
#1
Nov1012, 02:41 AM

P: 43

Okay, so I've just begun to have a grasp on the concepts of quantum chromodynamics. And what amazes me is that quarks never actually exist on their own as a single particle because of the strong interaction between them. I've read that when you try to pull apart a pair of quarks that is bonded together by a gluon, they act like a rubber band, and when they got into this "pulled limit", instead of breaking them apart, another pair of quarks is produced, and the old pair of quarks remain confined.
1. I would appreciate if someone could explain and discuss more on this awesome process/phenomena. I just think this is pretty interesting. 2. Do physicists work on trying to get around this quark confinement, in an attempt to find a way to break them apart? 3. So if quarks remain confined as composite particles (hadrons), how did particle physicists experimentally observed, discovered, and identified each type of quarks (up, down, top, bottom, strange, and charm) as a single particle? It's like they "poked" into a hadron and observed that there are those quarks inside, and identified there different types. Or something like that. Thank you very in advance. I apologize if I said anything wrong in the post, being a beginner. 



#2
Nov1012, 08:43 AM

Mentor
P: 11,239

http://en.wikipedia.org/wiki/Deep_inelastic_scattering 



#3
Nov1012, 09:04 AM

Mentor
P: 10,824

Heavy quarks (charm, bottom) dominate the properties of the hadron they are in, so you can study the quarks via their hadrons. The topquark even decays before it forms hadrons. 



#4
Jan413, 10:26 PM

P: 43

Quark Confinement and the discovery of quarks 



#5
Jan413, 10:32 PM

P: 43

How are the quarks "free" or not bound in hadrons in Quarkgluon plasmas? And what is exactly happening with the quarks and gluons inside these "plasmas"? 



#6
Jan513, 06:13 AM

P: 136

In Deep Inelastic Scattering you basically 'fire' beams of electrons or neutrinos at targets containing nucleons and observe the scattering angles and outgoing energies of the particles that have interacted. From these observations some of the properties of the particles that scattered them can be deduced via appropriate mathematics.
Electrons are assumed to interact by exchanging a virtual photon with a charged constituent of the nucleon  the term 'parton' is sometimes used for the latter. Neutrinos interact by the weak force (charged current), ie they exchange a virtual W boson with the target and in doing so become charged leptons which can then be easily detected. Repeating these experiments with different targets (ie different proportions of protons to neutrons) and comparing the resulting outgoing energy and scattering angle distributions allow the differences between these nucleons to be studied. It is found that, at lower energies, the beam particles behave as if scattered by the nucleon as a whole (Rutherford scattering) but at energies where the beam particle wavelength gets below the nucleon radius new behaviour emerges. This is consistent with the beam particles having been scattered by smaller individual partons. Assuming the proton to have two partons with charge +2/3 and one with 1/3, and the neutron to have one with +2/3 and two 1/3 fits very well with the observations. The maths of the interaction dynamics also allows the average proportion of the nucleon momentum carried by the 'struck' partons to be estimated, and the combined momenta of these charged partons is found to add up to only about 50% of that of the nucleon. The remainder therefore appears to be carried by the gluons that mediate the strong interactions between the charged partons (quarks). The other experiments we can do that involve quarks directly are highenergy particle/antiparticle collisions or similar, eg e^{+}e^{}. The incoming particles annihilate into a virtual photon or Z boson which can then pairproduce a quark and an antiquark. The latter rapidly hadronise as described in an earlier post (unless the quark/antiquark are tops, in which case they decay first). By comparing the rate of production of hadrons to that of (eg) μ^{+}μ^{} the charges of the different quark types can be further corroborated. In addition, resonance peaks (of higher than otherwise expected proportions of hadrons produced) are observed at specific energies. This is actually how many of the heavier mesons, and in particular those containing charm and bottom quarks, were discovered. A book I'd strongly recommend is 'The ideas of Particle Physics' by Coughlan, Dodd and Gripaios. This gives a great phenomological description of fundamental particles, interactions and experiments without going into too much maths  it assumes some knowledge of the relevant school maths and physics  but without dropping into the kind of woolly narratives often found in popular science books. 



#7
Jan513, 08:14 AM

Mentor
P: 10,824





#8
Jan1613, 09:48 PM

P: 43

Thank you and apologize if I may have asked or stated any wrong statements. 



#9
Jan1713, 12:51 AM

P: 43

And if all those electrons and nuclei that make up the atoms and molecules become "unbound", wouldn't that "destroy" the chemical elements/materials that make up the gas/plasma altogether? Because all that gas/plasma, as matter, is composed of an element, like say Hydrogen in the core of the sun, right? And that Hydrogen molecules are composed of atoms, which then are composed of electrons and nuclei. Did I understand this correctly? Sorry for my ignorance and I thank you for your patience. 



#10
Jan1713, 07:41 AM

Mentor
P: 10,824

If a neutrino hits a proton and you get a neutron plus a positron afterwards (those can be detected), you know that the incoming particle was an antielectronneutrino. In addition, it confirms that you can transform a proton into a neutron if one uptype quark (here: an upquark) is converted to a downtypequark (here: a downquark). [quoteWhat kind of new behavior is that? How is the interaction look like? [/quote] That is scattering at the individual components of protons  quarks or gluons. [quote]Is the math the framework of Quantum Chromodynamics?[/quote QCD allows to predict those functions, but to measure them you have to analyze the data (not with QCD). Most interesting LHC collisions are gluongluon interactions, so you can really see their effects in colliders. It is random, but you can only produce pairs if the energy is high enough for them. Therefore, LEP could not produce top+antitop, for example. 



#11
Jan2513, 09:31 AM

P: 56

Go to Frank Wilczek's homepage  He oughta' know about this stuff!  and find his paper titled, "QCD Made Simple".
Lotsa' nice info! CW 



#12
Jan2713, 08:07 AM

P: 136

Looking back at this again, a couple of quick followups:
Knowing the incoming beam energy and the energies and scattering angles of the outgoing particles, we can then calculate parameters such as [itex]Q^2[/itex], the square of the 4momentum transferred from the incoming particle to the nucleon, and [itex]\nu[/itex], the energy transferred to the target in the lab frame. A further, dimensionless, quantity of considerable import is [tex]x = \frac{Q^2}{2\nu M_N}[/tex] This turns out to be the proportion of the nucleon momentum carried by the parton 'struck' by the incoming beam particle. The fact that basic scattering behaviour reemerges at very high energies gave key evidence for 'asymptotic freedom' in QCD. Essentially, at these energies the quarks behave approximately as free particles, so for the actual scattering interactions we don't have to employ any model of the strong force. The nucleons can be modelled using a set of 'structure functions' [itex]F_i(x)[/itex] which describe the charge densities of partons carrying [itex]x[/itex] proportion of their momenta. 


Register to reply 
Related Discussions  
Quark confinement and meson confinement  High Energy, Nuclear, Particle Physics  39  
Quark Confinement  High Energy, Nuclear, Particle Physics  13  
Groups of quarks and confinement  High Energy, Nuclear, Particle Physics  4  
Tetraneutrons, quark confinement  High Energy, Nuclear, Particle Physics  1  
Quark Confinement  High Energy, Nuclear, Particle Physics  17 