The Color Glass Condensate and the Glasma
By Larry McLerran
The
HERA accelerator some years ago measured the gluon distribution
inside of protons. The surprising result was that the measured
number of gluons inside a fast moving proton was rapidly rising
as the energy increased. On the other hand, the size of a proton
is a very slowly increasing function of energy. This means that
the gluon density has to become large.
In the theory of strong interactions, Quantum Chromodynamics (QCD), the intrinsic strength of interactions becomes very weak when typical distances scales are very small, and this happens as the density increases. So how can the interaction be strong enough to make all those gluons? The answer is that the forces add coherently. This is a little like gravity where the intrinsic strength of gravity is weak in proton-proton interactions, but becomes strong in the interaction of your body with the earth, because there are many proton-proton interactions which add together with the same sign. Examples of systems which have high density and are highly coherent are superconductors, Bose condensates and lasers.
Such a highly coherent gluon distribution arises as the yet higher energy gluons cascade down to low energies inside a hadron. These high energy gluons have their natural times scales Lorentz time dilated. This is communicated to the low energy gluons which then evolve very slowly compared to their natural time scales. Ordinary glass has this property of slow motion: It is a liquid which flows so slowly that it looks like a solid on ordinary time scales.
These observations led theorists to name this high energy density matter the Color Glass Condensate (CGC). The CGC explains not only many features of HERA electron-proton scattering, but also high energy proton nucleus and nucleus-nucleus interactions. The earliest data from RHIC on the total number of produced particles can be simply understood with ideas from the CGC.
Figure 1. Two sheets of Colored Glass after their collision. The fast gluons in the sheets have not yet fragmented, but have become dusted with color electric and color magnetic charge. This produces longitudinal electric and magnetic fields, shown in the figure.
In interactions of very high energy nuclei, two Lorentz contracted sheets of Colored Glass collide, and shatter. A surprise was that the nature of the Color Glass Condensate changes in the earliest stages of the collision: The two sheets of Colored Glass pass through one another and are dusted with color electric and color magnetic charge. This results in big color electric and color magnetic fields along the collision axis. These fields are also strong as a result of coherence. This matter is called the Glasma, because it is intermediate between the Color Glass Condensate and the Quark Gluon Plasma (QGP). The QGP takes some time to make after the collision, and the Glasma fields have to decay and thermalize before this happens. The space-time picture of this collision and the production of matter has a resemblance to the space-time picture of the big bang.
There are very many questions both theoretical and experimental about the nature of the Color Glass Condensate and the Glasma. Do these forms of matter really exist? How does the Glasma thermalize? How do we best measure their properties?
The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
funded by the U.S. Department of Energy Office of Science. Hundreds
of physicists from around the world use RHIC to study what the
universe may have looked like in the first few moments after its
creation. What physicists learn from these collisions may help us
understand more about why the physical world works the way it does,
from the smallest subatomic particles, to the largest stars.