The Importance of Low-Energy Solar Neutrino Experiments

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The Importance of Low-EnergySolar Neutrino
Experiments

• Thomas Bowles
• Los Alamos National Laboratory

Markov Symposium Institute for Nuclear
Research 5/13/05
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Standard Solar Model
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Comparison of measured rates and Standard Solar
Model (After 30 years of effort)
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Flavor Content of the Solar 8B Neutrino Flux
Detecting Neutrinos in SNO
CC Interaction
Sensitive to electron neutrinos only
NC Interaction
Equally sensitive to all flavours
ES Interaction
Sensitive to all flavors, but most sensitive
to electron neutrinos
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What We Know

• Flux of 8B ns has a large non-ne component
• Survival probability Pee for En gt 5 MeV is
• essentially independent of En
• Pee for ns of lower energy (p-p) is larger
• There is no significant (gt 2s) D/N asymmetry

All observations are consistent with the
following hypotheses Mass-induced flavor
oscillations (with LMA as the favored solution)
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Neutrino Oscillations
If neutrinos have mass leptons can mix
Flavor eigenstates are a mixture of mass
eigenstates
States evolve with time or distance
The ne survival probability for two flavor mixing
is
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Reactor Neutrino Experiment
Terrestrial Neutrinos KamLAND is a 1 kton liquid
scintillator detector that observes from
a number of reactors in Japan at an
average distance of 180 km
(NOBS - NBG)/NEXP 0.611 0.085 (stat)
0.041 (syst)
KamLAND observes a significant deficit
of neutrinos and confirms solar neutrino
LMA neutrino oscillation solution
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Neutrino Properties

• What We Know
• There are 3 types of neutrinos ne , nm , nt
• Neutrinos have mass and oscillate
• Oscillation parameters (Dm2 and tan2q) known to
  30
• Neutrino masses are small
• 50 meV lt mn lt 2.8 eV (90 CL)
• Lower limit from atmospheric neutrino results
• Upper limit from tritium beta decay results
• Neutrinos account for at least as much mass in
  the Universe
• as the visible stars

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Neutrino Properties

• What We Dont Know - Neutrino Properties
• Are neutrinos their own antiparticles? (Majorana
  n)

• What is the absolute scale for neutrino mass?

• Is the mass scale normal ordered or inverted
  hierarchy?

• Are there sterile neutrinos?

• What are the elements of the MNS mixing matrix?

• Is CP / CPT violated in the neutrino sector?

• What We Dont Know - Neutrino Astrophysics
• Is the Standard Solar Model correct?

• What is the flux of solar neutrinos below 5 MeV?

• What is the flux of CNO neutrinos?

• What is the radial temperature distribution of
  the Sun?

• How do neutrino properties affect supernovae?

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Physics Program for FutureSolar Neutrino
Experiments (I)

• Directly observe the 99.99 of solar neutrinos
• that are below 5 MeV
• Direct test of solar models (p-p, 7Be, CNO)

Uncertainties in the solar neutrino
fluxes p-p 7Be CNO 8B Present 15 35 100 6 Wit
h present 12 8 100 4 generation dets Future
expts 1-3 2-5 10-20 2-4

• Measurement of CNO neutrinos provides an
  important test
• 1.5 of the Suns energy is from the CNO cycle
• CNO burning is crucial in first 108 yr
  convective stage
• Provides test of initial metallicity of the Sun

• Determine unitarity / dimension of n mixing
  matrix

Goal is to measure the flavor composition of the
p-p solar ns to 1 precision in a
model-independent manner Requires CC and ES/NC
measurement (assuming active oscillations)
Model-indep test for sterile ns using measured
oscillation parameters (p-p KamLAND)
? Can achieve 13 sensitivity (90 CL)
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Physics Program for FutureSolar Neutrino
Experiments (II)

• Use p-p neutrinos as standard candle
• Precision test for CPT violation comparing
• and

Model-dependent cross-check for sterile
neutrinos with 2 sensitivity (90 CL)
Measurement of the p-p rate to 1 provides
knowledge of q12 to allow a search for CPT
violation at a scale of 10-20 GeV
Compared to the present CPT test from the upper
limit on the mass difference in the kaon system
of 4.4 x 10-19 GeV
Various scenarios imply that the sterile
component of solar neutrino fluxes may be
energy dependent

