
Dark Matter Experiment Finds Unexplained Signal - pseudolus
https://www.quantamagazine.org/dark-matter-experiment-finds-unexplained-signal-20200617/
======
tech-historian
"Researchers say there are three possible explanations for the anomalous data.
One is mundane. Two would revolutionize physics."

The "mundane explanation" is contamination from inside the experiment.
Hopefully it's not that!

~~~
enkid
But realistically it's probably the mundane explanation, just like the faster
than light travel a few years ago.

~~~
lisper
Yep. You almost don't even have to look at the details. Any time you see a
headline that says some experimental result "could revolutionize physics" the
odds will be very much with you if bet against.

~~~
lou1306
Quasars, pulsars, and the Cosmic Microwave Background all started as
unexplainable experimental data: I know the most likely explaination is the
mundane one, but what if it is not?

~~~
lisper
Yes, that's true. The problem is that the more our knowledge converges on the
actual truth, the less likely new discoveries become. It has been 50 years or
so since the last really new thing was discovered in particle physics despite
heroic efforts. The more time goes by, the more likely it is that this is
because the next breakthrough is simply beyond our reach.

~~~
AnimalMuppet
Um, Higgs Boson?

Now, true, it was expected rather than a surprise. But it was still a
discovery.

~~~
at_a_remove
Visit
[https://en.wikipedia.org/wiki/Timeline_of_particle_discoveri...](https://en.wikipedia.org/wiki/Timeline_of_particle_discoveries)
and, since we are talking about subatomic particles, remove 1995's
antihydrogen and 2011's antihelium-4. Note that the criteria listed on the
page are a little generous. What's that graph look like, plotted on time?

1930s: Three subatomic particles

1940s: Two subatomic particles

1950s: Three subatomic particles

1960s: Three subatomic particles

1970s: _Four_ subatomic particles, that's going up but then ...

1980s: only one subatomic particle

1990s: One subatomic particle

2000s: Two subatomic particles, and that is if you are being generous about
including the quark-gluon fireball, which I wouldn't be

2010s: One subatomic particle

There are only so many to discover, if these are fundamental, and so the
timeline of discoveries per decade must trend down.

~~~
vl
But in the Standard Model each “generation” of particles is so much heavier
than the previous one, and in practice there is a limit on the power of the
accelerator we can build.

Even then there is no reason to believe that there is infinite number of
subatomic particles.

~~~
TeMPOraL
But what happens if you add _more_ energy? Do we know that past some point,
new particles will stop appearing?

~~~
at_a_remove
"Know" in the sense that we can be certain, one hundred percent, never doubt?
No.

In the sense that you would have to convert the Milky Way's entire mass into a
particle accelerator large enough, over the course of millions of years, to
get that much energy to probe even further, ain't nobody gonna do that. You
could go out even more, why not convert the Local Group? At some point you
must accept that we're not _going_ to undertake a billion year engineering
project to do that and so it is irrelevant.

"More" will run out as a useful strategy eventually.

However, there are other clues, which largely live in symmetry. How do we
"know" that there are three families of leptons and quarks? Even in the 1990s,
there were certain cosmological-level hints that the universe would look very
different if there were five; four was just barely allowable if you squinted
very hard.

Don't get me wrong, you can slam together a lot of energy in one place and get
funny things, but they won't be funny things that are new fundamental
particles, which are governed by different symmetries, which you will see if
you begin browsing the, ah, "zoo" of subatomic particles. For a particle, an
anti-particle (even if it is itself), as a symmetry example.

And so you would need new symmetries to explore, which brings us back to the
possible axion of the article, which was suggested by a symmetry. In general,
new particles are suggested either by symmetry or in some cases, simple
conservation, like the humble neutrino, which was originally a placeholder
particle to say, "Where is the spin in that reaction going?"

~~~
AnimalMuppet
> Even in the 1990s, there were certain cosmological-level hints that the
> universe would look very different if there were five; four was just barely
> allowable if you squinted very hard.

