# Neutrinos Interact!

I was quite a bit surprised when I saw this! Correct me if I'm wrong but I heard and read it somewhere that neutrinos are the particles that don't interact with the normal matter around us. This is what helps us to detect Supernovas before the light from them reaches us.

And now, I saw this pic and read this article about a project named 'Gargamelle' that used Neutrinos and interacted them with electrons in a Freon Plasma. Above is a Pic that was clicked and the white path in the black background is that of an electron...

I would really like to know, how did the neutrinos actually interact with the electrons?! Thanks!

Note by Abhineet Nayyar
5 years, 6 months ago

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Neutrinos almost do not interact with ordinary matter, but they do sometimes. Very very rarely. That's why a number of neutrino detectors have been built around the world in the past 40 years, the Gargamelle project in CERN being the first to successfully detect the weak neutral current. It's become a huge industry, billions have already been spent on neutrino detectors worldwide.

There are now four known fundamental forces in physics, which are 1) Gravity 2) Electromagnetism 3) Strong Force, and 4) Weak Force. The first three "hold things together", the strong force the one holding together atomic nuclei. The weak force instead plays a role in decay processes, first hypothesized by Fermi in 1933 to explain beta decay, in which a proton turns into a neutron and expelling an electron during the process. In the 1970s, it was theorized that subatomic particles could interact via the weak force, mediated by the still-theoretical Z-boson. This theory was the first step towards an unification between electromagnetism and the weak force, predicting the existence of W-bosons as well. Z-boson interactions are the only known means of neutrino elastic scattering, and the Gargamelle project was based on this interaction. It was almost immediately successful in detecting neutrino, a very fast verification of theory in particle physics. Results were announced in 1973.

The Z-bosons and W-bosons were later discovered at CERN in 1983, which won Weinberg, Salam, and Glashow a Nobel Prize in Physics.

- 5 years, 6 months ago

Okay, so what you mean is that since neutrinos dont carry a charge and are not massive enough for gravity, therefore they interact only with weak nuclear force, right?

- 5 years, 6 months ago

And maybe that for opposite spins(due to Exclusion principle), nuclear force is much more weaker, we are getting such a result. For same spins, it is all "quite" fine.

- 5 years, 6 months ago

What do you mean??

- 5 years, 6 months ago

That's right. Since neutrinos do actually have mass, some physicists have proposed that dark matter in the universe could be neutrinos, but that idea has since fallen out of favor. As of this time, there has been no measurement of gravitational attraction of neutrinos. Weak neutral current is it, our primary "link" with the ghostly neutrino world. Trillions of neutrinos are passing through all of us every second, and we're only able to glimpse their existence for the rarest and briefest moments.

- 5 years, 6 months ago

Alright, but have you read about SNEWS?? If neutral currents exist then how are we able to detect supernovas with the help of neutrinos. It is said that since the neutrinos dont interact with matter, so in a supernova explosion, the neutrinos from the explosion reach Earth before photons...sometimes hours earlier, and this helps us to detect them...

Check out this vid by TED ED if u can...thanks Link in the beginning...

- 5 years, 6 months ago

Early detection of supernovas is one of the great successes of neutrino detectors around the world, like this one, Kamiokande II, built deep underground in Japan, which I believe was the first to report SN 1987A.

Like the best astronomical telescopes, neutrino detectors tend to be quite large, in order to increase the odds of detecting any neutrino event.

- 5 years, 6 months ago

I'm talking about at the time of explosion. Imagine a Supernova happening right next to your place. Provided you survive the impact, There would be total chaos, all the particles would be thrown off. So, in these kinds of explosions, why don't neutral currents come into play, and slow down neutrinos??

It is clear that they don't slow them down cuz they are detected way before than the photons.

- 5 years, 6 months ago

Let me rephrase your question to make the question more clear. It is known that when a supernova goes off, the first burst of particles to leave it are neutrinos, hours or even days ahead of light--giving neutrino detectors enough time to alert telescope observatories. You are probably asking, "why doesn't weak neutral currents slow down the neutrinos as they burst outwards through the stellar mass?" The answer is that the vast majority of the neutrinos pass through matter analogously the same way light passes through glass, and since the supernova starts at the center "before the rest of the star knows about it", the neutrinos pass right through out into space. In contrast, light that is generated by fusion reaction at the core of our sun takes thousands of years to reach the surface of the sun, and then only 8 more minutes to reach Earth. Why? Because light photons strongly interact with free ions in the plasma of the sun, while neutrinos doesn't have anything much to strongly interact with on their way out of the star becoming a supernova. Interaction with matter via weak neutral current is a statistically rare event. In an effort to illustrate by analogy, let's imagine that every time a neutrino comes across any other particle such as protons, it pulls a slot machine to see if maybe a hundred wheels coming up all $7$s. If all $7$s, then the neutrino is slowed down. Otherwise it goes on unimpeded.

- 5 years, 6 months ago

Ermm in the Gargamelle experiment, about 200 events of neutral currents were detected and that too in a controlled environment. In this case, neither is the even man made, nor is it controlled, so don't you think the neutral currents should not be such a rare event?

