP: Future Experiments
Bruce Berger
PHENO 2004 – March 26, 2004

2. (Brief) Introduction
The well-established oscillations (solar, atmospheric) can be described
by three active, massive neutrinos and the “MNSP” (Maki-Nakagawa-
Sakata-Pontecorvo) matrix, analogous to the CKM matrix:
Mass hierarchy not completely known
Both solar and atmospheric oscillations are
well approximated by two-flavor mixing
because q13 is small, D(m23)2  D(m12)2
Two-flavor survival probability, e.g. solar ne
Bruce Berger
PHENO 2004 – March 26, 2004

3. Solar Neutrino Oscillations
The sun as a neutrino source:
Complicated energy spectrum
MSW Effect:
propagation through matter
modifies the ne survival probability
91% 7%
0.2%
0.008%
Below critical energy,
vacuum oscillations dominate
Above critical energy
matter effects dominate
Critical energy depends on Dm2,
~1.8 MeV for LMA
Spectral distortion near critical energy
Bruce Berger
PHENO 2004 – March 26, 2004

4. Radiochemical Experiments
ne capture on select radioisotopes:
Chlorine: ne + 37Cl  e– + 37Ar > 814 keV
Gallium: ne + 71Ga  e– + 71Ge > 233 keV
Detect decays of capture daughters
Sensitive only to integrated ne flux above threshold
Results:
Homestake (Cl): Cl/SSM = 0.34  0.03
SAGE+GALLEX/GNO: Ga/SSM = 0.54  0.03
(SSM is “Standard Solar Model”,
BP00: Bahcall/Pinsonneault,Astrophys. J. 555, 990, 2001)
Ray Davis
Bruce Berger
PHENO 2004 – March 26, 2004

5. Kamiokande/Super-K Water-Čerenkov, detects forward-scattered
electrons from neutrino-electron elastic scattering
nx + e–  nx + e–
Only sensitive to most energetic 8B solar neutrinos
Real-time detection of electron energy and direction
Flux result: SK/SSM =  0.015
Constant suppression:
no time variation or energy distortion detected
Koshiba
Masatoshi
Bruce Berger
PHENO 2004 – March 26, 2004

6. SNO Heavy-water-Čerenkov detector, 5 MeV threshold
Three different n detection modes:
CC (charged current) ne + D  p + p + e– ne only
NC (neutral current) nx + D  p + n + nx all three flavors!
ES (elastic scattering) nx + e–  nx + e– (same as Super-K)
Full SSM flux seen
5.3s appearance of nm,t in a ne beam
Ratio of CC to NC: strongly
constrains the mixing angle q12
CC/NC =   0.024
SNO also sees constant suppression
Bruce Berger
PHENO 2004 – March 26, 2004

7. q12, D(m12)2 from solar data SNO 95% allowed regions
overlaid with previous solar
neutrino measurements
In global fits with the most
recent SNO data, only
the LMA region survives.
Maximal mixing ruled out at 5.4s
Bruce Berger
PHENO 2004 – March 26, 2004

8. KamLAND Studies “solar”-type LMA oscillations in reactor antineutrinos
Reactor baselines on the order of the oscillation length
Source is a time-varying ensemble of reactors
Liquid scintillator calorimeter, sub-MeV threshold
Inverse b-decay: ne + p ® e+ + n
Coincidence signal:
prompt e+ annihilation (E = En  0.8 MeV)
delayed n capture (~190 ms) (E = 2.2 MeV)
No directional information
Detected ne spectrum
(no oscillations)
Inverse b-decay
cross-section
Reactor ne spectrum
Bruce Berger
PHENO 2004 – March 26, 2004

9. Antineutrino Rate Analysis
Observed (145.1 days livetime)
No-oscillation expectation 86.8  5.6 (syst)
Background 1  1
(Nobs–NBG)/Nno-osc =  (stat)  (syst)
(statistics above on 54 events)
Probability that 86.8 events would
fluctuate down to 54 is < 0.05%
“Standard” ne propagation
is ruled out at the
99.95% confidence level
Neutrinos and antineutrinos both see
effects consistent with the same mixing
parameters
curve, shaded region:
global-fit solar LMA
Bruce Berger
PHENO 2004 – March 26, 2004

10. Rate + Shape Analysis
Fit prompt (positron) energy spectrum above 2.6 MeV with
full reactor information (power, fuel, flux), 2-flavor mixing
Energy spectrum shape provides additional
constraints on oscillation parameters
Bruce Berger
PHENO 2004 – March 26, 2004

11. KamLAND mixing parameter constraints
Shape analysis further
constrains LMA parameters
LMAI (lower)
LMAII (upper)
Best-fit values of mixing parameters
are in the same region for both
neutrinos and antineutrinos
> test of CPT
Constraints symmetric about
tan2q=1 due to absence of
MSW effects
Under CPT:
KamLAND rate analysis
confirms LMA
rules out all other regions
Bruce Berger
PHENO 2004 – March 26, 2004

