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The Dark Matter Data Bonanza

The Dark Matter Data Bonanza

Posted: 07 Jun 2011 09:10 PM PDT
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After years of scepticism over the existence of dark matter, a growing 
number of experiments are revealing that dark matter was there all along


The universe is filled with mysterious invisible stuff that refuses to 
interact with light. It doesn't reflect, emit or absorb light. But 
astronomers know it is there because of its gravitational effect on the 
visible stuff. They call it dark matter.

But there is a problem. If dark matter exists (and on this blog we've 
looked at a number of alternative ideas), there ought to be a lot of it 
ought there. Astronomers estimate that 83 per cent of the mass of the  
universe should take this form. The rest, a mere 17 per cent, is visible.

So where is all this stuff? It should permeate the Solar System, the Earth 
and our environment. And yet when physicists look for it, they find zip.

At least, most physicists find nothing. For the last few years, one group 
of scientists have been shouting from the rooftops that they can see dark 
matter.

These guys have placed a giant lump of salt at the bottom of a mine in 
Italy.  This lump is a 250kg crystal of sodium iodide doped with thallium. 
The thinking is that a collision between an exotic particle and a nuclei in 
the crystal would generate a photon that can be picked up by sensitive 
light detectors nearby.

This experiment is called DAMA/LIBRA and its results are controversial. 
While particles of dark matter can certainly generate photons in the 
crystal, any other kind of particle can also generate light too. So the 
experiment also picks up cosmic radiation, thermal neutrons and background 
radioactivity. This makes the results extremely noisy.

There  is a way to separate the dark matter signal from all this 
background, however. As the Sun moves through the galaxy, it must also be 
moving through a sea of dark matter. And as the Earth moves around the 
Sun,  it will plough more quickly into the sea of dark matter at some times 
of the year and at other times more slowly.

So the dark matter signal ought to have an annual modulation.

This is exactly what the DAMA/LIBRA people say they can see. The dark 
matter signal peaks in May and then drops away. And this no weak tentative 
signal--the DAMA/LIBRA people say the statistical evidence is so clear that 
there is almost no possibility that they are mistaken.

But most astrophysicists have ignored and even ridiculed the DAMA/LIBRA 
result. The reason is that there are many other dark matter detectors at 
the bottom of other mines around the world that see nothing. Many of these 
are thought to be more reliable because they screen ought the background 
noise from cosmic radiation and so on.

They should only see the dark matter. But they don't.

Or at least they didn't. A few weeks ago, a team with a detector called 
CoGeNT at the bottom of a mine in Minnesota announced that it had gathered 
very similar evidence to the DAMA/LIBRA experiment. Their evidence of dark 
matter is not as statistically strong but it is modulated in exactly the 
same way, peaking in late April or early May.

Today, Dan Hooper at the Fermi National Accelerator Laboratory and Chris 
Kelso from the University of Chicago review the data from CoGenT and 
DAMA/LIBRA and say they are compatible with each other. "If the true phase 
peaks in early May, this would represent a modulation consistent with that 
reported by the DAMA/LIBRA collaboration," they say.

That's quite a statement given the scepticism that many researchers have 
showed towards the DAMA/LIBRA team.

But the evidence doesn't stop there. Hooper and Kelso also say that the 
type of dark matter that these results imply is consistent with other 
indirect evidence of dark matter that other experiments have seen. Things 
like the spectrum of gamma rays observed by the Fermi Gamma Ray Space 
Telescope and the haze seen by the WMAP spacecraft, thought to be generated 
by electrons near the centre of the galaxy emitting photons.

And there is more to come. Hooper and Kelso say that another experiment is 
on the verge of publishing detailed results that back up the 
DAMA/LIBRA-CoGenT claims. "The CRESST collaboration has reported the 
observation of an excess of events roughly consistent with that anticipated 
from a CoGeNT-like dark matter particle."

So the world of dark matter research has been turned on its head in just a 
few months.After years of negative reports, we suddenly have an avalanche 
of positive ones.

That makes it an interesting topic not just for physicists but also for 
psychologists studying group dynamics too. The process by which scientific 
ideas become scientific facts  has always been murky and strange.

But the truth is that it is as a susceptible to human foibles as any other 
field of endeavour and so just as likely to experience fads and fashions 
and sudden changes in opinion. It'll be interesting to see what historians 
of science make of this episode.

Ref: //arxiv.org/abs/1106.1066: Implications of CoGeNT's New Results 
For Dark Matter



08/06/2011

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