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Wild BunchFirst five-quark particle turns up

Physicists on three international teams haverecently spotted what’s most likely a long-sought subatomic particle known as a pen-taquark. It contains five components—fourquarks and one antiquark—which areamong the most fundamental bits of mat-ter yet known. No subatomic particledetected previously contains more thanthree of those building blocks.

“After 30 years of failing to find any con-vincing evidence for something that oughtto be there, this recent news is certainly metwith excitement,” says nuclear physicistAndrew M. Sandorfi of BrookhavenNational Laboratory in Upton, N.Y.

Although unusually complex, the new-found particle fits within the prevailing the-oretical framework of particle physics,known as the standard model. The newlydetected bit may be just the first memberof a family of pentaquark particles. The findalso underscores the possibility of discov-ering particles with four or six quarks.

To fathom how five quark components cancoexist within one particle, theorists expectto reconsider their models of the interactionsamong quarks and gluons, the particles thatbind quarks together. For instance, it’s pos-sible that the fivefold structure is not a spher-ical lump but rather a moleculelike arrange-ment in which a so-called kaon, which ismade of a quark and an antiquark, orbits aneutron, which is made of three quarks.

“Exactly what form of the theory makesit work now becomes very interesting,” saystheorist Peter D. Barnes of Los Alamos(N.M.) National Laboratory.

If the pentaquark can exist in labs today,it probably also was present in the very first,fiery moments of the universe, says TakashiNakano of Osaka University, leader of theteam that found the pentaquark at Japan’sSPring-8 synchrotron in Hyogo.

The experiments that appear to havebagged the elusive pentaquark weren’tdesigned to look for it. “This is an exampleof serendipity,” says Kenneth H. Hicks ofOhio University in Athens. He’s a member

of both the SPring-8 team and a group atthe Thomas Jefferson National Accelera-tor Facility in Newport News, Va., that hasconfirmed the SPring-8 finding.

In 1997, Russian theorist Dmitri Diakonovand his colleagues predicted the existence ofa pentaquark with a mass about 50 percentheavier than that of a hydrogen atom. Thetheorists then urged the SPring-8 team toreexamine data from an experiment in 2001that might have inadvertently made theparticle when it had gamma rays striking apiece of plastic.

When Nakano, Hicks, and their cowork-ers combed that data, they in fact foundsigns for about 20 pentaquarks. The teamis slated to present its evidence in anupcoming Physical Review Letters.

Inspired by the SPring-8 findings,researchers at the Jefferson lab and the Insti-tute of Theoretical and ExperimentalPhysics in Moscow double-checked old datafrom different sorts of particle collisions andnetted their own pentaquark candidates.

Besides its unprecedented quark count,a pentaquark is unusual in that it includesan exotic antiquark, the so-called strangeantiquark. The composite particle also con-tains two up quarks and two down quarks—the same ones found in ordinary matter.

Nakano says the SPring-8 team is nowanalyzing data from new collisions that weregenerated explicitly to make pentaquarks.The aim is to nail down the identity andproperties of this new entity. Meanwhile,the Jefferson lab has approved an experi-ment intended to boost its pentaquark pro-duction 20-fold, Hicks says. —P. WEISS

A Matter of TasteMutated fruit flies bypass the salt

People’s fondness for salty snacks reflectsa fundamental biological imperative. “Allcells, in order to survive, need salt,” says LeiLiu of the Howard Hughes Medical Insti-

tute at the University of Iowa in Iowa City.To keep themselves supplied with this crit-ical nutrient, animals have developed waysfor sensing sodium chloride and other salts.

By creating mutant fruit flies with animpaired capacity to taste salt, Liu and hiscolleagues have now identified several genesthat contribute to this crucial sensory sys-tem in insects. Liu suggests that the flyresearch could provide insights into howpeople taste salt. It may even lead to aneffective salt substitute to fight high bloodpressure and other conditions exacerbatedby today’s salt-rich diets, he speculates.

When it comes to salt, the fruit flyDrosophila melanogaster acts much like aperson. Food or water sources with too muchsalt repel the flies, but those with low con-centrations attract them. The flies can evendistinguish between sodium chloride, whichis typical table salt, and potassium chloride,which tastes even saltier to people.

To investigate how flies sense salt, Liu andhis colleagues drew upon past work indi-cating that certain cellular pores, known asepithelial sodium channels, act as salt recep-tors in mammals. These pores, throughwhich sodium ions flow into cells, are foundon rodents’ taste buds, for example.

Yet not all biologists are convinced thatthis sodium channel is a widespread salt-sensing receptor. For one thing, eventhough the sodium-channel-blocking drugamiloride impairs salt taste in rodents andsome other animals, this effect isn’t consis-tent among all species or even among dif-ferent strains of rodents. Moreover, no onehas shown clearly that the human tonguehas this particular sodium channel, notesSue C. Kinnamon of Colorado State Uni-versity in Fort Collins.

Scientists sometimes investigate the roleof an ion channel by inactivating the genesencoding the channel’s protein constituents.In mice, however, this particular sodiumchannel appears to do more than mediatesalt taste. Mice lacking the channel die asembryos or just after birth.

Seeking to do a similar gene-knockoutexperiment in fruit flies, Liu and his col-leagues identified insect genes that closelyresemble the mammalian ones that

HIGH FIVE Gamma rays hitting neutrons in carbon nuclei appear to have created exoticpentaquarks, which quickly shatter into neutrons and kaons.

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