"Long before it's in the papers"
June 07, 2011

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“Dark matter” may dress for the changing seasons

June 7, 2011
Courtesy of the University of Chicago
and World Science staff

It’s pe­r­haps not a sign of the pass­ing sea­sons as un­mis­tak­a­ble as spring show­ers, fall leaves or the ball at Times Square. Yet the en­ig­mat­ic dark mat­ter—a sub­stance that fills the un­iverse yet is in­vis­i­ble and so far un­iden­ti­fi­a­ble—also seems to show a sea­son­al rhythm, phys­i­cists say.

To be pre­cise, the elu­sive sub­stance does­n’t change its be­ha­vior, but the Earth does. As it moves around the Sun it trav­els at dif­fer­ent speeds, at dif­fer­ent times, with re­spect to what sci­en­tists be­lieve is the soup of dark-mat­ter par­t­i­cles all around.

It’s “a situa­t­ion si­m­i­lar to a car mov­ing through a cloud of gnats,” said Un­ivers­ity of Chi­ca­go phys­i­cist Juan Col­lar. “The faster the car goes, the more gnats will hit the front wind­shield.”

With John Or­rell of Pa­cif­ic North­west Na­t­ional Lab­o­r­a­to­ry in Rich­land, Wash., Col­lar has led an ex­pe­riment deep in Min­neso­ta’s Sou­dan mine aimed at de­tect­ing varia­t­ions in dark mat­ter sig­nals. They iden­ti­fied a sea­son­al varia­t­ion si­m­i­lar to one an Ital­ian project has been re­port­ing for more than a dec­ade, though sci­en­tists had been await­ing sep­a­rate tests giv­en the un­cer­tain­ties that plague mod­ern-day phys­ics ex­pe­ri­ments.

Sci­en­tists say the de­tected sig­nal varia­t­ion is ex­actly what the­o­reti­cians had pre­dicted if dark mat­ter turned out to con­sist of a sub­stance dubbed call Weakly In­ter­act­ing Mas­sive Par­t­i­cles, or WIMPs for short.

“We can­not call this a WIMP sig­nal. It’s just what you might ex­pect from it,” said Juan Col­lar, a phys­i­cist at the Un­ivers­ity of Chi­ca­go, who with Or­rell is sub­mit­ting the find­ings in two pa­pe­rs to the jour­nal Phys­i­cal Re­view Let­ters. The ex­pe­ri­ment in the mine is known as Co­her­ent Ger­ma­ni­um Neu­tri­no Tech­nol­o­gy, or Co­GeNT.

WIMPS might have caused the sig­nal varia­t­ion, but it al­so might be a ran­dom fluctua­t­ion, a false read­ing sparked by the ex­pe­ri­mental ap­pa­rat­us it­self or even some ex­ot­ic new phe­nom­e­non in atom­ic phys­ics, Col­lar said. Still, the re­sults may help give re­newed im­pe­tus to the dark-mat­ter search af­ter a recent hunt for WIMPs, de­scribed as the most am­bi­tious to date, turned up none.

Dark mat­ter is es­ti­mat­ed to ac­count for nearly 90 pe­r­cent of all mat­ter in the un­iverse, yet its ident­ity re­mains one of the big­gest mys­ter­ies of mod­ern sci­ence. Al­though dark mat­ter is in­vis­i­ble to tele­scopes, most as­tro­no­mers say they know it exists be­cause it ex­erts a strong gravita­t­ional in­flu­ence over ga­lax­ies that noth­ing vis­i­ble can ex­plain.

The­o­rists had pre­dicted that dark mat­ter ex­pe­ri­ments would de­tect an an­nu­al modula­t­ion be­cause of the rel­a­tive mo­tion of the Earth and sun with re­spect to the plane of the Milky Way gal­axy. The sun moves in the plane of the gal­axy on the out­skirts of one of its spir­al arms at 220 kilo­me­ters per sec­ond (136 miles per sec­ond). The Earth or­bits the sun at 15 kilo­me­ters per sec­ond (18.5 miles per sec­ond). Dur­ing win­ter, Earth moves in roughly the op­po­site di­rec­tion of the sun’s move­ment through the gal­axy, but dur­ing sum­mer, their mo­tion be­comes nearly in the same di­rec­tion. This align­ment in­creases Earth’s net ve­lo­city through a ga­lac­tic ha­lo of dark mat­ter par­t­i­cles, whose dis­tri­bu­tion sci­en­tists have in­ferred from many ob­serva­t­ions.

