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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$\rho$ and $\omega$ mesons decays

As far as I understand, typical decay for $\omega$ meson is into $3\pi$, while for $\rho^0$ is into $2\pi$. In fact they are quite similar particles (same spin, parity, similar masses). Why this ...
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Can we say tension is the ability to generate interference by this paper?

https://arxiv.org/abs/1206.2021 Abstract : We propose a method for measuring the string tension in gauge theories, by considering an interference effect of mesons, which is governed by a space-time ...
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What prevents $K^0$ from decaying into $K^+ e^- \overline{\nu}_e$?

Energetically it is just possible for a neutral kaon to decay into a charged kaon, electron and antineutrino, namely: $$K^0 \rightarrow K^+ e^- \overline{\nu}_e$$ Indeed a mass difference of a few ...
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(Coleman QFT notes) Physical vacuum has no one-meson content?

I'm looking at Lecture 14 in Coleman's QFT notes. In defining the renormalized field $\phi'(x)$ corresponding to the meson field $\phi(x)$, Coleman notes first that \begin{equation*} \langle k | \phi(...
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Identifying mesons with Goldstone modes

In QCD, due to the work of ‘t Hooft and Vafa-Witten, we know that confinement implies chiral symmetry breaking (David Tong’s gauge theory notes have a clear discussion of this). It is then said that ...
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In the history of the development of the standard model was there a version in which quark-antiquark pairs or flux tubes were the fundamental particle [closed]

Based on the observations of color confinement and no free quarks, I'm wondering if instead of a having quarks as fundamental particles, during development of the Standard Model was there ever a ...
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Reaction-Threshold Energy

I have this exercise: "Given the following reaction : $K^+ + n \rightarrow \pi^+ + \Lambda $ find : threshold energy of $K^+$, $E_{Kmin}$, to make the reaction happen. $E_{K^+}$ and $P_{\pi^+}$...
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Spin dependence of wavefunction of meson states bound by a certain potential

Lets assume two spin half particles, for example a charm quark and a charm anti-quark are bound by a spherical harmonic oscillator potential to form bound meson states. The possible ground state ...
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Conservation of energy in nuclear physics

In terms of whether a nuclear process is "allowed" or not, is conservation of energy relevant? If lepton number, charge, colour and baryon number are conserved, and there exists a vertex ...
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Mass counterterms in Yukawa-type meson-nucleon theory (Coleman's notes)

On page 102 of his QFT notes (https://arxiv.org/abs/1110.5013) Coleman uses the following reasoning for calculating the counterterms to the meson and nucleon masses in what he calls "quantum ...
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Feynman diagram

In this picture first diagram, can't the gluon be a $Z$ boson? Similarly, can't the $Z$ boson in the second diagram be a gluon?
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Why $R(D)$ is not measured in LHCb and only $R(D^*)$ is measured?

Measurement of $R(D)$ and $R(D^*)$ are important probe of lepton universality violation. They are defined as the ratios of branching fractions: $$R(D^{(*)}) = \frac{\mathcal{B}(B\rightarrow D^{(*)}\...
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Cause of Strong force

What is the origin and cause of the strong interaction which occurs between the nucleons? I have read that it is caused by the exchange of mesons but what then ultimately causes this meson exchange to ...
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Particles composed of square roots?

There are some particles which require square roots when telling their compositions. Mostly mesons, not baryons. e.g. $$ρ^0=\frac{u\bar{u}-d\bar{d}}{\sqrt{2}}.$$ Could you explain me how to make sense ...
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$\omega$ vs $\rho ^0$ meson decay into $e^+e^-$

The decay $\omega \rightarrow e^+e^-$ has a partial width that is around 10 times smaller than the partial width for the decay $\rho \rightarrow e^+e^-$. Why is this the case? The only difference I ...
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Finding the velocity of a bounded quark of a meson

How would you compute the velocity of one of the quarks of a meson, when we have no reference to a reaction or a collision? I initially thought we could use the conservation of energy and momentum to ...
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Why is the hadronic $\tau$ branching ratio into 2 pions prefered?

