"Long before it's in the papers"
January 27, 2015


Exotic antimatter finding may clarify cosmic symmetries

March 4, 2010
Courtesy of Brookhaven National Laboratory
and World Science staff

Phys­i­cists say they have de­tected the heav­i­est “an­ti-nu­cle­us” to date, a rare spec­i­men of a sort of mirror-image form of or­di­nary mat­ter.

The find­ing may shed light on cos­mic sym­me­tries, and asym­me­tries, that ex­plain why most of the an­ti­mat­ter orig­i­nally pro­duced at the birth of the uni­verse is gone, ac­cord­ing to sci­en­tists.

The STAR Detector (cour­te­sy Brook­ha­ven Nat'l Lab) 

An an­ti­par­t­i­cle is a var­i­ant of one of the nor­mal build­ing blocks of mat­ter that has equal weight, but is op­po­site in elec­tri­cal charge and cer­tain oth­er re­spects, to its “nor­mal” par­t­i­cle coun­ter­part. As a nu­cle­us is the co­re of an or­di­nary at­om, an an­ti-nu­cle­us is the co­re of an “an­ti-at­om.” 

The new­found an­ti-nu­cle­us al­so con­tains the first ex­am­ple of a smaller, equally ex­ot­ic com­po­nent build­ing block that phys­i­cists call an an­ti-strange quark.

The dis­cov­ery “may have un­prec­e­dent­ed con­se­quenc­es for our view of the world,” said the­o­ret­i­cal phys­i­cist Horst Stoe­cker, Vi­ce Pres­ident of the Helm­holtz As­socia­t­ion of Ger­man Na­tional Lab­o­r­a­to­ries. “This an­ti­mat­ter pushes open the door to new di­men­sions in the nu­clear chart — an idea that just a few years ago, would have been viewed as im­pos­si­ble.”

The find­ing, at the U.S. De­part­ment of En­er­gy’s Brook­ha­ven Na­tional Lab­o­r­a­to­ry in New York, may al­so help shed light on the work­ings of com­pact ce­les­tial ob­jects known as neu­tron stars, re­search­ers said.

The nu­cle­us of a nor­mal at­om on Earth con­sists of build­ing blocks called pro­tons and neu­trons, which in turn con­tain smaller com­po­nents known as quarks. These quarks ap­pear in two types, ar­bi­trarily called “up” and “down” va­ri­eties.

The stand­ard Per­i­od­ic Ta­ble of El­e­ments is a grid ar­ranged by num­ber of pro­tons, which de­ter­mine each chem­i­cal el­e­men­t’s prop­er­ties in its bas­ic in­ter­ac­tions with oth­er el­e­ments. 

The "3-D chart of the nu­clides." The fa­mil­iar Per­i­od­ic Ta­ble ar­ranges the el­e­ments ac­cord­ing to their atom­ic num­ber, Z, which de­ter­mines the chem­i­cal prop­er­ties of each el­e­ment. Phys­i­cists are al­so con­cerned with the N ax­is, which gives the num­ber of neu­trons in the nu­cle­us. The third ax­is rep­re­sents strange­ness, S, which is ze­ro for all nat­u­ral­ly oc­cur­ring mat­ter, but could be non-ze­ro in the co­re of col­lapsed stars. Anti­n­u­clei lie at neg­a­tive Z and N in the above chart, and the new­ly dis­cov­ered anti­nu­cle­us (ma­gen­ta) now ex­tends the 3-D chart in­to the new re­gion of strange antimat­ter. (cour­te­sy Brook­ha­ven Nat'l Lab)

But phys­i­cists al­so use a more com­plex, three-di­men­sion­ chart which adds in­forma­t­ion on the dif­fer­ing num­ber of neu­trons that can oc­cur in sam­ples of each el­e­ment. The 3-D chart al­so in­di­cates a num­ber known as “s­trangeness,” which de­pends on the pres­ence of so-called “s­trange” quarks. Nu­clei con­taining one or more strange quarks are called hy­per­nu­clei.

For or­di­nary mat­ter with­out strange quarks, the strange­ness val­ue is ze­ro and the chart is flat. Hy­per­nu­clei are charted on a sep­a­rate grid, which is shown as if hov­er­ing above the stand­ard ta­ble. The new dis­cov­ery of strange an­ti­mat­ter with an an­ti­strange quark—an “an­ti­hy­per­nu­cle­us”—marks the first en­try be­low the stand­ard grid, sci­en­tists ex­plain.

