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Title says it all.

Have they been seen in experiment or are they just theoretical things?

Do quarks really exist?

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    $\begingroup$ You can't see quarks. Something that behaves like quarks ought to, has been detected in experiments. Now they could be quarks, or they could be something that is just having a good laugh at our expense, but we just don't know $\endgroup$ – Pranav Hosangadi Sep 5 '13 at 5:47
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    $\begingroup$ How do you know that what you see exists ? You eyes may see water in the desert where none is to be found. They are only an instrument, a biological one, while quarks are seen with more complex man-built instruments. You may trust or distrust the readings, and confront them with other sources of information to check that they are an identifiable phenomenon, deserving its own name and existence, or only an illusion due to some other known phenomenon. Mirages exist, and actually have their own name and explanation, but they are not water. Similarly, quarks is a name for an identified phenomenon. $\endgroup$ – babou Sep 5 '13 at 9:19
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    $\begingroup$ You can also ask: do electrons exist? protons?... We cannot touch them, taste them, feel them, hear them or see them the way we see this web page. It is by nested proxy representations that we interpret from our experiments that we know atoms exist, nucleons exist, electrons exist, and yes, quarks exist. $\endgroup$ – anna v Sep 5 '13 at 9:58
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    $\begingroup$ LAUNCELOT: Look, my liege! ARTHUR: Camelot! GALAHAD: Camelot! LAUNCELOT: Camelot! PATSY: It's only a model. ARTHUR: Shhh! $\endgroup$ – Jeanne Pindar Sep 5 '13 at 13:56
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    $\begingroup$ To the OP: Define exist. Do you exist? Hello? The point is that these existence questions are always more about philosophy and language than they are about science. All science can give at the end of the day are useful models and abstractions for describing the world. If you choose to be one of these people who doubts the "true existence" (whatever that means) of everything then you can always get away with it. But to adapt a Feynman quote: those philosophers who couldn't decide whether the food in front of them was real or an illusion have perished through hunger. $\endgroup$ – Michael Sep 5 '13 at 23:41
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The first conclusive evidence for structure within the proton/neutron was from deep inelastic scattering. This shows there is structure within the particles that matches what we expect from quarks. As Pranav says, it's possible that something else may be going on that just looks like quarks at the energies we can generate, but this seems unnecessarily cautious and I suspect most of us accept that quarks exist.

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    $\begingroup$ Even if the quarks have a deeper structure at higher energy, that merely means that the quarks are artifacts of an effective theory. They are still quarks. They have the right charges, the right spin, and the right color-charge addition symmetry. They are quarks. (And to think I was recently told that I was very scientifically conservative...) $\endgroup$ – dmckee --- ex-moderator kitten Sep 5 '13 at 6:17
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    $\begingroup$ There are also alternative descriptions of the phenomena, like an AdS/QCD-dual string-based description, but the existence of such an alternative description also doesn't mean that quarks are wrong - they're one of the legitimate ways to describe the physics of the strong force. $\endgroup$ – Luboš Motl Sep 5 '13 at 6:22
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    $\begingroup$ @LubošMotl +1 -- Physics is about modeling reality, not describing how it "truly is", which is a philosophical question. $\endgroup$ – Bakuriu Sep 5 '13 at 11:40
  • $\begingroup$ To echo @dmckee's point: composite quarks are still quarks. Think about atoms. You can't "see" atoms either (STM doesn't count because how do you understand how STM works? By modelling it in terms of atoms: circular argument.). And atoms are composite objects, but nobody is using this as an argument that atoms don't exist. Composite atoms are still atoms (in the modern sense, not the ancient Greek sense, obviously). $\endgroup$ – Michael Sep 5 '13 at 23:33
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    $\begingroup$ @Lubos Motl and Bakuru. The only thing required of any physical (or other science) theory, is that is consistently and reliably explains the observed outcome of experiments, including experiments, never yet performed. In that sense, there is NO requirement, that science theories be unique or even make common sense; only that properly manipulated according to the rules of the theory, they correctly predict or explain the outcome. The wave / particle duality is an example of two quite different methodologies of explaining the same thing. sometimes one way is better. $\endgroup$ – user26165 Sep 6 '13 at 18:52
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Quarks do exist! However we are not able to seen them directly, since the strong energy force between them increases as we tried to separate them from each other. The Quark-gloun plasma is a hypothetically state of matter in which quarks and gluons are free to move. This state of matter may exist at high temperatures and density.

For the discovery of quarks, see this small video:

https://www.youtube.com/watch?v=Kv9NbS_GumA

So they do exist!

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