In the PDG the $\tau$ branching ratios are listed and Br$(\tau^- \rightarrow \pi^-\pi^0 \nu_\tau)\approx 25$ %, while Br$(\tau^- \rightarrow \pi^-\nu_\tau)\approx 11$ %. How can this be since the ...
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Questions about Yukawa Potential for Strong Force

Here is what I have understood about the strong force: This is a close range force which acts on two charges. The way it works is that a charge releases a meson which attracts the two charges together ...
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Flavor-changing neutral currents (FCNC) and leptons oscillations

I have some confusion regarding flavor-changing neutral currents (FCNC) processes and relative meson-antimeson oscillations. If the question has already been asked, I'm sorry, I didn't find what I was ...
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Gell-Mann–Oakes–Renner relation for heavier pseudoscalar mesons?

The Gell-Mann–Oakes–Renner relation between the pion mass and light-quark masses is the following, $$m_{\pi}^2=-\frac{2}{f_{\pi}^2}(m_u+m_d)\langle\bar \psi \psi\rangle,$$ where $f_\pi^2$ is the pion ...
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Why can't I assume the quarks inside a hadron move together?

Looking at the following Feynman diagram: Using conservation of energy, we can see that in the rest frame of $D^0$, the energy of $K^-$ is higher than its rest energy. Meaning, it is in motion. I ...
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How can charm mesons conserve charge?

If a charm meson can change from particle to antiparticle. Then how can charge be conserved if they have different charges. what am i missing?
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How to distinguish between $\pi$ neutrinos and $K$ neutrinos?

Consider the neutrinos produced by two decay channels: $$ \pi^+ \rightarrow \mu^+ +\nu_\mu\,, \qquad\qquad K^+ \rightarrow \mu^+ +\nu_\mu\,. $$ with $\pi$ decay being 10 times more likely than $K$ ...
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Penguin diagram for $K\rightarrow 2\pi$ process

Studying the $\Delta S=1$ effective theory, is well known that QCD corrections and renormalization, give rise to a set of possible effective operators, some of which are known as penguin operators. Is ...
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Can the $W^+$ boson couple an antistrange quark decaying into an antiup quark?

I am trying to figure out the Feynman diagram for the fully hadronic $K^+$ meson decay $K^+ \rightarrow \pi^+ + \pi^0$. I have drawn out my attempt below, but in order for this to work, I would need ...
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What exactly is longitudinal polarization?

See the title. I keep reading about longitudinal polarization in difference scenarios (muons, especially), however I have been unable to find a clear definition of what it is. I would appreciate an ...
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Can someone please explain to a layperson how we know that the $B$ meson decay into a muon doesnt then decay into an electron?

Apologies if this is a stupid question. I am a complete layperson with no formal scientific education. If the SM predicts that $B$ Mesons decay into either electrons or muons at a 1:1 ratio, but ...
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Mass term in chiral lagrangian and chiral lagrangian

Mesons in chiral lagrangian are described by the chiral action $U = e^{2i\pi^a T^a/f_\pi}$, \begin{equation} \mathcal{L}_{chiral} = \frac{f_\pi^2}{4} \text{Tr} \left( \partial^\mu U^\dagger \...
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Feynman diagram for the decay $\tau \rightarrow \nu_{\tau}\pi^+\pi^-\pi^-$

$$\tau \rightarrow \nu_{\tau}\pi^+\pi^-\pi^-$$ This is a possible decay mode of the $\tau$ lepton (see Wikipedia). Given that a neutrino is present this must be the weak interaction and therefore we ...
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Is There A Reason lepton flavor violation (LFV) In $B^+$ Meson Decays Can't Be Explained By $K+πº$ Decays?

