# What are mesons

1. Oct 17, 2004

### lakshmi

what are mesons

2. Oct 18, 2004

### reilly

They are particles, force carriers, bosons, and quite nicely described in any book on particle physics -- prrobably even in freshman physics --, on the standard model. and, of course, a quick trip to GOOGLE will tell you more than you want to know.

Regards,
Reilly Atkinson

3. Oct 18, 2004

### marlon

This is not entirely correct. Mesons are particles that are constituted out of two quarks(a quark and anti-quark). These mesons are not all force carriers. The lightest meson, which is easiest to make since it corresponds to the least amount of energy via E =mc², is called the pion. This pion is indeed a force carrier of the socalled residual strong force. This force keeps atomic nuclei together. The strong force keeps the two quarks of a meson together and is mediated by an elementary particle called the gluon. Keep in mind that a pion is NOT such an elementary particle since it is constituted out of two quarks. Three quarks together makes up another kind of particle which is called the baryons. Examples are the proton or neutron. These mesons and baryons are collectively called the hadrons because they all "feel" the strong force. Electrons are also elementary yet they do NOT feel this strong force. They are members of another particle group called the leptons. So basically all particles can be divided into hadrons(baryons and mesons) and leptons(electrons,...)

regards
marlon

4. Oct 18, 2004

### meteor

Some papers mention something called the gluonic meson, I wonder if this particle has been discovered. What characteristics has this particle?

5. Oct 18, 2004

### Orion1

Compound Carriers...

Why exactly are the Meson Pions residual strong force carriers, and the other Mesons, such as the Kaons are not? Conservation of Energy?

If the pion+,- carry charges, why are these charges not detected with the field charge inside a compound nucleus?

Only the pion0 is the carrier of the residual strong force?

Last edited: Oct 18, 2004
6. Oct 18, 2004

### chroot

Staff Emeritus
Kaons are composed of strange quarks. There is no way for an up or down quark to spontaneously become a strange quark, because that would violate the conservation of energy.
I have no idea what a "field charge" or a "compound nucleus" are.
No, all three pions participate.

- Warren

7. Oct 19, 2004

### marlon

Indeed all three pions participate and i also don't know what a compound nucleus or a field charge is. Pions are generated in a similar way as the electron in beta decay. They created out of the vacuum. Ofcourse such a process can only occur when enough energy is available. This energy is then converted into matter : the pion. Now there are two questions here :1) what is the necessary energy ?
2) Why pions ???

1) the energy comes from the strong force. Suppose one baryon comes this close to another baryon that the quarks of the first baryon can "feel" the quarks of the second baryon. These quarks will exhibit a linear potential between them that is weaker then the potential they feel in the baryon itself. As a consequence of this linear potential, energy rises when they move away from each other (so when the two baryons move away from each other). This energy will be converted into the lightest quark-anti-quarkpair : the pion.

2) why pions ???
Well, nature likes to be as lazy as possible. This means the least amount of energy will be used for conversion into matter. And via E = mc² you know that lighter particles correspond to lower energy-values, so they are more "easy" to make. Another fundamental ingredient is the fact that a PAIR of quarks is created and not just one quark. The reason for this is confinement (or the linear interquark-potential). For low energies (like the ground state) it will cost an "infinite" amount of energy two separate two quarks from each other because of the asymptotic freedom (the Nobel prize for physics this year, was awarded to the three scientists who described this phenomenon using renormalization group-theory and QFT). This means that the strength of the strong force (expressed by its coupling constant) get's bigger when energy decreases. So low energies correspond to very high coupling constants, thus very strongly bound quarks...These energy-dependent coupling constants are referred to as running-coupling constants or form factors of an EFFECTIVE field theory...

regards
marlon

8. Oct 19, 2004

### humanino

9. Oct 19, 2004

### humanino

The pions are indeed detected in the nucleons. They are here, and they are very important. There is a pion cloud in nucleons, as well as probably in all baryons. In the case of the proton for instance, we already know that an important part of the spin comes from contributions of the pion cloud.

10. Oct 19, 2004

### reilly

My take is very simple: any particle that can be emitted or absorbed by either a fermion or a boson is a force carrier. Within the hadron community, say as described by an SU(3) lagrangian, forces can be generated, for example, by octet meson exchange. This type of terminology goes back originally to Yukawa. And, it was used extensively in the 1960s, perhaps less so now -- so it has an honorable history.

Regards,
Reilly Atkinson

11. Oct 19, 2004

### Orion1

Pion Impiety...

Why are pions 'stable' inside a compound nucleus?

Given that there is a meson pion mediated residual strong nuclear force that is NOT fundamental, and these pions are 'stable' during these exchanges, and such compound nuclei only decay through weak bosons, there should also be a corresponding 'residual nuclear decay' which corresponds to compound nuclei with a disrupted pion exchange particle:

Residual Nuclear Decay:
$$\gamma_{be} + D \rightarrow p + n + \pi^0$$

In this example, a gamma ray equal to the Deuterium binding energy has 'disrupted' the pion from its pion cloud. The pion carries most of the energy of interaction during this nuclear reaction, resulting in nuclear dissociation. Is such a reaction a reasonable description for residual nuclear decay?

Reference:
http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html

Last edited: Oct 19, 2004