The bi­zarre par­t­i­cle was de­tected as a re­sult of high-speed col­li­sions of gold nu­clei at the Rel­a­tiv­is­t Heavy Ion Col­lider, the Brook­ha­ven lab­o­r­a­to­ry’s at­om smash­er. The re­sults were pub­lished March 4 on the on­line edi­tion of the re­search jour­nal Sci­ence.

The study of the new an­ti­hyp­er­nu­cle­us al­so yields a val­u­a­ble sam­ple of hy­per­nu­clei, and has im­plica­t­ions for our un­der­stand­ing of the struc­ture of col­lapsed stars, called neu­tron stars, re­search­ers said. “The strange­ness val­ue could be non-ze­ro in the co­re of col­lapsed stars,” said Jin­hui Chen, one of the lead au­thors, of the Shang­hai In­sti­tute of Ap­plied Phys­ics and a post­doc­tor­al re­searcher at Kent State Uni­vers­ity in Ohio. The new mea­sure­ments “will help us dis­tin­guish be­tween mod­els that de­scribe these ex­ot­ic states of mat­ter.”

The find­ings al­so pave the way for ex­plor­ing vi­ola­t­ions of fun­da­men­tal sym­me­tries be­tween mat­ter and an­ti­mat­ter that oc­curred in the early uni­verse, mak­ing pos­si­ble the very ex­ist­ence of our world, phys­i­cists added.

Smashups be­tween at­omic nu­clei at the col­lider are be­lieved to fleet­ingly re­pro­duce con­di­tions that ex­isted a mi­nus­cule frac­tion of a sec­ond af­ter the Big Bang, which sci­en­tists be­lieve gave birth to the uni­verse as we know it some 13.7 bil­lion years ago.

In both events, quarks and an­ti­quarks emerge with equal abun­dance, ac­cord­ing to phys­i­cists. At the lab­o­r­a­to­ry, among the col­li­sion frag­ments that sur­vive to the fi­nal state, mat­ter and an­ti­mat­ter are still meas­ured as close to equally abun­dant. In con­trast, an­ti­mat­ter ap­pears to be largely ab­sent from the pre­s­ent-day uni­verse.

“Under­stand­ing pre­cisely how and why there’s a pre­dom­i­nance of mat­ter over an­ti­mat­ter re­mains a ma­jor un­solved prob­lem of physics,” said Brook­ha­ven phys­i­cist Zhang­bu Xu, anoth­er one of the lead au­thors. “A so­lu­tion will re­quire mea­sure­ments of sub­tle de­via­t­ions from per­fect sym­me­try be­tween mat­ter and an­ti­mat­ter, and there are good prospects for fu­ture an­ti­mat­ter mea­sure­ments at RHIC [Rel­a­tiv­is­t Heavy Ion Col­lider] to ad­dress this key is­sue.”

In a sin­gle col­li­sion of gold nu­clei at the col­lider, many hun­dreds of par­t­i­cles burst out at the point of the crash. Most of these don’t ac­tu­ally come from the pre­vi­ously ex­ist­ing, col­lid­ing ob­jects as such. Rath­er, they are formed from the en­er­gy of the col­li­sion, by the con­ver­sion of en­er­gy in­to mass in ac­cord­ance with Ein­stein’s fa­mous equa­t­ion E = mc2

The par­t­i­cles leave tell­tale tracks in a de­tec­tor hooked up to the col­lider, called the STAR de­tec­tor. Sci­en­tists an­a­lyzed about a hun­dred mil­lion col­li­sions to spot the new an­ti­nu­clei, which aren’t di­rectly detecta­ble them­selves but are iden­ti­fa­ble through the byprod­ucts in­to which they dis­in­te­grate. Al­to­geth­er, 70 spec­i­mens of the new an­ti­nu­cle­us were de­tected.

STAR de­tec­tor sci­en­tists, who come from 54 in­sti­tu­tions in 13 coun­tries, say they should be able to disco­ver even heav­i­er an­ti­nu­clei soon. The­o­ret­i­cal phys­i­cist Stoe­cker and his team have pre­dicted that strange nu­clei around dou­ble the mass of the newly disco­vered state should be par­tic­u­larly sta­ble.

