All Questions
Tagged with conservation-laws particle-physics
208
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Can impact offset of an electron on an ion affect the resulting particle velocities?
A free electron, of mass $m_e$ and velocity $v_e$ collides with an ion of mass $m_i$ and velocity $v_i$. When they recombine, a photon of exactly the ionization energy $E_i$ will be emitted, moving in ...
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88
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Why does the mass term not violate particle number conservation in a free theory?
The Lagrangian of a free real scalar field theory is
$$ \mathcal{L} = \frac{1}{2} \partial_{\mu} \phi\; \partial^{\mu} \phi \; - \frac{1}{2} m^2 \phi^2. $$
If we decompose $\phi$ in terms of the ...
- 61
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46
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Threshold energy formula
In Krane's Introductory Nuclear Physics, in chapter 11, he uses the conservation of energy and momentum to derive the formula for the threshold energy of a reaction a + X → b + Y (with X being an ...
22
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2
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Have all the symmetries of the standard model of particle physics been found?
Background
The standard model of particle physics is entirely determined by writing down its Lagrangian or, equivalently, writing down the corresponding system of PDEs.
Every set of PDEs has a ...
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Why are two gluons needed for Feynman diagram?
Why do we need two gluons for the decay $$\pi^-+ p\rightarrow\pi^-+n+\pi^+\:\:?$$
If we have always the gluon $$\frac{1}{\sqrt{2}}(r\bar{r}-g\bar{g})$$ it should be possible with only one gluon
...
- 425
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612
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Is the number of electrons in the universe always the same?
My teacher just said the number of electrons in the universe always stays the same, that's not how I understood weak force interactions and electron capture. Is there some rule that states that the ...
- 341
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83
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Conservation of angular momentum in particles reactions
In particles or nuclear reactions the square modulus of total angular momentum (the sum of spin and orbital angular momentum) is conserved.
When calculating the density of states to compute cross ...
- 338
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150
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Conservation of momentum with antiparticle annihilation
I'm confused about how momentum can be conserved when a particle and its antiparticle collide. For example, if an electron and positron collide and annihilate to form two photons, then there should be ...
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47
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What does conservation of color charge mean for mixed states in QCD?
In quantum chromodynamics, in an interaction in which a quark and an anti-quark exchange a gluon, the color charge must be conserved. When we are talking about base states like $r\bar{b}$ it seems ...
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152
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About particle number non-conservation in Quantum Field Theory
There are already similar topics with interesting answers such as When particle number can change in quantum physics? but I still don't understand much.
I often read about the non-conservation of ...
1
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1
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Possible orbital angular momentum values in the weak decay of $B_s \rightarrow J/\psi+\phi$
So $B_s$ has $J=0$, and both the decay products have $J=1$. $J$ is conserved, so the joint final state must also have $J=0$, and I'm told the final joint state can have orbital angular momentum values ...
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Is spin conserved in a reaction between elementary particles?
I was making an exercise where I had to check if a decay is possible, so I checked if the baryon number, charge, energy, Lepton numbers, and spin are conserved. One of the decays is this one:
$\Sigma^...
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1
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52
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Isn't weak interactions necessarily violate strangeness?
The electron-proton elastic scattering process $$e^-+p^+\to e^-+p^+$$ can take place either via a photon exchange (electromagnetic interaction) or via $Z^0$ exchange (weak interaction). But isn't weak ...
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A very basic question about particle (leptons, hadrons) interactions
Suppose a particle reaction $A+B\to C+D$ is allowed in nature. Then, the reactions,
\begin{align}
A&\to \bar{B}+C+D,\\ \bar{C}+\bar{D}&\to \bar{A}+\bar{B},\\ B&\to\bar{A}+C+D,\\& \...
- 11.8k
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Why doesn't KE of annihilation electrons necessarily have to be equal to each other?
We know that when a high energy gamma ray(E >= 1022 keV because the total energy of 1 electron at rest and 1 positron at rest is 511 keV) passes near a high Z(atomic weight) atomic nucleus ...
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