Is dark matter hiding in the neutrino fog? | Even Bananas

0:00 Neutrino scientists might spend their whole

0:02 careers building beautiful and intricate machines

0:05 to try to see neutrinos and treasure every last one they find.

0:09 But then some dark matter scientist accidentally snags a couple

0:12 in their dark matter detector and is more disappointed than delighted.

0:17 Why do dark matter physicists think that nutrinos are the worst?

0:22 That's what we're talking about today on Even Bananas.

0:31 There's a lot of evidence of some kind

0:34 of unseen mass or dark matter filling the universe.

0:37 There are many theories about what it actually is,

0:40 but it's almost certainly nothing we know about, including neutrinos.

0:43 You might remember we've discussed that before in this video.

0:49 Nevertheless, our favorite ghostly particles can pose

0:52 a problem for certain dark matter searchers, particularly WIMPs.

0:57 These so-called weakly interacting massive particles,

1:01 are theoretical particles that have mass,

1:03 typically quite a lot of mass by particle standards,

1:07 and only interact weekly with nuclei.

1:10 WIMP hunters are not that different from nutrino hunters.

1:14 They wait for interactions in big sensitive

1:17 detectors that need protection from cosmic rays.

1:20 So they tend to be built deep underground.

1:22 Turns out a mile of rock can stop a lot of noise.

1:26 But can it stop a neutrino?

1:28 Clearly no.

1:29 Or else a lot of underground neutrino experiments would be off to a poor start.

1:34 But that means that dark matter detectors are always going

1:37 to have some neutrinos scooting through that might interact in the detector.

1:42 The real problems start when you can't tell the difference

1:46 between a wimp and a sneaky neutrino that shows up uninvited.

1:50 Speaking of sneakily showing up, though in this case very much invited, Dr.

1:55 Kimberly Paladino is a WIMP hunter who works on the Lux Zeppelin experiment,

2:00 which uses liquid xenon to search for dark matter.

2:03 Maybe she can help us understand exactly why

2:06 dark matter scientists think nutrinos are the worst.

2:09 Hi Kim.

2:10 Hi, Kirsty.

2:11 Now, surely you can tell the difference in your detectors

2:15 between these massive dark matter particles and almost massless neutrinos.

2:20 Well, it's not the mass that directly affects what we see,

2:23 but the momentum, which is the mass times the velocity.

2:26 And in that case, if you have a very low mass neutrino moving

2:29 at very high speeds and a high mass WIMP moving at a low speed,

2:34 you might come out with about the same momentum.

2:36 So, how would you ever know if you spotted

2:39 dark matter or if it's just a rogue neutrino?

2:42 In some cases, it's easier to tell than in others.

2:45 Some neutrinos rock up to a costume party dressed as dark matter,

2:49 but you don't have to look very hard to realize it's not a dark matter particle.

2:53 But other neutrinos have really great costumes,

2:56 so it can be much harder to tell.

2:59 So far, we've only seen a handful of nutrinos in bad costumes at our parties,

3:03 but eventually they're going to get pretty crafty.

3:07 This is what we call an exclusion plot,

3:09 and we use it to show where we've searched for dark matter in terms

3:12 of how heavy a particle would be versus how strongly it would interact.

3:17 As you can see, we've ruled out heavy

3:19 particles with fairly strong interactions with our current detectors.

3:23 Essentially, if a dark matter particle

3:25 had this mass and this interaction strength, we would have already seen it.

3:30 As our experiments get more sensitive, we're able to search over more options,

3:34 and this line gets lower and further across.

3:36 But at the bottom here, our detectors pick up more and more neutrinos.

3:42 Some of them, like these boron-8 neutrinos, which come from the sun,

3:45 are quite well constrained, so we can in some sense work around them.

3:49 But these atmospheric neutrinos,

3:50 which come from cosmic rays hitting our atmosphere,

3:54 those are the ones that are wearing really good disguises and will

3:57 look a lot like dark matter signals in a liquid xenon detector.

