# Definition of Elementary Particle

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1. Jun 5, 2015

### msumm21

Just wondering if there's a precise definition of what it means to be an elementary particle. I had assumed it was related to not being able to convert it into multiple "smaller" things, but then a photon is called elementary when it can be converted into smaller energy positrons and electrons.

2. Jun 5, 2015

### ZapperZ

Staff Emeritus
Not being "converted" to another form is not a criteria. It is whether a particle can be considered as not being composed of other smaller or more "elementary" entity, i.e. there's nothing else "inside" of it.

A conversion from one to another does not imply that original particle contained something else. After all, I can collide electrons with positrons and "covert" them into a zoo of other particles. It doesn't mean that electrons and positrons contain all of these things. So just because an elementary particle can be converted into something else does not mean that the original particle isn't an "elementary" particle.

Note that this is a very fluid concept, because if history has told us anything, it is that we keep finding new things. So what is "elementary" now may not be so in the future.

Zz

3. Jun 5, 2015

### msumm21

Thanks. What does it mean to say it is "composed of smaller particles." From the discussion above, we are saying it does NOT just mean you can do some experiment to get the "smaller" particles from the starting particle (i.e. "convert"), so how do I say A is elementary and B is not. Is there any rigorous/scientific distinction?

4. Jun 5, 2015

### ZapperZ

Staff Emeritus
A proton is composed of uud quarks. A proton is not an elementary particle. A quark (u, d, s, c, t, b) is elementary because we haven't found anything that makes up each of the quarks.

An electron is an elementary particle. We don't know of any internal structure and haven't found anything that makes up an electron.

Zz.

5. Jun 6, 2015

### vanhees71

A theoretical definition of "elementary particle" is that it can be described within a quantum field theory based on an irreducible local representation of the Poincare group (the emphasis is on "irreducible", defining what we mean by "elementary").

As said, this is a theoretical definition. In practice it's not fully valid, because you can describe also composite particle, quite well in terms of what we defined to be an "elmentary" particle as long as you don't look at it too closely, i.e., as long as you don't scatter it with other particles at an energy high enough to resolve its structure it has becose of its compositeness. That's in fact a gift of nature, because it allows us to describe the "relevant" low-energy degrees of freedom in terms of elementary quantum fields, socalled effective field theories.

An important example is the standard model of elementary particles. Almost nothing concerning matter in everyday form can be described using Quantum Chromodynamics, which is the part of the standard model that describes the strong interactions among quarks and gluons. Never ever we have found a free quark or gluon, which are thought to be elementary at any energies so far available to be studied on earth (including the largest energies available at the LHC at CERN). Instead, what we find in the very low energy regime, applicable to everyday life, are hadrons, i.e., (in fact pretty complicated not yet fully understood in all details) bound states of quarks and gluons, which are color neutral. So far we've seen quark-antiquark states (the mesons) and three-quark states (the baryons) bound together with a lot of other (virtual) quarks and gluons.

Fortunately, it's not necessary to describe the hadrons in all this details as long as you don't resolve this complicated bound-state structure. You can use effective models, where the hadrons are described by elementary quantum fields, using the (accidental) symmetries of QCD, most importantly the chiral symmetry in the light-quark sector.

Only if you use high-energy probes, you'll find out that the hadrons are composite objects. Historically the quarks where postulated by Gell-Mann and Zweig to understand the zoo of hadrons observed in the early 1960ies. At this time they didn't think that quarks with their -1/3 and +2/3 elementary charges exist at all but that you can use a mathematical scheme based on group theory to bring order in the zoo of hadrons. Somewhat later at SLAC by shooting high-energy electrons on protons one (Feynman) figured out that the cross section is compatible with the scattering on point-like particles making up the protons. The socalled parton model was born. Particularly it explained the socalled Bjorken scaling of cross sections.

The parton model had, however, a problem: One had to asssume that there exist hadrons which violate the Pauli principle. On one hand the quarks had to be clearly spin-1/2 particles which should obey the Pauli exclusion principle, on the other hand the parton model had to assume that there are bound states of these quarks that violate it (e.g. the $\Omega$ baryon consisting of three strange quarks).

The solution came with the idea that each quark comes in three "colors", which finally lead to the modern theory describing the strong interaction, quantum chromodynamics, which could also explain the deviations from Bjorken scaling. As a non-Abelian gauge theory it has the remarkable feature of "asymptotic freedom", i.e., the coupling constant becomes small if particles are colliding with high energy-momentum exchanges. This explains why one can consider the deep inelastic scattering of electrons observed at SLAC as scattering on quasi free partons within the hadrons. On the other hand it also explains why one cannot use perturbation theory of QCD at low collision energies: The coupling constant becomes large then, and perturbation theory becomes unreliable. That's why we have to use effective hadronic models to describe the interactions with hadrons at low energies or lattice QCD simulations which evaluate the QCD action on a discrete space-time grid without using perturbation theory.

6. Jun 6, 2015

### KylieVegas

Elementary particle is also the same as Fundamental particle, which means it is a particle that is NOT made up of other particles, it has nothing inside it. Example, the fundamental particle of light is photon, where as the fundamental or elementary particle of the higgs field is higgs boson, which are both found in the standard model along with the quarks and leptons :)

7. Jun 25, 2015

### msumm21

I don't understand the theory well enough to understand, but looks like there's a theoretical definition from Vanhees71 above.

Other explanations using phrases like "composed of" or "made up of" still trouble me because I don't know what they mean precisely. For example, I don't see how it's any more valid to say an atom is "composed of" electrons (and other things) than it is to say an electron is "composed of" photons (and other things). In both cases you can physically add and remove the second thing from the first. In both cases the first thing changes a bit when you add/remove the second thing to/from it.

8. Jun 25, 2015

### rootone

An electron is not composed of photons.
Photons can change the state of an electron energetically, but the photon is not then a 'component' of the electron,
The photon is gone, it no longer exists, and you just have a higher energy state electron.

9. Jun 26, 2015

### tom aaron

Great question.

I've always liked the term 'particle' with no qualification. The word elementary only has context within a specific use or model. We can only define an 'elementary' particle from using the tools at our disposal and these are limited. The Standard Model fits for the most part and electrons seem to be 'elementary' from observations to date.

10. Jun 26, 2015

### Staff: Mentor

@msumm21 This point by vanhees71 is the key point. If you scatter sufficiently long wavelengths off a nucleus then the resulting scattering looks the same as though the nucleus were a point particle. If you use shorter wavelengths then you eventually get a deviation from the point particle scattering. Once you see that deviation then you say that the nucleus is not an elementary particle and that it has some internal structure. Shorter wavelengths allow you to find details about the characteristic size of that internal structure.

An elementary particle is one where no scattering experiment to date has shown any evidence of internal structure.

11. Jun 27, 2015

### vanhees71

What do you mean by "the Standard Model fits for the most part"? If you know of any settled deviation of the Standard Model, you should publish it immediately. This would be a paper of highest interest with thousands of citations very soon. In fact, although we know that the SM cannot be the whole truth, there are no such observations. Maybe the muon (g-2) experiment or the NuTeV result on the Weinberg angle are examples for hints of physics beyond the standard model, but neither is really strong. The culprit here is hadronic physics: The radiation corrections due to the strong interactions are not calculable accurately enough to really establish a deviation between SM and experiment to a high enough confidence level.