Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 08/09/2018 11:03:30
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I decided to educate myself more about particle physics earlier this year and in doing so turn some random names into some actual knowledge and understanding.
I was surprised to learn that the matter in the University is formed entirely from down and up quarks and electrons. This means you can make protons, neutrons and atoms - and therefore everything that we can "see" - from just these species.
What I am struggling to comprehend, then, is what the other 4 species of quark (strange and charm, bottom and top) actually "do".
As far as I can make out from reading the literature, these are analogous in almost every way to a down and up quark but differ only in mass with the charm and top quarks being the more massive in ascending order and the strange and bottom quarks showing a similar pattern.
So, why do these other particles exist and what is/are their role(s).
Also, could we make a "heavy" proton from a couple of top quarks and a bottom quark?
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Atoms are constructed of two types of elementary particles: electrons and quarks.
Electrons occupy a space that surrounds an atom's nucleus. Each electron has an electrical charge of -1.
Quarks make up protons and neutrons, which, in turn, make up an atom's nucleus. Each proton and each neutron contains three quarks.
A quark is a fast-moving point of energy. There are several varieties of quarks. Protons and neutrons are composed of two types: up quarks and down quarks.
Each up quark has a charge of +2/3.
Each down quark has a charge of -1/3.
The sum of the charges of quarks that make up a nuclear particle determines its electrical charge.
Protons contain two up quarks and one down quark.
+2/3 +2/3 -1/3 = +1
Neutrons contain one up quark and two down quarks.
+2/3 -1/3 -1/3 = 0
The nucleus is held together by the "strong nuclear force," which is one of four fundamental fources (gravity and electromagnetism are two others). The strong force counteracts the tendency of the positively-charged protons to repel each other. It also holds together the quarks that make up the protons and neutrons.
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Thanks @Dave Lev - your reply is really helpful but it doesn't actually answer my question, which is that down and up quarks account for the matter in the Universe, but we also know of the existence of strange, charm, top and bottom quarks. They appear to be analogous but more massive than their down and up counterparts, and they appear to play no part in the structure of matter.
So what the hell do they do?!?
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At one level, science avoids questions like "what are they for?" and "what do they do?".
Science can detect some patterns, like there a 3 "generations" of quarks (purple) and 3 "generations of leptons (in green):
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We are most familiar with the stable members: up & down quark and the electron. These are the left-most particles.
Sensitive detectors can identify neutrinos (which are always oscillating); practical applications of these is currently limited to peering inside the Sun and exploding supernovae. We can also detect muons from cosmic rays, but practical applications are limited to peering inside pyramids.
The rightmost particles are unstable, and quickly decay into the stable particles to the left.
See: https://en.wikipedia.org/wiki/Generation_(particle_physics)
https://en.wikipedia.org/wiki/Standard_Model
The other quarks have very high energies, and can only be created for an instant in our most powerful particle accelerators. These emulate the extreme energies that existed for a moment during the Big Bang, when these heavy particles would have been stable (for a moment).
So, while they don't do much today, they would have played an important part during the Big Bang.
Maybe some advanced civilisation may be able to create and utilise these other quarks, but we can't today.
Some physicists think that these more exotic quarks may be stable under the extreme pressures at the center of a neutron star, preventing the neutron star from collapsing into a black hole. That's why it's important for LIGO to "weigh" colliding neutron stars.
Some physicists speculate that there may be other generations at even higher energies than the 3 we know, but these would exist at such high energies that we can't produce them today.
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Excellent account @evan_au - thank you!
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Also, could we make a "heavy" proton from a couple of top quarks and a bottom quark?
Just returning to this point - is this what you are hinting at in your reference to neutron stars, Evan?
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Well, Chris, it looks as though you have three possible answers:
Not a lot.
Nothing, for a very long time.
Don't know.
Not the most satisfactory answers, perhaps, but some good "hitch-hiker" level explanations so far.
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could we make a "heavy" proton from a couple of top quarks and a bottom quark?
