When protons strike a glancing blow

The Forward Proton Detector measured protons and antiprotons that scattered at extremely small angles with respect to the beam.

Not every interaction between a proton and antiproton at the Tevatron was a head-on collision. Some collisions would break apart one or both of the particles, perhaps creating rare forms of matter like top quarks or possibly Higgs bosons. However, elastic collisions did often occur. In these, both the proton and antiproton are left intact and no additional particles are created. The interaction rate of these glancing blows between a proton and antiproton cannot easily be calculated in the Standard Model. Experimental measurements of these interactions place important constraints on the models used to describe elastic collisions.

After an elastic collision at the Tevatron, the proton and antiproton would continue to travel nearly parallel to their own proton or antiproton beam. The scattered particles would leave the DZero collision hall while still within the beam pipe that bores through the middle of the detector, invisible to the active readout elements. To detect elastic collisions,the DZero collaboration installed special detector arrays within the beam pipe surrounding the DZero interaction point at four locations, at 23 and 31 meters in either direction down the beam line. During data taking, the elements of the Forward Proton Detector were inserted to within a few millimeters of the beam in order to detect elastically scattered protons and antiprotons.

Analyzers used the Forward Proton Detector to study the rate of elastic collisions between protons and antiprotons. The data used in this analysis were collected with dedicated Tevatron beam conditions, where only a single proton bunch and a single antiproton bunch were injected into the accelerator and brought into collision. This analysis extended measurements of the proton-antiproton elastic cross section to higher momentum transfers than ever before at the full Tevatron collision energy. The results also demonstrate a significant change in elastic collision rate past a certain amount of momentum transfer, consistent with the expectations from elastic collision models.

—Mike Cooke

These physicists made major contributions to this analysis.

As the databases coordinator, Jim Linnemann (Michigan State University) works with the Fermilab Computing Sector to maintain and ensure reliable access to information vital to the success of the DZero experiment, from detector calibration, trigger and luminosity data to slides and proceedings from conference presentations.