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Questions tagged [helicity]

In particle physics, helicity is the projection of the angular momentum onto the direction of momentum. For massless spin-1⁄2 particles, helicity is equivalent to the chirality operator multiplied by $\hbar/2$, so may be used for related chirality questions as well.

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5
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1answer
263 views

Derivation of conformal generators in spinor helicity formalism

I've been trying for some time to find the expressions for conformal generators of Witten's paper in perturbative Yang-Mills. Given $P_{\alpha \dot{\alpha}} = \lambda_{\alpha} \overline{\lambda}_{\...
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0answers
143 views

General Relativity and spin/helicity two massless particle

I usually read that GR can be thought of as the unique theory of a massless spin-2 particle (I think that this is the graviton). I know that GR is the unique theory that has: diffeomorphism ...
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2answers
53 views

Helicicity operator of a particle

How can one project the spin of a particle along the direction of momentum as they belong to different bases? Or is it allowed because there are infinitely many bases connected through similarity ...
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1answer
98 views

How strong are the experimental constraints on the neutrino helicity really?

It is commonly accepted that all neutrinos have left-handed helicity and all anti-neutrinos are right-handed helicity.$^1$ The experimental base comes from Goldhaber's famous experiment performed in ...
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1answer
123 views

Thinking about Chirality and Helicity

Here is my point about chirality and helicity: I thought that chirality is defined for field, But helicity is defined for particle state, you can really measure helicity though measure the spin and ...
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2answers
163 views

Photon carries spin angular momentum of $\hbar$

I know from numerous articles and also wiki (http://en.wikipedia.org/wiki/Spin_angular_momentum_of_light) that a photon carries spin angular momentum of $\hbar$. How to prove it mathematically? What ...
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2answers
570 views

What are the actual transformation properties of Dirac spinors $u_\sigma(p)$?

Let $u_\sigma(p)$ be a Dirac spinor. As far as I know, it transforms under changes of reference frame according to $$ u_\sigma(p)=S(\Lambda)u_\sigma(\Lambda p)\tag{1} $$ where the $\sigma$ label doesn'...
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0answers
52 views

Left-handed and right-handed helicity, can you explained well the phenomenon of chirality simply? [duplicate]

Chirality, helicity as a projection of its spin vector .. that becomes a kind of 'virtual spin'? I often confuse them: but the particles 'spin' on the right or left, or is the projection of .. what? ...
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2answers
317 views

Why can we approximate massive particles as massless or vice versa?

Our descriptions of massless and massive particles are very different. For example: Massless particles have only two polarizations, which we call helicities. Spin projection on axes different than ...
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0answers
542 views

Polarized Moller scattering cross section

When doing a computation of scattering cross sections of particles with spin, one usually averages over the initial spins and sums over the final ones. I'm a bit puzzled as to how to do the ...
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0answers
116 views

Is the helicity of photon Lorentz invariant? [closed]

If the helicity of a photon is $+1$ in an inertial frame, then is the helicity of this photon $+1$ in another inertial frame? The helicity operator is $$ h=\mathbf{S}\cdot\hat{\mathbf{p}} $$ with $$ ...
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1answer
996 views

Pion decay: why is the positron electron neutrino decay suppressed?

I'm reading through M. Thomson's Modern Particle Physics to better understand why the positron electron neutrino decay channel is suppressed relative to the muon and muon neutrino decay route. I know ...
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2answers
608 views

Spinor helicity formalism - original reference?

The spinor helicity formalism is a modern technique widely used in scattering amplitude calculations nowadays. However, it is hard to find a reference for who first came up with the formalism. Maybe ...
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1answer
232 views

Computation of theta-term from triangle diagram

The chiral $U(1)$ anomaly in QCD can be calculated exactly by one-loop Feynman diagrams, for example by the famous triangle diagram. I am currently performing the computation to get a better ...
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1answer
80 views

What is the QED model about half wave plate?

Surely classical electrodynamics (CED) has a good and well-known answer about the change of light polarization in a HWP (or Quarter wave plate). I tried to find how does this look like from point of ...
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1answer
801 views

Confusion with chirality eigenstates

In the Weyl/chiral basis, the four components of the Dirac spinor represent left-chirality spin up, left-chirality spin down, right-chirality spin up, and right-chirality spin down, respectively. When ...
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2answers
436 views

What is a linear polarized photon?

