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David Schmitz | Fermilab | USA

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To whom neutrinos may concern

Hi everyone.  This post is inspired by a letter I received recently written by a remarkable 6th grader who is working on a project about neutrinos.  He mailed his letter to the Office of Communications here at Fermilab and I was asked if I could put together a response. In his letter he asks three great questions about neutrinos and the Fermilab neutrino experiments, so I thought I would share my reply here on QD, as others might be interested.

1. How does Fermilab send beams of neutrinos up to the neutrino research lab in Minnesota?

2. How are neutrinos formed?

3. Why are neutrinos not classified as dark matter?

See, great questions, right?  Here were my thoughts…

Thank you for your letter with your three questions about the neutrino experiments being done here at Fermilab.  Indeed we do send a beam of neutrinos created here at the Laboratory in Illinois to a research laboratory about 450 miles away in northern Minnesota!

We couldn’t do this with just any particle.  Only neutrinos can travel straight through the Earth unaffected and arrive at a neutrino detector so far away.  This has to do with the way different kinds of particles interact.  There are three forces through which particles can interact (ignoring gravity since it is so weak for such tiny objects):

1.    the electromagnetic force – the force between charged particles
2.    the strong nuclear force – the force between quarks that holds protons and neutrons together
3.    the weak force – a force that acts between all particles but is much much weaker than the other two

A proton is capable of feeling the effects of all three forces – it has electric charge, is made of quarks, and feels the weak force like all particles.  The electron can feel the effects from the electromagnetic force and the weak force.  But the neutrinos can feel only the weak force since they are not charged and do not contain quarks.  This means that they can pass right through dense materials without interacting at all.  In comparison, a proton or electron would make it only a few feet or less before it gets absorbed by the surrounding material.  So we could never send a beam of electrons, for example, great distances through the Earth – they would all just interact and get absorbed in the rock and dirt.  But a beam of neutrinos will pass right through with only a very small number of them getting absorbed.

This same quality (lack of interaction with material or electric and magnetic fields) that makes the neutrino beam to Minnesota possible also makes it challenging to work with neutrinos experimentally.  To make a focused beam of charged particles like protons is relatively straightforward because you can control them directly with electric and magnetic fields.  Not so with neutrinos.

So how do we create  a neutrino beam here at Fermilab? We use the proton beam in the particle accelerator at Fermilab as a starting point.  Again, protons are easy to control.  We accelerate the protons with electric fields and guide them with magnets to collide them with a stationary, solid target.   In this case, the target is made of carbon, but other light elements can work as well, such as beryllium.  When the very energetic protons collide with the carbon nuclei in the target an interaction occurs that creates new types of particles.  The most common type is called a ‘pion’.  A pion is a combination of two different quarks (a proton is three, so a pion is just a rearrangement of the quarks that were available from the original protons in the beam and the carbon nucleus).  A very special quality of the pion particle is that is doesn’t last very long.  It travels along for about 20 billionths of a second before it spontaneously decays.

Decays?  What does that mean?  It means it converts into other particles, in this case a muon (the heavier, charged big brother of the electron) and a neutrino!  So, in summary:

protons + carbon    →    pions    →    muons + neutrinos

So, the way we create neutrinos to send to Minnesota is by using the proton beams available at Fermilab to create pions to create neutrinos.

It is important to note that the more protons you have to start with the more neutrinos you will get to do your neutrino experiments with, so a lot of effort goes into making beams with many many protons.  Since May 2005, Fermilab has delivered about 7E20 (700,000,000,000,000,000,000!!) protons onto our carbon target to create neutrinos that then go straight through the Earth’s crust to the lab in Minnesota!

Your final question, “Why are neutrinos not classified as dark matter?” is a really good question.  Technically, neutrinos are dark matter!  The term ‘dark matter’ refers to matter that does not interact with electromagnetic radiation.  It is this radiation in the form of visible light (as comes from stars) or other frequency ranges that we use to observe objects in our Universe.  The way we have ‘observed’ dark matter is by noticing the strong gravitational effects on objects we can see by some very massive object nearby that we cannot see.  These gravitational effects have been so strong that we now realize there is far more matter out in the Universe that we can’t see (dark matter) than we can.   Physicists now have evidence that 83 percent of all matter in the universe is made of dark matter.

So can neutrinos account for all the dark matter in the universe? When experimenters discovered about 10 years ago that neutrinos have mass (previously it was thought that they did not have any mass at all) people got excited thinking this may be a solution to the dark matter puzzle.  Could neutrinos be the mass that we could detect gravitationally, but not see?  We now know that, while neutrinos do have mass, and there are enormous numbers of them throughout the Universe, it is still not nearly enough mass to explain what we have seen in the astronomical observations.  Today we know that neutrinos account for less than one percent of the dark matter in the universe. That’s why people often say that neutrinos are not dark matter. But technically, they are.

So, there must be more dark matter, something other than just neutrinos, that we don’t yet know about.  And physicists are trying very hard to detect this matter directly.  They must interact very, very rarely like neutrinos, but are probably much heavier.   The direct detection of such dark matter particles will be a major and very exciting discovery in particle physics.

I hope this has answered your questions about neutrinos.  Please let me know if you have any further questions, and best of luck on your expert report!

David Schmitz