APS: Dark matter clues
The Fermi space telescope has reported seeing an excess of high-energy electrons that could hint at dark-matter annihilation in the universe. Full story is available here.
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May 05, 2009
APS 2009: Back to DC
I'm set to head to the airport in a bit, and so this is sayonara. As usual, I didn't get to half the sessions that I wanted to, but that's part of the appeal. Just to keep us on our toes, it looks like APS is having its next April meeting in February. But it will be in Washington, DC, my home, so I'll be happy to offer insider tours. I'm a journalist, so my rates are cheap!
May 03, 2009
APS 2009: Pierre Auger backs off claims for cosmic ray source
The mysterious origin of ultra-high energy cosmic rays is, it seems, still a mystery. Two years ago, scientists at the Pierre Auger Observatory in Argentina thought they had it solved. They published a paper in Science, based on two dozen particles, that there was a correlation with the location of Active Galactic Nuclei -- supermassive black holes that accelerate jets of material at near-light speed throughout the universe. At the time of the announcment, there was some doubt: The Hi-Res project, which scans the northern sky like Auger does the south, found no such correlation.
And now, today, Stefan Westerhoff, an Auger scientist from the University of Wisconsin at Madison, said that, based on new particle detections -- they have more than 50 now -- the correlation no longer holds. "The signal strength is certainly considerably weaker now," he told his audience. "This is certainly a disappointment."
But the correlation isn't so weak that they can give up. The 70% correlation between the cosmic rays and the AGN at the time of the Science publication has now dropped to about 40% -- considerably less, but not enough to support the null hypothesis. What could cause some particles to come from AGN, but not others? Westerhoff says it might have something to do with their composition. Maybe the protons come from the AGN, whereas higher mass cosmic rays, say iron nuclei, do not.
Westerhoff says this will be sorted out as they track more particles -- which can only come with more time and bigger detectors. If Pierre Auger is the size of Rhode Island, the proposed Pierre Auger North, not too far from Denver, would be the size of Massachusetts -- and Westerhoff showed a slide how they can get the necessary statistics in a decade or two. No offense to the lovely state of Colorado, but I say keep the cow pasture free of Cerenkov detectors, and give folks like JEM-EUSO a chance to stick their 2.5-meter camera on the space station.
Image: Pierre Auger
APS 2009: Earth-observing astronomy
In my previous post, I talked about how not all astronomers use photons. Well, neither do all astronomers look up from Earth or Earth orbit. A new telescope aims to look down at the Earth -- precisely so that it can see the sky.
JEM-EUSO proposes fixing a downward looking camera on the space station. It would stare at Earth -- primarily the oceans -- and watch for ultra high energy cosmic rays (charged particles, typically protons) as they hit. It would work very much like the Pierre Auger Observatory in Argentina, which detects fluorescent showers of secondary particles caused by the cosmic rays colliding with atmospheric particles, followed by a final flash as the particles hit detectors on the Earth. Auger astronomers have detected the most energetic things in the universe, and asserted with bare bones statistics that they probably emanate from the supermassive black holes at the centers of galaxies. But some want better proof than that given by a few dozen high energy cosmic rays, and, for that, they need more hits.
Looking down from the space station, JEM-EUSO could monitor an area between 50 to 250 times as large as Pierre Auger. The 5-year mission goal would be to detect at least 1,000 particles with greater than 7x10e19 eV energies. Particles like that would be probes of energy regimes 10 million times bigger than those explored by the LHC. Maybe JEM-EUSO could even detect a few particles with 10e21 eV energies. Because then we could use my favorite prefix in the world, and talk about the Zev scale: Zettaelectronvolt particle physics. Yoshiyuki Takahashi, of University of Alabama Huntsville, and Mark Christl, of Marshall Space Flight Center gave a talk on JEM-EUSO on Saturday. They say it would take 20 years for Pierre Auger and its successor, Pierre Auger North (proposed for nearby southeastern Colorado), to do what JEM-EUSO could do.
The total cost? $220 million -- and that includes the rocket ride that it needs. ESA and NASA were once enthusiastic. But then, with the space shuttle retirement, NASA couldn't find a ride for EUSO. ESA also liked it but its transport vehicle, the ATV, can't deal quite handle it. Thankfully, JAXA, the Japanese space agency, seems to like the idea. And it may have a rocket, the new H-IIB, to fly it up -- if test flights later this year prove successful.
