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Particle may combine nature’s building blocks in new way

June 18, 2013
Special to World Science  

A new­found par­t­i­cle may con­sist of four fun­da­men­tal build­ing blocks of na­ture, called quarks—pre­vi­ously thought to group only in twos and threes, phys­i­cists say.

The find­ings ap­pear in two pa­pers in the June 21 is­sue of the jour­nal Phys­i­cal Re­view Let­ters, and were al­so sum­ma­rized June 17 in Phys­ics, an on­line mag­a­zine of the Amer­i­can Phys­i­cal So­ci­e­ty.

“Par­t­i­cle phys­i­cists seem to have a pret­ty good han­dle on the fun­da­men­tal par­t­i­cles of the uni­verse, but there are some glar­ing holes in this un­der­stand­ing,” wrote Uni­vers­ity of Pitts­burgh phys­i­cist Er­ic Swan­son in that ar­ti­cle.

Quarks are a good ex­am­ple, he added. They’re the sim­plest known build­ing blocks of mat­ter, but sci­en­tists don’t un­der­stand why they group to­geth­er as they do—only in twos and threes, past ex­pe­ri­ence shows.

Three-quark com­bina­t­ions are pro­tons and neu­tron­s—the com­po­nents of the atom­ic nu­cle­us, which them­selves can clus­ter to­geth­er when they form that co­re of the at­om.

Quark pairs form short-lived par­t­i­cles called pi­ons.

Now comes “the pos­si­ble dis­cov­ery of a new par­t­i­cle con­tain­ing at least four quarks,” Swan­son wrote. Two sep­a­rate re­search groups “have seen ev­i­dence for this strange par­t­i­cle, called Zc(3900). Al­though the da­ta is open to oth­er in­ter­preta­t­ions, it’s clear that our un­der­standing of quarks has a long way to go.”

The two groups are known as the BE­SIII Col­la­bora­t­ion at the Bei­jing Elec­tron Pos­i­tron Col­lider, in Chi­na, and the Belle Col­la­bora­t­ion at the High En­er­gy Ac­cel­er­a­tor Re­search Or­gan­iz­a­tion in Tsukuba, Ja­pan.

Both labs smash to­geth­er elec­tri­cal charge-carrying par­t­i­cles called elec­trons and positrons to see what their de­bris con­tains. “Taken to­geth­er, the two col­la­bora­t­ions have un­cov­ered 466 events that ap­pear to have a Zc(3900) in their de­bris,” Swan­son wrote. This par­t­i­cle would it­self be short-lived, as it dis­in­te­grates in­to oth­er par­t­i­cles, he not­ed.


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A newfound particle may consist of four fundamental building blocks of nature, called quarks—previously thought to group only in twos and threes, physicists say. The findings appear in two papers in the June 21 issue of the journal Physical Review Letters, and were also summarized June 17 in Physics, an online magazine of the American Physical Society. “Particle physicists seem to have a pretty good handle on the fundamental particles of the universe, but there are some glaring holes in this understanding,” wrote University of Pittsburgh physicist Eric Swanson in that article. Quarks are a good example, he added. They’re the simplest known fundamental building blocks of nature, but scientists don’t understand why they group together as they do—only in twos and threes, past experience shows. Three-quark combinations are protons and neutrons—the components of the atomic nucleus, which themselves can cluster together when they form that core of the atom. Quark pairs form short-lived particles called pions. Now comes “the possible discovery of a new particle containing at least four quarks,” Swanson wrote. Two separate research groups “have seen evidence for this strange particle, called Zc(3900). Although the data is open to other interpretations, it’s clear that our understanding of quarks has a long way to go.” The two groups are known as the BESIII Collaboration at the Beijing Electron Positron Collider, in China, and the Belle Collaboration at the High Energy Accelerator Research Organization in Tsukuba, Japan. Both labs smash together electrical charge-carrying particles called electrons and positrons to see what their debris contains, he added. “Taken together, the two collaborations have uncovered 466 events that appear to have a Zc(3900) in their debris.” This possible particle is itself short-lived, as it disintegrates into other particles, he noted.