Why neutrinos "go through anything and everything"

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The ties that bind

To understand the bizarre behaviour of the neutrinos, it is best to start by examining the different ways that matter "sticks together".


Hubble M100.half.gif On the largest scale, everything is held together by gravity. All matter, all energy attracts other things by the gravitational "force" (or "interaction"), and all matter and energy is affected by it:
  • A galaxy is held together by the mutual attraction of the stars, gas, and dust that comprise it.
  • A star, against the enormous pressure created by the radiation streaming out from its interior, is held together literally "by its own weight".
  • The earth and the other planets stay in their orbits around the sun due to the gravitational attraction of the sun. This effect cuts both ways: an astronomer on a nearby star could know of the existence of Jupiter, the largest of the planets, by watching the "wobble" that it produces in the motion of the sun through the galaxy.
  • Gravity pulls you (or an apple, say) towards the centre of the earth - you need the ground to be "in the way" to stop you falling!

Yet gravity is actually the weakest of the forces. It dominates everything on a large scale because it always attracts: everything attracts everything else and there is no way to "shield" oneself from this effect.

The other forces which hold matter together are different: they form small clumps which are very tightly bound, but have only a weak effect on the world outside. The most famous of these clumps of matter is the atom:

Atom forces.75.gif

Atoms, and the electromagnetic interaction

An atom is held together by electromagnetic interactions. The tiny nucleus, containing 99.98% of the mass of the atom, defines its centre, and the attraction between this positively-charged centre and the negatively-charged electrons binds the electrons within the atom. (The cartoon of a carbon atom shown here is purely schematic - electrons do not travel in neat orbits within an atom and can't really be said to travel along a path at all - one just has to draw them somehow.) Objects with a positive charge would be repelled, rather than attracted, by the nucleus.

Atom forces.atom.75.gif Overall, an atom is electrically neutral - in a carbon atom, for example, there are six positively-charged protons in the nucleus and six negatively-charged electrons, giving a total charge of zero - and it only influences charged objects that are close enough to "see" that some parts of the atom are positive, others negative. From a long way away, electromagnetically speaking, you wouldn't know that the atom was there. Atoms can share electrons between each other and build larger structures or molecules - this is the business of chemistry - but on a large scale, everything always comes out neutral.

The tiny imbalances of charge which you can cause by friction have a dramatic effect: a comb, charged by brushing your hair on a dry day, can move pieces of paper and the like, and a large cloud, "charged up" by the motion of dust and ice particles in its internal winds, produces violent electric discharges - lightning. This is some sort of index of the strength of electromagnetic interactions compared to gravity: it takes an object the size of a planet to produce easily-noticeable gravitational effects.

A fast-moving charged particle, travelling through matter, interacts with its surroundings: its electric field "kicks" electrons out of the atoms in its path, and it steadily loses energy through this process until it comes to a stop. A light particle, like an electron, will sometimes lose a large fraction of its energy in one go, emitting a bremsstrahlung (German for "braking radiation") gamma ray if it passes through the intense electric field near an atomic nucleus. One way or another, the strength of the electromagnetic interaction does not allow a "naked charge" to last for long.

The nucleus, and the "strong force"

Atom forces.nucleus.75.gif The nucleus, ten-thousand times smaller than the atom which it dominates, is made up of positively-charged protons and electrically neutral neutrons. The electromagnetic repulsion between the protons would blow the nucleus apart, but a third type of interaction, the so-called strong nuclear force, holds the protons and neutrons together. The neutrons, participating in the strong nuclear interaction but not the electromagnetic, act as an extra "glue" within the nucleus. Nuclei contain at least as many neutrons as protons, and a nucleus with too few neutrons will be unstable, and will rapidy disintegrate.

Electrons and other leptons (such as muons and neutrinos) do not participate in the strong nuclear interaction. This accounts for the vast difference of scale within the atom:

  • the electrons, bound to the atom by the electromagnetic interaction only, are confined within a distance of a ten-billionth of a metre or so;
  • the protons and neutrons, bound together by the strong nuclear interaction, are confined within a distance of about a hundred-trillionth of a metre: ten-thousand times smaller.

