New possible signal of dark matter subject of debate

Dark matter detection experiment CoGeNT has seen a possible signal of dark matter, similar to the much-disputed DAMA/LIBRA collaboration result, it’s spokeperson announced yesterday at the American Physical Society April meeting in Anaheim, California. Whether or not it is really a sign of dark matter is still very much open to debate but it presents an intriguing possibility that is leading to heated discussion in the dark matter community.

The germanium heart of the CoGeNT dark matter detector.

The germanium heart of the CoGeNT dark matter detector.

CoGeNT spokesperson Juan Collar presented data that showed an excess of low energy interactions in their germanium crystal detector that couldn’t be explained by any known cause. Something seems to be hitting the germanium atoms and making them recoil. That something is a mystery, but possibly dark matter. The hits are coming at a rate of about 3 per day which is higher than expected based on other dark matter searches but is still consistent with the DAMA/LIBRA experiment. That experiment has been conducting its search for nearly a decade and claims to have seen a real signal of dark matter.

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Connecting the dark with the light

Just this month, the CDF experiment at Fermilab saw a bump in their data at an energy of 140-150 GeV suggesting that they had seen a new type of particle. But does it really exist and, if so, what is it?

The result was at the 3.2 sigma level, which in statistics means that it is about three standard deviations away from the null hypothesis–or about a 6 in 10,000 chance that the signal is just a statistical fluctuation. That’s a small chance but particle physicists have high standards when it comes to this sort of thing.

In particle physics, a 3 sigma result is often described as “evidence” for something but it takes a 5 sigma result to claim “discovery.” The trouble is, there are lots of 3 sigma results in particle physics that go away with more data, usually because of some unsuspected systematic error. That doesn’t change the likelihood that this result is correct but says that 3 sigma is not actually a discovery, just a promising hint.

So now we’ve dealt with the idea that this might not be anything, let’s play with a fun speculation advanced by Fermilab’s Dan Hooper at today’s plenary session at the American Physical Society April meeting.

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Hinting at dark matter

We haven’t seen dark matter yet. We haven’t, right? Sitting in a plenary talk at the APS April meeting yesterday I started to have my doubts.

Dan Hooper from Fermilab gave a great overview discussion of the attempts to detect dark matter covering the three major techniques: direct detection, where you see dark matter particles collide with nuclei; indirect detection, where you use telescopes to observe the gamma rays produced by dark matter annihilating; and collider detection, where you create the dark matter in something like the Tevatron or the Large Hadron Collider.

In the discussion, Hooper pointed out the various experiments which have seen hints of dark matter. Of these, only one is claiming to have definitively seen dark matter–the DAMA collaboration, which looks for seasonal variance in a signal representing the amount of dark matter hitting the Earth. Nobody doubts they have seen a signal, there is just debate about whether it is due to dark matter or some other, so far unexplained, effect.

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Antihydrogen trapped for 1000 seconds

A new experiment from the ALPHA collaboration, based at CERN, has created and trapped antihydrogen atoms for 1000 seconds, 6000 times longer than their previous attempts which trapped antihydrogen for 172 ms. Having antihydrogen trapped for this period allows the possibility of studying fundamental properties of antimatter in detail including the possibility of how it is affected by gravity. Although antimatter is believed to fall under gravity, that needs to be checked along with the other basic principles of physics.

The magnetic trap and detector used to trap antihydrogen for 1000 seconds.

The magnetic trap and detector used to trap antihydrogen for 1000 seconds.

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Wrapping a basketball

If you’ve ever tried to wrap a spherical present, you know it feels next to impossible to do it neatly. That’s because there is no way to map a flat surface, like a piece of wrapping paper, onto a spherical surface, like a basketball, while keeping all the angles and distances intact. Now physicists have characterized just how the attempt to wrap a sphere fails and shown the variety of patterns that the wrapping paper forms as you attempt to do the wrapping.