Co­GeNT has re­portedly de­tected an av­er­age of one WIMP par­t­i­cle in­ter­ac­tion per day through­out its 15 months of ope­ra­t­ion, with a sea­son­al varia­t­ion of ap­prox­i­mately 16 pe­r­cent. The re­sults could be con­sistent with those of the Ital­ian Dark Mat­ter, or DAMA, ex­pe­ri­ment. “We are in the very un­for­tu­nate situa­t­ion where you can­not tell if we are barely ex­clud­ing DAMA or barely in agree­ment. We have to clar­i­fy that,” Col­lar said.

In par­t­i­cle phys­ics, he fur­ther cau­tioned, agree­ment be­tween two or three tests does­n’t nec­es­sarily mean much. The “pen­taquark” par­t­i­cle is a case in point, col­leagues of Col­lar said. Early this cen­tu­ry, about 10 ex­peri­ments found hints of ev­i­dence for the pen­taquark, a par­t­i­cle con­sisting of five units called quarks, when no oth­er known par­t­i­cle had more than three. But as time went on, new ex­peri­ ments were un­able to see it.

Col­lar and his col­leagues have cal­cu­lat­ed the prob­a­bil­ity that their find­ing is a fluke to be one in 200. “It’s not an ex­act sci­ence yet, un­for­tu­nately,” Col­lar said. “But with the in­forma­t­ion we have, the usu­al set of as­sump­tions that we make about the ha­lo and these par­t­i­cles, their be­hav­ior in this ha­lo, things seem to be what you would ex­pect.”

Oth­er dark-mat­ter ex­peri­ments, in­clud­ing Xenon100, have not de­tected the sea­son­al sig­nal that Co­GeNT and DAMA have re­ported.

“If you really wanted to see an ef­fect, you could ar­gue that the Xenon100 peo­ple don’t have the sen­si­ti­vity to Juan’s re­sult,” said Ros­ner, who is not a mem­ber of the Co­GeNT col­la­bora­t­ion. “On the oth­er hand, they’ve done a num­ber of stud­ies of what their sen­si­ti­vity is at low en­er­gies and they be­lieve they’re ex­clud­ing this re­sult.”

Co­GeNT ope­rated from De­cem­ber 2009 un­til in­ter­rupt­ed by a fire in the Sou­dan mine in March 2011. Fif­teen months of da­ta col­lec­tion is a rel­a­tively brief pe­riod for a dark-mat­ter ex­pe­ri­ment. In fact, Col­lar and his col­leagues de­cid­ed to ex­am­ine the da­ta now only be­cause the fire had stopped the ex­pe­ri­ment, at least tem­po­rar­ily.

The fire did not di­rectly af­fect the ex­pe­ri­ment, but the Co­GeNT team has not been able to ex­am­ine the de­tector be­cause of clean-up ef­forts. The de­tector may no long­er work, or if it does work, it may now have dif­fer­ent prope­rties.

“This ef­fect that we’re see­ing is touch-and-go. It’s some­thing where you have to keep the de­tector ex­quis­itely sta­ble,” Col­lar said. If a sin­gle key char­ac­ter­is­tic of the de­tector has changed, such as its elec­tron­ic noise, “We may be un­able to look for this modula­t­ion with it from now on.”

The pu­ta­tive mass of the WIMP par­t­i­cles that Co­GeNT pos­sibly has de­tected ranges from six to 10 bil­lion elec­tron volts, or around se­ven times the weight of the co­re parts of nor­mal atoms. “To look for WIMPs 10 times heav­i­er is hard enough. If they’re this light, it be­comes a night­mare,” Col­lar said.

