Big World of Small Neutrinos

Neutrinos will find you!

Fig 1: Hubble image of the deep field.

Neutrinos are tiny particles, elementary building blocks of our Universe. They travel through the Universe with a speed close to the speed of light. They were first created in the Big Bang, in the beginning of the Universe, and continue to be created in nuclear reactions and particle interactions. From the very beginning, 14 billion years ago, to the present, neutrinos have played a crucial role in the evolution of the Universe and our existence. In every cubic meter of space at every instant we find about 330 millions neutrinos. On average there are roughly one billion times more neutrinos than protons in the Universe. There is no place in the world that cannot be reached by neutrinos. Billions of neutrinos pass through our body every second, and we don't even notice it. Neutrinos pass through matter mostly without any interactions and cause no harm on their way through the human body. Neutrinos are affected by only the weakest of nature's forces. Most of the time these particles travel unhindered, through space, matter, and time.

Where do they come from?
Fig 2: Particle Data Group (PDG) chart of the history of the universe.

Neutrinos we find in the Universe today were created in the Big Bang, in supernovae, or in fusion reactions inside stars like the Sun. Neutrinos are also created in the interaction of cosmic rays with the atmosphere of the Earth and emitted in the radioactive decay of elements inside the Earth. In addition, man-made neutrinos are produced in nuclear reactions inside nuclear power plants and in particle interactions at accelerators like the ones at Fermilab. (Both the MINOS and MiniBooNE experiment at Fermilab are neutrino experiments). Depending on the source and the environment in which neutrinos are produced, different types of neutrinos are created and their energies range from 0.0004 electron volt (eV) (Big Bang neutrinos) to 30x109 eV, or higher (at accelerators).

An egg sandwich ...

Neutrinos pass through matter mostly unaffected which makes it very difficult to detect them. The chance that a neutrino passing through matter causes an observable interaction is small...very small indeed. As Douglas Adams said, "The chances of a neutrino actually hitting something as it travels through all this howling emptiness are roughly comparable to that of dropping a ball bearing at random from a cruising 747 and hitting, say, an egg sandwich." However, neutrinos are very abundant particles in the Universe. Our Sun alone emits 2x1038 neutrinos every second. Using large-scale detectors (of 1000 tons or more) we can detect neutrinos and study their properties.

Discovery of the Neutrino

Fig 3: Continuous energy spectrum of electrons (beta particles) emitted in nuclear beta decay.

A new type of radiation. - Until the early part of the 20th century neutrinos were unknown. Only the study of nuclear decays revealed the existence of an unknown particle. In the early days physicists studied three types of radioactivity: alpha (charged helium), beta (electrons), and gamma (high-energy light) radiation. They found that both the alpha and gamma spectrum emitted in nuclear decays were discrete. In 1930, however, James Chadwick observed that the energy spectrum of electrons (the beta particle) emitted in nuclear beta-decay was continous. This observation could not be explained by the nuclear model of beta-decay at the time. In an attempt not to abandon the fundamental conservation law of energy Wolfgang Pauli postulated a new particle to explain Chadwick's observation. Three years later, Enrico Fermi called this new particle the neutrino and incorporated it in his theory of weak interaction.

Hunt for the Poltergeist
Fig 4: Poltergeist: Reines' experiment that discovered the electron anti-neutrino at the Savannah river reactor in 1956.

It took another 26 years before Frederick Reines and Clyde Cowan made the first experimental observation of neutrinos in their pioneering experiment at the Savannah River reactor for which they received the Nobel Prize in 1995. In their experiment they used the inverse of the beta-decay reaction which led to the postulate of the neutrino in 1930. Through inverse beta-decay (in which anti-neutrinos interact with protons to produce a positron and neutron) they detected anti-neutrinos from a nearby nuclear reactor. Their neutrino detector was placed at a distance of a few meters from the core of the reactor. Subsequent accelerator experiments found that there are three distinct types (or flavors) of neutrinos according to the type of charged particles found in the interaction. These were named the electron neutrino, muon neutrino, and tau neutrino (see below). For the discovery of the muon neutrino Leon Lederman, Melvin Schwartz, and Jack Steinberger received the Nobel Prize in 1988