There have been many particles created in accelerators which are not a mixture of Up and Down quarks - but they are all extremely unstable.
Protons & neutrons are examples of triquarks, but there are many other triquarks. Only the proton is thought to be maybe stable.
Particles made of two, four and five quarks are known, and particles of 6 or more quarks are theorised.
See: https://en.wikipedia.org/wiki/Isospin#Hadron_nomenclature
https://en.wikipedia.org/wiki/Quark_star#Other_theorized_quark_formations
your reference to neutron stars, Evan?
It is thought that in the extreme pressure of a neutron star (or the Big Bang), strange quarks could be stable, forming a "Quark Star" or "Strange Star".
There are even a few candidates for quark stars that look like odd neutron stars.
See: https://en.wikipedia.org/wiki/Quark_star
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Asking their role is similar to asking why they exist, which is a question addressed by metaphysics. The quarks you asked about form an large number of different particles (hadrons and baryons) which differ by the number and type of quark. Protons and neutrons are hadrons.
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What do strange quarks do?
The property of "strangeness" was named because of a family of subatomic particles formed in particle accelerators that seemed to last an extraordinarily long time (for a subatomic particle of their mass): around 10-8s for kaons, and 10-11s for hyperons.
Later this property of "strangeness" was associated with the strange quark.
See: https://en.wikipedia.org/wiki/Strangeness
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I used to know that .... a few decades ago. (sigh) I'm getting so old. :)
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could we make a "heavy" proton?
To make a triquark (like proton and neutron), you could choose any combination of 3 quarks out of 6 candidates (plus their antiparticles = 12 total).
That makes a total of 12 x 11 x 10 = 1,320 potential triquarks!
No wonder physicists used to refer to the products of particle accelerators as a "particle zoo", before the idea of quarks brought some order to this mess!
I am sure that some of these potential triquarks will be less stable than others - for example particles with fractional charges (like 11/3) have never been observed
- a triquark made of three 2/3 charge quarks will be less stable, since it has a nuclear charge of +2, and that must be very unstable.
- There will also be some neutral triquarks, like the neutron.
- And I am sure that there are some more subtle reasons why some of these potential combinations are not valid.
But in general, heavier triquarks will decay into lighter particles very quickly.
Even the neutron (which is only slightly heavier than the proton) decays into a proton (+ other particles) in around 618 seconds, when left by itself in a vacuum.
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Outside the nucleus, free protons are considered stable while free neutrons are not. The decay involves flavor change. That is the flavor change from down to up for one of the quarks in the neutron. This flavor change is via the weak nuclear force and results in the change of the neutron to a proton. This involves the emission of a W- boson. This boson then decays to an electron and an antineutrino. We also find flavour change in neutrinos. This is neutrino oscillation, which is similar to color change among quarks. In that both are 'oscillations'. What causes the neutron to be stable within the nucleus and unstable outside is very interesting. The majority of composite particles are short lived in the current era of the universe.
See also the neutron lifetime puzzle.
https://en.wikipedia.org/wiki/Free_neutron_decay
EDIT: On the neutron lifetime puzzle.
https://www.quantamagazine.org/neutron-lifetime-puzzle-deepens-but-no-dark-matter-seen-20180213/
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It may also be of interest to look at conservation and weak hypercharge as it relates to neutron decay.
https://en.wikipedia.org/wiki/Weak_hypercharge#Neutron_decay
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the neutron lifetime puzzle
I wondered why I kept seeing different figures for neutron lifetime.
Not the 9 second difference between beam and bottle experiments.
But the larger difference between half-life and mean lifetime...
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For mean lifetime and half life the following calculator is useful. Just input the required values to get a result.
https://www.calculator.net/half-life-calculator.html?type=1&nt=1&n0=2000&t=&t12=881.5&x=55&y=35
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For completeness it would be worth reading up on hypercharge. This is distinct from weak hypercharge.
https://en.m.wikipedia.org/wiki/Hypercharge