|Antiproton-proton scattering by the strong interactions can be non-diffractive (left) or diffractive (right). Both original particles, the proton and antiproton, are colorless.|
At the Fermilab Tevatron, protons and antiprotons were brought into collision at very high energies, equivalent to about 2,000 proton masses according to Einstein’s equation, E=mc2. In each collision, about 100 particles of different types are produced.
A small group at CDF has been studying what scientists call the diffractive production of jets, in which “ghost” particles help create these sprays of highly collimated particles. Exactly how are they produced?
The proton and antiproton each consists of three quarks bound by the strong force. Though the proton and antiproton are free to move inside a “bag” full of gluons and quarks, the gluons and quarks themselves are confined to each other in order to maintain something called color-neutrality.
Diffractive collisions, in the simplest case, are characterized by an outgoing antiproton, a region in which there are no particles (called a rapidity gap) and a particle cluster corresponding to the initial proton. The particle cluster is shown as the white circle in the top figure.
This kind of collision can be explained by the color-neutral exchange of a particle called a pomeron. With its vacuum-like properties, a pomeron can escape invisibly out of the quark-gluon bag like a ghost, strike the passing proton and give it an energy injection by allowing itself to be absorbed by the proton. The energy is used to create jets that faithfully obey the equation E=mc2.
The results of this experiment can be explained by a model (called DL in the figure below) at low-momentum transfers (t) between the incoming and outgoing antiproton by way of the escaping pomeron. However, the model does not explain the result for high-momentum transfers, where the data is constant. It will be interesting to see how the theory can be adapted to the high-momentum data.
These measurements are being repeated at the higher energies of the LHC to provide more discrimination among theoretical models.
|A scintillator fiber tracker (RPS) is used to observe diffractive events as a function of the momentum transfer between the incoming and outgoing antiproton.|
|These physicists were responsible for this analysis. From left: Michele Gallinaro, Dino Goulianos and Koji Terashi, all from Rockefeller University.|