Neutrino Experiments

Other ways of catching them. - In modern experiments, neutrinos are detected through a variety of techniques but some detectors still use the technique employed in the pioneering neutrino experiment by Reines and Cowan. With the quest for better and more precise particle detection, neutrino experiments have considerably grown in size. From the small pioneering experiments that could fit into one room, scientists now build neutrino detectors of the size of 10-story buildings, and larger. The size of and complexity of neutrino detectors make these experiments unique. They use state of the art sensor technology with large quantities of detector material (from a few tons to 50,000 t) in cleanroom environments. Most experiments are located deep underground to shield them from cosmic rays that could mimic a neutrino signal in the detector and interfere with the measurement. In certain classes of neutrino detectors, the interaction of the neutrino with a transparent detector medium creates light that can be detected by photodetectors. In radiochemical experiments, one identifies and counts invdividual atoms which have undergone a nuclear reaction with the neutrino.

Fig 5: Ray Davis' pioneering solar neutrino experiment at the Homestake mine used cleaning fluid to detect solar neutrinos through a radiochemical reaction.

After the initial discovery of neutrinos at the Savannah River reactor, scientists turned to the detection of astrophysical neutrinos from the Sun. For more than 30 years scientists have studied the flux of neutrinos produced in the fusion reactions inside the Sun. Measurements of the solar neutrino flux were first performed by Ray Davis in his experiment at the Homestake mine in South Dakota. He found that the number of solar neutrinos arriving on Earth was about half of what people predicted with solar models. Subsequent measurements by other experiments confirmed his result. The unexplained difference between the measured solar neutrino flux and model predictions lead to the 'Solar Neutrino Problem': the case of missing neutrinos from the Sun.

In 2002, the Sudbury Neutrino Observatory (SNO) resolved this puzzle. It demonstrated that neutrinos from the Sun were not missing but simply changed their flavor. On their way from the core of the Sun to Earth they transformed from one neutrino type into another type which was not detected by earlier experiments. The electron neutrinos produced in Sun had mostly transformed into muon or tau neutrinos. Through the use of heavy water as a target medium, the Sudbury Neutrino Observatory is the first and only solar neutrino experiment that could detect all neutrino flavors independently and determine their flavor change. SNO is located 2 km underground in a nickel mine in Sudbury in northern Ontario, Canada.

For their "pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos" Ray Davis who led the first solar neutrino experiment, and Masatoshi Koshiba, the principal investigator of the Kamiokande experiment that detected neutrinos from the Supernova 1987a, received the 2002 Nobel Prize in Physics, the highest honor of this discipline.

Fig 6: The Sudbury Neutrino Observatory (SNO) contains a 1000 tons of heavy water to detect solar neutrinos. Fig 7: In SNO an array of about 10,000 photodetectors detects light from neutrino interactions with heavy water.

Indications for the flavor transformation of neutrinos were also obtained by other experiments. Super-Kamiokande, a neutrino experiment, in Japan observed the apparent 'disappearance' of (muon type) neutrinos created in the interaction of cosmic rays in the atmosphere. And KamLAND, a reactor neutrino experiment, measured the 'disappearance' of (electron anti-) neutrinos produced in the nuclear reactions of a power plant. Several experiments are underway to confirm these results but all evidence points to a novel phenomenon amongst neutrinos, called neutrino oscillations (see below).

Fig 8: Inside view of the Super-Kamiokande (SK) neutrino detector in Japan during waterfill. 10,000 photosensors detect the light from neutrino interactions in 50,000 tons of water. Fig 9: The IceCube experiment at the South Pole uses the Antarctic ice shield as a detector and is looking for high-energy neutrino sources in the sky.

Other experiments, such as the IceCube project in Antarctica, use neutrinos from the sky to search for cosmic sources of some of the highest-energy particles in nature. IceCube is an international high-energy neutrino observatory being built and installed in the clear deep ice below the South Pole Station. It uses the Antarctic ice shield to detect high-energy neutrinos and discover new neutrino sources in the sky.

At present several neutrino experiments worldwide study neutrinos from the Sun, from the atmosphere, from nuclear reactors, accelerators, and from the inside of the Earth. Common to all neutrino experiments is that they use large volumes of target material (water, cleaning fluid, the antarctic ice) to detect these weakly, almost invisible particles. Over the last decade the field of neutrino physics has grown to become its own discipline and is one of the most vibrant fields of physics at the intersection of particle, nuclear, and astrophysics.

Neutrinos and their Properties: What do we know about these invisible particles?

