
Trouble Detected in Infamous Dark Matter Signal - dbasedweeb
https://www.quantamagazine.org/trouble-detected-in-infamous-dark-matter-signal-20180412/
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danharaj
> Certain isotopes of argon radioactively decay more or less depending on the
> season.

This sentence puzzled me a bit so i looked at the abstract of the referenced
paper: [https://arxiv.org/abs/1803.10110](https://arxiv.org/abs/1803.10110)

 _A hypothesis is proposed to explain the long-standing DAMA /LIBRA puzzle.
Introduced into the DAMA/LIBRA shielding is a purge gas of nominally high-
purity nitrogen, which under this hypothesis contains argon impurities. Argon
is introduced into the nitrogen purge gas either through leaks in the purge
gas plumbing, or through commercially-supplied bottled nitrogen, diffuses
through materials in the detector housings, and then comes in direct contact
with the DAMA/LIBRA detectors. These argon impurities can then lead to a
modulating 2.8 keV background under two scenarios. Scenario 1): These
impurities include the isotope 37Ar, which decays by electron capture,
emitting a 2.8 keV x-ray. These decays appear as single-site, monoenergetic
events in DAMA/LIBRA, and produce an annual modulation due to the variation of
neutron flux in the atmosphere and at the Earth's surface, which in turn leads
to a seasonal variation in 37Ar production from the reactions 40Ca(n,α)37Ar
and 36Ar(n,γ)37Ar. Scenario 2): Radon is also in the DAMA/LIBRA purge gas,
modulating seasonally at a rate below the current DAMA/LIBRA limits. When
radon or its short-lived daughters decay, the resulting beta, gamma, and
bremsstrahlung radiation cause stable 40Ar to be ionized within the copper
housings surrounding the NaI(Tl) detectors, resulting in characteristic 2.8
keV x-rays. Modulating backgrounds might also result from radon-induced
neutron or gamma-ray flux from the surrounding cavern, leading to a small
modulating background enhanced at low energy by the presence of 40Ar within
the copper housings. These two scenarios are straightforward to test through
assay of the purge gas as well as Monte Carlo and laboratory study of the
DAMA/LIBRA copper housings when excited by ionizing radiation._

~~~
FastCat
I had the same reaction when reading that. I'm frankly still confused. Isn't
the difference in radioactive decay rate only in the atmosphere where it can
be effected by ionizing radiation? Not in a sealed device under a mountain?

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stephensonsco
In the direct-search-for-dark-matter-experimental-community the DAMA results
have long been excluded and therefore ~discredited (see lots of papers from
LUX/XENON/PANDAX/CDMS/etc).

But theoretician's have to keep beating a dead horse to keep themselves
relevant (i.e. publish papers, get PR, and get funding). Kudos for the PR they
managed out of this but it's a yawn for the people actually hunting for DM.

------
ars
Am I missing something, or does this seasonal variation imply that the "sea"
of dark matter WIMP is stationary in the galaxy (which would cause the motion
of the sun around the galaxy to look like "wind").

But actually, if there were WIMP's in the galaxy, wouldn't they orbit in
precisely the same way as the Sun, leaving no relative motion? And thus no
seasonal variation?

There would not even be daily motion, since it's assumed dark matter is cold,
and thus the sun would capture them and they would orbit the sun in the same
way as the Earth.

~~~
cozzyd
The disc of the Galaxy forms due to the self-interactions ("frictional
forces") of ordinary matter. WIMPs shouldn't have those forces and therefore
the dark matter halo is probably spherical. If we assume that the dark matter
and ordinary matter started with the same angular momentum, then the
rotational velocity of stars in the disc should be much faster than the WIMP
halo.

~~~
ars
The only frictional forces of ordinary matter that matter on a galactic scale
are gravity. And that would affect WIMPs equally.

> If we assume that the dark matter and ordinary matter started with the same
> angular momentum, then the rotational velocity of stars in the disc should
> be much faster than the WIMP halo.

I don't see how this can be the case. The orbital period of both would have to
be identical in order to be in the same place.

