# What's the difference between helicity and chirality?

When a particle spins in the same direction as its momentum, it has right helicity, and left helicity otherwise. Neutrinos, however, have some kind of inherent helicity called chirality. But they can have either helicity. How is chirality different from helicity?

• I think this wikipedia article does a reasonable job in explaining the difference. If, after reading the article, you are still confused be sure to come back to ask further questions. – Marek Nov 19 '10 at 19:52

## 5 Answers

At first glance, chirality and helicity seem to have no relationship to each other. Helicity, as you said, is whether the spin is aligned or anti aligned with the momentum. Chirality is like your left hand versus your right hand. Its just a property that makes them different than each other, but in a way that is reversed through a mirror imaging - your left hand looks just like your right hand if you look at it in a mirror and vice-versa. If you do out the math though, you find out that they are linked. Helicity is not an inherent property of a particle because of relativity. Suppose you have some massive particle with spin. In one frame the momentum could be aligned with the spin, but you could just boost to a frame where the momentum was pointing the other direction (boost meaning looking from a frame moving with respect to the original frame). But if the particle is massless, it will travel at the speed of light, and so you can't boost past it. So you can't flip its helicity by changing frames. In this case, if it is "chiral right-handed", it will have right-handed helicity. If it is "chiral left-handed", it will have left-handed helicity. So chirality in the end has something to do with the natural helicity in the massless limit.

Note that chirality is not just a property of neutrinos. It is important for neutrinos because it is not known whether both chiralities exist. It is possible that only left-handed neutrinos (and only right-handed antineutrinos) exist.

• At first glance, they seem like very related concepts to me, actually! – Noldorin Nov 20 '10 at 21:20
• This is really a misleading/uninformative answer - what does "chirality is like your left hand versus your right hand" even mean? Especially when people often describe helicity in precisely the same way. – knzhou Aug 21 '19 at 23:47
• A better answer, as stated elsewhere on this site, is that chirality is a property of fields and helicity is the closely related property of particles. For more about how chirality of fields is related to helicity of particles see here. – knzhou Aug 21 '19 at 23:48

Helicity is easy to define; chirality is more subtle.

The helicity of a particle is the normalized projection of the spin on the direction of momentum. If the spin is more along the same direction of the momentum than against it, then the helicity is positive; otherwise it is negative.

Chirality is to do with the way the particle's properties transform when they are described with respect to one inertial reference frame or another. The difference between right-handed and left-handed is like the difference between contravariant and covariant 4-vectors, but now we are talking about spinors. For a massless spin half particle, the spin and momentum can both be extracted from a single spinor. When one transforms from one frame to another, one should use the ordinary Lorentz transformation for a right-handed spinor, and the inverse Lorentz transformation for a left-handed spinor. Thus chirality is an intrinsic property of such a particle, but one whose influence is only revealed in this subtle way. It influences how the spinor enters into the Weyl equation, for example.

Massive spin-half particles such as electrons have their spin and momentum described by Dirac spinors which are made of two Weyl spinors, one of each chirality.

What distinguishes a neutrino (treated here as massless) from an anti-neutrino is primarily its chirality. But whenever just a single 2-component spinor describes both the momentum and the spin, one finds that the helicity for such a particle can only take one value (and for the antiparticle it takes the opposite value). Thus the helicity and the chirality then have the same value, but it does not mean they are the same thing.

When a given type of particle can only have one helicity, one has a situation that does not respect parity (mirror-reflection) symmetry. This is at the heart of the breaking of parity invarience by the weak force.

Chirality and helicity are exactly the same thing in the massless limit. By this, I mean that either term can be used interchangeably in the massless limit (recall that a condition for massless particles, is that they necessarily move at velocity c). And yes you are correct in that there is no frame of reference where we can hypothetically boost beyond say a photon, and find that helicity has flipped. Having said that, there also exist massive particles with definite chirality. This has to do with spin and a process called quantum interference. The explanation has its roots tied deep in the understanding of QM and field theory. It's long winded and mathematical. But it's crucial to having a full understanding of the Standard Model. Look it up and research! And if you have anymore questions we will be happy to oblidge. Good luck.

• Your answer is absolutely correct I guess. I can't understand why it's downvoted!?!!! – Bastam Tajik Jul 15 '20 at 17:35

Helicity and chirality are not the same thing in the massless limit. They are unrelated. Helicity is an extrinsic physical property related to the alignment of spin and momentum; chirality is related to weak interactions. Chirality is more akin to electric charge or strong color charge than it is to momentum.

• Clearly some people don't like your answer, but I think it would be ok if expressed a bit more clearly (and perhaps fewer no's at the start). It is over-stating it to say that the two properties are unrelated, I think. I like the final sentence though; I would keep that. – Andrew Steane Feb 11 '20 at 9:50

So this is how I see it...

...particle spins,

if momentum direction = spin direction, helicity is positive, if momentum direction <> spin direction, helicity is negative...

chirality is complementary to helicity in that...

if the elements (momentum and spin) are reversed, the same outcome is observed, but is opposite in direction.