Solar Neutrinos

Learning about the core of the Sun

Guest lecture: Dr. Jeffrey MorgenthalerJan 26, 2006

Review

• Conventional solar telescopes– Observe optical properties of the Sun to test

standard model– Properties include: brightness, color,

spectrum, Doppler shifts of small patches– Doppler shifts indicate Sun is ringing like a

bell– “Tones” of bell allow probing of solar interior –

to a point (fig 4.9 in textbook)

Helioseismology

Neutrinos

• Basic scientific principle: ENERGY CONSERVATION– Energy in = energy out

• Wolfgang Pauli solved a problem using the principle of energy conservation– The answer to his problem: the neutrino

• The neutrino gave us another problem:– How to detect neutrinos!

A little Particle Physics

• Chemistry tells us atoms are made up of electrons, protons and neutrons

• Particle physics probes deeper into the nucleus finding more fundamental particles: leptons and quarks– Leptons: electron, muon, tau and associated

neutrinos, (all come in + and – varieties)– Quarks: up, down, charm, strange, top, bottom (all

come in matter and anti-matter varieties)– Protons and neutrons are built out of 3 quarks (combo

of up and down) with leptons waiting in the wings– Rules of interaction are complicated

Nuclear Reactions that involve Neutrinos

• Beta Decay (neutrino “discovery”)

• Inverse beta-decay (first neutrino detection in 1956)

• Common neutrino detection scheme

• P-P chain first step

• Boron 8 (occasional solar reaction)

P + P D + e+ + e

N P + e- + e

P + e N + e+

8B 2 4He + e+ + e

N + e P + e- + recoil

A look at the numbers

• 2 x 1038 solar neutrinos produced every second• Almost all make it out of the sun (weakly

interacting with matter)• Traveling very near the speed of light (8 min

travel time to earth)• 70 billion neutrinos per second in each 1 cm

square patch on earth• Idea: catch the neutrinos and see if they tell

us anything about the solar interior

Neutrino Detector History• First try in ~1963 (Barberton Ohio)

• Debugged technique – A Chlorine + neutrino radioactive Argon– Underground to reduce background from non-

neutrino events

• Showed bigger detector required

Homestake Construction (1966 photo)

Homestake Neutrino Detector Operation

• The Solar Neutrino Unit (SNU) = 1 neutrino interaction per second per 1036 detector atoms

• Problem: 1036 is a lot of atoms (about 240 million tons of cholrine)

• Homestake could afford ~1030 atoms• Result: Homestake counted about 2.5 neutrinos

per day (2.55 ± 0.25 SNU)• Based on standard solar model, expected 8 ± 1

SNU• “Solar neutrino problem”

If at first you don’t succeed…

• Kamiokande was built to look for the spontaneous decay of protons and bound neutrons– Works by detecting flashes of light

(Cherenkov radiation) from recoil of nuclei– Maximum speed of light waves is fixed in a

vacuum, but gets slower when traveling through something

– Shock waves happen when a particle travels faster than the maximum speed of a wave

Kamiokande

Kamiokande II

• Upgrade of Kamiokande

• Detected neutrinos from supernova 1987a– Started detecting solar neutrinos in 1988– Definitive direction information by 1991

• Measured ½ as many neutrinos as expected!– But was sure they were coming from the

Sun…

…try again…

Gallium-based neutrino detectors

• Use Gallium to detect neutrinos from p-p reaction

• GALLEX and SAGE used similar techniques to Homestake– Gallium + neutrino radioactive Germanium– Sweep Germanium out of system– Count radioactive decays

• Expected 132 SNU, measured ~75 SNU!

This is getting ridiculous

• Choose 1:– The standard solar model is wrong– We don’t (didn’t…) understand neutrinos

• The ability to predict many solar properties just starting from a ball of gas and letting nuclear reactions, diffusion, and convection take place suggests we need to look more closely at neutrinos

Neutrino Oscillations (MSW effect)

• Lincoln Wolfenstein first to have the idea, Stanilaw Mikheyev and Aleksei Smirnov refined

• Using the rules of quantum mechanics MSW predicts how neutrinos of different types (flavors) behave in the presence of matter– Electron, Mu, and Tau neutrinos mutate from one to

another

• Detectors sensitive to electron neutrinos miss others, hence the SNU shortfall

Superkamiokande

Late-breaking news

• The Japanese-American collaboration announced the detection of neutrino oscillations in 1998 by looking at neutrinos produced in the atmosphere

• Raymond Davis, Jr. (Homestake) and Masatoshi Koshiba (Kamiokande) shared the 2002 Nobel Prize in physics for their work in neutrino physics

Future of Neutrino Astronomy

• Sudbury Neutrino Detector (SNO)– Verified MSW

• Superkamiokande

• Amanda/ICE Cube

Amanda/Ice Cube

• Detects neutrinos converting in ice at the south pole

Summary

• Principle of CONSERVATION OF ENERGY led to proposal of neutrino by Wolfgang Pauli

• Neutrino flux from sun measured by several experiments (in units of SNU) fell short from solar model expectations (solar neutrino problem)

• Solar model proves reliable for many thing• Led to proposal of MSW (neutrino oscillations)

• Solar astronomy helped particle physics

Next Topic: Rotation of the Sun and Magnetic Field

• Review effect of sound speed on waves– Vacuum cleaner analogy– Waves propagate faster

towards center of Sun

• Refraction (bending) depends on wavelength– Bigger wheels on vacuum

cleaner

Differential rotation