## What actually happens inside a Large Hadron Collider.....how accurate is this?

I was wondering, perhaps KelseyMH could enlighten me further, how accurate the information is from this article. I realize it is slated for the general public so it may not be as detailed as it should be, but barring that, how good a job does it do?

Large Hadron Collider: how it works

Large Hadron Collider: how it works

Don't you wonder how a proton was discovered to be composed of

only 3 Quarks and about 20 glueons ?

If you shoot a bullet like another proton at the nucleus, sometimes it will interact with one of the valence quarks, sometimes with one of the gluons, and sometimes with one of a virtual quark-antiquark pair (the so-called "sea quarks"). By doing this over and over, and carefully analyzing the distributions of particles which come out of those collisions, you can measure the relative fractions of valence quarks, gluons, and sea quarks, as a function of energy.

The thing is, those measurements depend on the energy of the projectile. At low energies, you see almost exclusively the valence quarks, with a small fraction of gluons. At intermediate energies, the gluon fraction dominates, and at very high energies, the distributions all become more or less indistinguishable.

If you want to read about this in more detail, the search term is

deep inelastic scattering.It seems to have needed a lot of "patching" through the addition more particles, and as an abstract-concept it's rather complicated these days.

L

_{c}× SU(2)_{Y}× U(1)_{B}. The SU(2)×U(1) are the electroweak part: the neutral member of the SU(2) triplet and U(1) singlet mix and split into the Z^{0}and photon, leaving the charged W's. Color SU(3) automatically gives you the eight gluons from the simple octet representation.The part that seems a bit ad hoc are the matter fields. The fact that they are all doublets for spin (fermions), isospin (quarks)

andweak isospin (electron/neutrino) is a nice symmetry.Why there are three generations is not obvious (nor is the mass hierarchy explained), though it is in the minimum number needed in order to have a complex phase in the cross-generation mixing matrix. The fact that both quarks and leptons (neutrinos) have fully-populated mixing matrices is another nice symmetry.

The theorems fit the results and existing theory, so one thing inevitablly leads to another (particle. Or dimension if you're a String-theoreticist)

L

Historically, the late 1950's and 1960's were the "worst" time for any kind of particle physics theory: at each increase in beam energy, new particles were discovered, with seemingly arbitrary masses, decays which weren't related to one another. There seemed to be no organization, and no limit to the "particle zoo."

It was Gell-Mann and Feyman's discovery of symmetry-group representations (SU(3) flavor) in the zoo of baryons that led to the quark model and then to QCD as a theory with SU(3) (color) symmetry.

After that, the idea of applying group theory ideas in particle physics became more acceptable to theorists. Glashow, Weinberg and Salam did that with the unification of the electromagnetic and weak interactions, and the combination with QCD became the Standard Model.

L

Is the cross section area the main reason these objects are depicted as spheres ?

Like electrons, quarks are essentially pointlike entities: if you measure, for example, the angular distribution of electrons scattering off of quarks in nuclei, you get a 1/sin

^{2}distribution at all energies, with no flattening. That is the sign of two point-like charges being deflected.There's no dipole or higher multipole component, so the electric field is spherically symmetric.