Could you (or someone) be more specific here? Or give a pointer to where to
learn more?

~~~
at_a_remove
Short of me digging through boxes of old college textbooks and such, I must
wave my hands about as I struggle with memory. Here goes:

essentially the number of particle "generations" (with the first generations
of leptons being the electron, the positron, the electron neutrino, and the
electron anti-neutrino) has an effect on the abundance of the "light elements"
like Helium-4 during the early phase of the universe. This was pointed out as
a dependence in roughly the 1960s by Hoyle (yes, _that_ Hoyle) and Tayler.
Each generation would increase the density of the early universe, causing it
to expand faster, and in turn the earlier the transmutations of light elements
would have stopped, leaving excess neutrons to go around. This means more
Helium-4. So, measure the Helium-4 abundance and you can set some error bars
on the number of generations.

So the tau particle, in generation three, was predicted around 1971 and
finally detected about six years later. Of course the Nobel for this discovery
wasn't awarded until much later but everyone was on tenterhooks: could we find
a fourth generation? Could it exist? But we didn't have the measurements of
the early universe nailed down at the time.

Measurements of the abundance of these elements around the late eighties or
early nineties is in line with the number of generations being three, but it
could be four at the "couple of sigma" level of confidence. Indeed, originally
it was thought that four was an upper limit in the 1980s.

And so here we are. For a fourth generation, the equivalent of the electron
mass would be >> 45 GeV/c². Simply enormous. My guess is that further
observations have pushed the confidence of "three" much much higher since
then, but I haven't been paying attention.

Of course, the generations aren't confined to leptons (the "lightest"
particles), but also to the quarks. So the first generation has its mirror in
the up quark, the anti-up quark, the down quark, and the anti-down quark.
Again, one of those symmetries I brought up.

------
praveen9920
Experimental physicists journey is similar to startup founder

They have to figure out which "idea" they want to commit for a very long time.
They have to convince a rich person/org to fund their expensive experiment.
Work hard to get things right. Have patience until you get it right.

Success will give you recognition and money and in some cases probably Nobel
(being a unicorn). Failure will get you lot of learnings and probably some
recognition too ( for proving something wrong). Unfortunately, there is one
more state, being in limbo and not going anywhere.

As long as you are having fun in what you do, it is worth it.

Note: I am not an experimental physicist.

~~~
Herrin
I was an experimental physicist, and I'd say that's a reasonable comparison to
make.

The time scales are much longer, unfortunately. Instead of taking one or two
years to get a company off the ground, it can be more like five for an
experiment[1]. I'd attribute this to slower iteration. You end up throwing
away or rebuilding a lot of physical things, many of them bespoke, on the way
to a working apparatus.

Also, the compensation is much worse for experimental physics. For he first
decade of your career, you work as a grad student, then a post-doc, for
slightly above poverty wages. This does have the advantage of meaning you work
with very motivated people, but you're giving up a lot for that.

[1] The XENON experiments from the article (10, 100, and now 1 Tonne) have
been developed over about 15 years, for example

------
torbjorn
Am I the only one left with a befuddled feeling of missing context when it
comes to particle physics? Like what does it mean for dark matter to be
axions? "What's a quark?", Etc.

I feel like I have a context for understanding electrons and protons. I have
read A Brief History of Time. What do I read next in order to have this
article not sound like mumbo jumbo?

~~~
waterhouse
The first four paragraphs of the Wikipedia page on quarks seem decent:
[https://en.wikipedia.org/wiki/Quark](https://en.wikipedia.org/wiki/Quark)

I hadn't heard of axions before, but
[https://en.wikipedia.org/wiki/Axion](https://en.wikipedia.org/wiki/Axion) and
[https://en.wikipedia.org/wiki/Peccei%E2%80%93Quinn_theory](https://en.wikipedia.org/wiki/Peccei%E2%80%93Quinn_theory)
seem to be a starting point.

------
snarfy
With tritium's short half-life, it seems like it could be ruled out by simply
waiting.

~~~
SaberTail
Tritium is actually being continuously produced by cosmic rays smashing up
atoms in the detector. That's why these experiments are buried deep in mines,
but even the small amount that get through the earth can cause problems in a
sensitive detector.

[https://www.sciencedirect.com/science/article/abs/pii/S09276...](https://www.sciencedirect.com/science/article/abs/pii/S0927650517301895)

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jovial_cavalier
Very cool.

Another nothing burger anomaly being reported on by a journalist who
desperately wants to push the narrative that it will change the entire field
so you will click on the article.

I've gotta set up filters for "new particle", "unexplained signal," and
"potential breakthrough."

~~~
ojnabieoot
Wolchover is a great scientific journalist and I can't think of a less
clickbait-y headline than the one provided.

Please set up the filters and take your ignorant bitterness elsewhere.

~~~
jovial_cavalier
Great X Journalist is an oxymoron.

Journalism has no art to it. It's a simple algorithm

> find a story you can contort into something interesting

> interview an "expert" for two hours so you can quote mine one sentence about
> how remarkable the story is

> Publish bullshit article, wait for ad revenue

Maybe this story isn't bullshit, but when your only quote is from a guy who --
by your own admission -- was uninvolved with the research, and all he has to
say is "this could maybe be significant," you don't really have a story, do
you?

It's not Wolchover that I object to, it's the incentives that are set up to
make her subtly overstate the importance of anything she has to cover
(disclaimer: I don't know any of her other work).

95% of journalism should be book-keeping. Dryly reporting the quantitative
results of research. The fact that she has a quote like that suggests to me
that she was digging to make it something more. I don't think that's her
fault; it's just her job. But I have to call it out when I perceive it.

~~~
vdersar1
you do know that quanta is run by a non-profit. if you want book-keeping go to
arXiv.

pretty pathetic that you're this upset