12. Future SNO: Improved CC/NC measurement to further constrain tan2q
Holanda&Smirnov
SNO:
Improved CC/NC measurement to
further constrain tan2q
Improved day/night ratio better constrains
Dm2 in solar-only fits
Possible to see MSW distortions?
KamLAND:
Better measurements with more data
Distinguish LMAI vs. LMAII
Observe shape distortion, oscillation?
> published data are consistent
with constant supression
Scatter of 500-event MC
datasets generated with
common mixing parameters
Bruce Berger
PHENO 2004 – March 26, 2004

13. 7Be solar neutrino measurement?
Borexino
Players: Borexino, KamLAND
Goal is a direct measurement of the
solar 7Be neutrino flux
Tough measurement:
single ES event
need very low background to
statistically extract the signal
SSM 7Be prediction is at the ~10% level
> This measurement is not expected to improve
the determination of oscillation parameters
> Measurement can improve the SSM
Still an important check:
7Be neutrino energy is below the MSW transition
Bruce Berger
PHENO 2004 – March 26, 2004

14. pp neutrinos?
The pp solar neutrino flux is by far the largest (>90%)
and best-determined (~1%)
Measurement of pp neutrinos could give the best
solar neutrino oscillation parameter constraints
Even though the rates are high,
backgrounds are the key issue
R&D efforts on a variety of
approaches
M.Nakahata
Bruce Berger
PHENO 2004 – March 26, 2004

15. Atmospheric neutrino oscillation
Cosmic-ray showers in the earth’s
atmosphere produce nm, ne
In an underground detector,
atmospheric n’s from different
directions have different baselines
Detect neutrinos through charged-current
interactions in the detector: ne  e, nm  m
The q23 oscillation causes a deficit of upward-going nm
Best atmospheric oscillation measurement is from Super-K,
though multiple other experiments have seen the same effect
(Kamiokande, IMB, MACRO, Soudan-II, …)
Bruce Berger
PHENO 2004 – March 26, 2004

16. Super-K atmospheric measurement
Most recent analysis presented at NOON 2004 (C.Saji)
Super-K is working on a final analysis of the “SK-I” dataset:
improved analysis, MC, flux predictions, calibrations, cross-sections, …
SK-II is taking more data. Results improve as (data)1/2
Bruce Berger
PHENO 2004 – March 26, 2004

17. K2K “Atmospheric” oscillations have also been detected with
accelerator neutrinos:
K2K (KEK-to-Super-K): 250 km baseline
Data collection ongoing, new near detector
Bruce Berger
PHENO 2004 – March 26, 2004

18. Super-K L/E analysis
New, very interesting preliminary analysis of Super-K data
presented at NOON 2004 by M.Ishitsuka
Analyze data vs. L/E instead of zenith angle
Uses subset of events with
good L/E resolution
Dip at first oscillation minimum,
later maxima/minima smeared out
Oscillation preferred to other
explanations of deficit
Better Dm2 resolution than
standard zenith-angle analysis
(Note added May 26, 2004: preprint hep-ex/ now available)
Bruce Berger
PHENO 2004 – March 26, 2004

19. MINOS Long-baseline accelerator neutrino experiment
sending nm’s from the NuMI beam at Fermilab
past Madison to the Soudan mine
Detect neutrinos through CC interaction in
steel+scintillator detector with B-field
Tunable beam energies
Beam scheduled for April 2005
Already taking atmospheric neutrino data
> Can distinguish nm from nm, test CPT
Bruce Berger
PHENO 2004 – March 26, 2004

20. MINOS MINOS limits compared to current Super-K values
Better constraints for higher Dm2 – oscillation minimum at higher energy
Bruce Berger
PHENO 2004 – March 26, 2004

21. nt appearance Goal: observe nt appearance in nm beam
to confirm that the osciallation is to nt
CERN to Gran Sasso: CNGS beam optimized
for nt appearance – high enough energy
Detectors:
ICARUS: liquid Argon TPC
OPERA: lead emulsion
ICARUS cryostat
Bruce Berger
PHENO 2004 – March 26, 2004

22. Future Improved “atmospheric” parameter measurements from: MINOS
Continued Super-K operation
> Is mixing really maximal?
Confirmation of oscillation from Super-K L/E analysis
CPT test: MINOS comparison of nm, nm
Direct observation of nt appearance
Bruce Berger
PHENO 2004 – March 26, 2004

23. LSND? LSND reported a significant excess of ne in a nm beam (3.8s)
So far, this result has not been reproduced by any other experiment
Some parameter space is still left…
Bruce Berger
PHENO 2004 – March 26, 2004

