When the possible existence of an exotic new particle called a leptoquark was announced earlier this year by researchers in Germany, other physicists rushed to try and corroborate their results. But an extensive search at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, for evidence of similar leptoquarks has come up empty.
The negative search does not strictly rule out the possibility of these exotic hybrids between the two basic kinds of matter, say the Fermilab physicists who reported their results at a small meeting at Vanderbilt University in Nashville, Tennessee. But the analysis "leaves almost no room for the simplest leptoquark solution," says Herbi Dreiner, a theorist at the Rutherford-Appleton Laboratory in the United Kingdom.
The original announcement came in February from physicists at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany (Science, 28 February, p. 1266 ). Two detectors at DESY's HERA accelerator, which smashes antimatter particles called positrons into protons, had recorded more "hard," or violent, collisions than expected under physicists' current theory of the fundamental structure of matter, called the Standard Model. One possibility was that the collisions were spawning a particle that combines the properties of quarks--the building blocks of the proton--and leptons, such as the positron and the electron. By briefly materializing, then decaying in a spray of ordinary particles, a leptoquark might explain the seemingly violent collisions.
"The HERA results came out, and we really moved fast," says Henry Frisch of the University of Chicago, co-convener of CDF's "exotics" groups. The group then began to analyze data on the debris from 3 trillion collisions in Fermilab's Tevatron particle accelerator, which smashes together protons and their antimatter counterparts, antiprotons. These collisions might sometimes produce pairs of leptoquarks, which would then decay into ordinary particles, leaving a recognizable signature. That signature wasn't seen, says Frisch.
So the HERA group is still mulling over its anomalous collisions. "We'll continue to compare our ... data to Standard Model predictions" in search of the answer, says Bruce Straub of Columbia University, a HERA collaborator. The answer, when it finally comes, could be as mundane as minor tweaks in physicists' understanding of proton structure--or as electrifying as another new particle.