Knocking on Heaven's Door - Lisa Randall [46]
So long as the net charge of the new particles is zero, this particle creation from the energy in the proton doesn’t violate any known physical laws. For example, a positively charged proton cannot suddenly change into a neutral object when virtual particles are created.
This means that every time a quark—which is a particle that carries nonzero charge—is created, an antiquark—which is a particle identical in mass to a quark but with opposite charge—must also be formed. In fact, quark-antiquark pairs can both be created and destroyed. For example, a quark and antiquark can produce a photon (the particle that communicates the electromagnetic force), which in turn produces another particle/antiparticle pair. (See Figure 17.) Their total charge is zero, so even with pair creation and destruction, the charge inside the proton will never change.
[ FIGURE 17 ] Sufficiently energetic quarks and antiquarks can annihilate into energy that can, in turn, create other charged particles and their antiparticles.
In addition to quarks and antiquarks, the proton sea (that’s the technical term)—consisting of the virtual particles that are created—contains gluons as well. Gluons are the particles that communicate the strong force. They are analogous to the photon that is exchanged between electrically charged particles to create electromagnetic interactions. Gluons (there are eight different ones) act in a similar manner to communicate the strong nuclear force. They are exchanged between particles that carry the charge that the strong force acts on, and their exchange binds or repels the quarks to or from each other.
However, unlike photons, which carry no electric charge and therefore don’t directly experience the electromagnetic force, gluons themselves are subject to the strong force. So whereas photons transmit forces over enormous distances—so we can turn on a TV and get a signal generated miles away—gluons, like quarks, cannot travel far before they interact. Gluons bind objects on small scales comparable in size to a proton.
If we take a course-grained view of the proton and focus just on the elements carrying the proton charge, we would say that a proton is primarily composed of three quarks. However, the proton contains a lot more than the three valence quarks—the two up quarks and the lone down quark—that contribute to its charge. In addition to the three quarks responsible for a proton’s charge, inside a proton is a sea of virtual particles—that is, quark/antiquark pairs and gluons. The closer we examine a proton, the more virtual quark-antiquark pairs and gluons we would find. The exact distribution depends on the energy with which we probe it. At energies with which protons are colliding together today, we find a substantial amount of their energy is carried by virtual gluons and quarks and antiquarks of different types. They are not important for determining electric charge—the sum of the charges of all this virtual stuff is zero—but as we will see later on, they are important for predictions about proton collisions when we need to know exactly what is inside a proton and what carries its energy. (See Figure 18 for the more complicated structure inside a proton.)
More complete picture of a proton
[ FIGURE 18 ] The LHC collides protons together at high energy, each of which contains three valence quarks plus many virtual quarks and gluons that can also participate in the collisions.
Now that we have descended to the scale of quarks, held together by the strong nuclear force, I would like to be able to tell you what happens at yet smaller scales. Is there structure inside a quark? Or inside an electron for that matter? As of now, we have no evidence for such a thing. No experiment to date has given any evidence of further substructure. In terms of our journey inside matter, quarks and electrons are the end of the line—so far.
However, the LHC is now exploring