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April 15, 2011

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Ambitious search fails to find dark matter

April 15, 2011
Courtesy of UCLA
and World Science staff

A search for par­t­i­cles of the mys­te­ri­ous “dark mat­ter” thought to per­vade our uni­verse—de­scribed as the most am­bi­tious such ef­fort to date—has turned up none.

Most phys­i­cists ac­cept that dark mat­ter, though in­vis­i­ble, ex­ists, mostly on grounds that it ex­erts strong gravita­t­ional ef­fects in space that no vis­i­ble struc­tures can ex­plain.

Di­a­gram of the XENON100 dec­tor. (click here for larg­er ver­sion; cred­it: Zina Deretsky, Nat'l Sci­ence Foun­d­a­tion )
But sci­en­tists are at a loss for what dark mat­ter ac­tu­ally is. Searches for its bas­ic un­its or par­t­i­cles have yielded lit­tle, apart from new con­straints on what it can or can­not be. A 2008 study re­vealed pos­si­ble in­di­rect ev­i­dence of dark mat­ter high above Ant­arc­ti­ca, but the in­ves­ti­ga­tors said oth­er pro­cesses might al­so ex­plain their re­sults; con­firma­t­ions of the find­ings have not been an­nounced.

Lack of suc­cess in dark-mat­ter searches have fu­eled claims by some phys­i­cists that dark mat­ter does­n’t ex­ist at all. The con­ven­tion­al view, though, is that it com­prises about five-sixths of the ma­te­ri­al in the un­iverse. 

The new­est re­sults were an­nounced April 14 at the Gran Sasso Na­t­ional Lab­o­r­a­to­ry in It­a­ly, where a huge “dark mat­ter” de­tec­tor is housed al­most mile (1.6 km) be­neath a moun­tain west of Rome. It rep­re­sents the highest-sensiti­vity search for dark mat­ter yet, pro­po­nents say, with back­ground noise 100 times low­er than com­pet­ing ef­forts.

The lead­ing can­di­date to ex­plain dark mat­ter’s make­up is a pro­posed type of rel­a­tively mas­sive, fun­da­men­tal par­t­i­cle that in­ter­acts weakly with or­di­nary mat­ter and is called a WIMP, for weakly in­ter­act­ing mas­sive par­t­i­cle. WIMPs are be­lieved to have fea­tures that would ac­count for dark mat­ter’s known prop­erties; their ex­istence is al­so thought to make sense for in­de­pend­ent rea­sons. These hy­po­thet­i­cal par­t­i­cles were the tar­get of the new search.

The ex­pe­ri­ment con­sisted of a vat filled filled with de­tec­tion in­stru­ments and over 100 pounds (45 kg) of liq­uid xen­on, an el­e­ment. The idea was that a WIMP bounc­ing off a xen­on at­om would cause a flash of light, as well as a sec­ond flash oc­cur­ring when a charged par­t­i­cle called an elec­tron, knocked free from the xen­on at­om, is pushed to­ward the top of the de­vice by an elec­tric field.

While the ex­pe­ri­ment found no dark mat­ter in 100 days of test­ing, it has led to the most ac­cu­rate lim­its yet on the pos­si­ble mass of WIMPs and their prob­a­bil­ity of in­ter­act­ing with oth­er par­t­i­cles, said Un­ivers­ity of Cal­i­for­nia - Los An­ge­les phys­i­cist Kat­sushi Arisaka, a mem­ber of the re­search team. Oth­ers not­ed that they are now work­ing on a new Xen­on-based de­tec­tor that will be 100 times more sen­si­tive than the pre­vi­ous one, called XENON100.

De­spite the dif­fer­ences be­tween or­di­nary and dark mat­ter, cos­mol­o­gists be­lieve the two have been linked since the be­gin­ning of the un­iverse, with dark mat­ter play­ing a key role in the co­a­lesc­ing of par­t­i­cles in­to stars, ga­lax­ies and oth­er large-scale struc­tures.

Though dark mat­ter ex­erts a tan­gi­ble force on our gal­axy as a whole, ac­cord­ing to UCLA phys­i­cist Da­vid Cline, in­di­vid­ual WIMPs are hard to de­tect be­cause they in­ter­act only very weakly with nor­mal mat­ter. Small sig­nals that might come from WIMP de­tec­tions above ground would be drowned out by the cos­mic radia­t­ion that con­stantly bom­bards Earth’s sur­face, he added. The Xen­on ex­pe­ri­ments were bur­ied deep un­der­ground to elim­i­nate most of this back­ground noise. Dark mat­ter par­t­i­cles are ex­pected to pass through rock eas­i­ly, though most or­di­nary par­t­i­cles can­not.

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A search for particles of the mysterious “dark matter” pervading our universe has turned up none, though it is described as the most ambitious such effort to date. Most physicists accept that dark matter, though invisible, exists, mostly on grounds that it exerts strong gravitational effects in space that no visible structures can explain. But scientists can’t explain what dark matter actually is. Searches for its basic units or particles have yielded little, apart from new constraints on what it can or cannot be. A 2008 study revealed possible indirect evidence of dark matter high above Antarctica, but the investigators said other processes might also explain their results; confirmations of the findings have not been announced. Lack of success in dark-matter searches have fueled claims by some physicists that dark matter doesn’t exist at all. The conventional view, though, is that it comprises about five-sixths of the material in the universe that has mass. The newest results were announced April 14 at the Gran Sasso National Laboratory in Italy, where a huge “dark matter” detector is housed almost mile (1.6 km) beneath a mountain west of Rome. It represents the highest-sensitivity search for dark matter yet, proponents say, with background noise 100 times lower than competing efforts. The leading candidate to explain dark matter’s makeup is a proposed type of relatively massive, fundamental particle that interacts weakly with ordinary matter and is called a WIMP, for weakly interacting massive particle. WIMPs are believed to have features that would neatly account for dark matter’s known behavior; their existence is also thought to make sense for separate reasons. These hypothetical particles were the target of the new search. The experiment consisted of a vat filled filled with detection instruments and over 100 pounds (45 kg) of liquid xenon, an element. The idea was that a WIMP bouncing off a xenon atom would cause a flash of light, as well as a second flash occurring when a charged particle called an electron, knocked free from the xenon atom, is pushed toward the top of the device by an electric field. While the experiment found no dark matter in 100 days of testing, it has led to the most accurate limits yet on the possible mass of WIMPs and their probability of their interacting with other particles, said University of California—Los Angeles physicist Katsushi Arisaka, a member of the research team. Others noted that researchers are now working on a new Xenon-based detector that will be 100 times more sensitive than the previous one, called XENON100. Despite the differences between ordinary and dark matter, cosmologists believe the two have been linked since the beginning of the universe, with dark matter playing a key role in the coalescing of particles into stars, galaxies and other large-scale structures. Though dark matter exerts a tangible force on the galaxy as a whole, according to UCLA physicist David Cline, individual WIMPs are hard to detect because they interact only very weakly with normal matter. Small signals that might come from a WIMP detections above ground would be drowned out by the cosmic radiation that constantly bombards Earth’s surface, he added. The Xenon experiments were buried deep underground to eliminate most of this background noise. Dark matter particles are expected to pass through rock easily, though most particles of ordinary cannot.