* * *

It’s perhaps not a sign of the passing seasons as unmistakable as spring showers, fall leaves or the falling ball at Times Square. Yet the enigmatic dark matter—a substance that fills the universe yet is invisible and so far unidentifiable—also seems to show a seasonal rhythm, physicists say. To be precise, the elusive substance doesn’t act differently itself at different times, but the Earth’s movement around the sun does. This leads the world to travel at different speeds, at different times, with respect to what scientists believe is the surrounding broth of dark-matter particles. It’s “a situation similar to a car moving through a cloud of gnats,” said University of Chicago physicist Juan Collar. “The faster the car goes, the more gnats will hit the front windshield.” With John Orrell of Pacific Northwest National Laboratory in Richland, Wash., Collar has led an experiment deep in Minnesota’s Soudan mine aimed at detecting variations in dark matter signals. They identified a seasonal variation similar to one an Italian experiment has been reporting for more than a decade, though scientists had been awaiting separate tests given the uncertainties that plague modern-day physics experiments. Scientists say the detected signal variation is exactly what theoreticians had predicted if dark matter turned out to consist of a substance dubbed call Weakly Interacting Massive Particles, or WIMPs for short. “We cannot call this a WIMP signal. It’s just what you might expect from it,” said Juan Collar, a physicist at the University of Chicago, who with Orrell is submitting the findings in two papers to the journal Physical Review Letters. The experiment in the mine is known as Coherent Germanium Neutrino Technology, or CoGeNT. WIMPS might have caused the signal variation, but it also might be a random fluctuation, a false reading sparked by the experimental apparatus itself or even some exotic new phenomenon in atomic physics, Collar said. Still, the results may help give renewed impetus to the dark-matter search after a hunt for WIMPs, described as the most ambitious to date, recently turned up none. Dark matter is estimated to account for nearly 90 percent of all matter in the universe, yet its identity remains one of the biggest mysteries of modern science. Although dark matter is invisible to telescopes, astronomers are sure it exerts a strong gravitational influence over galaxies that nothing visible can explain. Theorists had predicted that dark matter experiments would detect an annual modulation because of the relative motion of the Earth and sun with respect to the plane of the Milky Way galaxy. The sun moves in the plane of the galaxy on the outskirts of one of its spiral arms at 220 kilometers per second (136 miles per second). The Earth orbits the sun at 15 kilometers per second (18.5 miles per second). During winter, Earth moves in roughly the opposite direction of the sun’s movement through the galaxy, but during summer, their motion becomes nearly in the same direction. This alignment increases Earth’s net velocity through a galactic halo of dark matter particles, whose distribution scientists have inferred from many observations. CoGeNT has reportedly detected an average of one WIMP particle interaction per day throughout its 15 months of operation, with a seasonal variation of approximately 16 percent. Energy measurements are consistent with a WIMP mass of approximately 6 to 10 times the mass of a proton. These results could be consistent with those of the Italian DArk MAtter (DAMA) experiment, which has detected a seasonal modulation for years. “We are in the very unfortunate situation where you cannot tell if we are barely excluding DAMA or barely in agreement. We have to clarify that,” Collar said. In particle physics, he further cautioned, agreement between two or three experiments doesn’t necessarily mean much. The “pentaquark” particle is a case in point, colleagues of Collar said. Early this century, approximately 10 experiments found hints of evidence for the pentaquark, a particle consisting of five quarks, when no other known particle had more than three. But as time went on, new experiments were unable to see it. Collar and his colleagues have calculated the probability that their finding is a fluke to be one in 200. “It’s not an exact science yet, unfortunately,” Collar said. “But with the information we have, the usual set of assumptions that we make about the halo and these particles, their behavior in this halo, things seem to be what you would expect.” Other dark-matter experiments, including Xenon100, have not detected the seasonal signal that CoGeNT and DAMA have reported. “If you really wanted to see an effect, you could argue that the Xenon100 people don’t have the sensitivity to Juan’s result,” said Rosner, who is not a member of the CoGeNT collaboration. “On the other hand, they’ve done a number of studies of what their sensitivity is at low energies and they believe they’re excluding this result.” CoGeNT operated from December 2009 until interrupted by a fire in the Soudan mine in March 2011. Fifteen months of data collection is a relatively brief period for a dark-matter experiment. In fact, Collar and his colleagues decided to examine the data now only because the fire had stopped the experiment, at least temporarily. The fire did not directly affect the experiment, but the CoGeNT team has not been able to examine the detector because of clean-up efforts. The detector may no longer work, or if it does work, it may now have different properties. “This effect that we’re seeing is touch-and-go. It’s something where you have to keep the detector exquisitely stable,” Collar said. If a single key characteristic of the detector has changed, such as its electronic noise, “We may be unable to look for this modulation with it from now on.” The putative mass of the WIMP particles that CoGeNT possibly has detected ranges from six to 10 billion electron volts, or around seven times the weight of the core parts of normal atoms. “To look for WIMPs 10 times heavier is hard enough. If they’re this light, it becomes a nightmare,” Collar said.