Questions tagged [mesons]

Μesons are unstable hadronic subatomic particles of integral spin, and hence bosons. They are composed of one quark and one antiquark, and bound together by the strong interaction to a physical size of roughly one fermi.

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19
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3answers
9k views

What does it mean that the neutral pion is a mixture of quarks?

The quark composition of the neutral pion ($\pi^0$) is $\frac{u\bar{u} - d\bar{d}}{\sqrt{2}}$. What does this actually mean? I think it's bizarre that a particle doesn't have a definite composition. ...
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Are there any Baryons that have quark-antiquark combinations?

I'll be honest, I only have a Highschool education, so there might be something obvious I'm overlooking. However particle physics is of massive Interest to me. My question is, I know there are ...
13
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1answer
185 views

Probability of forming mesons vs baryons

When a heavy quark hadronizes it has some probability of forming a meson vs forming a baryon. I suspect there is a well known branching ratio for each type of hadron. Does anyone know what the ...
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1answer
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Why is the charmed eta meson its own antiparticle, but the neutral kaon is not?

I have a limited understanding of antiparticles, so this may be why I am unable to explain why certain mesons are their own antiparticles, while others are not. My understanding is that antiparticles ...
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5answers
10k views

Why is the decay of a neutral rho meson into two neutral pions forbidden?

Why is the decay of a neutral rho meson into two neutral pions forbidden? (Other modes of decay are possible though.) Is it something with conservation of isospin symmetry or something else? Please ...
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2answers
566 views

What's the difference between a meson and a glueball?

There is a blog post by Matt Strassler about the structure of hadrons. He contrasts the "conventional" picture of hadrons shown below: with one that he considers to be more accurate: From the ...
10
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1answer
190 views

Are mesons color polarized?

The binding of quarks in mesons baffles me. It's an Occam's Razor thing. Since a meson is a colorless, the simplest way to bind its two quarks together is to use a $U(1)$ Cartan subalgebra of $SU(3)$. ...
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2answers
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How do we measure meson decay constants?

I'm trying to understand how people actually measure decay constants that are discussed in meson decays. As a concrete example lets consider the pion decay constant. The amplitude for $\pi ^-$ decay ...
9
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1answer
214 views

$B$-meson naming convention

An unbarred $B$-meson contains $\bar{b}$ (an anti-bottom quark), whereas a barred $\bar{B}$-meson contains $b$ (a bottom quark). What is the historical reason for this hellish naming convention?
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2answers
862 views

Quantum field theory meson scattering calculation (scalar yukawa theory)

Please see this question for a clear background of the notation I use. My issue is that I want to use Wick's theorem to calculate the amplitude of meson $\psi(p_1)\psi(p_2)\rightarrow\psi(p_1')\psi(...
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2answers
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Why do quarks tend to bond in groups of three?

Why do quarks tend to bond in groups of three? I understand why they might in bond in groups of three to create a net charge of zero (i.e. neutrons), but I don't understand why they form protons and ...
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3answers
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What is the difference between a charged rho meson and a charged pion?

They both seem to have the same quark content: $$\rho^{+} = u\bar{d} = \pi^{+}$$ and $$\rho^{-} = \bar{u}d = \pi^{-}$$ What is different about the two?
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1answer
854 views

Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation?

The decay of heavy quark/antiquark pairs (say $c\bar{c}$, $s\bar{s}$) is supposedly 'suppressed because of the Zweig/OZI rule', see for instance Phi meson. And they certainly have a longer lifetime ...
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2answers
717 views

If mesons were stable could they form atoms?

if there were stable enough mesons similar to protons and neutrons could they capture electrons to form exotic elements. if not why is this not possible?
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Why are all mesons unstable?

My question is: why are all mesons unstable? Shouldn't the lightest mesons like the pion be stable because they are made from the lightest quarks (and anti-quarks)? Or could the they annihilate (the ...
6
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1answer
2k views

Derivation of Gell-Mann Okubo relation for mesons

In SU(3) quark model of hadronic structure one assumes that mass splitings between hadrons is due to difference between masses of $s$ quark and $u,d$. This is modeled by perturbation Hamiltonian $$ \...
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1answer
1k views

Does the color of a quark matter in a meson?

QCD and confinement specify that hadrons must be color-neutral. My understanding is that this means you can have mesons (quark + antiquark) or baryons with 3 quarks, one of each color: Red+green+blue=...
6
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1answer
240 views

String breaking in QCD

I have trouble understanding string breaking in QCD. I have read an article on arxiv (http://arxiv.org/abs/hep-lat/0505012), and I still don't understand what truly happens. My understanding of ...
6
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1answer
194 views

How are the 'physical' isospin zero states determined?

