Neutrinos: What we’ve learned and what we still want to find out

Jessica Clayton

Astronomy Club

November 10, 2008

Neutrinos, they are very small,they have no charge and have no mass†,and do not interact at all.

†Almost no mass.

John Updike (2003)

Back to the Basics

The Standard Model

Something’s missing…

Neutron proton electron

?Conservation of energy, momentum and angular momentum require that something else exists.

Beta decay

Birth of a Particle

1930: Wolfgang Pauli predicts that there is another particle involved in beta decay

First theories about neutrinos were soon after written by Enrico Fermi

Fermi coined the term neutrino - meaning “little neutral one”

+ p n + e+

Discovery! In 1956, Fred Reines and Clyde Cowan

detected the neutrino via inverse beta decay

e-

2 photons in opposite directions

CdAnother photon, 5 x 10-6 sec later

Predictions for neutrinos from the sun

4p He + 2e+ + 2 e

+ energy

Protons in the sun fuse to form helium

In the process, neutrinos and energy are released.

Ray Davis and John Bahcall formed a team to study this prediction in 1964.

Underground in South Dakota… Ray Davis built a neutrino detector one

mile underground in the Homestake Mine

Large tank of cleaning fluid, C2Cl4 Cl + -> Ar + e-

Count the number of Ar atoms to find the number of neutrinos

The Solar Neutrino Problem

It was 1968. Three possibilities:

1) problem with detector

2) problem with solar theory of fusion and neutrino production

3) something is wrong with the Standard Model.

The number of neutrinos measured by Davis was only 1/3 of what Bahcall predicted.

Davis and Bahcall at Homestake. Photo from nobelprize.org.

Searching for answers… Kamiokande detector was built in Japan

and detects about half of the neutrinos that Bahcall predicted.

GALLEX, SAGE and Super-Kamiokande confirmed the deficit in neutrinos over different energy ranges

… but still, the theory doesn’t match the observations…

http://www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html

SNO breakthrough in 2001 The Sudbury Neutrino Observatory

could only measure one flavor of neutrinos, e.

Kamiokande was sensitive mostly to e , but also to and .

Results were combined to come up with the total number of solar neutrinos and the number of solar e.

Neutrinos change flavors!

1/3 of solar neutrinos are electron flavor by the time they get to Earth

The “missing” electron neutrinos oscillate into or .

In order to change flavors, neutrinos must have a non-zero mass. That doesn’t fit into the Standard Model as we know it!

Vindicated… after 40 years.

Bahcall made his first predictions about the number of neutrinos produced by the sun in the mid-1960s.

The Solar Neutrino Problem was born with Davis’ first results in 1968.

Neutrinos were studied by several experiments - and were measured from a supernova in 1987

In 2001, SNO results confirmed that neutrino oscillations occur.

Supernova 1987a Neutrinos were detected from

Supernova 1987a by Kamiokande and IMB

Credit: C Burrows (ESA/STScI), HST, NASA

Within 12 seconds, Kamiokande saw 12 events (6-35 MeV) and IMB saw 8 events (19-39 MeV).

First optical observations were the next day.

Neutrinos in our midst…

Earth’s radioactivityNuclear reactors

Human body

atmosphere

Accelerators

sun Supernova 1987a

Big Bang

A trillion neutrinos pass harmlessly through your body every second!

A New Window on the Universe

NRAO

Radio image of the Crab Nebula

Anglo-Australian Obs.

Optical image of the Crab Nebula

Ultraviolet Imaging Tel.

Ultraviolet image of the Crab Nebula

Chandra X-ray Obs.

X-ray image of the Crab Nebula

Gravitational waves?

Neutrinos?

NEUTRINO

COSMIC RAY PROTON

PHOTON (LIGHT)

STAR OR

GALAXY ?

INTERSTELLAR DUST

Neutrinos travel in a straight lines.

Because they have no electric charge, they are not deflected by magnetic fields in space.

What’s accelerating neutrinos? Gamma-ray bursts (GRBs)? Active Galactic Nuclei (AGN)?

What’s the mass of each flavor of neutrino? What’s the value of the oscillation

parameters? Are neutrinos and anti-neutrinos the same

thing?

Neutrinos: many open questions

Neutrinos: what we think now Neutral (no charge) Tiny, non-zero mass 3 flavors, which oscillate Very tiny cross-section, meaning that

they don’t like to interact with matter Promising new way of studying the

Universe

“A particle that is almost nothing may tell us everything about the Universe.”

Christine Sutton

Why do we study neutrinos?