The first observation of neutrinos at CERN's Large Hadron Collider | Hacker News Reader

The first observation of neutrinos at CERN's Large Hadron Collider

phys.org

129 pointswglb24 comments

Have I read over it, or does the article not answer the main question I expect most people have with such a headline: how?

The only descriptions I see are "advanced equipment" and that it's 2 meters and has to filter muon background noise. I thought neutrino detections were done with a few cubic kilometers of ice, but there's no mention of that. Does the higher rate of production/concentration simply let them do it with a small device, or is there actually a new invention here?

I haven't read the full article yet but the abstract has some hints: I think what they're doing is leveraging the fact that they know where and when the neutrinos were created to narrow down the search in the actual neutrino detector itself.

Because they can see the associated muon, they know when to look for a correlated neutrino signal.

Between that and the extremely hot source, they can get away with a relatively small active detector volume.

Again that's just based on my reading of the abstract, I need to see the full article to validate that guess.

relevant pre-print is here: https://arxiv.org/pdf/2305.08665.pdf

that'll be my evening reading :)

This appears to be the reason:

> Until now no neutrino produced at a particle collider has ever been directly detected. Colliders copiously produce both neutrinos and anti-neutrinos of all flavors, and they do so in a range of very high energies where neutrino interactions have not yet been observed. Nevertheless, collider neutrinos have escaped detection, because they interact extremely weakly, and the highest energy neutrinos, which have the largest probability of interacting, are predominantly produced in the forward region, parallel to the beam line. In 2021, the FASER collaboration identified the first collider neutrino candidates 13 using a 29 kg pilot detector, highlighting the potential of discovering collider neutrinos in LHC collisions.

> forward region, parallel to the beam line

What collides here exactly? I always thought collider do throw a particule A in the direction of particule B while B travels in the direction of A, with measure instruments all around but not blocking one of the particule path. The photographie 0 make me wonder if they also crash particules directly into their instruments?

0 https://scx2.b-cdn.net/gfx/news/hires/2023/the-first-observa...

Well, you sort of have to have the particle collide with something in order to detect it. When a photon collides with your retina, you see a flash of light (it causes a protein to twist, which generates an electrical signal, which is sent to your brain). The problem is the neutrinos tend to pass right through without getting absorbed by anything. No absorption, no change, no detection.

So the neutrino has to collide with something to get detected. Given that previous neutrino detections require a large vat of heavy water underground, while the current results are from a little box, the salient question is what did they do differently (and is it applicable elsewhere). The article completely ignores this.

The experiment is described in this[1] article. Relevant quote:

The detector is positioned on the beam collision axis line-of-sight (LOS) 480 m from the ATLAS collision point (interaction point 1, IP1) in an unused service tunnel, TI12. [...] A huge number of neutrinos are produced in LHC collisions via hadron decays, and their flux is collimated along the beam collision axis.

So what they did differently is to place the detector in a spot which has much higher neutrino flux than your average spot on earth. Thus the small detector volume is compensated for by having more neutrinos pass through it in a given time.

[1]: https://arxiv.org/abs/2207.11427

Neutrinos pass through the entire planet from space, so hopefully they will be able to see the background neutrinos as well as the neutrinos from their collisions.

If this was a brimstone and fire depiction for those with a religion vent, neutrinos would be likened to ash, the other particles would be various sizes of burning embers, some large but glowing gently (beta), others small and burning brightly (alpha).

Its all contained in a giant underground magnetic donut that would have Homer Simpson salivating which accelerates the particles, like a rail gun or steam catapult on an navy aircraft carrier or a catapult launcher for roller coasters, and it keeps accelerating these particles, like a child with ADHD on sodium benzoate can accelerate a fidget spinner, until the particles collide, like the crescendo of a fantastic firework display at a country's official New Years eve display watched through computer screens that's somewhat reminiscent of the 1979 Atari Inc Asteroids computer game when an asteroid explodes.

The electrical demand for the magnets is great but short term like the Death Star blowing up Alderaan, so whilst its CO2 footprint could be likened to a large Cruise Liner over the year, it generates this footprint over extremely small periods of time when running these experiments like something out of Weird Science when Kelly LeBrock comes to life!

So the electrical infrastructure is highly capable.

The electrical companies need advance knowledge to make sure they have ordered in enough coal and gas, and topped up the hydroelectric dams, when they run their experiments, otherwise the neighbourhood experiences something of blackout like in the movie Batteries Not Included.

https://en.wikipedia.org/wiki/Neutrino

SuperK (not the raving form of Ketamine being consumed down the Dog and Duck pub on weekdays) https://en.wikipedia.org/wiki/Super-Kamiokande#Description

> Until now no neutrino produced at a particle collider has ever been directly detected.

Is this considered different from e.g. the OPERA experiment because in that they dump the proton beam from the SPS into a target? So what you're observing here are neutrinos from the actual beam crossing? And is this interesting because you could conceivably start to correlate the actual collision that creates the neutrino with its eventual detection?

They seem to point to high flux and high neutrino energy offsetting the weakness of neutrino interaction, with the neutrinos being the highest energy recorded in a lab environment.