Consider the light mesons. Since $3 \times \bar{3} = 8 + 1$, the states should be grouped into $\mathfrak{su}(3)$ octets and singlets. In the case of the spin zero states (the pseudoscalars), the ...
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1answer
173 views

Semileptonic decays of the $B_c$ meson

I am struggling with calculating the exclusive semileptonic $B_c^+\rightarrow J/\psi l^+\nu_l$ decay. I learnt that the amplitude is given by a product of the leptonic current $L^{\mu}$ and the ...
5
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1answer
510 views

Is there a tb meson?

I was wandering around the particle date group page for meson and couldn't find a meson for top-bottom, which from symmetry you would expect. Q1: Is this because it hasn't been found? Q2: There is ...
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Chiral WZW term for Hadron Supersymmetry?

Remarkably, an approximate supersymmetry not in the color- but in the flavor-sector is experimentally observed: hadron supersymmetry, relating mesons (the bosons) to baryons (the fermions). The most ...
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3answers
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Building the meson octet and singlet

I am very lost in this topic. I understand that there are $3\times 3$ possible combinations of a quark and an anti-quark, but why should one decide arbitrarily (that's how it appears to me) that one ...
4
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1answer
882 views

How can mesons have spin greater than 1?

My understanding was that a meson, being made of a quark and an antiquark (spin 1/2) could only have spin 1 or 0, by addition of angular momentum states. I just saw an article (http://arxiv.org/pdf/...
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2answers
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What is a sigma meson? [duplicate]

I have seen some articles talking about the role of sigma meson in nuclear force, but more articles omit it and only mention pion, rho and omega (e.g., Wikipedia). What is this sigma meson?
4
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1answer
207 views

Why are there both scalar and vector mesons?

I understand that the term "vector meson" denotes a meson particle (composite particle with a quark and an antiquark) which transforms as a vector under the Lorentz group. Since both the quark and the ...
3
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2answers
353 views

Meson as hadron and boson

In wikipedia page about hadrons the following image appears: I can understand why the intersection between hadrons and fermions are baryons, as a way to say a baryon is a kind of hadron composed of ...
3
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1answer
121 views

Why are the lightest mesons (Goldstone bosons) pseudoscalar?

I get that the lightest mesons are generically the (approximate) Goldstone bosons of spontaneous chiral symmetry breaking in the quark sector. I understand that they are spin-0, but why are they ...
3
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1answer
65 views

Positive Kaon decay into $\pi^+$ and $\pi^0$ suppressed

I am looking at the reactions of positive Kaons and am looking at $$K^+\to\pi^++\pi^0$$ Since the strangeness is not conserved, this has to be a Weak decay. When I try to make the Feynman diagrams ...
3
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1answer
420 views

Rho and Omega meson mixing

I am new to particle physics. So this might seem to be a trivial question. I read about $\omega$ and $\phi$ meson states being written in terms of octet and singlet state. In the same way, can $\...
3
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1answer
229 views

Scalar pseudo-Goldstones from hypothetical $SU(3)_V$ symmetry breaking in QCD

The eight light pseudoscalar mesons of QCD are the pseudo-Goldstone bosons of the spontaneously broken chiral (axial) $SU(3)_A$ quark flavor symmetry. If we consider the hypothetical case of also ...
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2answers
284 views

Measurement on a particle described by a quantum superposition of quark states

Some particles are described as a quantum superposition of quark states. For example, the pion meson: $$ |\pi^0⟩=\frac{|u\bar u⟩−|d \bar d⟩}{\sqrt{2}} $$ Is it possible to design an experiment giving ...
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2answers
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$\mathrm{\rho^0}$ meson decay via the weak interaction?

Of course, the $\mathrm{\rho^0}$ meson can decay in $\mathrm{\pi^{+}\ \pi^{-}}$ through the strong interaction. Using Feynman diagrams, I cannot understand why the same decay couldn't happen through ...
3
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1answer
157 views

How can matter and anti-matter coexist in a meson?

Whilst learning about the nature of subatomic particles, I came along a pion zero meson. What really stumbled me was that the quark configuration of the particles was up and anti-up or down and anti-...
3
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1answer
108 views

How to build tetra-quark mesons symmetry group?

