# How can we say a particle is a particle or an antiparticle

1. Jan 12, 2014

### FVS

Now we have 2 particle with all identical properties excepts for their opposite electric charge. So how can we say which one is antiparticle and which one is particle?
It didn't seem to be problem to me before, I just thought we call which one we discover first is particle and the other antiparticle. But when they say: In particle physics, mesons (/ˈmiːzɒnz/ or /ˈmɛzɒnz/) are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction (Wikipedia). It really confuses me. So, you guys, I hope someone can help me fill the knowledge I lack here. Thank you :)

2. Jan 12, 2014

### Simon Bridge

Welcome to PF;
By default the regular matter particle is the one that you and I are made of and anything that annihilates with them is the antiparticle. Pretty much as you suspected. The distinguishing characteristic is the annihilation.

This description only applies above the quark level - so you can get a meson and an anti-meson wiht the quark-antiquark flavors the opposite way around. One is matter because that is what was found first or it was convenient to someone to label them that way around at the time. These are just labels, it is the relationship that counts.

It turns out that dividing all objects into pure matter and pure antimatter is a bit naïve.
eg.
positronium is made of an electron and a positron: is positronium matter or antimatter?
some particles can be their own antiparticle

You'll have to work through this to get to a more sophisticated understanding of the role antiparticles play in physics.

3. Jan 12, 2014

### Yashbhatt

Then don't they annihilate each other ?

4. Jan 12, 2014

### bhobba

Its unstable and they annihilate each other after a short amount of time.

Thanks
Bill

5. Jan 12, 2014

### Staff: Mentor

For elementary particles:

There are two groups of 6 quarks, one group is called "particles" and the other is called "antiparticles". This is arbitrary, but the groups are not: As an example, an up-quark can be transformed into a down-quark in the weak interaction, but never into an anti-down quark.

The same applies to the leptons, there are two groups of 6. Again you have a choice how to label those groups. The electron is the most important lepton in our world, so the group with the electron is called "particles" and the other group is called "antiparticles".
There are hints that this definition is unfortunate in the way that quarks ("particles") and antileptons ("antiparticles") could be transformed into each other (and antiquarks and leptons into each other).

The bosons (gluon, W, Z, photon) have no meaningful way to get sorted in those two groups.

Baryons have either 3 quarks or 3 anti-quarks (more precise: as valence quarks), so they get the same name as those quarks. An antiproton is an "antiparticle" as it has 3 antiquarks.
For mesons, those groups are meaningless. There is no "antipion", for example.

6. Jan 12, 2014

### Staff: Mentor

I think one can say that the $\pi^+$ and $\pi^-$ are antiparticles of each other, but one can't say that one is "matter" and the other "antimatter" because each contains one quark and one antiquark.

7. Jan 12, 2014

### Staff: Mentor

I agree.

8. Jan 12, 2014

### Simon Bridge

positrons and electrons can exist together without annihilating - but they will annihilate if they come too close.

Since they have opposite charges, they are attracted to each other - but so are protons and electrons, yet those can exist as a composite objects like an atom without falling into each other.

Same with positronium.

In practice, it is very difficult to set up positronium so it lasts very long - the chance they will annihilate increases as they get closer together. But we can get them to last long enough to detect them - you can look it up.

The view that everything is either matter or antimatter is a simplistic one - the selection of which particles to call matter is an historical accident similar to the assignment of the sign for electric charge. Once made, that choice leads to the classifications we have now for quarks and leptons, and, inevitably, to particles composed of both matter and antimatter.

The process involves finding out which combinations annihilate and which particles decay into which other particles. When you see the families summarized into tidy tables it can be difficult to see how the classifications came about.

9. Feb 7, 2014

### Yashbhatt

So, the force which keeps electrons in place is the same force which prevents annihilation in the case of positronium?

10. Feb 7, 2014

### Staff: Mentor

Annihilation does happen in positronium. I don't understand your question.
Both systems are examples of the electromagnetic interaction.

11. Feb 10, 2014

### Simon Bridge

... depends what you mean.
If you are asking if the attraction between the opposite charges stops them from annihilating then the answer is "no" - this force actually makes it more likely they will annihilate, by drawing them closer together.

If, on the other hand, you are asking about the force that allows them to exist as a composite object - then you've answered your own question: it is the separation that reduces the chance of annihilation - whatever keeps the two separate will prevent the annihilation - so that's a "yes".

It just remains to understand how such composite objects can exist without collapsing in on themselves. Other examples are atoms and molecules (at one scale) and stars/galaxies etc (at another scale). You understand that such situation can exist and be stable for measureable lengths of time right? The exact "how" of it is off-topic for this thread. If you honestly don't know how planets go around the sun without crashing into it, or (more appropos) an electron can exist in a bound state with a proton without "crashing into" it then you should ask the question in another thread.

@FVS: anything more needed?

12. Feb 11, 2014

### Yashbhatt

This is my question. I know how the earth stays in orbit. It stays in orbit because it has a velocity. It is similar to projectile motion but in this case there is nothing to slow down the earth. So is it the same with electrons?

13. Feb 11, 2014

### ZapperZ

Staff Emeritus
You seem to be skipping A LOT of responses directed at you and your questions.

To make it clear:

1. positronium has a finite but SHORT lifetime

2. The positron and electron eventually will annihilate each other.

3. How they could exist even for a short period of time? It is very much like a Hydrogenic atom, except that in this case, the center of mass of the system isn't the same, and that when the electron-positron get too close to each other, they go Poof!

I believe, as has been stated, your question has been addressed. Now please allow for the topic to be consistent with what the OP has asked! Your questions have taken over the original question of this thread!

Zz.

14. Feb 11, 2014

### Simon Bridge

@Yashbhatt: I agree with Zapper Z: please start a new thread for these questions.

15. Feb 17, 2014

### Yashbhatt

@ZapperZ and @Simon Bridge I apologize. . . I will post them in another thread.