The reasons are complicated, and perhaps not entirely known yet. You can see a brief overview of what is known in the Wikipedia article "Color confinement".
In the ordinary sense of the word ... No, because you can't even have free quarks at temperatures below about 2,000,000,000,000 K, where they exist with other quarks and gluons in a kind of plasma or "soup". You can figuratively regard quarks below that temperature as being "frozen" (bound with other quarks) into hadrons.
Protons and neutrons are composite particles make up of up and down quarks. There are two up quarks and one down quark in a proton, and two down quarks and an up quark in a neutron.
There are no quarks in an electron. Electrons are leptons, which are not made of quarks. Further, the electron is a fundamental particle, just like the quark. That means that neither quarks or electrons are made up of other particles, per the Standard Model of particle physics.
As far as we know, quarks are the smallest particles to exist. We have found 6:- Top- Bottom- Up- Down- Strange- CharmEach of the quarks also has a property called color charge. These are red, green, and blue. Particles that are constituted of quarks must be color neutral. In all, there are 18 different possible types of quarks if one does not count antimatter. When antimatter is taken into account, that number increases to 36.
There are six types of quarks, with three "color charges" each. There are the up, the down, the charm, the strange, the top, and the bottom quarks, each with a "red" charge, a "blue" charge, or a "green" charge. If you want to count antimatter, then there are six antiquarks, each with "anticolor charges." The anticolors are Antigreen, antired, and antiblue. These are each represented by cyan, yellow, and magenta. So, counting antimatter and color charge, there are 36 quarks.
Actually quarks can exist freely.
No, it is not possible ,a proton is the combination of two up and one down quarks but quarks do not exist freely.
In an atom, the neutrons and protons are made up of up quarks and down quarks. Strange quarks, charms quarks, top quarks, and bottom quarks also exist, but do not play as much of a role in the structure of an atom.
Every particle that should exist if quarks are real has been found. No particle that should not exist if quarks are real has ever been found. The results of in-elastic scattering between hadrons fits perfectly with quark. The hypothesis of charm quarks predicted experimental results that were almost perfectly replicated with the discovery of the psi-particle. In other words, the quark hypothesis matches observation perfectly. That's about as strong of evidence as can exist.
In the ordinary sense of the word ... No, because you can't even have free quarks at temperatures below about 2,000,000,000,000 K, where they exist with other quarks and gluons in a kind of plasma or "soup". You can figuratively regard quarks below that temperature as being "frozen" (bound with other quarks) into hadrons.
In the Standard Model, quarks are fundamental particles. As such, they are not made up other particles. They either exist as quarks, or not at all. Like all matter they can be converted into energy, but quarks are the building blocks of hadrons like the proton and neutron. Links can be found below to learn more.
No, they are now solid fact. Quarks are the fundamental particles which make up the protons and neutons of everything around us, as well as some other crazy particles such as the Higgs boson. There are some bosons (things made up of quarks) which are merely theoretical; we know they muct exist but have not yet found one, but quarks are definitely NOT fictional
Quarks have not been observed to exist separately - they are "confined" within larger particles such as protons and neutrons, that are made up of several quarks (3 each, in the case of protons and neutrons).
Yes, quarks exist, and they have been demostrated in particle accelerators in the high energy physics laboratory.
Protons and neutrons are composite particles make up of up and down quarks. There are two up quarks and one down quark in a proton, and two down quarks and an up quark in a neutron.
There are no quarks in an electron. Electrons are leptons, which are not made of quarks. Further, the electron is a fundamental particle, just like the quark. That means that neither quarks or electrons are made up of other particles, per the Standard Model of particle physics.
Quarks, which are fundamental particles, all "suffer" from an extension of one of their basic characteristics (color) called color confinement, and this has a consequence. Quarks are never found free in space anywhere outside a hadron (like a proton or neutron) which they make up. Quarks, which are the bulding blocks of hardons, simply cannot exist outside the particles in which they are those building blocks. Links can be found below for more information.