The HyperCP Experiment

The HyperCP experiment (Fermilab Experiment E-871) seeks to shed new light on the mysterious matter/antimatter asymmetry known as CP violation. The only effect known that distinguishes matter from antimatter in an absolute sense, CP violation has been postulated (by the Russian physicist Andrei Sakharov) to have played a key role in how the Universe came to be made of matter rather than antimatter or pure energy. The HyperCP experiment is carrying out a detailed and precise comparison between the decay properties of the unstable subatomic particles known as hyperons and their antiparticles. (Hyperons are particles composed of three quarks, at least one of which is a strange quark.) The experiment is sensitive to hyperon/antihyperon differences down to about one part in 10,000. If a difference is detected, it will be evidence for new types of CP violation that may have contributed significantly to the evolution of the matter Universe in which we live.

A schematic diagram of the experimental apparatus is shown here. The 800 GeV proton beam from Fermilab's Tevatron accelerator comes in at the left of the diagram and strikes a metal target in which the hyperons are produced. A curved magnetic channel selects hyperons of the desired momentum. A large fraction of the hyperons decay within the 13-meter vacuum pipe, producing "daughter" particles that traverse the multi-wire particle detectors labeled C1 through C9. The detectors measure the particle trajectories to sub-millimeter precision, allowing the decay properties of the hyperons or antihyperons to be accurately determined. Illustrated on the diagram is a typical decay of a Xi hyperon into a Lambda hyperon and a pi meson, followed by the decay of the Lambda into a proton and a pion. Every few hours of running, the polarities of the Hyperon and Analysis electromagnets are simultaneously reversed to switch between hyperon and antihyperon operation.

The experiment is being conducted by a group of about 35 physicists and students from 9 institutions. The data-taking runs occupied several months during 1997 and 1999. A data acquisition system of unprecedented bandwidth (to which the IIT group made major contributions) allowed the recording on magnetic tape of a sample of some billions of hyperon and antihyperon decays. The data are being analyzed on massive parallel-processing workstation arrays (or farms). The main analysis passes on the 1997 and 1999 data samples each took about a year. Extraction of physics results from both samples is now in progress.

Extensive detail on the HyperCP experiment may be found in the Fermilab proposal, a 450-kB PostScript file, supplemented by a 23-MB PostScript file of figures. The proposal detailing the second run may be found here.