Dr. Winston Roberts, Hadron Physics

Dr. Winston Roberts

Hadron Physics

Let us begin by examining the figure on the left that represents the 'spectrum' of the hydrogen atom. In this spectrum, each line represents a state or 'energy level' in which the hydrogen atom can exist, and the arrows indicate possible transitions between these energy levels. When a hydrogen in one energy level makes a transition to a lower energy level, it emits energy in the form of photons, and the wavelength (or color) of the emitted light is completely determined by the energy difference between the two levels. The spectrum of light shown below arises because the atom 'falls' from various higher states to the second lowest state. The lowest state is called the ground state.

Atomic physicists study the properties of atoms. This means that they seek to understand the observed spectra of atoms (such as the hydrogen spectrum discussed in the previous paragraph). They try to understand all aspects of this spectrum: where the energy levels are, how quickly do the transitions between different energy levels take place, and how all of this can be understood in the theory that describes the electromagnetic interaction, Quantum Electrodynamics or QED.

In some sense, hadron physicists try to do for hadrons what atomic physicists do for atoms. There are a number of similarities between the two fields, but some significant differences as well. Just as there can be many 'excited states' of an atom (an excited state is any state above the ground state), so, too, can there be many excited states of hadrons. The proton and neutron are the ground state, and there is a large family of excited nucleons. One can draw a spectrum of 'lines' representing the excited states of hadrons, but unlike the spectrum of hydrogen, or any other atom, this spectrum would not have a 'maximum' energy like the hydrogen spectrum, but the energies of the states would keep rising, perhaps forever. An example of what part of the spectrum of excited nucleons is expected to look like is shown on the right. Many more states are expected to exist at higher energy.

Just as in atoms, there can be transitions among the states in the hadron spectrum. Many of these transitions involve the emission of photons, just as with atoms. However, hadrons can also emit other hadrons in their transitions. For instance, many of the states in the nucleon spectrum shown would have transitions to the ground state nucleon with the emission of a meson, such as a pion (&pi) or eta (&eta), or even a rho (&rho), which is an excited version of the pion. In addition, in contrast with atoms, the time scale for many of these transitions is very short, so short that the existence of the excited hadrons can only be inferred. This complicates the task of hadron physicists.

One of the main difficulties in hadron physics is that it is not yet known how to perform realistic calculations starting from Quantum Chromodynamics (QCD). At present, the only methods that are closely related to QCD are the 'lattice' simulations. In these simulations, the continuum of space, as well as that of time, are divided up into discrete points. The simulations on the page that discusses color were obtained using lattice calculations.

But why does it matter that we do not (yet) know how to calculate the properties of the spectra of QCD? First, QCD is unique, in that it is the only interaction that leads to confinement. Second, confinement is responsible for about 99% of the matter that we see around us. The u and d quarks have masses that add up to about 1% of the mass of the proton or neutron. This means that 99% of the mass of a nucleon comes about because the quarks are confined. Hadron physicists are working to understand how confinement comes about, and how it generates the mass of the nucleon. In addition, we are trying to understand the excited states of hadrons, and whether states like the pentaquark and other exotic hadrons, can exist, and what their properties might be.