Questions tagged [quarks]

The quark model postulates that the hadrons such as protons and neutrons have an inner structure; quarks.

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19 views

Why is it that the Callan-Gross relation predicts that quark has spin 1/2?

I'm studying deep inelastic scattering, and currently at the part where they say the Callan-Gross relation: $$F_1 (x) = \frac{1}{2x} F_x (x)$$ where $F_1$ and $F_2$ are the dimensionless structure ...
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Asymptotic freedom and momentum exchange

Why is the momentum exchange very high for low distances? For sufficiently short distances or large exchanges of momentum https://en.wikipedia.org/wiki/Asymptotic_freedom While I think about ...
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How does the introduction of the charm quark suppress FCNC?

I did some reading on the GIM mechanism today, and simply fail to understand how it works. I understand how the CKM-matrix can be used to do the basic calculation of the probability of, say, observing ...
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Why is $J/\psi$ not being able to decay into $\pi^+\pi^-\pi^0$ (Ozi-rule)?

There are already (very similar) questions like What is the precise statement of the OZI Rule? and Why does charmonium (and phi mesons) not decay via quark and antiquark annihilation? but, to be ...
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How does Isospin work?

I have been trying to find a good source to explain the concept of isospin thoroughly but everything I have found seems to gloss over it. I have found sources that tell me it is a vector quantity but ...
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Physical significance of the decomposition $3\otimes\overline{3}=8\oplus 1$ in meson classification

Under the assumption of $SU(3)$ flavour symmetry of early days, proposed by Gell-Mann and Neeman, among other irreducible representations, the meson octet was obtained. This result from the $SU(3)$ ...
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Why do mesons decay?

Why do mesons decay shortly while baryons are more stable? And why do mesons decay, meaning that there are unstable, while they formed because meson is more stable than the two corresponding quarks? ...
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55 views

How does a quark $u$ or $d$ turn into one another?

The pseudo-explanation that the proton is stable because it has a lower mass than the rest is incomplete, because a particle formed by three u: uuu would have still a lower mass. I know that some ...
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How can we reconcile the many different models in QCD that make inconsistent assumptions/claims?

For example, we have quark models, shell models, cluster models, etc. which seem to admit to different interpretations regarding nuclear structure and dynamics. Is there a way to resolve the fact that ...
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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 ...
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Theta-dependence of QCD quark condensate for $m=0$

I'm trying to understand the $\theta$-dependence of the following expression for the quark condensate in QCD, $$ \langle \bar{\psi}\psi\rangle = - \Sigma \cos(\theta)$$ taken from Eqs. (5) and (7) ...
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If quarks don't exist individually how can we say baryons are made up of three quarks?

We know the composite subatomic particles are made up of odd number of quarks (at least 3 for baryons ) or combination of quarks and antiquarks (mesons). My question is if they don't exist ...
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Do Quarks decay into Photons at high enough temperatures?

At high enough energy, particles lose coherence and are ripped apart into a Quark-gluon plasma where the individual quarks can break free of the atomic nucleus and fly around unencumbered. A black ...
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How much color charge does a quark carry? [duplicate]

If we look at QED, we can say that an electron carries the charge $$Q_\text{electr.}=-e = -1.602\cdot 10^{-19}\text{ C}$$ so how can we extend this to quarks? Would, for instance, $$ Q_\text{red} = ...
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How are quarks elementary when they can become leptons? [duplicate]

From a recently reignited [casual] curiosity into particle physics thanks to the Fermilab YouTube channel, I read about the g-2 experiment, followed by muons, naturally. Muons, it turns out have short ...
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204 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{...
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Why do we think of a nucleus as nucleons held together by a strong force rather than a cluster of quarks? [duplicate]

The implication of this point of view is that protons and neutrons retain their structure in a nucleus. That is the 3 quarks making up nucleons still closely interact and interactions with quarks from ...
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Is there a direct evidence for the fact that quarks have fractional charge? [duplicate]

Is there an evidence for quarks having fractional charge? Besides the fact that we know there must be 3 of them inside the nucleons, and the only way that this could happen is if they have indeed ...
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What happens if matter particle and antimatter particle both carry no charge were to collide elastically?

I read that particle carrying charge is able to interact with photon so that when electron and positron collides they may be converted into pure photons, then what about neutron-antineutron collision? ...
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43 views

How can pure leptonic interactions be mediated by the weak force?

$\mu^+ \rightarrow e^+ + \nu_e + \bar \nu_\mu$ Consider the above decay, it is given in my lecture notes as a weak force interaction. I thought the weak force was only involved when quarks change ...
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What are direct experimental evidences that quarks exist? [duplicate]

Now we have very established model of quarks explaining fundamental strong interaction. What are experimental proofs for existence of quarks and what is the name of physicist which made them?
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Most of the momentum of a proton is carried by the quarks

I saw this in wiki https://en.wikipedia.org/wiki/Antiproton#Modern_experiments_and_applications "the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the ...
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Why do we have 3 quantum colours?

I understand the need to invoke colour as another quantum state to explain the observation of uuu ddd and sss baryons. I just wanted to know if there was some other property which explains why we only ...
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How is the charge conserved at the up to anti-down vertex?

I can see how the charge is conserved in the overall diagram but I can not understand how it is conserved at the up to anti-down vertex.
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Is the (range of the) strong force the same for nucleons and quarks?

In high school, I learnt that the strong force held the nucleus together. It had a very short range and was repulsive at small separation distances to prevent the nucleons from collapsing. Now I am ...
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Can muons decay into quarks?

