February 25, 1997
Surprising New Particle Appears, or on the Other Hand, Maybe Not
By DAVID KESTENBAUM
hysicists in Germany may have detected a bizarre hybrid particle called a leptoquark that, if it turns out to be real, could topple the reigning scientific model of how the world is put together.
For two decades, particle physicists have sorted the smallest, apparently indivisible subatomic particles into two exclusive categories, quarks and leptons. Quarks are what protons and neutrons are made of. Leptons are particles like electrons and neutrinos. Together, leptons and quarks make up the atoms that form all of the material world.
The leptoquark -- half-lepton, half-quark -- has lived for almost two decades only in the dreams and equations of theoretical physicists as an object that would overturn the Standard Model, a theory that has been dominant for so long that a generation of physicists has known nothing else. The Standard Theory does not predict leptoquarks, but the tracks of a leptoquark may now have been seen in a particle accelerator in Germany.
"A leptoquark would have such enormous consequences for our understanding of physics that I don't dare to dream of it," said Dr. Ralph Eichler, a physicist at the Institut fur Teilchenphysik in Switzerland and a spokesman for one of two international teams, with more than 400 physicists each, that reported the leptoquark results on Wednesday at the accelerator, the Deutsches Elektronen-Synchrotron in Hamburg, Germany. The results of the work were submitted for publication Monday.
But the DESY physicists are wary of claiming too much. "What we're seeing might just be a random fluctuation," said Dr. Allen Caldwell, a physicist at Columbia University who is the spokesman for the other team, "like getting seven heads in a row when flipping a coin."
"We won't really know until we take another year's worth of data," Caldwell said.
The new results, the teams say, could also be evidence for new particles within the quarks, an equally exciting possibility. Dr. Guido Altarelli, a physicist at the CERN laboratory in Switzerland, said the data could also be interpreted as evidence for an even heavier particle, a leptoquark or something else, that has not yet been observed.
The Standard Model describes the world from the ground up, from the architecture of atoms to the reactions that power the sun. Although it does not include gravity, it is the closest thing physicists have to a "theory of everything."
"I'm positive the Standard Model is incomplete," said Dr. Joseph Lykken, a physicist at Fermilab in Batavia, Ill., in an interview. "It just leaves too many questions unanswered." He added, "I expect we will find a crack in it sometime this decade."
A new particle like the leptoquark would be such a crack. Leptoquarks arise, Lykken said, in theories that try to unite leptons and quarks into a larger, simpler structure. The Standard Model will not fall so much as it will be consumed by a more complete theory, he predicted.
"If the leptoquark is real," Lykken said, "it will really be a shock to the whole community."
At the DESY accelerator, a ring of magnets accelerates protons and positrons (the electron's antimatter counterpart) to close to light speed and has them collide. The magnets occupy an underground tunnel nearly four miles in circumference. The DESY instrument is the only proton-positron collider in the world, and it is ideal for making leptoquarks, if they exist.
In most collisions, the positron bounces off the quarks within the proton, which breaks up to form a shower of other particles. The two research teams, which include collaborating groups from 19 countries, monitor the collisions at different points around the ring, each using a house-sized particle detector weighing thousands of tons.
After examining the data from millions of collisions over the last few years, the teams found that some positrons had emerged at surprisingly high energies and large angles -- a greater number than predicted by the Standard Model. In these events, Caldwell said, the positron "almost makes a U-turn." He added, "We only expect very few of these events per year."
One explanation for that would be the momentary formation of a leptoquark. The leptoquark would have survived only a fraction of a second before decaying back into a positron and a quark.
Because there were only about 10 of these collisions, the teams said it was impossible to draw definite conclusions about what they might be. The odds that those collisions were produced them by chance, they said, is only about 1 in 200. But by particle-physics standards, that is still well short of the 1-in-10,000 standard usually required to establish firmly that a new particle has been detected.
The two DESY research teams analyzed their data in isolation and did not compare their results until last week, and they were excited to find that they had a lot in common. "We thought we were alone," said Dr. Guenter Wolf, a physicist at DESY, "but then we found they had seen the same thing."
If the presence of a leptoquark is established, it would be the heaviest subatomic particle yet observed, weighing almost as much as a lead atom. Previous experiments searching for leptoquarks had not observed any but could have missed one this heavy.
But DESY physicists are choosing their words with lawyer-like caution, saying leptoquarks are only one possible explanation for the data. The history books contain more than a few reports of discoveries that turned out to be false.
DESY will begin collecting more data in March. By that time, the physicists will be able to rule out or confirm that there has been an excess of strange collisions. If leptoquarks are real, physicists say, they should also turn up at other accelerators. Fermilab's Tevatron would probably be the first to detect such a particle when it resumes those types of experiments in 1999.
Other Places of Interest on the WebOverview of Research at DESY, the Deutsches Elektronen-Synchrotron in Hamburg (all pages in English) Physics at HERA Recent Results from HERA Speculation on leptoquarks from ZEUS analyses
Copyright 1997 The New York Times Company
