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Richard Ruiz | Univ. of Pittsburgh | U.S.A.

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What are Sterile Neutrinos?

Sterile Neutrinos in Under 500 Words

Hi Folks,

In the Standard Model, we have three groups of particles: (i) force carriers, like photons and gluons; (ii) matter particles, like electrons, neutrinos and quarks; and (iii) the Higgs. Each force carrier is associated with a force. For example: photons are associated with electromagnetism, the W and Z bosons are associated with the weak nuclear force, and gluons are associated with the strong nuclear force. In principle, all particles (matter, force carries, the Higgs) can carry a charge associated with some force. If this is ever the case, then the charged particle can absorb or radiate a force carrier.

SM Credit: Wiki

Credit: Wikipedia

As a concrete example, consider electrons and top quarks. Electrons carry an electric charge of “-1″ and a top quark carries an electric charge of “+2/3″. Both the electron and top quark can absorb/radiate photons, but since the top quark’s electric charge is smaller than the electron’s electric charge, it will not absorb/emit a photon as often as an electron. In a similar vein, the electron carries no “color charge”, the charge associated with the strong nuclear force, whereas the top quark does carry color and interacts via the strong nuclear force. Thus, electrons have no idea gluons even exist but top quarks can readily emit/absorb them.

Neutrinos  possess a weak nuclear charge and hypercharge, but no electric or color charge. This means that neutrinos can absorb/emit W and Z bosons and nothing else.  Neutrinos are invisible to photons (particle of light) as well as gluons (particles of the color force).  This is why it is so difficult to observe neutrinos: the only way to detect a neutrino is through the weak nuclear interactions. These are much feebler than electromagnetism or the strong nuclear force.

Sterile neutrinos are like regular neutrinos: they are massive (spin-1/2) matter particles that do not possess electric or color charge. The difference, however, is that sterile neutrinos do not carry weak nuclear or hypercharge either. In fact, they do not carry any charge, for any force. This is why they are called “sterile”; they are free from the influences of  Standard Model forces.

Credit: somerandompearsonsblog.blogspot.com

Credit: somerandompearsonsblog.blogspot.com

The properties of sterile neutrinos are simply astonishing. For example: Since they have no charge of any kind, they can in principle be their own antiparticles (the infamous “sterile Majorana neutrino“). As they are not associated with either the strong nuclear scale or electroweak symmetry breaking scale, sterile neutrinos can, in principle, have an arbitrarily large/small mass. In fact, very heavy sterile neutrinos might even be dark matter, though this is probably not the case. However, since sterile neutrinos do have mass, and at low energies they act just like regular Standard Model neutrinos, then they can participate in neutrino flavor oscillations. It is through this subtle effect that we hope to find sterile neutrinos if they do exist.

Credit: Kamioka Observatory/ICRR/University of Tokyo

Credit: Kamioka Observatory/ICRR/University of Tokyo

Until next time!

Happy Colliding,

Richard (@bravelittlemuon)

 

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4 Responses to “What are Sterile Neutrinos?”

  1. QM says:

    How can sterile neutrinos be produced if they don’t carry any force, and are not affected by any force other than gravity? And how do they differ from gravitons?

    • Richard Ruiz says:

      Hi QM,

      You have two good questions.

      1. I am glad you caught that. The subtly is that sterile neutrinos are sterile only with respect to the Standard Model forces. If sterile neutrinos do exist, then they can still interact via other hypothetical forces. If not the Standard Model Higgs, then some additional particle must also exist to generate a sterile neutrino masses. Depending on the mass of this mass-generating partner, it can decay and produce a pair of sterile neutrinos.

      2. Gravitons are the force carrier of gravity and is spin-2. Sterile neutrinos are matter particles and is spin-1/2.

  2. Pythagore says:

    Nice article. From the questions and answers to QM I would like to have some additional explanations from the author.

    Regarding question 1 and the article content. It seems your description of the sterile neutrino and associated particles is in fact dark matter while you say in the article it is improbable the sterile neutrino is dark matter.

    To me, it sounds if we are facing new particles outside the Standard model which go undetected because they do not interact through any of the known forces isn’t this in fact exactly the definition of dark matter? Hence the sterile neutrino is just one of them.

    Thansk for enlightening my understanding.

    • Richard Ruiz says:

      Hi Pythagore,

      I was ambiguous in my post. Yes, sterile neutrinos would qualify as dark matter. In fact, ordinary neutrinos *are* dark matter: they are chargeless, colorless, weakly interacting, massive particles. I should have been more precise in saying that sterile neutrinos are not likely to be the major component of dark matter that is responsible for our cosmology. Sufficiently heavy neutrinos decay and sufficiently like neutrinos are too energetic to form structure. This middle-of-the-road option neutrinos is possible, but not easy to accommodate in models.

      – Richard

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