According to Dirac a 'linear' polarized photon is a superposition of left and right rotating photons. Here is a puzzling aspect of this superposition. There are dichroic materials which can absorb ...
3
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1answer
417 views

Schwartz's book: Spinor-helicity formalism

I'm trying to learn the spinor-helicity formalism from Schwartz's QFT book. His equation 27.44 is describes the annihilation of an electron(1)-positron(2) pair to a muon(3)-antimuon(4) pair. He ...
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1answer
316 views

How can neutrinos have both mass and helicity?

If a neutrino has mass it must travel at less than the speed of light. So how can it possess helicity, which can change depending on relative velocity?
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1answer
552 views

If a neutrino and his antineutrino has different helicity how can it be a possibility that it is a Majorana particle?

All antineutrinos observed thus far possess right-handed helicity (i.e. only one of the two possible spin states has ever been seen), while neutrinos are left-handed. Notwithstanding that it is said ...
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2answers
580 views

Helicities in electron-positron annihilation

Consider the massless limit of a process in which an electron-positron pair annihilates into a virtual photon - the final state doesn't matter. If the electron is massless (or if the energy is high ...
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3answers
10k views

Helical motion of charged particle in external magnetic field

When an electron moves in uniform external magnetic field, with velocity not perpendicular, I understand that the magnitude of force is only due to the perpendicular component of velocity and is ...
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1answer
2k views

What is polarisation, spin, helicity, chirality and parity?

Polarisation, spin, helicity, chirality and parity keep confusing me. They seem to be related, but exactly how they are related is unclear to me. Can someone maybe give a short overview about what ...
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2answers
102 views

Which particles have helicity?

I know electrons have helicity but can particles which are not fundamental i.e.protons have helicity?
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1answer
95 views

Helicity amplitudes and muon/electron currents

I have the following helicity spinor: $$ u_R=\sqrt{E} \begin{pmatrix} c \\ se^{i\phi} \\ c \\ se^{i\phi} \end{pmatrix} $$ We take $s=\sin\theta/2$ and $c=\cos\theta/2$. Also, $E$ and $\phi$ are ...
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1answer
922 views

Angular Momentum of a Photon

Why is it that the angular momentum of a photon is $\hbar$, irrespective of its energy? I encountered such a claim in a text about Raman spectroscopy. Is there an explanation for this using basic ...
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0answers
329 views

Spin and polarization, QM vs QFT

On page 34 of A. Zee's book QFT in a Nutshell, he states: I expect you to remember the concept of polarization from your course on electromagnetism. A massive spin 1 particle has three degrees of ...
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2answers
333 views

Why is the Spin of the photon neglected?

We know photons have spin s=1. However, in Nuclear physics, the conservation of angular momentum in case of Gamma transitions is employed as follows: $$\vec J_i=\vec J_f+\vec L$$ where $J_i$ is the ...
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3answers
316 views

Does orbital angular mometum has no meaning for single photons?

In the quantization of free electromagnetic field, it is found that the left-circularly polarised photons corrsponds to helicity $\vec{S}\cdot\hat p=+\hbar$ and right-circularly polarised photons ...
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139 views

Bound state of $\mu^- \mu^+$ . Why non-relativistic muon states?

Peskin & Schroeber page 148 discuss the creation of a $ \mu^+ \mu^-$ resonance state . Equation (5.44) describes the creation of a Spin Up bound $ \mu^+ \mu^-$: $$ \mathcal{M} = \sqrt{2M} \int \...
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1answer
97 views

Spin angular momentum

Spin angular momentum is defined as $\Sigma^{i}= 1/2 e^{ijk} \sigma_{jk}$ . Thus, I can write $\Sigma^{1}$ as $[\gamma_{2},\gamma_{3}]$ . I want some insights on this definition. Also, Can anybody ...
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0answers
300 views

Dirac Spinors as Eigenvalues of Helicity Matrix

I am trying (unsuccessfully) to verify this relation regarding the helicity of Dirac spinors: $$ { \sigma }_{ \vec { p } }u_{ r }\left( \vec { p } \right) =\frac { \vec { \Sigma } \cdot \vec { p } ...
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0answers
185 views

Spinor helicity formalism, exact form of the spinors

I am trying to understand how to perform computations with the spinor helicity formalism, I am studying on this review http://arxiv.org/abs/1308.1697. I have stumbled upon a problem though, in pag. ...
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1answer
521 views

Scattering Amplitude Not Invariant under Little Group?

