Twice-charmed particles spotted? (Physics).Physicists have spied hints of two new members of the family of subatomic particles that includes the proton and neutron. Known as baryons This is a list of baryons, which are the family of subatomic particles each made of three quarks. See also quark model. Antiparticles are not listed in the table; however, they simply would have all quarks changed to antiquarks, and their baryon number, , 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). 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 SELEX Systematic Evolution of Ligands By Exponential Enrichment SELEX Segmented Large X (baryon spectrometer) , scientists at the Fermi National Accelerator Laboratory Fermi National Accelerator Laboratory (Fermilab), physical science research center located near Batavia, Ill., est. 1968 as the National Accelerator Laboratory, renamed 1974 in honor of Enrico Fermi. It was built on the site of the former village of Weston. (Fermilab) in Batavia, Ill., have uncovered signs of two different, doubly charmed baryons Charmed baryons are a category of composite particles comprising all baryons with at least one unit of charm (quantum number). In quark model terms, charmed baryons consist of three quarks, one or more of which is a charm quark. . In the experiment, a beam of high-energy protons striking a beryllium beryllium (bərĭl`ēəm) [from beryl ], metallic chemical element; symbol Be; at. no. 4; at. wt. 9.01218; m.p. about 1,278°C;; b.p. 2,970°C; (estimated); sp. gr. 1.85 at 20°C;; valence +2. 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 down quark n. Abbr. d A quark with a charge of - 1/3 , a mass about 20 times that of the electron, and a downward spin. It is a component of protons and neutrons. See Table at subatomic particle. . 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 big bang Model of the origin of the universe, which holds that it emerged from a state of extremely high temperature and density in an explosive expansion 10 billion–15 billion years ago. 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.--P.W. |
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