[P L A N E T   S C I E N C E]

         May the fifth force be with us?

 [Image] AN EXOTIC heavy particle may have made its debut
         at a particle accelerator in Hamburg.
         Researchers say it could mark the birth of an
         entirely new physics. If the so-called
         "leptoquark" exists, it implies that a
         previously unknown force is at work, one that
         may open the way to a unified theory of all
         nature's forces.

         The results from DESY, the German Electron
         Synchrotron, could signify one of several
         things, the researchers believe. They could
         represent a curious marriage between quarks and
         leptons, the two families of fundamental
         particles which make up all matter.
         Alternatively, they could mean that quarks and
         leptons are not fundamental particles after all,
         but are made up of even smaller particles.

         "If these results turn out to be true, this
         discovery would be on a par with the discovery
         of the electron or DNA," says Robin Marshall of
         the University of Manchester, who was involved
         in the work. "It would be the most breathtaking
         physics of my career."

         The findings have emerged from ZEUS and H1, two
         experiments that have been running at DESY over
         the past four years. Physicists have been
         monitoring high-speed collisions between
         anti-electrons or positrons, from the lepton
         family of particles, and protons, which are made
         of quarks. Detectors pick up the showers of
         particle debris thrown out when a positron hits
         a quark in a proton, or one of the "gluons" that
         bind the quarks together.

         Existing theories about the four known forces in
         nature predict how the particle debris should
         emerge from the collisions. Physicists would
         expect an occasional head-on collision from
         which a positron and a quark recoil at sharp
         angles with enormous energy. "The events are
         spectacular," says Roger Cashmore of the
         University of Oxford, a member of the ZEUS team.

         Both DESY teams could have expected just one of
         the most violent collisions to occur during each
         of their four-year experiments. Instead, they
         have seen around four in each. One explanation
         for this frequency, they say, is that a positron
         teamed up with a quark inside the proton to form
         a leptoquark before spitting out a high-energy
         positron as it decayed.

         "The leptoquark is a bizarre object that we
         don't understand completely," says Marshall.
         Because none of the four known forces in
         nature--gravity, electromagnetism and the strong
         and weak nuclear forces--would be capable of
         holding together a lepton and a quark, a new
         force must be at work. The existence of a fifth
         force would revolutionise physics.

         Such a force would be welcomed by physicists
         searching for grand unified theories which can
         explain how all the forces in nature acted as
         one just after the birth of the Universe. Some
         unification theories rely on the existence of a
         mysterious force that could have glued together
         quarks and leptons in the fireball of the big
         bang.

         Other explanations are possible, say the
         researchers. A positron could briefly have stuck
         to a gluon within a proton to create a
         "leptogluon". This would also imply that a new
         force was at work. Alternatively, quarks or
         electrons may have some kind of substructure,
         such as families of yet smaller particles inside
         them. Cashmore says the violent scattering might
         suggest that positrons are occasionally striking
         one of these smaller particles inside a quark,
         for instance, and bouncing back sharply.

         A less likely theory is the appearance of a
         particle from the "supersymmetric" world.
         Supersymmetric theories suggest that every
         particle in nature has a partner in a mirror
         world. If they exist, these mirror images would
         also help to unify nature's forces. The prime
         suspect in the DESY experiments would be the
         supersymmetric partner of the top quark,
         according to theorist Joe Lykken of Fermilab,
         near Chicago. "But it would be unlikely to decay
         back into a quark and positron," he adds.

         The researchers warn that the effect may turn
         out to be a mirage. Predictions about the number
         of these violent collisions that could be
         explained by normal physics are merely
         probabilities. Although the chances are
         tiny--about 1.4 per cent--it is possible that
         everyday physics has dealt this improbable hand.
         "You have to ask if it's simply because God is
         being unkind to us," says Cashmore.

         The uncertainty may vanish over the next year,
         however. ZEUS and H1 will start again in March,
         and researchers expect to double the amount of
         data they have gathered by the end of the year.
         If they manage to record around 20 of the most
         violent collisions, they may be able to confirm
         why the surplus occurs. "But we may be stuck
         with this until the next generation of particle
         accelerators is running," says Marshall.

         Hazel Muir

         From New Scientist, 1 Mar 97

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