~~~
cozzyd
Normal matter loses energy through self-interactions (collisions), which is
radiated away. Because the emission is isotropic, angular momentum is
conserved even though the total energy isn't. This tends to make the normal
matter collapse into a disk, which must rotate faster than the original halo
to conserve angular momentum. Dark matter, which hasn't dissipated any energy
(gravity is conservative), can move around more, never decaying into the
orbits that the normal matter does.

~~~
ars
> never decaying into the orbits that the normal matter does.

Exactly my point - it's either in the same orbit as the sun, in which case the
orbital speed is the same, or it's not, in which case there's no wind
interacting with the detector.

You can't have it both ways.

~~~
stephensonsco
The dark matter is a gas not a solid. It's not orbiting the sun, it's orbiting
the galaxy (technically, not even the galaxy, it's orbiting inside it's own
halo mostly). The orbits of the particles would be highly irregular, going in
all directions, many with very eccentric orbits, etc. The sun is moving
through the "rest frame" of the galaxy at a ~constant speed, but the earth is
orbiting the sun so it has fractionally varying relative speed compared to the
rest frame (speeding up and down every year relative to the rest frame).

------
mjevans
Is this a good summary:

Results of current dark matter (effect) detection experiments have yielded
different than expected results.

These results don't seem to fit any current single theory about dark matter*
(it would be nice if there was a nice short enumeration of theories and WHY
they didn't fit with the combined set of observations).

Therefore we should spend more money, building more and different detectors in
different areas run by different groups, and collectively gather more and
varied data so that 'we' can attain a more clear understanding.

\--

However, aside from some of the changes in 'sensitivity' for existing
detectors yielding *"new data" (unexpected results) (this seems like low-
hanging, confirmation of theory, modification of existing experiments).
Lacking from the article is a proposal of what, in theory, the new detectors
might detect and /why/ they might be detecting that, or how different outcomes
from the proposed experiment might provide support for or against current
theories.

~~~
ajross
I think the bigger part of the story is that the criticism involves a
plausible scenario for terrestrial contamination that requires no new physics
at all. Needless to say that needs to be ruled out before we spend more time
than necessary pontificating about dark matter.

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dschuetz
Yes, I see the trouble. The team "strongly arguing" that they have found the
source of the signal while not sharing their raw data.

Evidence for dark matter is a big deal so who wants to give away credit for
that? Or, who wants to share inconsistent data when the results are literally
earth shattering? Visible matter interacting with some invisible forces only
at certain conditions! Sensational!

Perhaps the Aether Hypothesis is true after all?

------
banachtarski
Personally, I think any experiment that publishes plots and not actual raw
data should be defunded and decommissioned immediately.

~~~
stephensonsco
So you think the LHC should "publish" 100 petabytes of data?

What you are stating isn't practical because of cost.

But I'll definitely go with the idea that "if you want to make a claim as big
as finding dark matter and be believed, then releasing your data is probably a
good idea".

~~~
sgt101
It's worse ! The data for the Higgs _can 't_ be published because... it was
destroyed!

The aggregate, processed signals are all that is retained in the LHC, the raw
data was gone before it could be analysed.

Also they used a bloody hokey boosted classifier for the detection but that's
bye the bye now apparently. And there were 12 events out of about 1 trillion,
so all good there too...

~~~
thiagotomei
That is not true, at least for CMS. All the RAW data taken in pp collisions
during 2011 and 2012 - ie the Higgs boson discovery dataset - has been saved
in tape. As a general rule, we never delete RAW collision data that actually
make it into permanent storage. Of course, data that didn't pass the real-time
selection to be recorded in the first place is irrevocably lost.

~~~
sgt101
So all the data apart from the data that was thrown away?

