By Mike Albrow, June 2006
A new type of particle reaction has been observed in CDF: The colliding proton and antiproton fly through losing only 0.5% of their energy, which appears in the central detector as a pair of high energy photons, or gamma rays. The unusual, and rare, feature is that no other particles are produced in the collisions. Only three events have been detected, but they are very clean, with a background of at most 0.2 events. The search for this unusual process began in 2001, when some CDF physicists explored the possibility that the Higgs boson, if it exists, could be produced by an almost identical process. Theoretically the Higgs field fills the vacuum, and it should be possible to “excite the vacuum” into a real Higgs particle in a glancing collision of a proton and antiproton. Some theorists had tried to estimate the probability of this happening, but there was a very wide spread in their predictions. Some would even have allowed the Higgs to be found this way at the Tevatron, if the scattered protons could be well measured. It was important to test these calculations, and it was realized that the same process should give events with only two photons, which could test the theory and could perhaps be seen at the Tevatron. Physicists in CDF were able to install a “trigger” to select and record such events. Only one theory group, at Durham University, calculated this and predicted such a small production rate that only about 1 event should be found in a trillion collisions. The three events found by CDF agree with this prediction. This means that the similar process by which a Higgs boson can be produced with no other particles must also happen (if the Higgs boson exists), and could be seen at the LHC (but not at the Tevatron!) this way if the scattered protons can be measured. Then the Higgs boson mass, spin and other properties could be measured, and there is an R&D project investigating this.
The same data sample also contained 16 events with high momentum electron-positron pairs (and no other particles), agreeing with a theory (Quantum ElectroDynamics) in which photons from the electric field of the protons interact. This is the first time photon-photon collisions have been seen in a hadron collider.
The analysis was the thesis topic of Andrew Hamilton (Univ. of Alberta).