Are all neutrinos left-handed?

Just like people, neutrinos can be right-handed or left-handed. Southpaws will rejoice to hear that unlike in the human population, where left-handedness is not the norm, all the neutrinos that we have ever seen are in fact left-handed. Symmetrically, all of the antineutrinos that scientists have ever seen have been right-handed.

It may seem odd to imagine particles as having “handedness.” Handedness refers to two properties of a particle: the direction of its spin and how it relates to the direction the particle is traveling. If you curl your hands into fists and extend your thumbs toward one another, you’ll see examples of right-handed and left-handed particles. They have the same spin, represented by the direction your fingers curl, but have opposite directions of travel, represented by where your thumb points.

Another demonstration: If you curl your hands into fists and extend your thumbs towards the ceiling, you’ll see examples of right-handed and left-handed particles.  They have opposite spin (because your left fingers are curling clockwise and your right fingers are curling anticlockwise), but have the same direction of travel (which way your thumb is pointing).

Once again, neutrinos turn out to be an anomaly. Other particles such as the quarks and the other three leptons (the electron, muon, and tau) have both left-handed and right-handed versions of both the matter particle and their antimatter partner.

This leads to the question: Where are all the right-handed neutrinos and the left-handed antineutrinos? It remains a mystery, but scientists suspect that—because we haven’t seen them yet—if these right-handed neutrinos exist they will be very different from the left-handed neutrinos we know and love. Perhaps they are much heavier, or do not interact via the weak force but instead interact only via gravity (so-called “sterile neutrinos”). In fact, right-handed neutrinos are a good candidate for the sterile neutrinos that have been hinted at through various experiments but not yet discovered. Many experiments are trying right now to see whether these sterile neutrinos really do exist.

When scientists talk about handedness and neutrinos, they are actually talking about two different but related things: helicity and chirality. Despite helicity being the easier concept to understand, when scientists talk about neutrinos being left handed, they are probably talking about chirality. The two terms often get interchanged, in part because there’s a lot of overlap.

Helicity is a property similar to spin, energy, or momentum—it is a conserved quantity, but it depends on the reference frame. Helicity is essentially what was described in the basic section: It’s how the spin relates to the motion of travel. Helicity is not an unchangeable property; it varies depending on how you look at the particle. Imagine a left-handed neutrino by holding out your left fist. If your body is stationary and your arm moves to the right, it’s still a left-handed neutrino. That is, the direction and spin (the curl of your fingers) are what a left-handed neutrino would have. But if you hold your left arm stationary and move your body to the right, your fist appears to move to the left. It now has the spin and direction of a right-handed neutrino.

On the other hand, chirality is an intrinsic, fundamental property of the particle. It doesn’t depend on the reference points or perspective. Neutrinos can be left-chiral or right-chiral. Physicists have seen only neutrinos born in a left-chiral state, and only right-chiral antineutrinos.

So chirality is built in, but helicity is a matter of perspective. Something that could overtake a neutrino—say, a speedy photon—would see the neutrino’s helicity flip. But since humans don’t spend a lot of time moving at the speed of light, there is a lot of overlap between helicity and chirality. Hence, confusion and free use of the two terms when it comes to handedness.

neutrinos leaving trails through a striped square

Credit: Symmetry Magazine / Sandbox Studio, Chicago

If neutrinos were massless and traveled at the speed of light, as scientists initially thought and as the Standard Model predicts, then their helicity and chirality would match up. But when scientists discovered the particles had mass, they were surprised that right-chiral neutrinos were nowhere to be found. Because scientists have never seen right-chiral neutrinos, they can conclude that if they do exist, they are very different from left-chiral neutrinos. They might be very heavy, not interact through the weak force, or both.

This question of left-handedness or left chirality is particularly interesting if neutrinos are Majorana particles: particles that can function as their own antiparticles. If neutrinos are Majorana particles, then the only difference between neutrinos and antineutrinos would be their chirality. Left-chiral neutrinos would be what we currently call “neutrinos,” and right-chiral neutrinos would be what we call “antineutrinos.”

Majorana neutrinos could help explain many questions in neutrino physics. In addition to helping us understand where the right-chiral neutrinos are, it would open neutrinos up to getting their mass in a different way from all other particles. Neutrinos are incredibly light compared to all other particles with mass, which get their heft by interacting with the Higgs field. Scientists want to know why the Higgs field affects neutrinos so little—but it could be that neutrinos get their tiny masses through an entirely different mechanism.