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108

Free neutrons in flight are not deflected by electric fields. Objects which are not deflected by electric fields are electrically neutral. The energy of the strong proton-neutron interaction varies with distance in a different way than the energy in an electrical interaction. In an interaction between two electrical charges, the potential energy varies ...


22

Suppose that the strong nuclear force were instead caused by Coulomb interactions. Since we know how strong the binding energies are (of the order of $\sim 1\ \text{MeV}$, as can be gleaned by say, looking at a table of alpha particle energies) and how far apart the nucleons are (about a proton radius, or $a_p\sim1\ \text{fm}$) we know how charged the ...


21

No, a three-quark baryon can not be be made out of two quarks and one anti-quark (and vice versa) as this would necessarily give the particle color. Each quark carries one of three colors (red, blue, green) and each anti-quark respectively carries anti-color. Color is an additive quantity when constructing particle and the result must be color-neutral, i.e. ...


19

Pentaquarks contain three quarks and a quark-antiquark pair, and they are baryons, since baryons are defined as having an odd number of valence quarks.


18

Rob's answer is the simplest and probably best, but let me add another approach. We know that nuclei are made out of protons and neutrons. Protons repulse each other, but somehow, if you get them close enough, they stick together extremely strongly. This already suggests that there is another force in play! So even if you completely ignored neutrons, you ...


14

Short answer: yes. You should look in to the history behind the dark matter hypothesis. It started not from the examination of cosmology and the CMB, but from the motion of galaxies in clusters and stars orbiting around galaxies. See, the vast majority of ordinary matter in every galaxy is contained in the gas between the stars, not the stars themselves. ...


10

But the Sun in our solar system also is in the state of plasma and yet doesn't act like a blackbody Wrong, the sun radiation is approximately fitted as a black body. The word "black body" does not describe the frequencies, but the assumption that it absorbs all radiation falling on it and re-emits it. Here is the sun, and it fits the black body formula ...


5

As Richard Feynman pointed out in his lectures "The Character of Physical Law", the ultimate test to decide whether or not a theory is correct is the experiment. Rob correctly stated there is strong evidence suggesting the null interaction between a neutron and some external electric influence. Measurements about masses and electric charges of several atomic ...


4

Colour is independent of flavour. An up quark can be red, green or blue. Since gluons also carry colour, the colour of a quark isn't fixed. When a blue quark interacts with a green quark (of whatever flavour) they do so via a gluon that carries "blue-antigreen" (or green-antiblue) colour, and this has the effect of swapping their colours: The blue quark ...


4

Yes, there are some oddball baryons that contain four quarks and one anti-quark. They're called pentaquarks. These could be thought of as a baryon with three quarks plus a meson with a quark anti-quark pair, but sticking to each other more than they should. There are few hadrons that survive more than a tiny fraction of a second, and only the proton and ...


4

They do form, but don’t last long. Delta baryons can have three up quarks (for the $\Delta^{++}$) or three down quarks (for the $\Delta^-$). These baryons are unstable and last only a few trillionths of a trillionth of a second.


4

A proposal for the holographic dual of QCD is given by the so-called Witten-Sakai-Sugimoto model $[1,2,3]$. The Witten background is a solution of a consistent truncation of Type IIA supergravity (where only the metric, the dilaton and $F_{(4)}$ are switched on) and it corresponds to $N_c$ D4-branes wrapped around $S^1$. Intuitively this $S^1$ gives an IR ...


4

I think your misunderstanding is precisely that you think that $U(1)$ gauge symmetry in SM can be associated to any quantum number such as baryon or lepton number. No, it can not. The $U(1)$ coming from $SU(3)_C\otimes SU(2)_L\otimes U(1)_Y$ is related to a quantum number called hypercharge, $Y$ We say that baryon and lepton numbers are symmetries in a '...