I was thinking about the issue while reviewing my group theory notes. One can construct mesons with a nonet as an octet and a singlet, $SU(3)\otimes SU(3) = 8\oplus \bar{1}$. In a same way but for $qq$...
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0answers
66 views

Anomalous K-K*-γ interaction and similar D-D*-γ and B-B*-γ?

In trying to better understand the chiral-anomaly WZW-term interaction of the heavy vector mesons, I have trouble finding much relevant literature, but maybe I am looking in the wrong place. One ...
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0answers
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Overall symmetry of pion wave function

Mesons are bosons, therefore their wavefunction must be symmetric under particle exchange. Overall, the meson wave function ($\text{WF}$) has the following contributions: $$\text{WF} = \lvert \text{...
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4answers
521 views

How to understand the makeup of neutral pi and eta mesons?

I know that mesons are bosons made up of quark-antiquark pairs. But when I see the list of mesons, I can see that the makeup of neutral pions and eta mesons are noted in a strange way. $$\pi^0=(u\bar{...
2
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2answers
118 views

Can mesons be in $b \overline{b}$, $r \overline{r}$, $g \overline{g}$ states?

Can a meson be in a pure $b \overline{b}$, $r \overline{r}$, $g \overline{g}$ state or does it have to be in the $\frac{1}{\sqrt{3}}\left(b \overline{b}+r \overline{r}+g \overline{g}\right)$ state? ...
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1answer
173 views

What is the matter anti-matter asymmetry in the decay of the neutral K meson?

In 1964 Cronin and Fitch discovered that there is asymmetry in the weak decay of the neutral K meson. What is this asymmetry and what causes it?
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1answer
132 views

Why do we need the strong interaction if we have Yukawa's meson theory?

How is Yukawa's theory of mesons different from the standard model's strong interaction? I mean if Yukawa's theory is complete then why do we need quarks and gluons?
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1answer
74 views

Probability decay for rho meson

I have the following putative set of decays for the $\rho^0$ meson: $$\begin{array}{l} \rho^{0} \rightarrow \pi^{0} \pi^{0}:\text{STRONG} \\ \rho^{0} \rightarrow \pi^{-} \pi^{+}:\text{STRONG} \\ \rho^{...
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1answer
141 views

Why is usual matter made up of $n,p$ and $e^-$?

We have six leptons and six quarks. Yet most of what we see around us is made of only neutrons, protons and electrons. For $\mu^-$ and $\tau^-$, I think the reason is that these are unstable particles ...
2
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1answer
52 views

What's the mass of $D_1(2430)^{\pm}$?

What's the mass of $D_1(2430)^{\pm}$? The standard reference http://pdglive.lbl.gov/ParticleGroup.action?init=0&node=MXXX035 only shows the data for $D_1(2430)^0$ but not for $D_1(2430)^{\pm}$. ...
2
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1answer
70 views

How does the nuclear interaction keep protons and neutrons together?

The strong force holds atoms hadrons together by using gluons to change the color charge of quarks and making the hadrons color nutral. But how does the nuclear force keep protons together, I heard ...
2
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2answers
680 views

Wavefunctions of (Pseudo)-scalar/vector meson and Tensor meson

Two quarks can bound to form a meson, and the wavefunction consists of several quantum degrees of freedom: $$ \psi=\psi(\text{flavor})\psi(\text{color})\psi(\text{orbital})\psi(\text{spin}) $$ ...
2
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2answers
1k views

Why can omega meson not decay into two charged pions?

Why is the decay of the $\omega$ meson similiar to the neutral $\rho^{0}$ meson into two charged $\pi$ prohibited? My first thought was that it has something to do with isospin conservation in the ...
2
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1answer
1k views

Feynman diagram for $\overline{K}\,\!^0$ antimeson production on the quark-level

I've recently stumbled upon a physics problem concerning $\overline{K}\,\!^0$ antimeson production. In this particular example, colliding a $\pi^-$ meson with a stationary proton yields a $K^0$ meson ...
2
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1answer
37 views

How is the spin of an unstable meson or baryon measured?

Can bubble chambers measure the spin of particles? Can you just use conservation of spin to deduce it or can you not infer much this way because composite particles can carry angular momentum? It ...
2
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1answer
45 views

Does tau decay to rho meson + tau neutrino?

In the particle data group table: http://pdg.lbl.gov/2020/tables/rpp2020-sum-leptons.pdf ,only $\tau \rightarrow \pi + \nu_{\tau}$ is documented. But does $\tau \rightarrow \rho + \nu_{\tau}$ as well? ...