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


“Tantalizing hint” of dark matter particles

April 16, 2013
Courtesy of TAMU
and World Science staff

A new analysis of some old data has turned up what a scientist calls a “tan­ta­liz­ing hint” of par­t­i­cles the­o­rized to make up dark mat­ter, a mys­te­ri­ous com­po­nent of the uni­verse.

Phys­i­cists said they found with 99.8 cer­tain­ty a par­t­i­cle of a type widely the­o­rized to com­pose dark mat­ter. They hope to go on work­ing and at­tain 99.9999 per­cent cer­tain­ty, mean­ing the chance of er­ror is one in a mil­lion or less.

A 4-inch de­tec­tor in its mount be­ing fab­ri­cat­ed by phys­i­cist Ru­pak Ma­ha­p­a­tra and his team.

Dark mat­ter is a mys­te­ri­ous sub­stance that ac­cord­ing to many stud­ies floats in vast amounts through­out the uni­verse—but is de­tect­a­ble only through its gravita­t­ional pull. It’s in­vis­i­ble. 

Some as­tro­no­mers dis­pute its ex­ist­ence al­to­geth­er, and some newer stud­ies seem to back up these skep­tics.

Still, most main­stream as­tro­no­mers sub­scribe to cal­cula­t­ions show­ing that dark mat­ter com­prises about a quar­ter of the con­tent of the uni­verse. And al­though its na­ture re­mains un­ex­plained, a wide­spread view is that dark mat­ter con­sists of theor­etical par­t­i­cles called weakly in­ter­act­ing mas­sive par­t­i­cles, or WIMPs.

The new find­ings come from an in­terna­t­ional sci­en­tif­ic col­la­bora­t­ion known as Su­per Cry­o­gen­ic Dark Mat­ter Search, or Su­perCDMS. A pa­per on the work is to ap­pear in the re­search jour­nal Phys­i­cal Re­view Let­ters and is posted on­line.

WIMPs, whose existence is thought to make sense for sep­arate reasons also, would be very hard to de­tect be­cause they rarely in­ter­act with nor­mal mat­ter—hence their name. But sci­en­tists be­lieve one of them oc­ca­sion­ally bounces off the nu­cle­us of an at­om. This could leave be­hind a small amount of en­er­gy ca­pa­ble of be­ing tracked by cer­tain in­stru­ments, in­clud­ing one mount­ed on the In­terna­t­ional Space Sta­t­ion, called the Al­pha Mag­net­ic Spec­trom­e­ter.

The Su­perCDMS group makes use of a de­tector a half-mile un­der­ground at the Sou­dan mine in north­ern Min­ne­so­ta, man­aged by the Un­ited States De­part­ment of En­er­gy’s Fer­mi Na­t­ional Ac­cel­er­a­tor Lab­o­r­a­to­ry. The ex­pe­ri­ment uses soph­is­t­icated tech­nol­o­gy to search for the rare re­coil of dark mat­ter par­t­i­cles against ex­treme­ly cold ger­ma­ni­um and sil­i­con.

The lat­est anal­y­sis rep­re­sents da­ta gleaned from the larg­est ex­po­sure with sil­i­con de­tectors dur­ing a rel­a­tively early phase of the over­all ex­pe­ri­ment, called CDMS-II, ac­cord­ing to high-en­er­gy phys­i­cist Ru­pak Ma­ha­p­a­tra of Tex­as A&M Uni­vers­ity, who fab­ri­cates de­tectors used in the work.

“This re­sult is from da­ta tak­en a few years ago us­ing sil­i­con de­tectors ma­n­u­fac­tured at Stan­ford [Uni­versity in Cali­fornia] that are now de­funct,” Ma­ha­p­a­tra said. “In­creased in­ter­est in the low mass WIMP re­gion,” that is, in light­er WIMPS, “mo­ti­vated us to com­plete the anal­y­sis of the sil­i­con-de­tector ex­po­sure.”

