What is Mesons: Definition and 57 Discussions

In particle physics, mesons ( or ) are hadronic subatomic particles composed of an equal number of quarks and antiquarks, usually one of each, bound together by strong interactions. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one femtometer (1×10−15 m), which is about 0.6 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Heavier mesons decay to lighter mesons and ultimately to stable electrons, neutrinos and photons.
Outside the nucleus, mesons appear in nature only as short-lived products of very high-energy collisions between particles made of quarks, such as cosmic rays (high-energy protons and neutrons) and baryonic matter. Mesons are routinely produced artificially in cyclotrons or other accelerators in the collisions of protons, antiprotons, or other particles.
Higher-energy (more massive) mesons were created momentarily in the Big Bang, but are not thought to play a role in nature today. However, such heavy mesons are regularly created in particle accelerator experiments, in order to understand the nature of the heavier types of quark that compose the heavier mesons.
Mesons are part of the hadron particle family, which are defined simply as particles composed of two or more quarks. The other members of the hadron family are the baryons: subatomic particles composed of odd numbers of valence quarks (at least 3), and some experiments show evidence of exotic mesons, which do not have the conventional valence quark content of two quarks (one quark and one antiquark), but 4 or more.
Because quarks have a spin 1/2, the difference in quark number between mesons and baryons results in conventional two-quark mesons being bosons, whereas baryons are fermions.
Each type of meson has a corresponding antiparticle (antimeson) in which quarks are replaced by their corresponding antiquarks and vice versa. For example, a positive pion (π+) is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion (π−), is made of one up antiquark and one down quark.
Because mesons are composed of quarks, they participate in both the weak and strong interactions. Mesons with net electric charge also participate in the electromagnetic interaction. Mesons are classified according to their quark content, total angular momentum, parity and various other properties, such as C-parity and G-parity. Although no meson is stable, those of lower mass are nonetheless more stable than the more massive, and hence are easier to observe and study in particle accelerators or in cosmic ray experiments. The lightest group of mesons is less massive than the lightest group of baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher-energy phenomena more readily than do baryons. But mesons can be quite massive: for example, the J/Psi meson (J/ψ) containing the charm quark, first seen 1974, is about three times as massive as a proton, and the upsilon meson (ϒ) containing the bottom quark, first seen in 1977, is about ten times as massive.

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  1. E

    I Axial Version of the Phi Meson

    I only know of the Phi vector meson which is ##s\bar{s}## which has J=1, P=-. I am looking for its axial vector version (dont know by what name it should go) ie J=1, P=+. More specifically I am looking for any data associated with it eg its LCDAs, decay constant, mass. I am not sure I can see...
  2. E

    I What Do the Names and Numbers of Pseudovector Mesons Mean?

    I am looking for some detailed information on pseudovector mesons, mainly on their masses and quark contents. Also I am unfamiliar with the naming conventions of many of these mesons like K1(1270), K1(1400), a1(1260), b1(1235), f1(1420), h1(1380). What do these numbers and letters mean and ? I...
  3. James1238765

    I Is the superposition of mesons (such as pion) a new particle?

    There are 36 hadron composites composed of 2 quarks selectable from the set ##[u, d, c, s, t, b, \bar u, \bar d, \bar c, \bar s, \bar t, \bar b]## satisfying the condition of having total charge = ##[-1, 0, 1]##. However, the superposition states of pure hadrons are sometimes also listed as new...
  4. M

    I Why is the mass of the singlet meson higher than the octet mesons in QCD?

    Hello! I am reading some group theory applied to QCD and they show how by using the lightest 3 quarks in the ##l=0## state we get 9 pseudoscalar mesons and 9 vector mesons. The difference in masses between the 2 classes is explained by the existence of a spin-spin interaction, which seems like a...
  5. P

    A Is there a vibration energy term associated with mesons?

    Hello, I am learning about solutions of the Schrodinger equation including the term of rotational energy (i.e. L^2 /2I, in its quantized form) and I was wondering if there should be another quantum term describing vibrational states of hadrons or any other composite subatomic particle, in...
  6. CricK0es

    Why are vector mesons more massive than pseudoscalar mesons?

