Sarafina Nance @starstrickenSF · Sep 10

one of the coolest particles in the Universe is invisible.

it’s super low mass.

100 trillion pass through your body every single moment.

it travels at nearly the speed of light.

it has no electric charge.

it flummoxed scientists for decades.

𝘁𝗵𝗲 𝗻𝗲𝘂𝘁𝗿𝗶𝗻𝗼.

Sarafina Nance @starstrickenSF · Sep 10

my favorite story about the neutrino is the Solar Neutrino Problem...

to start the story, let’s review how the Sun generates energy.

the Sun—and stars in general—fuse elements in their core. that energy is released in the form of starlight, which we see from earth!

Sarafina Nance @starstrickenSF · Sep 10

neutrinos are byproducts of the nuclear fusion process taking place in the core of the star.

they travel to the surface of the star, pass through the outer layers, fly through the interstellar medium, reach the earth, fly through us, and continue on their journey..

Sarafina Nance @starstrickenSF · Sep 10

based on the solar luminosity, the distance to the sun, and the amount of energy of each neutrino, we can calculate the expected neutrino flux from the sun.

in other words, we can estimate the number of particles/second/cm^2 we should see.

and the answer shocked scientists.

Sarafina Nance @starstrickenSF · Sep 10

to measure the neutrino flux, scientists build HUGE underground detectors.

because the interaction cross section of neutrinos is SO small, we’d need a HUGE detector to get a high enough interaction rate.

Sarafina Nance @starstrickenSF · Sep 10

detectors are filled with water etc bc 𝘵𝘩𝘦 𝘴𝘱𝘦𝘦𝘥 𝘰𝘧 𝘭𝘪𝘨𝘩𝘵 𝘪𝘯 𝘢 𝘮𝘦𝘥𝘪𝘶𝘮 𝘭𝘪𝘬𝘦 𝘸𝘢𝘵𝘦𝘳 < 𝘵𝘩𝘦 𝘴𝘱𝘦𝘦𝘥 𝘰𝘧 𝘭𝘪𝘨𝘩𝘵 𝘪𝘯 𝘢 𝘷𝘢𝘤𝘶𝘶𝘮.

that means it’s possible for particles to 𝐦𝐨𝐯𝐞 𝐟𝐚𝐬𝐭𝐞𝐫 𝐭𝐡𝐚𝐧 𝐭𝐡𝐞 𝐬𝐩𝐞𝐞𝐝 𝐨𝐟 𝐥𝐢𝐠𝐡𝐭.

Sarafina Nance @starstrickenSF · Sep 10

for particles that move faster than the speed of light, light is emitted in a cone around the direction of travel. this is called Cherenkov radiation!

Sarafina Nance @starstrickenSF · Sep 10

enter the first solar neutrino detector: the Homestake mine detector.

this is a 600m ton tank (!!!) of C₂Cl₄ 1500m below ground in South Dakota.

when the neutrinos interact with the chlorine, they produce an isotope of radioactive argon. then it gets removed from the tank..

Sarafina Nance @starstrickenSF · Sep 10

scientists wait to watch the argon decay into other electrons, which is the signal we’re looking for!!

great. so. we’ve measured our neutrinos. now what?

Sarafina Nance @starstrickenSF · Sep 10

here’s the problem. the measured values of the neutrinos were 1/3 of what theory predicted.

live view of scientists at the time: https://t.co/omC7JJectu

Sarafina Nance @starstrickenSF · Sep 10

so scientists did what all good scientists do, and did follow-up experiments on detectors like the Soviet-American Gallium Experiment and Super Kamiokande.

they all measured a deficit of 1/3 of the expected value of neutrinos.

Sarafina Nance @starstrickenSF · Sep 10

SO. either the sun is producing far fewer neutrinos than expected, or neutrinos are somehow getting lost on their way to earth.

Sarafina Nance @starstrickenSF · Sep 10

enter the Sudbury Neutrino observatory, a 1,000 ton detector 7,000 feet underground.

now this detector is a little different. it uses heavy water D₂O. why does that make a difference..? https://t.co/eaciYqrwF0

Sarafina Nance @starstrickenSF · Sep 10

WELL— and here’s what’s cool— the heavy water can separate neutrinos into their three distinct flavors:

𝗺𝘂𝗼𝗻. 𝗲𝗹𝗲𝗰𝘁𝗿𝗼𝗻. 𝘁𝗮𝘂.

Sarafina Nance @starstrickenSF · Sep 10

the electron neutrino is detected via a charged current.

muon and tau neutrinos are detected via elastic scattering by bouncing off electrons.

finally, neutral current allows the neutrino to break up a D₂O molecule. this works for ALL neutrino flavors.

Sarafina Nance @starstrickenSF · Sep 10

and we finally figured it out...

the 𝗧𝗢𝗧𝗔𝗟 number of neutrinos agreed with theory.

the number of 𝗲𝗹𝗲𝗰𝘁𝗿𝗼𝗻 neutrinos is 1/3 of the total.

which means.. neutrinos from the Sun 𝗖𝗛𝗔𝗡𝗚𝗘 𝗙𝗟𝗔𝗩𝗢𝗥𝗦 en route to earth, morphing into each other.

Sarafina Nance @starstrickenSF · Sep 10

even weirder: the Standard Model of physics— the theory of fundamental particles and how they interact— predicts that neutrinos should be massless.

but for this neutrino oscillation to happen, they must have SOME very little mass.

Sarafina Nance @starstrickenSF · Sep 10

which means we learn two things:

1) neutrinos oscillate into different flavors (muon, electron, tau) as they fly through interstellar space

2) the Standard Model is wrong. neutrinos must have mass.

Sarafina Nance @starstrickenSF · Sep 10

this discovery revolutionized particle physics, earning the 2015 Nobel Prize in Physics. clearly there is physics behind the Standard Model..and we’re just at the beginning to learn what that’s like.