24. LSND? LSND can’t fit into the MNSP picture along with the solar and
atmospheric oscillations as the third oscillation: the mass differences
don’t add up
If LSND is right, we need another oscillation somehow…
CPT violation?
> Consistent results for neutrinos, antineutrinos in solar sector
> Two different mass differences in atmospheric sector strongly
disfavored by Super-K
Additional sterile neutrinos?
> 3+1 (one sterile) disfavored
> 3+2, etc. possible
If LSND is correct, things get very interesting
> Fully testing LSND is an experimental priority
Bruce Berger
PHENO 2004 – March 26, 2004

25. MiniBooNE
MiniBooNE is a short-baseline accelerator neutrino experiment
running at Fermilab that can definitely confirm or rule out LSND
See Sam Zeller’s talk!
two-ring event
Bruce Berger
PHENO 2004 – March 26, 2004

26. q13, dCP In the MNSP picture, we should have a third oscillation,
characterized by q13, D(m13)2  D(m23)2
How big is q13?
Experiment: Currently only have limits
Theory: No firm predictions
> Some models suggest q13 should be within the range of the
next generation of experiments
> Global three-flavor fits favor q13 just below current limits…
If q13 is small, why? Symmetry?
If q13 is large enough, we can also try to measure CP violation
in the lepton sector, characterized in MNSP by dCP
> CP violation could be large: can it explain the
matter/antimatter asymmetry through leptogenesis?
Bruce Berger
PHENO 2004 – March 26, 2004

27. Reactor q13 limits
The best limits on q13 come from short (~1km) baseline
reactor neutrino measurements that saw no flux deficit.
High-precision measurement: 2.7% systematic error at CHOOZ:
Bruce Berger
PHENO 2004 – March 26, 2004

28. Reactor measurement of q13
Goal is to try to measure the subdominant q13 oscillation in
ne disappearance
Measurement requires control of systematics to the 1% level:
How do we do better than CHOOZ?
Use two detectors to cancel flux uncertainties
The disappearance effect depends only on q13:
no matter effects, ambiguities from dCP
detector 1
detector 2
Bruce Berger
PHENO 2004 – March 26, 2004

29. Reactor measurement of q13
Double-CHOOZ: approved
Fast, cheap approach
sin22q13 down to 0.03
Options in the US?
Braidwood (Illinois)
> flat site, vertical access
Diablo Canyon (California)
> horizontal access, more overburden
Need reactor company agreement…
Other sites: Daya Bay (China),
Kashiwazaki (Japan), Krasnoyarsk (Russia),
Angra dos Reis (Brazil)
Braidwood
Diablo Canyon
Bruce Berger
PHENO 2004 – March 26, 2004

30. Sensitivity to q13
Bruce Berger
PHENO 2004 – March 26, 2004

31. Off-axis accelerator q13
With the right accelerator beam, we can measure q13 , dCP
in nm  ne appearance
Rate depends on multiple unknown parameters:
q13, dCP, and matter effects (hierarchy)
Bruce Berger
PHENO 2004 – March 26, 2004

32. Off-axis accelerator q13
A single measurement doesn’t
unambiguously measure q13
Parameter degeneracies can
be resolved by:
Running in both neutrino
and antineutrino modes
Running at different baselines:
multiple experiments are
complementary
sin22q13
~sind
~cosd
Bruce Berger
PHENO 2004 – March 26, 2004

33. Off-axis accelerator q13
Two experiments (not a complete list of proposals!):
T2K: Tokai-to-Super-K
high priority in Japan
Proposed NuMI off-axis experiment
Soudan
Bruce Berger
PHENO 2004 – March 26, 2004

34. Longer term: neutrino superbeams
Example: Brookhaven-to-Homestake
A wideband, high-flux, on-axis beam
allows parameter measurement by
observing multiple oscillation peaks
Potential for very good measurements
Improved detectors, beams necessary
Bruce Berger
PHENO 2004 – March 26, 2004

35. Conclusions
Neutrino Oscillations is a rich experimental field, with a variety
of sources (sun, atmosphere, reactors, accelerators) and detection
techniques
Two different oscillation effects are well-established
> Experiments ongoing to improve the parameter measurements
And verify that the oscillation interpretation is correct
LSND signal: MiniBooNE is key
> If LSND signal is real, things get very interesting
Big goals:
> Measure q13 oscillation
> Measure CP violation in neutrino sector
Other questions:
> Which hierarchy?
> Absolute mass scale?
> Are neutrino’s Dirac or Majorana?
Non-oscillation experiments are also a big part of the field:
neutrinoless 2b decay, cosmological limits, etc.
Bruce Berger
PHENO 2004 – March 26, 2004