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New collider promises to transform physics

Aug. 21, 2008
Courtesy UCSC
and World Science staff

Phys­ics is poised to en­ter un­known ter­ri­to­ry with the startup of a mas­sive new par­t­i­cle smash­er—the Large Had­ron Col­lid­er—in Eu­rope, sci­en­tists say. The first beam of pro­tons, fun­da­men­tal com­po­nents of atoms, is sched­uled to start speed­ing through the ma­chine Sept. 10.

The white curve marks where the tracks of the Large Had­ron Col­lider lie un­der­ground. (Cour­tesy CERN)


A par­t­i­cle col­lider has a sim­ple bas­ic pur­pose: to smash to­geth­er atoms or their part­s—the so-called fun­da­men­tal par­t­i­cles of na­ture—to find out what’s in­side them. 

The re­sults are of­ten sur­pris­ing and seem­ingly il­log­i­cal, but have re­vealed plen­ti­ful in­sights in­to na­ture over dec­ades. Phys­i­cists are hop­ing for more an­swers from larg­er, stronger col­liders.

The Large Had­ron Col­lider, or LHC, would test hotly de­bat­ed the­o­ries as it pro­duces moun­tains of da­ta. 

Po­ten­tial break­throughs in­clude an ex­plana­t­ion of what cre­ates mass and what is the mys­te­ri­ous “dark mat­ter” that makes up most of the mass in the uni­ver­se, phys­i­cists say. More ex­ot­ic pos­si­bil­i­ties in­clude ev­i­dence for new forc­es of na­ture or hid­den ex­tra di­men­sions of space and time.

“We don’t know what we’ll find,” said Abra­ham Sei­den, di­rec­tor of the San­ta Cruz In­sti­tute for Par­t­i­cle Phys­ics at the Uni­ver­s­ity of Cal­i­for­nia, San­ta Cruz, a U.S. par­ti­ci­pant in the proj­ect. About half the U.S. ex­pe­ri­men­tal par­t­i­cle-phys­ics com­mun­ity has fo­cused its en­er­gy on the col­lider’s two larg­est par­t­i­cle de­tec­tors, called AT­LAS and CMS, ac­cord­ing to Sei­den.

LHC is huge in eve­ry way—its size, the en­er­gies to which it can ac­cel­er­ate par­t­i­cles, the amount of da­ta it would gener­ate, and the size of the in­terna­t­ional col­la­bora­t­ion in­volved in it. The pow­er­ful beams of par­ti­cles are to cir­cu­late around the 27-km (16.8-mile) un­der­ground tube at CERN, the Eu­ropean par­t­i­cle phys­ics lab based in Ge­ne­va. Af­ter some test­ing, the beams are to cross paths in­side the de­tec­tors to make the first col­li­sions.

Sci­en­tists say the de­bris from those crash­es—show­ers of sub­a­tom­ic par­t­i­cles—will rev­o­lu­tion­ize our un­der­stand­ing of na­ture. A key hoped-for mile­stone is disco­very of the Higgs bos­on, a hy­po­thet­i­cal par­t­i­cle that would fill a gap in the “s­tan­dard mod­el” of par­t­i­cle phys­ics by en­dow­ing fun­da­men­tal par­t­i­cles with mass. This should oc­cur by 2010, Sei­den said, if the Higgs ex­ists at all; na­ture may have found an­oth­er way to cre­ate mass. “I’m ac­tu­ally hop­ing we find some­thing un­ex­pect­ed,” he said.

The Higgs is part of a frame­work called elec­tro­weak the­o­ry, which pro­poses a deep un­­ity and sym­me­try among cer­tain forc­es and par­t­i­cles, but al­so claims this sym­me­try was “bro­ken” long ago so that it’s not ob­vi­ous. The LHC will re­veal how this “sym­me­try break­ing” oc­curred, said phys­i­cist How­ard Ha­ber at the uni­ver­s­ity. But de­tect­ing the Higgs is hard, he added, be­cause oth­er events can mimic its pre­dict­ed sig­nals.

Ev­i­dence for an­oth­er ma­jor the­o­ry, “supersym­me­try,” could al­so show up in the col­li­der data. This the­o­ry pre­dicts each known par­t­i­cle has a mas­sive, un­seen “su­per­part­ner.” The scheme cre­ates a tidy cor­re­spond­ence be­tween two fam­i­lies of par­t­i­cles, those that make up mat­ter and those that trans­mit forc­es. As a bo­nus, supersym­me­try pre­dicts the ex­ist­ence of par­t­i­cles that could be the dark mat­ter—a stuff as­tro­no­mers say they have de­tected through its gravita­t­ional ef­fects, but that oth­erwise seems in­vis­i­ble.