Physicists at Moriond 2021 reported New results on theoretically clean observables in rare $B$-meson decays from LHCb 3.1 sigma away from the SM prediction of lepton universality in the $B+→K+μ+μ- $vs....
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How to construct Feynman diagram for decay of pseudoscalar $\phi$ meson?

I am struggling to understand the construction of the Feynman diagrams for the following decays: The answer is given as follows: I do not understand how the quark anti-quark pair in each diagram is ...
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How to treat semi-relativistic bound states?

I’m interested in semi-relativistic bound state, positronium, bottomonium, charmonium, etc. One way to treat such bound states in relativistic QFT is Bethe-Salpeter equation: $$(\Box+m^2)(\Box +m^2) \...
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How have we proved that pions are mesons?

How have we specifically proved that pions are mesons? Meaning what experiments or calculations have we done to prove that. In addition, how do we know that muons and taus are fundamental particles (...
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Why does neutral/EM meson decay scale as the cube of mass?

For beta decay of mesons, the heavy quark approximation and surely other techniques can be used to show that the decay width scales as the quintic power of mass. This can be checked from pdg files ...
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Neutral $B$-meson oscillations

I would have a question about an Equation in Mark Thomson's book "Modern Particle Physics". On page 397, Eq. (14.62), he arrives at $$\vert B\left( t\right)\rangle = \frac{1}{2}\left[ \...
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Why is the pattern of full decay widths of $\Upsilon(1S)–(2S)–(3S)$ mesons inverted?

Upsilon mesons are bound states of b-quark and anti-b-quark, of $J^{PC} = 1^{– –}$. First three states are located below $B\bar{B}$ threshold so they decay either via OZI-suppressed strong transitions ...
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How does the process $\bar{K}^0+p \to \Lambda^0+\pi^+$ work?

I was asked to find an energy for the $\bar{K}^0+p \to \Lambda^0+\pi^+$ interaction for an exercise and I got curious about how the Feynman diagram for this process would look like. In terms of quarks,...
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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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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 ...
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Meson operator treated as an elementary field in effective action?

I have seen in many papers, with this as the earliest example I can find, that the meson chiral order operator $M_j^{\,\,\,i}(x)=\overline{\psi}^i(1+\gamma_5)\psi_j(x)$ is treated as an elementary ...
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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 ...
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Meson masses difference

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 ...
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Number of $\pi ^+$ and $\pi^-$ in collision experiments

My first idea is that number of $\pi ^+$ and $\pi ^-$ mesons in collision of heavy ions with some target made of heavy nuclei should be close to 50/50. But after that I start to think that mass of $d$ ...
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$b$ mesons lifetime

I read about a technique called $b$-tagging in which one can check if a $b$ meson was produced in a collision by looking for a jet which has a secondary vertex within it. The reason is that b mesons ...
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Has there been research done on the dependence of particle decay to its immediate surroundings?

The meson particle, Kaon, can decay into pions or leptons + neutrinos. Each decay has its own probabilities as measured and confirmed during experiments. Has there been any research into the ...
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How to write quark composition of $\rm SU(3)$ mesons?

In $\rm SU(2)$, taking up quark and down quark as a doublet we can easily apply the isospin ladder operators to write the combination of 2 quark or 3 quark (baryon) systems. In $\rm SU(3)$ quark model,...
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Why the mesons $\pi^0$ and $\eta$ are "degenerate"

I don't know anything about elementary particles, but reading the book by Zeidler on p158, table 2.8 one can observe the quark content of baryons and mesons. It is strange that the $\pi^0$ meson has ...
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Is the decay of a neutral rho meson into a neutral pi meson and a photon forbidden?

Is the decay of a neutral rho meson into a neutral pi meson and a photon forbidden? It doesn´t look like parity conservation or charge conservation is violated but I haven´t found this decay is ...
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How is the neutral kaon stable?

A neutral kaon is a mix of a particle and antiparticle, in layman's terms. In layman's terms, how is this possible without there being annihilation?
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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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