This extra interaction also means that a "free" particle which is strongly interacting - a proton, say, or a pion - will not travel very far in matter, no matter how much energy it possesses. Sooner or later the particle will interact with a nucleus, losing most of its energy either in a violent scattering, or in producing a spray of lower-energy particles: this happens about every 70 metres in air, and within only 17 cm in iron. A neutron, which has no electromagnetic interaction with its surroundings, will also be stopped in this way.

Atom forces.proton.75.gif The proton, on a scale of about a thousand-trillionth of a metre, is itself a bound structure, containing three quarks. It is the strong force that keeps the quarks confined: at this level it's called simply the strong force, or sometimes the colour force because of its peculiar three-charge structure. It has nothing at all to do with colours of light, of course. (And for the sake of making full-colour pictures, I haven't tried to "colour in" the quarks "correctly" in any of the cartoons. One would have to make electrons and "protons as a whole" colourless, for a start. Then there are other difficulties ... but I digress.)

It works like this: a structure carrying one unit each of the three different charges, which we label red, green and blue for convenience, will be "colour-neutral" or "white", and such a structure can be stable. The proton is such a "colourless" structure, while each of the quarks carries a colour charge.

What we previously called the "strong nuclear force" is a residual force, the left-over "clingy-ness" between the colour-charged constituents of the protons and neutrons. Between protons in a nucleus, this clingy-ness is about twenty times as strong as the electromagnetic interaction; at distances much larger than that, a proton or neutron looks completely "colourless" or "colour-neutral" and there is no strong interaction at all.

(As usual, the cartoon of the proton shouldn't be taken too seriously. While there are certainly three quarks confined within the proton, the proton is much more complicated than that. For a start, the strong interaction is mediated by quanta [units or "particles"] called gluons - "pieces of force" if you like - which are themselves colour-charged, and which carry much of the momentum, and most of the spin, of the proton. The strong interactions within a proton, or any other hadron, are lively and complicated.)

Neutrinos and the "weak nuclear force"

Neutrinos do not participate in any of this:

  • As leptons, they do not feel the strong force, and so they are not bound into the nucleus.
  • Electrically neutral, they are not bound in the atom by the electromagnetic interaction.

They thus have no place in the small-scale, stable structures of matter. Neither will they interact with matter when they pass through it: since they have no charge, they don't lose energy by displacing electrons or scattering from nuclei, and since they don't feel the strong force, they don't ever "run into" a nucleus the way a neutron eventually does.

The only way a neutrino can interact is through the so-called weak nuclear force, the last of the four types of interaction. The more colloquial term "force" is inappropriate: the weak nuclear interaction does not bind anything together, or bend the path of particles in flight. It is the way in which one of the fundamental particles changes into another.

Betadecay1.75.gif For example: when taken out of a nucleus, a neutron is slightly unstable. Within about fifteen minutes it will decay into a proton, an electron, and an anti-neutrino. At the level of the fundamental particles, one of the down quarks in the neutron changes into an up quark, emitting a W-minus - one of the quanta of the weak interaction - as it does so. Almost immediately the W-minus turns into a pair of particles: an electron, and an electron-type anti-neutrino.

(It was the imbalance in energy and momentum caused by this otherwise invisible particle which caused Wolgang Pauli to propose that something of the sort might exist. That was in 1931; neutrinos were finally detected in 1956.)

This sort of re-shuffling of quark identities is very common during nuclear reactions, and most neutrinos are produced in this way: in nuclear reactors, in the centre of the sun, and so on. For example, in one of the reactions which provides energy in the core of the sun, two protons collide to produce a deuteron, a proton and a neutron bound together by the strong nucelar force: one of the protons must change into a neutron for this to work. A weak nuclear interaction does the job, in a charge-flipped form of beta-decay: a proton turns into a neutron, emitting a W-plus, which then turns into a positron (an anti-electron, positively charged) and an electron-type neutrino.