* * *

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Physicists say they have detected the heaviest “anti-nucleus” to date, a rare specimen of a mirror-image-like form of ordinary matter. The finding may shed light on cosmic symmetries, and asymmetries, that explain why most of the antimatter originally produced at the birth of the universe is gone, according to scientists. An antiparticle is a variant of one of the normal building blocks of matter that has equal weight, but is opposite in electrical charge and certain other respects, to its “normal” particle counterpart. As a nucleus is the core of an ordinary atom, an anti-nucleus is the core of an “anti-atom.” The newfound anti-nucleus also contains the first example of a smaller, equally exotic component building block that physicists call an anti-strange quark. The discovery “may have unprecedented consequences for our view of the world,” said theoretical physicist Horst Stoecker, Vice President of the Helmholtz Association of German National Laboratories. “This antimatter pushes open the door to new dimensions in the nuclear chart — an idea that just a few years ago, would have been viewed as impossible.” The finding, at the U.S. Department of Energy’s Brookhaven National Laboratory in New York, may also help shed light on the workings of compact celestial objects known as neutron stars, researchers said. The nucleus of a normal atom on Earth consists of building blocks called protons and neutrons, which in turn contain smaller components known as quarks. These quarks appear in two types, arbitrarily called “up” and “down” varieties. The standard Periodic Table of Elements is a grid arranged by number of protons, which determine each chemical element’s properties in its basic interactions with other elements. But physicists also use a more complex, three-dimensional chart which adds information on the differing number of neutrons that can occur in samples of each element. The 3-D chart also indicates a number known as “strangeness,” which depends on the presence of so-called “strange” quarks. Nuclei containing one or more strange quarks are called hypernuclei. For ordinary matter without strange quarks, the strangeness value is zero and the chart is flat. Hypernuclei are charted in a in a separate grid, which is shown as if hovering above the standard table. The new discovery of strange antimatter with an antistrange quark—an “antihypernucleus”—marks the first entry below the standard grid, scientists explain. The bizarre particle was detected as a result of high-speed collisions of gold nuclei at the Relativistic Heavy Ion Collider, the Brookhaven laboratory’s atom smasher. The results were published March 4 on the online edition of the research journal Science. The study of the new antihypernucleus also yields a valuable sample of hypernuclei, and has implications for our understanding of the structure of collapsed stars, called neutron stars, researchers said. “The strangeness value could be non-zero in the core of collapsed stars,” said Jinhui Chen, one of the lead authors, of the Shanghai Institute of Applied Physics and a postdoctoral researcher at Kent State University in Ohio. The new measurements “will help us distinguish between models that describe these exotic states of matter.” The findings also pave the way for exploring violations of fundamental symmetries between matter and antimatter that occurred in the early universe, making possible the very existence of our world, physicists added. Smashups between atomic nuclei at the collider are believed to fleetingly reproduce conditions that existed a minuscule fraction of a second after the Big Bang, which scientists believe gave birth to the universe as we know it some 13.7 billion years ago. In both events, quarks and antiquarks emerge with equal abundance, according to physicists. At the laboratory, among the collision fragments that survive to the final state, matter and antimatter are still measured as close to equally abundant. In contrast, antimatter appears to be largely absent from the present-day universe. “Understanding precisely how and why there’s a predominance of matter over antimatter remains a major unsolved problem of physics,” said Brookhaven physicist Zhangbu Xu, another one of the lead authors. “A solution will require measurements of subtle deviations from perfect symmetry between matter and antimatter, and there are good prospects for future antimatter measurements at RHIC [Relativistic Heavy Ion Collider] to address this key issue.” In a single collision of gold nuclei at the collider, many hundreds of particles burst out at the point of the crash. Most of these don’t actually come from the previously existing, colliding objects as such. Rather, they are formed from the energy of the collision, by the conversion of energy into mass in accordance with Einstein’s famous equation E = mc2. The particles leave telltale tracks in a detector hooked up to the collider, called the STAR detector. Scientists analyzed about a hundred million collisions to spot the new antinuclei, which aren’t directly detectable themselves but are identifable through the byproducts into which they disintegrate. Altogether, 70 specimens of the new antinucleus were detected. STAR detector scientists, who come from 54 institutions in 13 countries, say they should be able to discover even heavier antinuclei soon. Theoretical physicist Stoecker and his team have predicted that strange nuclei around double the mass of the newly discovered state should be particularly stable.