Particle Physics


Sterile Neutrinos: The Ghost Particle’s Ghost

What do you call the ghost of a ghost?

If you’re a particle physicist, you might call it a “sterile neutrino.” Neutrinos, known more colorfully as “ghost particles,” can pass through (almost) anything. If you surrounded the Sun with five light years’ worth of solid lead, a full half of the Sun’s neutrinos would slip right on through. Neutrinos have this amazing penetrating capability because they do not interact by the electromagnetic force, nor do they feel the strong nuclear force. The only forces they feel are the weak nuclear force and the even feebler tug of gravity.

The Perseus galaxy cluster, one of 73 clusters from which mysterious x-rays, possible produced by sterile neutrinos, were observed. Credit: Chandra: NASA/CXC/SAO/E.Bulbul, et al.; XMM-Newton: ESA

When Wolfgang Pauli first postulated neutrinos in 1930, he thought that his proposed particles could never be detected. In fact, it took more than 25 years for physicists to confirm that neutrinos—Italian for “little neutral ones”—were real. Now, physicists are hunting for something even harder to spot: a hypothetical ghostlier breed of neutrinos called sterile neutrinos.

Today, we know of three different “flavors” of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos (and their antimatter equivalents). In the the late 1960s, studies of the electron-type neutrinos emitted by the Sun led scientists to suspect that they were somehow disappearing or morphing into other forms. Measurements made in 1998 by the Super Kamiokande experiment strongly supported this hypothesis, and in 2001, the Sudbury Neutrino Observatory clinched it.

One of the limitations of studying neutrinos from the Sun and other cosmic sources is that experimenters don’t have control over them. However, scientists can make beams of neutrinos in particle accelerators and also study neutrinos emitted by man-made nuclear reactors. When physicists studied neutrinos from these sources, a mystery presented itself. It looked like there weren’t three kinds of neutrinos, but rather four or perhaps more.

Ordinarily, this wouldn’t be cause for alarm, as the history of particle physics is full of the discovery of new particles. However, in 1990, researchers using the LEP accelerator demonstrated convincingly that there were exactly three kinds of ordinary neutrinos. Physicists were faced with a serious puzzle.

There were some caveats to the LEP measurement. It was only capable of finding neutrinos if they were low mass and interacted via the weak nuclear force. This led scientists to hypothesize that perhaps the fourth (and fifth and…) forms of neutrinos were sterile, a word coined by Russian physicist Bruno Pontecorvo to describe a form of neutrino that didn’t feel the weak nuclear force.

Searching for sterile neutrinos is a vibrant experimental program and a confusing one. Researchers pursuing some experiments, such as the LSND and MiniBoone, have published measurements consistent with the existence of these hypothetical particles, while others, like the Fermilab MINOS team, have ruled out sterile neutrinos with the same properties. Inconsistencies abound in the experimental world, leading to great consternation among scientists.

In addition, theoretical physicists have been busy. There are many different ways to imagine a particle that doesn’t experience the strong, weak, or electromagnetic forces (and is therefore very difficult to make and detect); proposals for a variety of different kinds of sterile neutrinos have proliferated wildly, and sterile neutrinos are even a credible candidate for dark matter.

Perhaps the only general statement we can make about sterile neutrinos is that they are spin ½ fermions, just like neutrinos, but unlike “regular” neutrinos, they don’t experience the weak nuclear force. Beyond that, the various theoretical ideas diverge. Some predict that sterile neutrinos have right-handed spin, in contrast to ordinary neutrinos, which have only left-handed spin. Some theories predict that sterile neutrinos will be very light, while others have them quite massive. If they are massive, that could explain why ordinary neutrinos have such a small mass: perhaps the mathematical product of the masses of these two species of neutrinos equals a constant, say proponents of what scientists call the “see-saw mechanism”; as one mass goes up, the other must go down, resulting in low-mass ordinary neutrinos and high-mass sterile ones.

Now, some astronomers have proposed sterile neutrinos could be the source of a mysterious excess of x-rays coming from certain clusters of galaxies. Both NASA’s Chandra satellite and the European Space Agency’s XMM-Newton have spotted an excess of x-ray emission at 3.5 keV. It is brighter than could immediately be accounted for by known x-ray sources, but it could be explained by sterile neutrinos decaying into photons and regular neutrinos. However, one should be cautious. There are tons of atomic emission lines in this part of the x-ray spectrum. One such line, an argon emission line, happens to be at 3.62 keV. In fact, if the authors allow a little more of this line than predicted, the possible sterile neutrino becomes far less convincing.

