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

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In summary, mesons are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction.
  • #1
FVS
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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 :)
 
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  • #2
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
Simon Bridge said:
positronium is made of an electron and a positron.

Then don't they annihilate each other ?
 
  • #4
Yashbhatt said:
Then don't they annihilate each other ?

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

Thanks
Bill
 
  • #5
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.For hadrons:
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.
 
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  • #6
mfb said:
There is no "antipion", for example.

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
jtbell said:
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.
I agree.
 
  • #8
Yashbhatt said:
Then don't they annihilate each other ?
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.

jtbell said:
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.

I'll add my agreement there.
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.
 
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  • #9
Simon Bridge said:
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.
So, the force which keeps electrons in place is the same force which prevents annihilation in the case of positronium?
 
  • #10
Yashbhatt said:
So, the force which keeps electrons in place is the same force which prevents annihilation in the case of positronium?
Annihilation does happen in positronium. I don't understand your question.
Both systems are examples of the electromagnetic interaction.
 
  • #11
Yashbhatt said:
me said:
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.
So, the force which keeps electrons in place is the same force which prevents annihilation in the case of positronium?
... 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.

I think the original question has been answered in spades.
@FVS: anything more needed?
 
  • #12
Simon Bridge said:
...
how such composite objects can exist without collapsing in on themselves.
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
Yashbhatt said:
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?

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
@Yashbhatt: I agree with Zapper Z: please start a new thread for these questions.
 
  • #15
@ZapperZ and @Simon Bridge I apologize. . . I will post them in another thread.
THANKS for the answers.
:thumbs:
 

1. How do we determine if a particle is a particle or an antiparticle?

In particle physics, particles and antiparticles have opposite electric charges. Therefore, one way to determine if a particle is a particle or an antiparticle is by measuring its charge. If the charge is positive, it is a particle, and if the charge is negative, it is an antiparticle.

2. Can a particle and an antiparticle be identical?

Yes, a particle and an antiparticle can have the same mass, spin, and other quantum numbers. The only difference between them is their electric charge, which is opposite. For example, an electron and a positron (antiparticle of an electron) have the same mass and spin, but the electron has a negative charge while the positron has a positive charge.

3. How are particles and antiparticles created?

Particles and antiparticles can be created through various processes, such as particle collisions or radioactive decay. In particle accelerators, high-energy collisions between particles can create new particles and antiparticles. In radioactive decay, a particle can spontaneously transform into its antiparticle counterpart.

4. What happens when a particle and an antiparticle meet?

When a particle and an antiparticle meet, they can annihilate each other, resulting in the release of energy in the form of photons. This process is called annihilation and is an essential concept in particle physics. The opposite of annihilation is pair production, where a particle and an antiparticle are created from the energy of photons.

5. Can a particle be its own antiparticle?

Yes, some particles, such as photons and neutrinos, are their own antiparticles. This means that a photon can interact with another photon, and a neutrino can interact with another neutrino, resulting in annihilation or pair production. However, most particles and antiparticles are distinct entities with opposite charges.

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