Fig 10: Particles in the Standard Model of particle physics: The Standard Model contains 3 neutrinos of definite flavor, and a set of corresponding anti-particles.

Neutrinos in the Standard Model. - After 5 decades of research we have started to develop an understanding of the basic properties of these mysterious particles that fill the entire Universe. Neutrinos are fundamental particles, building blocks of matter that were long assumed to be massless. They are neutral and carry no charge. Recent neutrino experiments have shown, however, that neutrinos have a tiny mass, much smaller than that of electrons: melectron neutrino < 0.0000059 melectron. Neutrinos come in three types, called flavors. In the Standard Model of Particle Physics they belong to the family of leptons (the interest of this conference). According to the charged leptons produced with the neutrinos they are referred to as electron neutrino, muon neutrino, or tau neutrino. The three different neutrinos are complemented by anti-neutrinos, a set of mirror particles. Anti-particles have the same mass as particles but inverse characteristics. Neutrinos are special in that they can be their own anti-particles. At present we do not know if neutrinos and anti-neutrinos are distinct, or in fact the same particle. Experiments searching for neutrino-less double beta decay (which can only occur of if the neutrino and its anti-particle are identical) may tell us the answer in the future.

Fig 11: What is the Universe made of? - Stars and luminous matter only account for 0.5% of the Universe. Neutrinos make up 0.3%.

As much as all the stars in the Universe. - While the individual mass of every neutrino is tiny compared to the mass of other particles such as a neutron or proton, the combined mass of all neutrinos in the Universe is nearly as much as the mass of all luminous stars. Neutrinos make up for about 0.3% of the total matter in the Universe while stars contribute approximately 0.5%. This leaves a large part of the Universe unaccounted for. Most of the content of the Universe is not understood. From the motion of galaxies physicists and astronomers have observed the gravitational effects of 'dark matter' but they do not understand its nature or what it is made of. And the force behind the recently observed acceleration of the Universe is known as 'dark energy'. Despite their small contribution to the overall content of the Universe neutrinos play a crucial role in the evolution of the Universe. Freely streaming through the cosmos they affect the formation of large-scale structures in the Universe, play a central role in the energy release of supernovae, and are central to nuclear decays, and particle interactions.

Fig 12: The periodic change of neutrino flavor from one type into another is referred to as neutrino oscillations.

A chameleon. - The observed flavor change of neutrinos - the transformation of one type into another - is a result of their massive nature. They are pointlike particles but not massless. Neutrinos continously evolve on their journey through the Universe through a quantum mechanical phenomenon known as neutrino oscillations. In free space or through the additional interaction with matter, neutrinos have the ability to spontaneously change their type. A priori we cannot tell the fate of individual neutrinos but on average a certain fraction of the neutrinos we detect will look different (have a different flavor) compared to the time they were created. As a result, neutrinos created in the Sun, for example, won't look the same to physicists when they are detected on Earth.

Future of Neutrino Physics: Open Questions

Fig 13: The matter/anti-matter asymmetry (or baryon asymmetry) in the Universe is one of the outstanding questions. Neutrinos may hold the key to understanding the dominance of matter in our world.

After almost 50 years since the discovery of the neutrino, we have started to appreciate the role of neutrinos in our world. From the radioactive decay of atoms to the evolution of the Universe, neutrinos are central to our understanding of particle physics and astrophysics. The observation of massive neutrinos and the discovery of neutrino oscillations requires us to revise our understanding of the fundamental building blocks of matter, and we are poised to understand the laws of nature that govern this particle. It may also be that neutrinos hold the key to understanding the matter/antimatter asymmetry in the Universe. In the Big Bang matter and anitmatter was created in equal amounts and to date physicists have failed to explain the dominance of matter. Violation of the symmetry of time is known to exist. If time violation occurs amongst neutrinos, it may be our best bet yet to explain the dominance of matter and resolve one of the fundamental mysteries of the Universe.

Often we are asked about the purpose of devoting significant effort, time, and resources to studying fundamental particles. Fundamental particles such as neutrinos are created in the Big Bang, they make up our Universe, and may hold the explanation to our existence. - As astrophysicist Hubert Reeves puts it: "We are Earth people, people of the Milky way, daughters, and sons of the Universe. Our roots are in the stars."

Karsten Heeger
Chamberlain Fellow, LBNL Physics Division