Thus the signal is a bit sketchy and could easily disappear with a better understanding of more prosaic sources of x-ray emission. This is not a criticism of the teams who have made the announcement, but an acknowledgement of the difficulty of the measurement. Many familiar elements emit x-rays in the 3.5 keV energy range, and though the researchers attempted to remove those expected signals, they may find that a fuller accounting negates the “neutrino” signal. Still, the excess was seen by more than one facility and in more than one cluster of galaxies, and the people involved are smart and competent, so it must be regarded as a possible discovery.

It is an incredible long shot that the excess of 3.5 keV x-ray from galaxy clusters is a sterile neutrino but, if it is, it will be a really big deal. The first order of business is a more detailed understanding of more ordinary emission lines. Unfortunately, only time will tell if we’ve truly seen a ghost.

Go Deeper
Author’s picks for further reading

NOVA: The Ghost Particle
Learn more about neutrinos at this companion site to NOVA’s 2006 film “The Ghost Particle.”

Résonaances: Signal of neutrino dark matter
Particle physicist Adam Falkowski comments on the possible link between sterile neutrinos and the observed galactic cluster x-ray signal.

Scientific American: Cosmic Mismatch Hints at the Existence of a “Sterile” Neutrino
Clara Moskowitz explains how sterile neutrinos could resolve a clash between measurements of the cosmic microwave background radiation and galaxy clusters.

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Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Anonymous

      Since neutrinos don’t interact via the electromagnetic force, how do physicists affect how they move in the lab e.g. cause them to form a stream?

      • Don Lincoln

        We form a beam of charged pions and aim the pion beam. Pions decay into muons and muon neutrinos. Strip out the muons and you are left with a neutrino beam

    • Matt McDade

      Because our particle universe is getting smaller and smaller, seemingly only limited by our technology, is there an end in sight? Is there a theory to the “pixelization” of particles?

      • Don Lincoln

        It is believed at a size scale of 10^-35 meters that we will have to take into account the quantization of space and time. Note that that number is very dependent on a specific theory and the real number might be different. However our current theoretical prejudice is that there is a size scale below which it doesn’t make sense to explore…simply because quantized space means that there are no sizes smaller than the smallest bit of space.

    • disqus_cqIF80ifOp

      I presume that there is some synchrotron radiation at 3.5 keV. Has that been subtracted out? Neutrinos were invented to give physicists ulcers. Theorists are too creative for the good of the sport; just like JFK conspiracy theories, they can’t ALL be true. Maybe they should be referred to as the particle physicists’ full employment act. 😉 Occam’s razor ought to favor right handed neutrinos over a 4th flavor.

    • George Raina
    • George Raina

      By definition the sterile neutrino does not participate in the electromagnetic and
      weak interactions, only the gravitational force gives its mass. There should be
      one strange neutrino that changes the diffraction patterns of the
      electromagnetic oscillations leaving the low frequencies side of the Planck
      Distribution Law with non-compensated high frequency side. Since the neutrino
      oscillation and the general weak interaction this sterile neutrino can be
      oscillate to another measurable neutrino.

      The change of the temperature at the Big Bang was the main source for this
      asymmetry and the creation of the dark matter by the Baryogenesis. Later on
      also the weak interaction can change the rate of the dark matter, but less
      influencing it, see the temperature changes of the dark side in the Planck
      Distribution Law.

      The Weak Interaction basically an electric dipole change and transferring the
      electric charge from one side of the diffraction pattern to the other side. If
      there is no other side (dark matter), the neutrino oscillation helps to change
      the frequency of the electromagnetic oscillations, causing real diffraction
      patterns and leaving the non – compensated side of the Planck Distribution
      curve for the invisible Dark Matter.

    • Clarifier

      Will the mass-energy balance relation for the proton decay has to be
      revised? affecting the masses of the positron, and neutron?, or will it
      be just a correction to the kinetic energy term?

    • Joseph Connolly

      Thanks Don. Twenty years ago I didn’t think I stood the ghost of a chance of understanding any astrophysics. Now thanks to you , Brian Greene, Hawking, Tyson and others with books written for us without math backgrounds I’m slowly getting a small grasp of some of it.

      • Alone: bad. Friend: good!

        That sounds great but make sure you question everything. Even Don says there are no sacred cows.

        While in physics classes in college I would always think about what I was learning and decide for myself what was right and wrong, i.e…
        That sounds correct
        That’s impossible.
        That’s stupid.
        That’s crazy