Twice-charmed particles spotted?

Physicists have spied hints of two new members of the family of subatomic particles that includes the proton and neutron. Known as baryons, all such particles contain a trio of more fundamental particles, called quarks.

Although protons and neutrons are made of only so-called down and up quarks, researchers at high-energy particle colliders have for years been detecting exotic baryons containing beefier quarks called strange and charm quarks (SN: 8/25/01,

p. 116: Window Opens into Strange Nuclei). The scientists have often created baryons containing two and even three strange quarks, but no one had ever detected a baryon containing more than one charm quark, at least no one thought so.

Sifting through the data of a now-defunct experiment called SELEX, scientists at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill., have uncovered signs of two different, doubly charmed baryons.

In the experiment, a beam of high-energy protons striking a beryllium target spawned a second beam of so-called sigma-minus baryons. Those, in turn, struck nuclei in copper and diamond targets, says Fermilab’s Peter S. Cooper, a member of the SELEX team.

The data suggest that two new types of baryons may have emerged from this nuclear pinball machine. One type is made of two charm quarks and an up quark; the other is made of two charm quarks and a down quark. SELEX scientists presented evidence of about 8 of the first particles and 16 of the second on May 31 at a Fermilab seminar.

Such baryons may have existed naturally only briefly after the Big Bang that started the universe.

One surprising result–SELEX’s yield of 10 to 100 times as many doubly charmed baryons as expected–has “the theorists . . . sharpening their pencils,” Cooper says.

Fermilab’s Harry W.K. Cheung notes that a high-energy photon experiment called FOCUS didn’t yield doubly charmed baryons among an abundance of singly charmed ones, a result that throws some doubt on the reanalysis of the SELEX data. Or, Cheung says, it could imply that SELEX has revealed an unforeseen way of making those twice-charmed baryons.

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