What is a ‘neutrino’ and why would we want to build a factory to produce them?

(Andrew Phillips)

What is the neutrino and where did it come from?

The neutrino is a fundamental particle, as is the electron. This means that it has not yet been broken down into any smaller constituents and therefore is considered to be part of the universe’s family of basic building blocks. It was originally believed to be mass-less and merely comprised of pure kinetic energy but now physicists are of the opinion that it indeed has a mass but only of small magnitude. It has no charge, a very low reaction cross-section (there is a very low probability that it will react with any other matter) and only really reacts via the ‘weak’ interaction which has an extremely short range. This was the reason for the name given to it by Wolfgang Pauli in 1930 when he proposed the existence of this elusive particle (neutrino: ‘little neutral one’). In fact, even after predicting its existence, Pauli was doubtful that scientists would ever successfully detect it by experiment. So the neutrino actually ‘existed’ in the scientific world for around 26 years as a purely theoretical particle whose purpose was to explain away the energy discrepancy observed in beta decay experiments. It was only in 1956 that neutrinos were experimentally detected for the first time and so physicists all around the world working in the neutrino field could breathe again; their reputations still intact!

But why?

There exist even greater motives to power the search for neutrinos though, reaching beyond beta decay experiments. More knowledge about the nature of these neutrinos would provide valuable pieces to several rather complex puzzles that have been perplexing physicists for many years now. Here are a few of the major ones:

1) The Sun produces neutrinos in the fusion reactions which take place in its core. Using their current knowledge of the workings of the Sun and neutrinos, theoreticians predict a certain flux of neutrinos emitted from the Sun. So why have experiments only successfully detected around half the neutrinos previously expected? This problem is currently thought to be linked to neutrino oscillations but is still under investigation.

2) Astrophysicists have observed an orbital angular velocity in stars rotating around galaxies that is higher than predicted using gravity and mass calculations. This has caused some scientists to challenge the theory of gravity. However, since it has been decided that neutrinos have mass, they could be a part of the ‘dark matter’, currently held responsible for this effect, and hold potential to explain this extra orbital velocity.

3) Cosmic rays are very high-energy particles, which have been observed to have been penetrating the Earth’s atmosphere. These particles interact with atmospheric atoms to produce a ‘shower’ of new particles (some of which are neutrinos) which can be detected on the Earth’s surface or in the atmosphere directly. A great deal of mystery still surrounds this issue and there is belief that some of these incoming cosmic rays could in fact be neutrinos.

4) Another point which is not so much of a puzzle but a key to a higher understanding of the universe’s origins: it is believed that a large quantity of neutrinos were generated during the Big Bang and once the cooling process began, many were left to drift throughout the universe. This neutrino ‘sea’ has now cooled to approximately 2K and learning more about these neutrinos would give insight into the processes that took place in the early universe and led to the existence of all the matter we see around us today.

So how can we detect neutrinos if they barely interact with other matter?

Ironically, even though neutrinos are some of the smallest particles to have ever been studied, the detectors constructed to find them are some of the largest to have ever been built. The problem lies in the low reaction cross-section (as mentioned earlier), not lack of abundance. In fact there are billions of neutrinos passing through your body every minute but they just slip by like ghosts, unaffected. This giant melee of energies is of little use in terms of scientific observation; with three neutrino ‘flavors’ in circulation we can learn little about the properties of a neutrino. ‘Flavor’ is simply a term used to define the interaction characteristics manifested by that particular neutrino. For example, in most circumstances electron-neutrinos (n_e) are involved in reactions with electrons and muon-neutrinos (n_m) are involved in reactions with muons. The third neutrino is a tau-neutrino (n_t) and is, unsurprisingly, involved in reactions with the tau particle. This means that one needs collimated beams with known flavors so that we can study time-dependent effects such as oscillations (see later).

Machines to generate and detect these beams have been proposed in several locations worldwide. For example, Fermilab (US) have a project underway, in conjunction with which the US Department of Energy has agreed to fund a mile long detector to be built half a mile underground in the Soudan mines. There is also a facility (NGS) under construction at CERN which will send a beam through the Earth’s crust to a detector in Gran Sasso, Italy. No accelerating tunnels are necessary due to the fact that the neutrinos are very unlikely to be involved in any reactions on their journey.

The detector to be built in the Soudan mines contains many plastic scintillation fibres and can only detect neutrinos indirectly via the particles emitted in process known as reverse beta-decay, should the neutrino happen to collide with an atomic nucleus. This process generates a flash of light which signifies a neutrino event. With a pure beam, such as the one proposed for CERN, it would be possible to observe the neutrino flavor both at launch and detection points in order to confirm any change in flavor, hence oscillation that might take place.

Neutrino oscillation was the experimental evidence that confirmed the neutrino’s mass. It is due to a phenomenon know as the ‘quantum- mixing’ of flavors, so that there is a discrete probability that you will find any one neutrino with a certain flavor at a certain time. Beams of neutrinos will therefore be in oscillation between flavors on the journey from source to detector.

What happens now?

It would seem as though the only scientific certainty that has confirmed its place in the future of this research is the insatiable quest for knowledge which is human curiosity. The mysterious and complex nature of neutrino physics, with so many dependent theories resting in the balance, provides the perfect bait for hungry minds.

One especially mind-blowing consequence of this research was the discovery of neutrino mass. This linked the particle nicely into a particularly cutting-edge field of research: the Higgs particle. The Higgs particle has been proposed as the mechanism by which all matter acquires mass, the stronger the Higgs field interaction, the larger the mass. In its previous mass-less state, the neutrino wouldn’t have been a candidate for investigation in this area of physics.

With neutrino factories under construction, the hunt continues and so one can only await further revelations from the search into our inner universe of the unimaginably small.

References

· NuMI-MINOS- Neutrino Physics

http://www-numi.fnal.gov/public/neutrinophysics.html

· Minnesota Technology

http://www.itdean.umn.edu/itbp/technolog/mayjun99/neutrino.html

· Neutrino History

http://wwwlapp.in2p3.fr/neutrinos/aneut.html

· Neutrino Flavor

http://hep.bu.edu/~superk/osc.html