• Provide improved precision of mixing angle

Future p-p solar neutrino experiments offer the
best prospect for improving our knowledge of q12
? Low-energy solar neutrino expts must be part of
any full study of sterile neutrinos

• Search for n magnetic moment with improved
• sensitivity (contribution ? 1/Te)

Qsolar required to determine mn in 0n-bb decay
? Expect sensitivity of 10-11 mB
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p-p Solar Neutrino ExperimentsPhysics Goals
fTotal fActive fSterile
Search with sterile neutrino components with an
order of magnitude improved sensitivity
Future Sensitivity
Present limits
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Next-Generation Solar Neutrino Experiments
What is required of future experiments
Measurement of ne fluxes Source To match To
match To match current expts projected
expts LMA prediction p-p 15 12
2 7Be 35 8 5 CNO 100 100 100
pep 100 100 2 8B 6 4
6
Mixing parameters To match current limits on
tan2q 3 p-p accuracy To match projected SNO,
KamLAND limits 2 p-p accuracy
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Future Experiments - Borexino

• Looks at solar 7Be line (862 keV)
• Precision measurement of q12
• Will provide test of SSM for 7Be flux
• Possible future extension to p-p neutrinos

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p-p Solar Neutrino Experiments
Charged-Current Experiments LENS, MOON Goal
Measure ne component of p-p (7Be) with 1-3
(2-5) accuracy Elastic Scattering
Experiments CLEAN, HERON, TPC, XMASS Goal
Measure ne / nm, nt component of p-p (7Be) with
1-3 (2-5) accuracy
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CC p-p Experiments LENS
Spokesman Raju Raghavan
40 tons In target in 400 tons scintillator Modular
design with In cells surrounded by non-In
cells (2000 tons scintillator)
Fundamental problem 115In beta decay
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CC p-p Experiments LENS
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LENS Count Rates

• Design Parameters (assumed)
• 40 tons In
• ? 480 tons InLS, 4 kton non-InLS
• 4 years of running (5 calendar years)
• Detection efficiency 22 for p-p, 57 for
  7Be, CNO
• 300 MeV/pe scintillator, 3 m attenuation length
• No backgrounds
• Calibrated by 8 MCi 51Cr source

Source Statistical Accuracy p-p 2.3 7Be
2.8 CNO 5.8 pep 11.8
Issue estimated cost 140M
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CC p-p Experiments MOON
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CC p-p Experiments MOON
Issue Double beta decay background!
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ES p-p Experiments HERON
Spokesman Bob Lanou
5,000 events/yr (10 ton fid. Vol.) BP00 SSM
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Low Energy Solar Neutrino Fluxes
Bahcall, Gonzalez-Garcia, Pena-Garay,
hep-ph/0204194
Ga ? SNO ? KamLAND ? BOREXINO ? BP00
Ga Ga CNO SNO
KamLAND BOREXINO
Expt X-Sect. SSM CC Expt
Expt Sterile
? ? ? ?
? ? ?
0.05 0.01
0.00 fpp 1.05 (1
0.11 0.007 0.05 0.04
) - 0.08 - 0.02

- 0.02 1.05 (1 0.15)
? Dedicated pp Experiments required to make
Improvements.
Flux Predictions for a pp Elastic Scattering
Experiment 0.697 0.023 (100 keV) 0.693
0.024 ( 50 keV)
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Low Energy Solar Neutrino Fluxes
SAGE Results 69.6 4.4/-4.3 (stat) 3.7/-3.2
(syst) SNU
GALLEX GNO 70.8 ? 4.5 (stat) ? 3.8 (syst) SNU
Progress in determining the flux of low-energy
solar ne can only be achieved in the next decade
by improved Ga measurements
SAGE 1990-2003
The Gallium experiments should continue to
operate until they are systematics limited
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The Russian-American Gallium Experiment
It has been my experience that SAGE has proved to
be a perfect example of the value of
international scientific collaborations
The SAGE collaboration has provided the means for
achieving a significant scientific result
It has been my privilege and honor to play a role
in SAGE
I am extremely grateful to the many people who
have made SAGE a success -
Without all of their support the success and
recognition that we have received in the world
scientific community would not have been
possible.
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