“This is cer­tainly very ex­cit­ing, but not fully con­vinc­ing,” he added, not­ing that in par­t­i­cle phys­ics, a find­ing is called a “discover­y” only if it meets the 99.9999 per­cent stand­ard, al­so called five-sig­ma. 

“At three-sig­ma,” the cur­rent cer­tain­ty lev­el, “you have a hint of some­thing,” he ex­plained. “At four-sig­ma, you have ev­i­dence. At five-sig­ma, you have a discovery.”

“In med­i­cine, you can say you are cur­ing 99.8 per­cent of the cases, and that’s OK,” he went on. But “when you say you’ve made a fun­da­men­tal discovery in high-en­er­gy phys­ics, you can’t be wrong. Giv­en the mon­ey in­volved—$30 mil­lion in this case—it has to be ex­tremely pre­cise. With a 99.8 per­cent chance, that means if you re­peat­ed the same ex­pe­ri­ment a few hun­dred times, there is one chance it can go wrong. We want one out of a mil­lion in­stead.”

“We just need more da­ta to be sure,” he added. “For now, we have to live with this tantalizing hint of one of the biggest puzzles of our time.”

* * *

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Scientists have turned up what they call a “tantalizing hint” of particles theorized to make up dark matter, a mysterious component of the universe. Physicists said they discovered with 99.8 certainty a particle of a type widely theorized to compose dark matter. They hope to go on working and attain 99.9999 percent certainty, meaning the chance of error is one in a million or less. Dark matter is a mysterious substance that according to many studies floats in vast amounts throughout the universe—but is detectable only through its gravitational pull. It’s invisible. Some astronomers dispute its existence altogether, and some newer studies seem to back up these skeptics. Still, most mainstream astronomers subscribe to calculations showing that dark matter comprises about a quarter of the content of the universe. And although its nature remains unexplained, a widespread view is that dark matter consists of a type of particle called a weakly interacting massive particle, or WIMP. The new findings come from an international scientific collaboration known as Super Cryogenic Dark Matter Search, or SuperCDMS. A paper on the work is to appear in the research journal Physical Review Letters and is posted online. WIMPs are theoretically very hard to detect because they rarely interact with normal matter—hence their name. But scientists believe one of them occasionally bounces off the nucleus of an atom. This could leave behind a small amount of energy capable of being tracked by certain instruments, including one mounted on the International Space Station, called the Alpha Magnetic Spectrometer. The SuperCDMS group makes use of a detector a half-mile underground at the Soudan mine in northern Minnesota, managed by the United States Department of Energy’s Fermi National Accelerator Laboratory. The experiment uses sophisticated technology to search for the rare recoil of dark matter particles against super-cold germanium and silicon. The latest analysis represents data gleaned from the largest exposure with silicon detectors during a relatively early phase of the overall experiment, called CDMS-II, according to high-energy physicist Rupak Mahapatra of Texas A&M University, who fabricates detectors used in the work. “This result is from data taken a few years ago using silicon detectors manufactured at Stanford that are now defunct,” Mahapatra said. “Increased interest in the low mass WIMP region,” that is, in lighter WIMPS, “motivated us to complete the analysis of the silicon-detector exposure.” “This is certainly very exciting, but not fully convincing,” Mahapatra said, noting that in particle physics, a finding is called a “discovery” only if it meets the 99.9999 percent standard, also called five sigma. “At 3-sigma,” the current certainty level, “you have a hint of something,” he explained. “At 4-sigma, you have evidence. At 5-sigma, you have a discovery.” “In medicine, you can say you are curing 99.8 percent of the cases, and that’s OK,” he went on. “When you say you’ve made a fundamental discovery in high-energy physics, you can’t be wrong. Given the money involved—$30 million in this case—it has to be extremely precise. With a 99.8 percent chance, that means if you repeated the same experiment a few hundred times, there is one chance it can go wrong. We want one out of a million instead.” “We just need more data to be sure,” he added