The CDF Collaboration

April 2003

A collision between a proton and an antiproton at the Fermilab Tevatron Collider is usually a spectacular event, in which about 100 new particles are produced flying outward from the interaction point into all directions. It is a perfect example of energy converting into mass! If a two-dimensional photograph could be taken from a 90 degree angle relative to the direction of the colliding beams, an event would look like a beautiful fireworks explosion. Such photographs can actually be produced by a computer display of data collected by detectors that sense and record the position of each particle as it moves through the detector volume surrounding the interaction point. If the tracks of different particles are represented by different colors, a slide show of collision events rivals the best pyrotechnics show, particularly since the particles form helical patterns as they curl up in the strong magnetic field usually applied within the detector volume.

However, unlike the patterns in fireworks explosions, there is a substantial fraction of collision events in which one or more large angular regions, as measured from the beam direction, have no particles at all! In fact, about 25% of events consist of just two outgoing particles, the original proton and antiproton, which are elastically scattered into small angles. Such collisions are thought to be caused by the exchange of a "pomeron" between the proton and antiproton.

The pomeron is a hypothetical particle, whose name honors the Russian physicist Pomeranchuk for his theoretical advances in high energy scattering in the 1960's. In our current theory of strong interactions, Quantum ChromoDynamics (QCD), the pomeron is thought to consist of quarks and gluons, called partons, in a combination that has the quantum numbers of empty space (vacuum). Partons carry a "color" quantum number, which is the strong interaction analogue of the electric charge of the electromagnetic (EM) force. When charged particles are accelerated, they emit radiation. In QCD, the radiation of a color-carrying parton consists of particles, mostly pi-mesons. The partons in the pomeron are in a colorless combination, which does not radiate when exchanged, explaining how it is possible to have elastic scattering at very high energies without any production of other particles. Elastic scattering is called "diffractive", because the angular pattern formed by a beam of particles scattered off a target particle is similar to the well known diffraction pattern of photons scattered by a small black disc.

Pomeron exchange can also lead to processes in which one or both of the colliding particles is excited into a high mass object by the energy absorbed from the pomeron and immediately dissociates into a bundle of particles. In our two-dimensional pattern of such collisions, the particles from proton or antiproton dissociation will cover an angular region in the forward or backward proton direction, or in both these regions if both the proton and antiproton dissociate, while the rest of the event is empty of particles. In technical jargon, the angular areas without particles are called "pseudorapidity gaps" or "rapidity gaps." The events with gaps look like elastic scattering between particles and/or particle clusters and they are all referred to as diffractive. What they have in common is that they are all produced by pomeron exchange.

Although diffraction has been under study since pre-QCD time, the partonic nature of the pomeron and the mechanism for diffraction are not yet well understood. There are two obstacles to interpreting the results of experiments on diffraction. First, the formation of the rapidity gap is a QCD area which cannot be reliably handled by the QCD techniques developed to deal with partonic collisions in which large momentum is exchanged in the transverse direction; and second, a rapidity gap formed by pomeron exchange may be filled by particles produced by another exchange between colored partons, which would radiate particles in all directions. The probability that no such exchange takes place represents the "survival probability" of a diffractive gap.

In a previous experiment, we measured the fraction of events with a central rapidity gap in proton-antiproton interactions at collision energies of 630 and 1800 GeV. The fraction of events with a gap at the lower energy was found to be higher, presumably because of a larger survival gap probability due to the smaller number of available partons that could be exchanged and fill the gap. While this general trend was expected, the calculation of the survival probability remained a controversial theoretical issue.

In the present experiment, we studied events with two rapidity gaps, one on the antiproton side and the other in the central region of the particle bundle recoiling against the antiproton. We actually selected events with a forward (leading) antiproton, which guarantees the presence of a rapidity gap associated with it.We then argued that the presence of this gap on the antiproton side is proof that no color-parton exchanges occurred in the event, which would have filled this gap. Therefore, the formation of the central gap must also be free from survival probability effects. To quantify these effects we measured the fraction of events with a central gap in the selected events and compared it with the same fraction measured in events with no forward gaps (minimum bias events). We found that the fraction is larger in the events with a leading antiproton, as expected.

Our results provide the opportunity to theorists to compare with data calculations of the ratio of two-gap to one-gap rates without the complications arising from survival probability effects. It is our hope that these results will advance the understanding of the QCD nature of diffraction, which is intimately related to the partonic structure of hadrons and to the puzzling question of quark confinement.

A copy of the paper is available from hep-ex/0101036.

For further information contact Dr. Konstantin Goulianos or Dr. Mary Convery