4

Take a look at the pseudoscalar mesons, a well understood family, since they comprise the pseudogoldstone bosons of spontaneous chiral symmetry breaking in 3 light flavor QCD. Flavor SU(3) arranges the octet in this pattern, and the singlet is $\eta'$, off the picture, since it has no isospin or strangeness, so U or V spin, to have any of its properties ...


3

They do! All three types of neutrinos are around us in great abundance and they are absolutely part of "ordinary matter" (as opposed for example to dark matter). The reason neutrinos do not combine with other particles to form something like atoms is because they do not have any electric charge. Yes. The standard reference website with the most up-to-date ...


3

I will just cite the introduction of a (relatively) recent paper by Carr et al.: arXiv:1607.06077 Since PBHs formed in the radiation-dominated era, they are not subjectto the well-known big bang nucleosynthesis (BBNS) constraint that baryons can have at most 5% of the critical density. They should therefore be classed as non-baryonic and from a dynamical ...


2

There are two dark matter problems. Most of the gravitating matter in the universe is "dark" and cannot be detected by the radiation it emits, but its presence is deduced by studying the dynamics of galaxies and clusters of galaxies. From analysis of the cosmic abundances of helium, deuterium and lithium, it can be inferred that most of the gravitating ...


2

We should start by making clear what cosmologists mean by baryonic matter because it isn't the same as what particle physicists mean by baryons. The light elements hydrogen, helium and small amounts of lithium and beryllium were formed in a process called Big Bang nucleosynthesis (BBN), and it's the matter formed by BBN that is referred to as baryonic ...


2

You should be able to answer your question directly by directly inspecting the baryon wavefunctions in your textbook. They are the unique solution to the constraints you mention, so, since they are color singlets, they must be color antisymmetric: consequently they are spin-flavor-space symmetric. Lowest mass baryons are in an S state, parity +, so, ...


2

No. Baryon number violation is one of the Sakharov conditions for transitioning from a universe with only radiation to a universe with more baryonic matter than baryonic antimatter, along with CP violation and thermal disequilibrium. However, there are so far no observed processes that change the baryon number of a closed system. If you're thinking of ...


2

OK, here is a schematic mathless picture that should fix your visualization. To start with, at very short distances, far shorter than a fermi, the quarks have small masses of the order of 2.5 MeV for the u and 5 MeV for the d, the up and down quarks of the nucleons. Masses convert left-chiral quarks to right-handed quarks and vice versa. Both types exist ...


1

The user MannyC has given an excellent answer addressing the baryon construction. I would like to add some comments mostly to spark some discussion and for completeness. Hopefully, the comments are interesting. In the following, I will also skip many steps, but if you are interested, please feel free to ask or comment. Since we are discussing fundamental ...


1

You are not getting any responses because this is a topic that is included in all courses implicitly, at all levels of courses. Searching on Google I found this specific for resonances, which of course includes two body resonances, since they are the simpler to write down mathematically: The answer of mine here , may help.


1

Why aren't Delta and Omega particles stable? It is a general rule, that a quantum mechanical state decays to the lowest energy level allowed by conservation of energy and quantum numbers. In the case of the Delta the lowest energy state is the proton, because it can decay to it via the strong interaction, conserving baryon number. In models beyond the ...


1

Delta half lives are around $5 \times 10^{-24}$, which is 11+ orders of magnitude less than nanoseconds. They are so short that one generally discusses the width ($\Gamma$) of the resonance, given in terms of the mean life ($\tau$) by: $$ \Gamma \tau \approx \hbar $$ Deltas are the 3/2-isopin version of the 1/2-isopspin nucleon, roughly and excited state, ...


1

Up down symmetry I interpret as : in hadronic reactions and in decays where hadrons are involved, if an up quark is involved in the calculation, the corresponding reaction where symmetrically a down quark is involved should have the same crossection and decay times. Why? Take the quark example of $Σ$ baryon , it has a $Σ^+$ going to $uus$ and a $Σ^-$ ...


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