Gluons in our future
July 30, 2014
If one were to eavesdrop on the conversations that we have with our visitors – students of all kinds, politicians or the general public – one might come to the conclusion that we here at Jefferson Lab are crazy about glue/must run a glue factory. It is, indeed, the goal of one of the major experiments that we are to pursue following the upgrade to attempt to measure particles that can tell us more about glue. But what sort of glue are we referring to?
In the ‘60s, theoreticians trying to construct a theory of strong interactions postulated that the putative colored quarks, not yet discovered, interacted by the exchange of vector bosons called gluons. We can describe the gluons as being much like photons. Electromagnetic interactions can be described as proceeding by the exchange of photons. Two electrons interact by passing photons between them. This is analogous to two ice hockey players interacting by one of them passing a puck to the other.
But there is a vital difference between photons and gluons. Photons are electrically neutral, so their interactions with each other don’t play much of a part in the world (Our theorist friends say they are only higher-order corrections). Not so for gluons. They have color (strong interaction) charge just like quarks. So they can interact with each other. We think that the quarks are bound and confined in hadrons (mesons and nucleons) by the gluon exchanges. If that is true of quarks, it should also be true for gluons. So are there hadronic states that are made entirely of gluons? Are there hadronic states that have the characteristics (quantum numbers) they have because they have bound gluons in their genes? This is the search that will be led by the GlueX experiment in our new Hall D.
As an aside, some of you may have noticed a little grey Miata with the license plate GLUON8. At least in some states, adding a number to a personalized license plate reduces the cost. But given that the owner, Mike Pennington, is head of our theory team and one preoccupied by strong interactions, I am sure that the "8" is a cute addition indicating that from the symmetries of the strong interaction, we have an octet of gluon states.
The gluons that were theorized in the ‘60s were electrically neutral, and thus are not directly accessible via the electromagnetic scattering for which we are famous, but electron scattering would start to give us hints of them. In 1968, the first deep-inelastic, electron-scattering experiments were performed at SLAC and DESY, and the SLAC data were interpreted as the manifestation of the quarks. From these experiments, one can deduce the total fraction of the momentum of a proton that is carried by the quarks. The answer was “about 50%”; the other half was carried by gluons. The magnitude was quite surprising, but it would not be the last surprise about the gluons.
In the quark parton model, the parton distributions of the quarks are independent of the violence of the interaction; they have no structure. However, if they emit gluons, then we expect that the distributions would be modified as the interactions become more violent. This “scale breaking” was another manifestation of the presence of gluons, which was observed in the mid-‘70s using electron, muon, and neutrino scattering.
Now, if gluons are emitted by quarks, or if they appear in hadron decays, there were a few processes in which the effects should be apparent, even if the gluons do not appear as free particles. Several groups found these effects in the late ‘70s. The prizes went to experimenters working on electron-positron colliders (Working on one of the other experiments, I was not so happy with that, but you can't win them all.)
Earlier, we mentioned that using electron scattering, we can see the momentum distributions of the quarks, and through measurements of the evolution of those distributions, we can deduce the gluon momentum distributions. In the ‘90s, the experiments H1 and ZEUS at the HERA (DESY) electron-proton collider found, again surprisingly, at least at the time, that at low momentum fractions, the gluon distribution grew dramatically. Changes in the parton (quark and gluon) distributions can come about from emission of gluons by quarks. But if the gluons can interact with each other, they must recombine as well as split. So this increase observed at HERA cannot go on forever. We say that eventually there must be saturation.
All this action happens when the momentum fractions carried by the quarks or gluons are small, less than 10% of that of the target nucleon. Above 10%, we really feel that our 12 GeV upgrade is in the cat-bird seat, but below 10%, we need more energy. So, we are designing the Electron Ion Collider. We intend to build a hadron accelerator for the protons and nuclei, which we will collide with our 12 GeV electrons coming from CEBAF. The initial version of the collider is called the Medium energy Electron Ion Collider (MEIC).
The collider would extend beyond the current Jefferson Lab site, so we have been working with the City of Newport News, as well as the Oyster Point and Tech Center developers, to ensure a bright future for Jefferson Lab, so all of us may co-exist comfortably when the new collider is built.
Maybe the gluons will hold together our future as well as the quarks in a hadron??