On the rare occasions that a neutrino (or an anti-neutrino) interacts with matter, it does so through a similar process:


Suppose that the neutrino is of the electron-type. It turns into an electron, emitting a W-plus, which strikes a down quark and turns it into an up quark: a neutron (two down plus one up) has been turned into a proton (one down plus two up quarks). The process is something like a forced beta-decay, induced by the neutrino.

This inverse-beta process is almost unbelievably rare. A neutrino will pass through air, water, solid rock, even metal, as if it were not there: if the whole universe were filled with water, for example, neutrinos would travel 100 light years (on average) before interacting in this way. Therefore most of the neutrinos produced by the sun travel straight through the earth, which is a mere twelve-thousand kilometres thick. Most, but not all: a tiny fraction of the neutrinos interact with the rock (or air, or whatever).

There is no "magic" to this, but rather the relative strength of the interactions: of the three types of interaction, strong, electromagnetic and weak, the weak interaction is by far the most feeble - in this context, this means "the least likely to actually occur" - and this is the only type of interaction of which neutrinos are capable.

Once again gravity

Well, not quite. We've been ignoring gravity. Everything is subject to the gravitational interaction - absolutely everything, including neutrinos. One would never suppose this to be relevant, except that neutrinos are extremely common, as common as electromagnetic radiation: light, radio waves, etc. We expect there to be between 300 and 600 neutrinos in every cubic centimetre of space, "left over" from the Big Bang, and long since thinned-out and cooled-down as the universe has expanded.

If neutrinos are massless, then only the miniscule kinetic energy of these relic neutrinos would act on their surroundings by gravity. But if neutrinos have a mass - even a hundred-thousandth of the mass of the electron, otherwise the lightest of the particles - their gravitational influence may be helping to hold the universe together.

Dark Matter

There seems to be more matter in the universe than the matter we can see: the "missing" material goes by the somewhat dramatic name of dark matter. For example, in a spiral galaxy like our own Milky Way, which rotates about its centre, the speed of rotation provides a measure of how much matter the galaxy contains - the faster the rotation, the more mass which must be present to hold the galaxy together. (By way of comparison: if the moon took two weeks to complete its motion around the earth rather than one month, the earth would have to be four times as massive as it actually is, in order to hold the moon in its present orbit.)

When we do measure the rate at which our galaxy rotates, and compare this with the rate we expect by counting up all the stars, gas and dust, we find that the galaxy is rotating too fast. There is matter there that we can't see - probably in a large, spherical cloud extending out past the visible "rim" of the galaxy - and the same is true of other galaxies, clusters of galaxies and so on.


The diagram above shows the "rotation curve" for our galaxy. The vertical axis shows the speed of rotation (in kilometres per second), the horizontal axis shows the distance from the centre of the galaxy (in kiloparsecs, a unit equal to 3262 light-years, or 31 thousand million million kilometres).

The dotted line shows the speed of rotation we'd expect if the visible matter was all there is. The solid line shows the actual speed of rotation.

There are other more subtle reasons to suppose that there is dark matter out there - and there may be more than one "type". In any case, neutrinos-with-mass are one of the important candidates, and the dark matter problem is one of the driving forces behind the search for neutrino oscillations - which can only take place if neutrinos have mass. This is especially true of an experiment like NOMAD, which is most sensitive to neutrinos with masses which are "cosmologically interesting", as they say.

Whether neutrinos make a significant contribution to the dark matter remains to be seen. If true, it would be remarkable that these elusive particles should play so vital a role on so large a stage.

About this document ...

This material was originally prepared by Bruce Yabsley for a presentation at the MLC School for Girls, Burwood, in July 1996.

The galaxy image is a Hubble Space Telescope picture of the spiral galaxy M100, provided by the Space Telescope Science Institute (operated by the Association of Universities for Research in Astronomy, Inc., from NASA contract NAS5-26555), and is reproduced with their permission. The full-size image and others are freely available (with acknowledgement) from their Web site.

The rotation curve for our galaxy is taken from here, one of the Web pages maintained by the UK Dark Matter Collaboration.