    Problem Statement: Why are vector mesons more massive than pseudoscalar mesons? Not any sort of set problem, just reading but I can't find an answer or explanation Relevant Equations: * It's going to be something to do with the spin-spin interactions for J=0 and J=1. But then I don't see how...
  7. N

    B Time Dilation Problem with Mesons

    At the suggestion of some of the members here, I am reading the book Space-Time Physics by Wheeler. The last problem in the first chapter states: "In a given sample of mesons, half will decay in 18 nanoseconds (18 x 10-9)), measured in a reference frame in which the mesons are at rest. Half of...
  8. C

    I Strong force as exchange of mesons, or of quark and antiquark

    The (residual) strong force between nucleons can be desribed as - The exchange of a meson (from a nucleon to the other), as in picture b) - The exchange of a quark and an antiquark: in picture a) one nucleon "gives" a quark and receive an antiquark and it's the opposite for the other I do...
  9. P

    A Why does sd replace sγ5d ee in the amplitude of meson decay?

    is the amplitude of pure leptonic decay of pseudo scalar meson(say K→ee), while is the amplitude of the semileptonic decay of K→π ee. Kindly explain why does sd replace sγ5d ee in the amplitude?
  10. Soffie

    B Can Leptons decay into mesons?

    I presume they cannot as they are fundamental particles? But then a meson can decay into a lepton?
  11. edguy99

    I Exploring the Absence of Down/Anti-Down Mesons: A Supposition in Meson Lists

    In list of Mesons like this you see charm/anti-charm, strange/anti-strange and bottom/anti-bottom. You don't see up/anti-up or down/anti-down. Instead you see up/anti-down or (up/anti-up and down/anti-down) in supposition. Why no down/anti-down meson?
  12. Sophrosyne

    I Why are gluons considered to be elementary particles?

    Gluons are often depicted as fundamental particles in the Standard Model. But in looking at their mechanism, it seems they are not really fundamental particles in the sense that they are fundamental, indivisible, building blocks. They are mesons- a composite quark-antiquark pair, where their...
  13. B

    Decay Widths of Neutral Vector Mesons

    Homework Statement Neutral vector mesons with ##J^{PC} = 1^{−−}## include the ##\phi## (mφ = 1020 MeV), ##J/\psi## (mJ/ψ = 3100 MeV), and ##\Upsilon## (mΥ = 9460 MeV), with quark content ss¯, cc¯, and b ¯b respectively. The decays of these mesons go largely to hadronic final states (jets)...
  14. Xico Sim

    I Mesons and baryons written in terms of quarks

    Hello, guys. I have not understood what it means when one writes ##\pi^+=u\bar{d}##, for example. I though it simply meant that the ##\pi^+## meson was composed of one up-quark and one anti-down-quark. However, that doesn't explain what writing ##\pi^0=\frac{1}{\sqrt{2}}(d\bar{d}-u\bar{u})##...
  15. Garlic

    How are neutral pi mesons formed and what is their composition?

    Hello, I understand the fact that pi mesons are superpositions of uubar and ddbar. But what happens if a decay only produces a single type of meson? I am not sure of this, but omega baryon decaying into xi-minus and a neutral pion seems to produce ddbar mesons, is it so that only a single type...
  16. websterling

    Mesons other than quark-antiquark states?

    Mesons: hadrons with integer spin, usually quark-antiquark states I came across this statement in a pdf of a powerpoint. Unfortunately the presentation is so badly formatted (missing images, content overlapping content, etc.) that it's hard to follow and I can't see if he ever describes mesons...
  17. Dilatino

    Confused about isospin conservation in rho to pion decays

    The decay \rho^0 \rightarrow \pi^- \pi^+ occurs with a probability of 100\%, whereas the decay \rho^0 \rightarrow \pi^0 \pi^0 does not occur in nature, due to isosphin conservation. I don't understand this. Looking at the Isospin and its third component ¦I,I_3\rangle we have in the first...
  18. Dilatino

    Why can the omega meson not decay to 3 neutral pions?

    What reasons prevent the decay $\omega \rightarrow \pi^0 \pi^0 \pi^0$ from happening?
  19. A

    Why Mesons Can't Be Made of 2 Quarks Alone

    As far as I know a Baryon is made of three Quarks (eg uud, udd etc) and a Meson of two Quarks, a Quark/Antiquark pair. As I am not a student / scholar in Physics but very deeply interested in this field, I couldn't find any explanation, why a Meson is omly made up by a Quark/Antiquark pair. What...
  20. binbagsss

    Parity formulae, orbital angular momentum, mesons

    So a particle has intrinsic parity ##\pm 1 ## . The parity of a system of particles is given by product of intrinsic parities and the result is: ##(-1)^l ## (1). Questions: 1) How does this result follow? and what exactly is ##l## here? so it's the orbital angular momentum, so say a particle...
  21. Safinaz

    What Causes the Observed Excess in the Semileptonic Decay Rate of B Mesons?

