Particle smasher may reveal extra dimensions
Feb 08,2008 00:00 by World-Science

When the world’s most pow­er­ful par­t­i­cle smash­er starts up this sum­mer, ex­ot­ic new par­t­i­cles may of­fer a glimpse of the ex­ist­ence and shapes of ex­tra di­men­sions, says a group of phys­i­cists.

Ex­tra di­men­sions are a pre­dic­tion of string the­o­ry, a mod­el of the un­iverse pop­u­lar among some sci­en­tists that de­scribes na­ture’s fun­da­men­tal par­t­i­cles as ti­ny vi­brat­ing threads of en­er­gy.

Engineers check the electronics at the Large Hadron Collider (Image courtesy CERN)

String the­o­ry of­fers rel­a­tively sim­ple ex­plana­t­ions for dis­par­ate phe­nom­e­na and claims to re­veal hid­den un­­i­ties among na­ture’s forc­es. But math­e­mat­ic­ally, it all works out only if you add six or sev­en ex­tra di­men­sions of space in­to the equa­t­ions, be­yond the three fa­mil­iar ones. 

Ex­plain­ing the ap­par­ent in­vis­i­bil­ity of these di­men­sions be­yond, the­o­rists say they’re curled up in­to ti­ny spaces.

In a new stu­dy, re­search­ers say the tell­tale sig­na­tures of a new class of sub­a­tom­ic par­t­i­cles could help test these ide­as and dis­tin­guish be­tween pos­si­ble shapes of the di­men­sions.

Much as a mu­si­cal in­stru­men­t’s shape de­ter­mines its sound, the shape of these di­men­sions de­ter­mines the prop­er­ties and be­hav­ior of our vi­sible un­iverse—with its three space di­men­sions plus one time di­men­sion, said phys­i­cist Gary Shiu of the Uni­vers­ity of Wis­con­sin-Mad­is­on.

“The shape of the di­men­sions is cru­cial be­cause, in string the­o­ry, the way the string vi­brates de­ter­mines the pat­tern of par­t­i­cle mass­es and the forc­es that we feel,” said Shu, lead au­thor of a pa­per on the sub­ject in the Jan. 25 is­sue of the re­search jour­nal Phys­i­cal Re­view Let­ters.

Pin­ning down that shape should fur­ther our un­der­stand­ing and pre­dic­tions of our four-di­men­sion­al world, Shiu added. “There are myr­i­ad pos­si­bil­i­ties for the shapes of the ex­tra di­men­sions out there. It would be use­ful to know a way to dis­tin­guish one from an­oth­er and per­haps use ex­pe­ri­men­tal da­ta to nar­row down” the pos­si­bil­i­ties.

Such ex­pe­ri­men­tal ev­i­dence could ap­pear in da­ta from a new par­t­i­cle ac­cel­er­a­tor, the Large Had­ron Col­lider, Shiu con­tin­ued. It’s sched­uled to beg­in op­er­at­ing lat­er this year near Ge­ne­va.

An ac­cel­er­a­tor smashes atom­ic nu­clei head-on at nearly the speed of light, cre­at­ing new, en­er­get­ic and very un­sta­ble par­t­i­cles. These quickly dis­in­te­grate or “de­cay” in­to show­ers of de­tect­a­ble, lower-en­er­gy ones. Char­ac­ter­is­tic pat­terns of de­cay serve as fin­ger­prints of the fleet­ing ex­ot­ic par­t­i­cles and, pos­si­bly, the shape of the un­seen di­men­sions, Shiu ex­plained.

With col­leagues at his school and the Un­ivers­ity of California-Berke­ley, Shiu pro­poses in the new study that the sig­na­ture pat­terns from par­t­i­cles called Kaluza-Klein (KK) gravi­tons can dis­tin­guish among dif­fer­ent pro­posed “ge­ome­tries” for ex­tra di­men­sions.

How? Shiu com­pares the ef­fect to a dark room where pat­terns of sound res­o­nat­ing off the walls can re­veal the room’s shape. Sim­i­lar­ly, KK gravi­tons are sen­si­tive to the ex­tra-di­men­sion­al shape and, through their be­hav­ior and de­cay, may re­veal clues to that, he ar­gued. The new study shows that in sim­ula­t­ions, even small ge­o­met­ric varia­t­ions lead to vis­i­ble dif­fer­ences in KK grav­i­ton sig­na­tures, said Bret Un­der­wood, a col­league at Shi­u’s uni­vers­ity.

Based on this, Shiu said, “At least in prin­ci­ple, one may be able to use ex­pe­ri­men­tal da­ta to test and con­strain the ge­om­e­try of our un­iverse.” Last year, Shiu and Un­der­wood re­ported that clues to di­men­sion­al ge­ome­tries might al­so be vis­i­ble in pat­terns of  radia­t­ion left over from the Big Bang. The new work com­ple­ments the pre­vi­ous ap­proach, they say.

“The more hints we get, the bet­ter idea we have about the un­der­ly­ing physics,” said Shiu. Added Un­der­wood, “If the cos­mol­o­gy and par­t­i­cle phys­ics da­ta agree, it’s an in­dica­t­ion we’re on the right track.”

Courtesy University of Wisconsin-Madison and World Science staff