Muon decays are almost always written as $$\mu^- \rightarrow e^-+ \bar\nu_e +\nu_\mu.$$ The reason given on wikipedia is that one of the product neutrinos of muon decay must be a muon-type neutrino ...
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Why is the neutron magnetic moment negative?

I understand that the magnetic moment is due to the quarks, but specifically why is it negative? Is it due to the two down quarks or something?
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Besides the up and down quark, what other quarks are present in daily matter around us?

Protons and neutrons, which are found in everyday matter around us, compose of up and down quarks. Are the other two generations of quarks, i.e. $c,s,t,b$ quarks found in everyday matter around us? I ...
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Mass splitting for leptons and down quarks in $SU(5)$ model?

In the minimal $SU(5)$ GUT model, the masses of the leptons and the corresponding down-type quarks must be equal. i.e. $m_e=m_d$, $m_\mu=m_s$, $m_\tau=m_b$. Whereas this is approximately correct for ...
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What Does Up-Down Asymmetry Mean?

There is strong experimental evidence (reported on in the linked paper), from more than one high energy physics experiment, that up-down asymmetry is present in the decays of certain charmed baryons. ...
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Can someone explain the $ttH$ Coupling?

IS the $ttH$ an actual particle? If so, how can it form? I though the life of the top quark was way to short to form a particle. If it is not a particle, what is meant by coupling.
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Does an antiparticle leave a different (from the normal particle) mark in a bubble chamber experiment

Experimentalists usually have experiments where they scatter particles superheated transparent liquid, thus checking for the particle's traces. These particles can be for example quarks, and ...
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Strange Quark Matter

I am watching a video on neutron stars. The video is discussing the collision of 2 neutron (quark) stars and the ejection of strangelets. That if one of these strangelets comes into contact with ...
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How many quark flavor quantum numbers are really needed?

Several authors I've consulted recently use six separate flavor quantum numbers for the six observed flavors of quarks when constructing their QCD Lagrangians That seems excessive when three "family" ...
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How can weak interactions not conserve strangeness if the SM always conserves energy?

I have read these questions: Weak Interaction and Strangeness transformation Weak decay iff or if strangeness changes? Understanding type of force interaction in particle decays The decay of the ...
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How would the structure function of protons be different if quarks had spin 0?

I saw this question in a practice exam. I know that the structure functions are not linearly independent but that $F_2(x)=2xF_1(x)$ when the energy is in the order of GeV. The question asks what ...
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Computing the decay width for Top quark decaying to bottom quark and W boson

I have been trying to calculate the decay width for the $t \rightarrow bW^+$ decay. After having done this numerous times, I can not figure out how the result $$\Gamma = \frac{g^2m_t^3}{64\pi m_W^2}\...
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Why is the up quark is lighter than the down quark, while top is heavier than bottom?

Up, charm and top seem to be similar in terms of electric charge (+2/3e) while down, strange and bottom have also same charge (-1/3e). In the second and third family, positively charged quark has ...
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Can a quark quantum tunnel outside a nucleus?

I read that a particle wavefunction can extend over a thin barrier and there is a probability of finding it over the barrier even though it doesn't have the necessary energy, what about quark?
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How and why is the pentaquark stable if it consists of 4 quarks and an antiquark?

If matter and antimatter annihilate each other, why is the pentaquark stable since quarks are matter and the antiquark is antimatter?
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How does hexagonal boundary arise in $SU(3)$ representation?

I am having trouble trying to understand the hexagonal boundary in $t_3$ and $y$ representation of $SU (3)$. I tried to work it out like we did in $SU \left( 2 \right)$ but got not luck. Could someone ...
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Why are mesons only made of a quark-antiquark pair? [duplicate]

Why can't we have a meson made of two quarks, why does there have to be the antiquark too?
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How do we know that gluons have no electric charge?

Since the W boson carries electric charge and there is no a priori reason that massless electrically charged bosons cannot exist, I'm wondering if the lack of gluon electric charge has been confirmed ...
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Why is the proton (uud) lighter than the $Δ^0$ (uud) baryon?

Neutron has quark composition udd with spin $\frac 12$. $\Delta^0$ baryon has quark composition udd with spin $3 \over 2$. On Wikipedia it says that $\Delta$ baryons have mass of approximately $1232 ...
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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-...
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If a particle decay has quark annihilation will it always emit a photon?

From what I can gather the decay $\pi^0 \rightarrow \gamma \gamma $ emits two photons because $\pi^0 $ has quark content $(u\bar u-d\bar d)/\sqrt{2} $, and the $u$ annihilates with $\bar u$, and $d$ ...
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How do leptons differ from one another?

All the leptons that I have come across have their native (matter) charges of -1 relative to the charge of an electron. Since they are all fundamental and are not made up of quarks, you can't ...
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If the combinations, in the context of particles which have identical interactions, are not orthogonal to each other?

I read from here If there are two or more particles which have identical interactions, then they may be interchanged without affecting the physics. Any (complex) linear combination of these two ...
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Quark flow diagram for $\rho^0 \rightarrow \pi^0+\gamma$

So $\pi^0=\frac{1}{\sqrt{2}}(u\bar{u}-d\bar{d})$ and $\rho^0=\frac{1}{\sqrt{2}}(u\bar{u}-d\bar{d})$ except with a higher angular momentum, this angular momentum is taken by the photon. How do I ...
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Difference between $W^-$ and $\pi^-$

Maybe it's a very naif question, but what is the difference between a $W^-$ and a $\pi^-$? I mean they both change a $d$ into a $u$ right? $d \rightarrow u W^- \quad \text{and} \quad d \rightarrow ...