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China's PandaX Project Looks For Dark Matter In the Heart of a Marble Mountain
the_newsbeagle writes "Chinese engineers love their superlatives: Biggest dam, fastest train, etc etc. Now they've constructed the deepest underground dark matter detector beneath a mountain in Sichuan province. Such dark matter seekers have to be buried deep to shield them from cosmic rays, because that radiation would be picked up by the detector and could be confused for radiation generated by dark matter. Other dark matter detectors are similarly subterranean: LUX, in the United States, is at the bottom of an abandoned mine in South Dakota, and a European effort called XENON lies below the Gran Sasso mountain. The Chinese researchers hope their PandaX detector will finally reveal the much-hypothesized, never-seen dark matter particles known as WIMPs."
Actually, XENON isn't a European project, it's an international collaboration with leadership in the United States and members in Europe and China. The device is in Europe, but that's sort of incidental. Here's the membership: XENON-100
We're talking about that one decimal place before the decimal point: current observations indicate more than five times as much dark matter as known matter. It's not just a little bit of round-off error in how much dust there is between stars, but apparently most of the stuff in the universe (at least, before you get to "dark energy," which is "twice as big" again, but far more poorly understood).
Ah, so less than one decimal place then. Even closer than I thought.
Sounds like what you're talking about are compact halo objects, e.g. brown dwarfs, black holes, etc. The broad consensus is that there is nowhere near enough of this material to explain the high velocity of stars in the outer parts of the galactic disk/s. Also we have observations such as the bullet cluster which are very difficult to explain without proposing invisible gravitating material.
Having said that, you might be right, maybe we'll find out.
I do not want your cheap brainburning drugs. They are useless for work. And I am a working man today.
I can't help to feel a bit of both schadenfreude and melancholy. Being a westerner myself, I wonder if Neil Stephenson were more of a prophet than a futurist.
We just had a story about "people doing something on a computer" last week. Not much has changed about the existence of computers since last week. No need to post articles bothering with the trivial details between one computer system and another --- it's all the same, once you've seen one. Only alert me when something new happens in the world.
In case anyone was wondering, WIMPs are Weakly Interacting Massive Particles.
A marble mountain? Here I've been under that impression that both granite and marble had a detectable amount of radioactivity of their own, so even given 20 miles of the stuff, there would still be a background count contaminating the data.. Can someone fact check me on that?
Cheers, Gene
> Chinese engineers love their superlatives
I wonder where they learned *that* from?
Max.
What collider experiments do look for is "missing mass" --- if the amount of stuff coming out doesn't add up to the stuff going in, minus known detector inefficiencies (indicating some new "invisible" particle being produced). So, people are looking for dark matter production --- anything that results in mass/energy being converted to unknown/undetectable forms --- in a systematic manner. You may not specifically be looking for "spacetime foam," but anything that doesn't mimic "ordinary" particles in firing detectors (thus would already be visible in existing DM searches) in sufficient quantities will be found.
We can make estimates of the amount of dust out there based on the light we see from distant stars. Where there is dust, it will scatter light passing through it (and modify the spectrum). There are lots of open questions about how much and what kind of dust is out there --- this isn't a "solved" problem --- however, best estimates plus known uncertainties don't put this within range of explaining dark matter. So, we still need dark matter to "make the math work out."
Just about every kind of rock has background radiation of its own, which must be dealt with (some more than others). However, radiation from rocks is typically easier to deal with than cosmic rays from space --- it's lower energy stuff that can be blocked by a few extra layers of extra lead/copper shielding (carefully screened for even lower radioactivity), instead of energetic particles that go through hundreds of meters of material unhindered. You have to worry about things like radon (radioactive gas) seeping out of the rocks and getting into the equipment; but, these are known effects to watch out for deal with by proper ventilation/sealing.
...a cosmology news story so a bunch of Slashdot armchair physicists without a lick of physics education past high school can tell the science community they've been doing it wrong for the last 70 years.
Because of Big Bang nucleosynthesis. Long story short, we can look at how the universe formed from evidence left behind in the cosmic microwave background radiation, and see that it is impossible for normal baryonic matter to explain dark matter. That, and the Bullet Cluster makes little sense without dark matter.
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
Same reason that these kinds of experiments often use "ancient lead" [aspera-eu.org] that has been buried under the seas for thousands of years: Stuff that has been underground for thousands or millions of years is vastly less likely to have been made radiative from the sun or other cosmic sources.
Newly-mined lead is not radioactive due to the sun or cosmogenic sources, but because of isotopes of lead which are produced from decays of uranium and thorium (which already existed from the supernova remnants that formed our current solar system) --- see here. So, you don't want lead that you've just mined from millions of years deep underground --- that stuff is still pretty hot. What you want is stuff that was mined by the Romans two millenia ago, separated from the underground crud full of uranium/thorium, and left to cool off since then.
To explain the rotational speed of galaxies.
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.
I know that the detector tank in the bottom of Homestake is lead shielded, but that lead is very old, no newly mined lead in it. It had to be at least 100 years old to even be considered for recycling into that shielding. I used to live in Rapid City in the '60's, even have a wife I still miss buried there, but in those years, Homestake, 50 miles away in Lead, SD was an actively producing gold mine. And environmental disaster as it struggled to remain profitable, it eventually had to close, and I am glad that another use has been found for its extended underground.
The Lead/Deadwood area tried to survive on tourists, but I imagine much of that allure has faded after the state raided and closed the Pink Lady in the '70's, the countries oldest continuously operated whorehouse. The girls were clean, checked daily to keep them that way, and they contributed 5 to 7 million a year to the local charities. When they had the liquidation sale, somebody wanting a piece of history had to bid $50,000 just to get the front door. End of an era as it had been there, a fully functioning, locally respected member of the community for over 140 years. I felt a little sad at the passing of a legend.
Cheers, Gene
That's a far better explanation than mine, thanks. My wet ram, at nearly 80, doesn't always recall the scale of the age, just that it was old and they paid a high premium for it because it was old & well "cooled".
Cheers, Gene