*“Within a nucleon there are uncounted sea quarks and only three tiny valence quarks, and these valence quarks are responsible for the residual strong interaction with other nucleons. Why are there countless sea quarks and exactly three valence quarks left over, above and beyond them? It feels a little like having 3 in the numerator and infinity in the denominator. What is the hidden structure that presumably keeps the numerator and denominator in balance?”*
Protons and neutrons are bound by the strong nuclear force, which is mediated by gluons and described by the theory of quantum chromodynamics (QCD). Unlike photons, which mediate the electromagnetic force, gluons can interact with each other. It is this self-interaction that generates around 99% of the visible mass in the universe, while only the remaining 1% is provided by the intrinsic masses of quarks. One of the outstanding puzzles in the understanding of the theory of the strong interaction is its behavior when the quarks are spinning and don’t carry much of the momentum of the proton. This is called small x. Another term in particle physics is Q2. Q2 is the invariant mass of the photon, and represents the resolution at which the proton is probed. It’s been found that at small x the proton's content is dominated by gluons. Small x means a gluon carries a small fraction of the proton's momentum, which means that the momentum is shared among many gluons. However, there is a puzzle: extrapolations of the present measurements to ever-smaller x yield an ever-rising gluonic cross-section. Thus, the gluonic part of the cross section will eventually become larger than the total cross section. Such unlimited growth is mathematically impossible; there has to be a mechanism that tames this rise. This mechanism is called saturation. Put simply, small x means the proton contains many gluons, all with a small momentum fraction. In quantum mechanics small momentum means large wavelength. This means that the wave functions of the gluons start to overlap, and two overlapping gluons can recombine into one, hence reducing the number of gluons. There are many models and calculations describing these phenomena, but there is as of yet no conclusive measurement of saturation. Such a measurement requires probing the proton at sufficiently small x and Q2 that saturation phenomena manifest. This has been the problem in past accelerator experiments, since they were unable to access the required small x and Q2. There are two ways to make these saturation phenomena accessible via experiments: to increase the energy in the collisions, by building new electron-hadron accelerators; or to use heavy ions instead of protons. When an electron collides with an ion at high energy, it does not see a ball of A nucleons. Rather, the ion appears Lorentz contracted into a “pancake”. At small x, the wavelength of the probe exceeds the size of the nucleus, such that the probe cannot distinguish between the individual nucleons. It therefore interacts with many nucleons at once, which has the effect of increasing the saturation scale, i.e. the value of Q2 below which saturation occurs, by a factor proportional to A1/3. On Sat, Apr 27, 2013 at 12:40 AM, Eric Walker <[email protected]> wrote: > On Fri, Apr 26, 2013 at 12:11 AM, Harry Veeder <[email protected]>wrote: > > A few experiments conducted before this showed >> ambiguous evidence: two protons emerged from the decay but one >> couldn’t tell that the protons had not been thrown out one at a time >> or both at the same time randomly from the whole Ne-18 or from a >> single lump. >> > > A diproton is something to think about. It gives rise to or is indirectly > related to the following novice questions: > > - In 3He, which is stable, electrostatic repulsion is felt between two > nucleons, and the strong interaction is felt equally between all nucleons. > In deuterium, which is stable, there is no electrostatic repulsion, and > the strong interaction is felt equally between both nucleons. In a > diproton, which is unstable, there is electrostatic repulsion, and > presumably the strong interaction is in affect to the same extent as > between the nucleons in a deuterium atom. Is the lack of stability of the > diproton due to a slight imbalance between electrostatic repulsion and the > residual strong force, or is it due to the combination of valence quarks > between the two nucleons not being quite right? > - Within a nucleon there are uncounted sea quarks and only three tiny > valence quarks, and these valence quarks are responsible for the residual > strong interaction with other nucleons. Why are there countless sea quarks > and exactly three valence quarks left over, above and beyond them? It > feels a little like having 3 in the numerator and infinity in the > denominator. What is the hidden structure that presumably keeps the > numerator and denominator in balance? > - A nucleus that decays, such as a diproton or tritium, is an unstable > nucleus transitioning to a more stable state. Perhaps there will be a beta > decay (tritium) or a fission (the diproton). Is there an analytical way to > work out the anticipated stability of a given set of nucleons in advance, > or is this something that can only be determined experimentally? > > Eric > >