    Hi there, Since some time I started to study the semilptonic decay of B meson : ## B \to D \tau \nu ## that there is an excess in the value R (D) = ## \frac{ Br ( B \to D \tau \nu) } {Br (B \to D l \nu) } ## in BABAR experiment than the SM expectations- See for example arXiv:1302.7031v5- Now I...
  22. K

    Quark Content of these resonances

    Hi. I was reading an introductory book about particle physics and I visited the PDG for an exercise. I found this... http://pdg.lbl.gov/2014/tables/rpp2014-tab-mesons-strange.pdf On page 9, the data of K1(1270) is given. How do I figure out it's quark content? If I look at the data for K*(892)...
  23. wolfspirit

    Hadron interaction via strong yet k mesons decay through weak

    Hi, I have been getting a little confused; the textbook (and from what i can see on-line) are saying that Hadrons only interact through the strong interaction (along with the electromagnetic force if charged Q) and yet K Mesons decay through the weak interaction. If my understanding is correct...
  24. J

    CP Operators and Mesons: Investigating Boundaries

    Hi, I am currently going over this and got me thinking about a scanario where you have A -> BC Where A is S = 0, L=0, B is S = 1 L=0, C is S=1 L =0 (I'll use S = intrinsic spin, L = angular momentum, J = Total Angular momentum, |L-S|=< J =< L+S) Maybe such a decay doesn't exist, but I'm just...
  25. H

    Difference between heavy and light mesons

    I am not clear with the difference of heavy and light mesons; 1. what is their difference? 2. how can we treat their decay properties? 3.for instance many people describe them using the Bethe-Salpeter equation at 4-dimension or at 3-dimension. which one is the better method? To help me...
  26. H

    Vertex Function & Wave Function of Mesons: Exploring Their Relation

    I need to know the relation of wave function and vertex function of mesons. Any one give me an explanation of them. Are they same? in some journals their expressions are the same and in other papers the wave function expresses as a function of the vertex function of meson. Which one is the...
  27. M

    Special Relativity: Photoproduction of Mesons?

    Homework Statement 12-16 K + mesons can be photoproduced by the reaction γ + p → K+ + Λ where the initial proton is at rest in the lab. From the conservation laws, discover if it is possible for either the K+ or the Λ to be at rest in the lab, and for what photon energy (in terms of...
  28. 4

    What exactly makes Mesons unstable?

    As the title says- what is it that makes all mesons unstable? I know some quarks are less stable than others obviously, but why is there no stable meson? What is it about any combination of quark and anti-quark that makes a particle decay nearly immediately? In a related topic- what exactly...
  29. G

    Can G-Parity Explain the Dominance of Vector Current in Tau Decay?

    I'm studying the \tau \rightarrow \rho \; \nu_{\tau} decay. I'm asked to calculate the decay width, using a parameterization of the matrix element of the hadronic current. I actually find a matrix element of the form: \left<\rho |\; \bar{u} \gamma^{\mu} \left( 1-\gamma^{5} \right ) d \; |...
  30. S

    Rare decays of mesons for new physics

    Why the semi-leptonic decays of mesons involving neutrinos are consider as clean signal for new Physics. As for as I think these are free from hadronic complexities (because we do not know exactly about strong interactions) and electromagnetic interference (because neutrinos do not couple to...
  31. B

    Psuedoscalar Mesons - why is there an eta and an eta prime?

    The pseudoscalar mesons have J^P = 0^- They form a nonet: for S = ±1, I (isospin) = 1/2 and so there are two particles for each value of strangeness. This account for 4 particles: the ground-state Kaons. For S=0, I can be 0 or 1. I=1 gives a triplet: \pi^\pm \mbox{ and } \pi^0. For...
  32. T

    How do mesons not self-destruct?

    Hello physicists, I was speaking to my friend about mesons when I realized that surely a meson would annihialate at creation due to the fact that it has a quark and anti-quark. I know it doesn't, but I'm wondering why. Clarrification on this would be great. Thanks, Joe
  33. L

    Mean lifetime of pi(+) mesons - Special Relativity

    Homework Statement The proper mean lifetime of a \pi^{+} mesons is 26ns. (a) (i) What is the mean lifetime of \pi^{+} mesons traveling with \beta=\frac{v}{c}=0.73? (ii) What distance is traveled at this velocity during one mean lifetime? (iii) What distance would be traveled in the...
  34. W

    The particle/antiparticle pairs in mesons as well as three quarks in baryons?

