Comment by _hark | Hacker News Reader

Comment by _hark | Hacker News Reader

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I worked on this experiment as an undergrad ~10 years ago during my freshman year! We built a Cherenkov radiation detector, focusing magnets, and did tons of simulations.

This is all from memory, but I remember the beamline setup was to get protons from the accelerator there, smash them into a target, which produced various charged particles which could be focused with the magnets, sent down a long pipe where they would decay into neutrinos et al. Then, there's a near detector and a far detector (far detector deep underground in South Dakota). The aim is to measure the neutrino flavors at both detectors to better understand the flavor oscillations (and look for asymmetries between neutrino/anti-neutrino oscillations, hopefully to help explain the matter/antimatter asymmetry in the universe).

The particular bit I worked most on was studying the effects of adding an additional solid absorber at the end of the beamline, which was needed to absorb all the particles that didn't decay in the pipe. It would produce more neutrinos that were unfocused, so would affect the near-far flavor statistics (since these would be detected at the near detector but not the far since they were unfocused, ruining the statistics). It was a great intro to doing physics research :-)

Fascinating. But I (non-physics person) don’t understand how the neutrinos won’t interact with atoms in the earth’s crust and their trajectory be deviated in the long journey between the 2 sites.

For typical solar neutrino energies, and given Fermilab and South Dakota are about 750 miles apart, around one in 10^15 neutrinos would interact with the ground (more specifically, the nucleus of an atom in the ground).

It would take about 10^14 miles (17 light years) of solid lead to block 50% of neutrinos.

Imagine that you are a tiny, tiny, tiny neutrino. You are a million times smaller than an electron. You are drifting through space, and the only other component of space that you can interact with is the nucleus of atoms. But atoms also have electron shells that repel each other, so the nuclei have to be centered inside electron clouds that refuse to go anywhere near each other. The nucleus of each atom is only 1/25,000 the width of the electron cloud, or the "full width" of the atom.

In other words, travelling through solid lead, for neutrinos, is like flying through mostly empty space.

From the perspective of a neutrino, in solid lead, if each lead atom was centered on a star in the Milky Way galaxy, each lead nucleus would be the diameter of Jupiter's orbit. Everything else would be pure emptiness.

Continuing with this blown-up scale, you would need to fly your spaceship for 10^27 lightyears before you have a 50% chance of flying within the Jupiter orbit distance of a star.

The universe is only 10^11 light years wide, so you would need to fly through 10^16 universes before you have a 50% chance of passing within the Jupiter orbit of a star. And this is assuming the entire universe is as dense as the milky way's disk around us.

Addendum: 1,000km of neutron star material would block 50% of solar energy neutrinos.

Unfortunately, a neutron star larger than 22km diameter collapses into a black hole.

Lovely explanation! Thank you! It is simultaneously crazy that neutrinos can pass through so much matter unhindered, yet intuitive that when something is so small and can only interact with such a small fraction of the volume it is passing through - it can indeed pass unhindered.

I believe they might, but it's all a question of how large or small the probability is. And in the case of neutrinos, my lay-person understanding is that the probability of interaction is incredibly small.

Anytime I hear about neutrino-related numbers my mind is blown. I have a hard time truly wrapping my head around these numbers... for eg.: "At the surface of the Earth, the flux is about 65 billion (6.5 × 10^10) solar neutrinos, per second per square centimeter." (wiki intro)

It's a big part of what makes them hard to detect. They interact with almost nothing. (source: i'm not a physics person either, but i watch a lot of physics videos)

How far away is the far detector?

800 miles.

https://lbnf-dune.fnal.gov/how-it-works/detectors-and-comput...

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