4:01 That means that as our detectors get bigger and more sensitive,

4:04 they won't be able to distinguish any dark matter signals from all the nutrinos.

4:09 That's what we call the "neutrino fog".

4:11 At the moment, we're okay, but it's going to be a problem in the future.

4:16 Okay, but can't you use some statistics to figure out

4:19 if there's dark matter there as well as the neutrinos?

4:24 Demystifying it with maths.

4:26 It's a nice idea, but the statistics aren't in our favor here because

4:31 we're expecting to see lots of neutrinos

4:33 and relatively few dark matter interactions.

4:35 Suppose you expect to spot about five neutrinos in a year.

4:38 If you see 10 interactions in your detector, you might get excited.

4:42 It's really unlikely that all 10 are neutrinos.

4:46 But now imagine you're expecting to see about a thousand neutrinos

4:49 and you detect a thousand and five interactions in your detector.

4:53 That's still about a thousand.

4:55 So you aren't going to be able to tell if those extra five

4:58 are dark matter particles or just

4:59 a random fluctuation in the number of neutrinos.

5:02 In order to definitively say that we found a dark matter signal in this case,

5:06 we'd need at least 30 extra interactions.

5:08 And at that point, looking at our exclusion plot,

5:11 we know that dark matter won't interact very often.

5:14 That's why the neutrino fog is a problem.

5:17 Now, does that spell the end of dark matter detectors?

5:20 Can't you just put your fog lights on and keep driving?

5:23 Well, there's not much you can do to shield against neutrinos.

5:26 They're just going to keep coming through

5:28 and any detector sensitive enough will pick them up.

5:31 But there's still a lot of searching we can do

5:33 with current detectors and the next generation of detectors, too.

5:36 And we never know when a dark matter interaction might happen.

5:39 Maybe next week, maybe next year, maybe in 5 years time.

5:43 If we get lucky, we might never need to worry about the neutrino fog.

5:47 Plus, there are ongoing efforts researching technologies

5:49 that could help us solve this problem.

5:51 Even if it does spell the end for this kind of search,

5:56 other dark matter candidates like axons are fogfree.

5:59 Maybe dark matter sciences loss is neutrino sciences gain.

6:02 Are there interesting things you can

6:05 find out about nutrinos from your detections?

6:07 Absolutely.

6:08 Because dark matter experiments are optimized to search for dark matter,

6:11 they're set up differently to neutrino experiments.

6:14 So experiments like Lux Zeppelin can measure energy deposits a thousand

6:18 times lower than the lowest energies neutrino experiments like DUNE can spot.

6:22 It's even possible atmospheric neutrinos that hide the dark matter will show

6:27 us some new neutrino physics such

6:29 as sterile nutrinos or unexpected nutrinoagnetic moments.

6:33 Sounds like it's a good time to be looking for ghosts.

6:36 Thanks, Kimberly.

6:37 Thanks, Kirsty.

6:38 [music] Great to spook to you.

6:42 So, in the future, the neutrino fog really is going

6:44 to limit the ability of wimp detectors to search for dark matter.

6:48 At that point, WIMP hunters will have to go back to the drawing board.

6:52 But we're not at that level yet.

6:54 And in the meantime, maybe, just maybe,

6:58 they'll spot a WIMP and perhaps even shed some light on neutrinos.

7:03 Would you rather spot dark matter or find some cool new nutrino physics?

7:08 Let us know in the comments.

7:10 And don't forget to like and subscribe.

7:12 Otherwise, you might find you've missed the next episode of Even Bananas.

7:17 Mist, as in fog, as in neutrino.

7:22 Oh, fog-get it.

7:24 Fun fact, it was way back in 2007 that physicist Joselyn Monroe

7:29 first realized neutrinos were going to be a problem for dark matter searches.

7:34 Back then, Monroe called it the neutrino floor.

7:37 But now, scientists have a bit more hope

7:40 and have started to call it the neutrino fog.