Examples of the kinds of patterns that are created when a flat sheet is placed on an adhesive sphere.

Examples of the kinds of patterns that are created when a flat sheet is placed on an adhesive sphere.

 

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Insects swim through the air

 

Reconstruction of forward flight in a fruit fly. The wings beat in horizontal arcs, and wing-tip trajectories for two strokes are shown in dark blue.

Reconstruction of forward flight in a fruit fly. The wings beat in horizontal arcs, and wing-tip trajectories for two strokes are shown in dark blue.

High-speed filming of everybody’s favorite fruit fly, Drosophila melanogaster, have shown that the insects paddle their way through air much as many aquatic animals do to achieve fast forward speeds. Previously, people believed that most insects used lift forces instead of drag forces.

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Bacterial back-and-forth isn’t useless for movement

 

This bacterium, Helicobacter pylori, is able to move about using its flagella. However, what use is the purely back and forth movement of other kinds of bacteria that have no means of continued forward propulsion?

This bacterium, Helicobacter pylori, is able to move about using its flagella. However, what use is the purely back and forth movement of other kinds of bacteria that have no means of continued forward propulsion?

Some bacteria undergo a strange looking kind of motion where they head in one direction and then completely reverse direction to get back to where they started. This seems like a fairly pointless exercise and like it wouldn’t contribute to movement. However, physicists have now shown that this type of motion does contribute to movement by enhancing how rapidly the bacteria diffuse through a liquid.

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New test of wave-particle duality

Young's double slit experiment usually measures the intensity of light but an alternative formulation, measuring polarization, also shows wave-particle duality.

Young's double slit experiment usually measures the intensity of light but an alternative formulation, measuring polarization, also shows wave-particle duality.

Physicists are all familiar with Young’s double-slit experiment and its variants. One key variant involves trying to look at which of two slits a photon travels through and then seeing what the effect of looking has on the interference pattern beyond the slits. The more information about the path that is available, the less there are interference fringes.

One way to think about this is that if light is acting as a wave then it passes through both slits and you get an interference pattern. If light is acting as a particle then it can only pass through one slit at a time and you expect no interference pattern. There are also all the stages in between. The more the light acts like a wave, the stronger the interference pattern.

In a new paper, Mayukh Lahiri shows that this wave-particle duality shows up in the polarization of light in interference experiments as well. The details follow the usual sorts of calculation of this kind of experiment but the result is interesting. Read the rest of this entry »

Understanding LED efficiency “droop”

A process called indirect Auger recombination is responsible for the drop-off in efficiency of LED lighting at high powers.

A process called indirect Auger recombination is responsible for the drop-off in efficiency of LED lighting at high powers.

When LEDs are operated at high power their efficiency drops–it’s called efficiency droop. The reason has been a matter of debate for some time now as the theoretical predictions haven’t matched the experimental measurements. However, a new study shows that a typically ignored cause of efficiency loss is responsible for the droop.

The finding applied to nitride LEDs, the kind that seem to have most potential for general lighting applications. If droop can be minimized then the whole efficiency goes up which means either brighter LEDs or less power needed to run them at required illumination levels. Read the rest of this entry »

A new form of carbon–lighter and strong

 

Different views of the lattice structure of T-carbon, where diamond atoms are replaced with tetrahedra of carbon.

Different views of the lattice structure of T-carbon, where diamond atoms are replaced with tetrahedra of carbon.

Carbon comes in a variety of forms, or allotropes. The best known are graphite, amorphous carbon, and diamond, but there are also graphene, carbon nanotubes, fullerenes, and then the really odd predicted forms of one-dimensional sp-carbyne, two-dimensional sp-sp2-graphyne, and three-dimensional sp-sp3-yne-diamond.Now scientists can add another predicted form to the mix, once again with some pretty impressive properties. The new form, called T-carbon, has the structure of diamond if you replace each diamond carbon atom with a carbon tetrahedron. Read the rest of this entry »