Signs of new five-quark particle.

Physicists have long wondered why no elementary particle discovered so far contains more than three of the fundamental building blocks known as quarks. Several recent findings have suggested particles with greater quark numbers (SN: 12/13/03, p. 381), but other data haven't supported some of those findings.

Now, some physicists at the HERA par title collider at Germany's Deutsches Electronen-Synchrotron (DESY) laboratory in Hamburg have unveiled evidence for a five-quark particle--only the third such pentaquark tentatively sighted. But other researchers at DESY looking at other data say they detect no trace of the purported pentaquark.

At HERA, smashups between protons and either electrons or positrons create many particles harboring exotic quarks known as charm quarks. Theorists had predicted the existence of pentaquarks containing charm quacks, but the first two pentaquark candidates didn't include charm quarks.

Because charm-quark-containing particles are so readily produced at HERA, researchers belonging to what's known as DESY's H1 team decided to seek evidence of pentaquarks containing charm quarks in data from past HERA runs. Such retrospective searches have uncovered both previous hints of pentaquarks (SN: 10/18/03, p. 245).

Combing data from millions of collisions that occurred between 1996 and 2000, the H1 researchers found evidence for about 50 appearances of a pentaquark containing a charm quark, says team spokesman Max Klein. The team will report its findings in an upcoming Physics Letters B.

Another DESY team, called Zeus, looked fruitlessly for pentaquarks in a different set of records of similar collisions in the HERA collider. Scientists know too little about how pentaquarks might form for the Zeus team's findings to dismiss the H1 team's interpretation of its particle collisions, Klein says.