Neutrinos from cosmic sources are incredibly rare. If you want any real chance of detecting them you need large detectors. The neutrinos from this CERN experiment are much more concentrated. That allows them to use much smaller detectors.

my understanding is the opposite. neutrinos fill the universe similar to photons, they are just extremely hard to detect because of their weak interaction with other matter. So detections are rare, not neutrinos themselves.

Light fills the universe, but a telescope will be blinded if you shine a flashlight into it.

right, because of light's interaction with matter which is different from neutrinos

a quick google estimates that a human has about 100 trillion neutrinos pass through their body every second, but only about 25% chance that even a single one will interact with any atom in your body over a lifetime.

it is a detection issue, not a quantity issue.

No, the telescope will be blinded because you shine at it with a lot of the kind of photons it has been constructed to detect, the actual space filling photons are only visible in the radio band which (optical) telescopes don't see and so aren't bothered with.

The situation with neutrinos is exactly analogous: IceCube looks for high energy cosmic neutrinos, not low energy background neutrinos. The latter are only of interest to dark matter searches.

its really not though. Detection is fundementally different. Neutrinos have no electric charge. the only thing you have to work with is the weak interaction. you have to literally wait for the low probability event of a neutrino-neuclei collision or detect by a second hand effect.

Dude, the only important difference is the interaction cross section, in practice virtually all particle detections happen via secondary effects: for example CCD chips also observe the secondary interaction effect (of a electron being kicked out), and with gamma ray detectors you often don't see the entire secondary effect because the photon exists the detector before being fully absorbed.

The Wikipedia article is a decent intro to the general challenge: https://en.wikipedia.org/wiki/Neutrino_detector

The paper this article is referencing includes a citation to a much longer paper about FASER itself that explains more about how it is designed and why: https://arxiv.org/pdf/2207.11427.pdf

The gist of it seems to be this was the first attempt to put a neutrino detector n nthe director line of sight of a particle collider where they're produced. The LHC's other detectors have blind spots where the neutrinos are produced. The general challenge with neutrinos is they don't interact via the electromagnetic force. This is what is meant when we say they just "go through" normal matter as if it wasn't there. Because mostly, it isn't. Matter is overwhelmingly empty space. It's the large interaction radius of the electromagnetic force that causes matter to behave as if it's solid. Electrically-charged particles do not need to be remotely near each other to interact. Neutrino mass is too negligible to detect anything gravitationally, so that leaves us only with the weak force, but that has a tiny interaction radius. They need to make a more or less direct collision, and the things colliding are unimaginably tiny. It's like throwing a bunch of baseballs into interstellar space and seeing if they ever hit anything. But also, the more energy a neutrino has, the more likely it is to interact, and also more likely to have the so-called "charged current interaction" in which it transforms into a regular charged lepton that can be detected. It needs a lot of energy to convert into mass to do that because it has so little mass of its own.

So I think it's mostly those two things, somewhat higher concentration and greater energy. But probably mostly the energy. There are already billions of neutrinos passing through every cubic centimeter of the earth every second. The sun produces way more of them than we can ever produce ourselves.

I'm not sure this is really the first detection like the article claims, too. The FASER paper says they detected neutrinos from LHC run 2 in 2018.

Detection is via quite a simple method if I understand correctly - in the paper, section 7. It's basically a big camera. It's hundreds of stacked layers of tungsten and photo emulsion. Big heavy stuff because it (presumably) has a higher interaction cross section, but this obviously isn't practical to build at scale. There are also several downstream instruments to measure particle properties once they exit the emulsion block, and "veto" layers to ignore muons which get into the detector, but neutrino+tungsten is what causes the magic to happen. The layers are also stacked in such a way that different flavours can be identified.

You don't detect neutrinos, you detect the things they create when they interact with other atoms (in this case Tungsten). These particles then interact with the emulsion to form tracks. You can read more about it here: https://cerncourier.com/a/nuclear-emulsions/

3-4 times a year they take all the emulsions out and process them. Apparently that's often enough and they can see 10^6 tracks per cm2.

On IceCube: it's a telescope. It's designed to locate the source of extra-galactic or other extremely high energy neutrinos. The flux of these neutrinos is lower and the larger the detector the better - not just for detection probability, but for angular resolution on the sky. Ice happens to be a good medium for Cerenkov radiation (other neutrino detectors work by looking for the ~blue light that the interaction products emit as they travel). The other option for a detector that large is under the sea, as there is no light after a few hundred metres (ANTARES is over 1km down), but there's more background/biological radiation and the sea moves... IceCube is also shearing a small amount, but significantly less. Despite the harsh environment, it's an incredibly reliable detector - cold and dry is probably nicer than salt water.

This comment section was a quick favorite. Thanks to all the smart folks here who fleshed this article out with uncommonly detailed flair. The plebs like myself should raise a glass.

> This work has shown that high energy experiments can also study neutrinos, and so has brought together the high-energy and high-intensity frontiers.

Can someone explain to a layman the difference of HE and HI experiments?

High energy refers to the energy imparted to particles, high intensity refers to the number of particles (presumably of the particle beams), at least if the terminology is anything similar to synchrotrons.

So a high intensity 3GeV beam would be one where the average particle energy is 3GeV, with a larger number of particles comprising the beam than a low/medium intensity one.

High intensity: lots of things. High energy: the things punch hard.

inb4 hft.

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