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Particle smasher may reveal extra dimensions

Feb. 1, 2008
Courtesy University of Wisconsin-Madison
and World Science staff

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.”

* * *

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When the world’s most powerful particle smasher starts up later this year, exotic new particles may offer a glimpse of the existence and shapes of extra dimensions, said a group of physicists. Extra dimensions are a prediction of string theory, a model of the universe popular among some scientists, which describes nature’s fundamental particles as tiny vibrating threads of energy. 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 four-dimensional universe, according to University of Wisconsin-Madison physicist Gary Shiu, lead author of a paper appearing in the Jan. 25 issue of the research journal Physical Review Letters. “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. 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 set of 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. In an accelerator, smashing atomic nuclei head-on at nearly the speed of light can briefly create new, energetic and very unstable particles. These quickly disintegrate or “decay” into a shower 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 University of California- Berkeley, Shiu proposes in the new study that the signature patterns from particles called Kaluza-Klein (KK) gravitons can distinguish between different proposed “geometries” or 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 cosmic 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.”