Physics Analysis

HERA Physics

HERA collides two fundamental constituents of matter, the electron (or its anti-partner, the positron) with the proton. During the collision, the highly energetic electron emits a virtual photon which probes structures deep inside the proton. The process is therefore called "deep inelastic scattering" (DIS). In most of these collisions the proton breaks up into a multitude of particles. In the past, the study of DIS reactions at SLAC led to the discovery of quarks and of asymptotic freedom. This, together with the later observation of the gluon in electron-positron collisions at DESY established QCD as the theory of Strong Interactions.

The HERA experiments extend the range of DIS measurements by several orders of magnitude in the variables, x and Q². x denotes the fraction of the proton momentum carried by the interacting quark and Q² denotes the virtuality of the photon emitted by the incoming electron. The main QCD result of HERA is the observation, for the first time in DIS, of the rapid rise of the proton structure function with decreasinq x and of a substantial amount of diffractive scattering. Another important measurement is the direct determination, in one experiment, of the decrease of the strong coupling constant α_s with increasing Q², i.e the direct observation of asymptotic freedom.

Today, QCD is the well established and experimentally tested theory of strong interactions. However, the understanding of QCD is excellent in the short distance limit only, i.e. when quarks and gluons are close to each other. At larger distances, QCD predicts confinement, a property of the strong force which prevents quarks and gluons from ever leaving the proton and being observed "naked". How confinement shapes nuclear matter around us is, however, barely understood.

The observation of the quickly rising proton structure function together with diffraction at large Q² (or small distances) together with the measurements at small Q² (or large distances) opens new approaches to understand QCD. These phenomena are presently the focus of theoretical investigations. The aim is to determine properties of the gluon and quark emission processes, i.e. QCD evolution. These studies also deal with high density effects and their dependence on the interaction size.

In the "naive" picture the proton is made up of up and down quarks. Other quarks such as strange, charm or beauty are not considered "constituents" of the proton. The HERA data indicate and QCD indeed predicts the presence of "dynamically generated" heavy quarks in electron-proton collisions. Therefore, how they appear and at what rate provides stringent tests of QCD and our understanding of proton structure.

Last but not least, the high center-of-mass energy at HERA reaches to the limits of our current knowledge of the fundamental interactions in nature. The search for supersymmetric particles, exotic leptoquarks, right-handed currents and many other even more esoteric possibilities is a constantly ongoing enterprise.