Takahashi says JAXA will make a final decision on JEM-EUSO in September, and it could launch sometime in 2013. It would be nice to get a real astrophysical experiment on the space station. Nobel prize winner Sam Ting seems to have found a way to get his $1.5 billion AMS up to the station on the shuttle's last flight. Why can't these guys fly a novel experiment that costs a fraction as much?
Image: JEM-EUSO
APS 2009: The muon shadow of the Moon
Astronomers typically use photons of some sort to figure out what's happening up there. Sure, some astronomers look for cosmic rays (which are not rays but in fact charged particles like protons), and eventually, gravitational waves are going to be important. But light is the way 99% of astronomy has been done. Now, a new window on the heavens is about to open -- and the window goes through the center of the Earth.
The $271 million IceCube project, stuck in the Antarctic ice, would be the largest observatory to use neutrinos -- chargeless, high-energy and nearly massless elementary particles. IceCube is a cubic kilometer array consisting of strings of detectors dropped down into ice cores made with hot-water drills. The detectors see the secondary effect of neutrinos colliding with atoms of ice. Neutrinos are terrific for astronomers because, lacking a charge, they aren't bent by galactic magnetic fields. They may provide some of the only evidence of what happens in the center of supernovae. But they are incredibly elusive -- they only rarely interact with matter. Thus the need for a cubic kilometre telescope.
On Saturday, Laura Gladstone of the University of Wisconsin showed that, based on data with only half of the telescope's 86 strings installed, IceCube was working pretty well: It could see the Moon's shadow. Common neutrinos particles called muons rain down on Antarctica -- some 9 million a lunar month land on IceCube. A much smaller signal was observed when the Moon passed overhead and absorbed some of those muons.
But what Gladstone and her colleagues really want to do is look for the neutrinos that, speeding through the Earth, come up and hit IceCube from underneath. That way, the Earth would blot out most of the common, lower energy muons that fill the galaxy, and instead filter for the zippy high-energy neutrinos that could be coming from pulsars and supernovae, maybe even extragalactic sources. Michael Baker, in the previous talk, said he didn't quite have the statistics yet to show any point sources in his through-the-Earth looking glass. They'll have to wait a little longer; IceCube is expected to have all 86 strings installed by 2011.
Image: NSF
May 02, 2009
APS 2009: Fermi seeing dark matter's signal?
Peter Michelson, head of the Large Area Telescope team on Fermi, the gamma ray telescope formerly known as GLAST, gave the opening talk this morning. He went through all the amazing things that it has found in its first 8 months: gamma-ray only pulsars, milli-second pulsars, and active galactic nuclei. But he saved the news for last: Fermi, like two other experiments PAMELA and ATIC, is seeing way too many electrons and positrons all around us -- which could be an indirect signal from the annihilation or decay of dark matter, the stuff that makes up up to a quarter of the mass of the universe, but has yet to be detected directly as a particle.
Last year, the PAMELA and ATIC teams showed rises in positrons and electrons -- far more than are expected to be in the diffuse galactic background. Now, Fermi, shows not just a rise, but a bump --- centered around 300 or 400 gigaelectronvolts. That bump could mark the center of mass for a dark matter particle, such as a WIMP (Weakly Interacting Massive Particle). The results of the different experiments are not exactly the same (Fermi counts the total electrons and positrons, whereas PAMELA can distinguish between the two), but they seem to be compatible. Michelson isn't ready to rule out a conventional source just yet -- the extra particles could be generated from nearby pulsars, and then whipped into a diffuse background by galactic magnetic fields -- but he says that there is a good chance that they could be observing new physics. "Exciting stuff," he says. Symmetry Breaking has their take on the discovery here. More news to come -- after I learn some more myself.
[Editor's update: that full story is now available here.]
APS 2009: Denver
Well here we are in Denver, at the American Physical Society April meeting, which this year just so happens to be in May. Denver, the mile-high city, should be in springtime bloom, all alpine sunshine and wildflowers. But it is colder than a Bose-Einstein condensate. Oh well -- there's so much good stuff going on in the basement of the Sheraton here that I probably won't be leaving the hotel. There are 1,025 preregistered attendees -- about the same as last year, says Don Wise of APS. But the number of abstracts are down slightly: 1,089, compared to 1,194 from last year.
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