I am trying to make sense of scattering amplitude recently. In some literature people say that if some number of massless particles collide together, one can theoretically express the scattering ...
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1answer
460 views

Helicity angle of electron in $B^0 \rightarrow K^{*0} \gamma$, with $\gamma \rightarrow e^+ e^-$?

I haven't been able to find the exact definition of a helicity angle, and the ones that I found don't apply to this particular case, because they'd require to boost to the electrons' 4-momentum to the ...
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0answers
123 views

Parke-Taylor formula in the $n=4$ simple case

I am trying to do ex. 2.23 of http://arxiv.org/abs/1308.1697. I have chosen as reference spinors $q_1,q_2 = p_3$ and $q_3,q_4 = p_1$. Therefore if I compute $A^4[1^- 2^- 3^+ 4^+]$ the computation ...
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1answer
821 views

How can we measure chirality in experiments?

Chirality is a concept quite different from helicity. These two concepts only happen to have the same numerical value for massless particles. I understand that we can measure helicity, but how can we ...
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1answer
207 views

Is Helicity an intrinsic property of massive Neutrinos?

Hyperphysics states that, unlike an electron, the helicity of a neutrino is invariant because we cannot change to a reference frame where it is different: This and subsequent experiments have ...
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2answers
4k views

Why photon only have helicity other than spin? [duplicate]

When learning angular momentum in quantum mechanics, a spin 1 particle have 3 states. Then I saw from sakurai's modern quantum mechanics that photon's two polarization are just like spins, but with ...
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1answer
517 views

What are Maximally Helicity Violating (MHV) Amplitudes?

Definition of MHV amplitudes on Wikipedia: In theoretical particle physics, maximally helicity violating amplitudes are amplitudes with n external gauge bosons, where n-2 gauge bosons have a ...
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2answers
450 views

Angular momentum of light

Can someone explain the classical angular momentum in electromagnetic theory of light? If I shine elliptically polarised em wave on a black disc it rotates. I would like to know how to calculate ...
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2answers
3k views

How can helicity be conserved but chirality not?

I read in a book that for $\beta$-decay the electrons have always been found to have an expectation value for their helicity of $h=-v/c$. Then ist is said in the book, that it follows from this ...
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0answers
250 views

General relativity from helicity 2 massless field theory by using Deser's arguments

Recently I have discovered the method of constructing of GR from massless field with helicity 2 theory. It is considered here, in an article "Self-Interaction and Gauge Invariance" written by Deser S. ...
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1answer
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What is chirality?

I actually wanted to make the title as "What is the difference between chirality and helicity"? But I didn't do that because I don't understand properly what chirality is. I have gone through this ...
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1answer
428 views

An identity for spinor helicity formalism

I have a question about the spinor helicity formalism from arXiv:1308.1697 Denote the massless spin-1/2 fermions as Eqs. (2.10)-(2.11) in that paper $$v_+(p)= \begin{pmatrix} |p]_a \\ 0 \end{...
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1answer
934 views

Chirality, helicity and the weak interaction

From what I'm understanding about Dirac spinors, using the Weyl basis for the $\gamma$ matrices the first two components behave as a left handed Weyl spinor, while the third and the fourth form a ...
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3answers
1k views

Addition of spin angular momentum for massless particles

How do I add the spin angular momentum of massless particles, like photons, where only the transverse polarizations are allowed? If all three polarizations were allowed, this would be an easy ...
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0answers
253 views

If a photon is a boson and has spin 1, shouldn't it have 3 spin orientations since spin 1 is a triplet? [duplicate]

I've gotten used to the fact that a spin can be described by its total spin and its $z$-component. And I've learned that a particle (really, anything) with spin 1 forms a triplet with three possible ...
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0answers
269 views

How do I prove the equivalence of chirality and helicity operators acting on a massless Dirac spinor?

I have massless Dirac equation and chirality and helicity operators which are given as $$ \hat {P}_{ch}\Psi = \gamma_{5}\Psi, \quad \hat {P}_{h}\Psi = \frac{(\hat {\mathbf S} \cdot \mathbf p)}{|\...
3
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1answer
660 views

The question about Lorentz invariance of the helicity quantum number for the massless particles

I need to show that helicity is Lorentz invariant (under the proper Lorentz transformation) for the massless particles. I heard about most frequently used argument which contains an idea of ...