Supersym­me­try is in many ways a more ex­cit­ing pos­si­bil­ity than the Higgs, said the­o­rist Mi­chael Dine at the uni­ver­s­ity: “by it­self, the Higgs is a very puz­zling par­t­i­cle, so there have been a lot of con­jec­tures about some kind of new phys­ics be­yond the stand­ard mod­el. Supersym­me­try has the eas­i­est time fit­ting in with what we know.”

Iron­ic­ally, the­o­rists say the LHC will be a huge ad­vance even if noth­ing turns up, pre­cisely be­cause cur­rent the­o­ries so strongly de­mand that cer­tain things should. “If noth­ing were found be­yond what we know to­day, that would be so rad­i­cal, be­cause it would be in vi­ola­t­ion of a lot of ex­tremely fun­da­men­tal prin­ci­ples,” Dine said.


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Homepage image: Simulation of a collision event that results in a detection of the Higgs boson. (Courtesy CERN)









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Physics is poised to enter unknown territory with the startup of a massive new particle smasher—the Large Hadron Collider—in Europe this summer, scientists say. The first beam of protons, fundamental components of atoms, is scheduled to start speeding through the machine Sept. 10. A particle collider has a simple basic purpose: to smash together atoms or their parts—the so-called fundamental particles of nature—to find out what’s inside them. The results are often surprising and seemingly illogical, but have revealed plentiful insights into nature over decades. Physicists are hoping for more answers from larger, stronger colliders. The Large Hadron Collider, or LHC, would test hotly debated theories as it produces mountains of data. Potential breakthroughs include an explanation of what creates mass and what is the mysterious “dark matter” that makes up most of the mass in the universe, physicists say. More exotic possibilities include evidence for new forces of nature or hidden extra dimensions of space and time. “We don’t know what we’ll find,” said Abraham Seiden, director of the Santa Cruz Institute for Particle Physics at the University of California, Santa Cruz, a U.S. participant in the project. About half the U.S. experimental particle-physics community has focused its energy on the collider’s two largest particle detectors, called ATLAS and CMS, according to Seiden. LHC is huge in every way—its size, the energies to which it can accelerate particles, the amount of data it would generate, and the size of the international collaboration involved in it. In September, powerful proton beams are to start circulating around the 27-km (16.8-mile) underground tube at CERN, the European particle physics lab based in Geneva. After some testing, the beams are to cross paths inside the detectors to make the first collisions. Scientists say the debris from those crashes—showers of subatomic particles—will revolutionize our understanding of nature. A key hoped-for milestone is discovery of the Higgs boson, a hypothetical particle that would fill a gap in the “standard model” of particle physics by endowing fundamental particles with mass. This should occur by 2010, Seiden said, if the Higgs exists at all; nature may have found another way to create mass. “I’m actually hoping we find something unexpected,” Seiden said. The Higgs is part of a framework called electroweak theory, which proposes a deep unity and symmetry among certain forces and particles, but also claims this symmetry was “broken” long ago so that it’s not obvious. The LHC will reveal how this “symmetry breaking” occurred, said physicist Howard Haber at the university. But detecting the Higgs is hard, he added, because other events can be confused with such a detection. Evidence for another major theory, “supersymmetry,” could also show up. This theory predicts each known particle has a massive, unseen “superpartner.” The scheme creates a tidy correspondence between two families of particles, those that make up matter and those that transmit forces. As a bonus, supersymmetry predicts the existence of particles that could be the dark matter—a stuff astronomers say they have detected through its gravitational effects, but that otherwise seems invisible. Supersymmetry is in many ways a more exciting possibility than the Higgs, said theorist Michael Dine at the university: “by itself, the Higgs is a very puzzling particle, so there have been a lot of conjectures about some kind of new physics beyond the standard model. Supersymmetry has the easiest time fitting in with what we know.” Ironically, theorists say the LHC will be a huge advance even if nothing turns up, precisely because current theories so strongly demand that certain things should. “If nothing were found beyond what we know today, that would be so radical, because it would be in violation of a lot of extremely fundamental principles,” Dine said.