    Hi, Does anyone out there know the rules for mesons when it comes to the particle/antiparticle combinations? Is it simply like an up and anti-up quark that makes up a meson or is it like up and anti-down or down and anti-up? Or can it even be other combinations of the six quarks? Also I have...
  35. Vaibhav089

    What's the reason behind the existence of mesons?

    I've read that mesons consist of a quark and an antiquark. So, here's my question. Why don't the quark and the anti-quark annihilate with each other (like they ususally do)? For example, the pi(0) meson consists of the up and the anti-up quark and the eta meson consists of the down and the...
  36. L

    Decoding the Notation: Understanding B Meson Decay Amplitude in Particle Physics

    This question is mostly about how to interpret notation used in Particle Physics. At lowest order the rate of b\rightarrow s\gamma is proportional to \langle B_p|b^\dagger b|B_p\rangle where p is either u or d labels and b, b+ are creation annhilation operators. However, I am unable to...
  37. R

    Is there a representation diagram for fermions like the 8 fold way for mesons?

    I was wondering if there was something similar to the 8 fold way representation used on mesons for the fermions?
  38. L

    Decay of Upsilon(4S) into two B mesons in laboratory frame.

    Homework Statement B mesons can be created through the reaction chain e+e- → \Upsilon(4S) → B+B- by colliding beams of electrons and positrons head-on at centre-of-mass energy equivalent to the mass of the \Upsilon(4S) resonance. a) If the electron beam energy is 8GeV, show that a positron...
  39. M

    Particle Physics: Why are Mesons a type of Gauge Boson?

    If a Meson is a Hadron that contains a Quark and an Anti-Quark which are both Fermions then why are Mesons a type of Gauge Boson?
  40. G

    What Factors Affect the Lifetime of B+ and Bc Mesons?

    I'm trying to understand why the Bc meson (anti b and c) has a lifetime of 0.46 picoseconds whilst the Bs, B+ and B- have lifetimes of ~1.6 picoseconds. Is this a density of states thing? or to do with the CKM elements? Thanks for any help.
  41. J

    Why Do No Stable Mesons Exist?

    So mesons consist of two quark. I am wondering why no stable mesons exist?
  42. fluidistic

    Calculating Mean Life of Mesons at Rest

    Homework Statement The half life time of mesons \mu with velocity 0.95 c has been obtained experimentally as 6\times 10^{-6}s. Calculate the half life of those mesons in a system in which they are at rest. Note: It shouldn't be "half life" but something like "mean life" or something like...
  43. D

    What is the gyromagnetic ratio of spin-1 mesons?

    Hello all. Would anybody know where I can find data on gyromagnetic ratios/magnetic moments/g-factors of mesons? I find everything about nucleons, electrons and muons, but that's about where it stops. Even the Particle Data Groups is silent about it. Or is there any reason why it is not possible...
  44. CarlB

    How many mesons and baryons are there?

    That is, how many different states are there assuming you distinguish two particles as different if they have different quantum numbers or different masses. For example, the \Delta^-(1232),\Delta^0(1232),\Delta^+(1232),\Delta^{++}(1232) are four different states. And these are all different...
  45. F

    CP violation: why not in K*, B* vector mesons?

    Experimentalists have measured CP violation in neutral K and B mesons. Everybody is unhappy that so far there are only 2 systems so for which these measurements are possible. (Neutral D mesons are predicted to show such a small effect that it is probably unmeasurable for a long time.) All...
  46. O

    Understanding Eta & Eta Prime Mesons

    I am confused by eta meson and eta prime meson ?what is the difference between the two ones ? who can tell me what are they made up ? please help me ! thank you very much !
  47. G

    How many W mesons required for D+ decay?

    Homework Statement The decay D^{+} \rightarrow D^{0} \pi^{+} is not possible via a weak interaction which involves one virtual W+ meson. How many virtual W mesons are required? Homework Equations D0 has quark content c-ubar Pi+ has quark content u-dbar I'm not sure what D+ has...
  48. D

    Mesons and bosons, same or different?

    Mesons are bosons, but are all bosons mesons e.g. intermediate vector bosons (w and z bosons in the weak interaction)?
  49. N

    How many mesons have been discovered?

    I've been looking for a list of all the mesons there are, but cannot seem to find it online. Anyone know just how many mesons have been discovered? A link to a page that shows the names of each meson would also be appreciated.