- 5 years, 6 months ago

I'm not sure what you are asking. There's approximately ${10}^{33}$ molecules of water in Super-Kamoikande neutrino detector, and something like ${10}^{16}$ neutrinos pass right through it every second, so that's something like in the order of ${10}^{50}$ possible neutrino interactions with matter every second (assuming each neutrino has equal probability of interacting with any water molecule in the tank). Yet, just $200$ were detected? That seems like an extremely rare event.

- 5 years, 6 months ago

No, i get that but we all know that in a supernova, the matter is not just water molecules, or not just Freon Plasma, or not just electrons. So, I just think, that there must be some bigger reason rather than ignoring it on the basis of it being a rarity.

- 5 years, 6 months ago

Okay, I'm guessing that you think there is some "bigger reason" why neutrinos don't interact with matter much besides the rarity of neutron-matter interaction via weak neutral currents. Am I right?

Particle physics, such as experiments being done at CERN Large Hadron Collider, is dominated by search for [usually] extremely rare particle interaction events. For example, it took months of continuous running of opposing beams of protons colliding with each other, with petabytes of data collected, before finally a very rare few events signaled the existence of the Higgs boson. This is typical. Particle physicists work with extremely rare events, much like how astronomers wring the most information from the most impossibly faint sources of light. Or most impossibly mere variations in light, as with the search for extrasolar planets.

- 5 years, 6 months ago

No, see, I get that the event is a rarity, but lemme ask you...In the Sun, the energy is produced by a process known as Beta decay...in which a neutron converts into an electron and a proton and a NEUTRINO...Now these neutrinos interact via the neutral currents with the Baryons and the electrons to produce energy that makes the Sun glow...Now, if these currents can be a part of the Sun's mechanism, then, why not that of a supernova? Even though, it is a more catastrophic event than the burning of Sun's fuel.

- 5 years, 6 months ago

Okay, I think I see what the problem is. It's easy to produce neutrinos, but once we have free neutrinos, they then very rarely interact with matter. In other words, it isn't symmetrical, it's not like what happens when you have molecules in solution in chemistry, when at all times there are molecules breaking up and forming in equilibrium. Yes, we're getting neutrinos from the sun all the time, it's actually a steady source of neutrinos with which physicists try to tease out some interesting properties of neutrinos, such as the fact the mass of neutrinos oscillate through 3 different masses. For this reason, efforts to measure this phenomenon, being conducted worldwide on such a large scale, is called the "Neutrino Oscillation Industry". It's a big deal. Go look this one up.

- 5 years, 6 months ago

Yea, i have read it...The masses being the 3 Lepton flavours, right? But, I still don't get it...What prevents the large number of neutrinos to not cause large number of neutral currents?

- 5 years, 6 months ago

To better understand that, you need to have a really good understanding of particle physics and quantum field theory. It's difficult to explain in laymen terms, but "current" in "weak neutral current" is nothing like electric or water current. It's not about something moving. It's something that is graphically represented by use of Feynman diagrams, see these links which briefly explains this

Feynman diagrams do not actually represent how particles move, they are just shorthand ways of expressing the complex mathematical descriptions of particle interactions. It allows the physicist to set up the integrations in which to compute probability amplitudes of certain interactions occurring. Notice that Feynman diagrams are sketched on a 2D something, but there are no x-y dimensions given. That is actually a key feature.

- 5 years, 6 months ago

Oh well thanks!! For this discussion..i hope i didnt waste up your time...guess i have a long path to cover, lol...Thanks anyways:)

- 5 years, 6 months ago

Sir,good explanation.But what is neutrino elastic scattering?

- 5 years, 6 months ago

This is one of the rare instances in particle physics where it is exactly what it sounds like--neutrinos are scattered, i.e., bounced, without any loss of total kinetic energy of the particles involved in the neutrino-matter interactions.

- 5 years, 6 months ago

But how can they not lose any energy?t

- 5 years, 6 months ago

Well maybe the collisions are elastic in nature or maybe that energy is converted into mass, which is less likely, as energy is still lost...

- 5 years, 6 months ago

A lot of particle interactions are elastic. As a matter of fact, that is precisely how neutrinos were predicted to exist, when Pauli in 1930 suggested that such ghostly particles "balanced the energy table" in beta decay, i.e. accounted for missing momentum. In inelastic scattering, lost kinetic energy has to be accounted for somewhere, it doesn't just vanish--and the smaller the particles involved, the harder it is to convert it into things like heat energy.

- 5 years, 6 months ago

Sir, I had read an article on neutrinos which said that neutrinos are faster than light! Is it really true? Or, is it just another pile of junk drawing foolish people's attention?

- 5 years, 6 months ago

Heeeey...it has a very nice explaination and is mostly what this discussion is about.. Have a look at the vid by TED-ED about supernovas on youtube, and u might get an idea!! Otherwise, you can always ask! :)

CHEERS!!!!

- 5 years, 6 months ago

Oh! okay thanks! I can't believe it's true. $\text{Thou shall not be faster than the light!}$. But neutrinos are faster than the . . . . . .!

- 5 years, 6 months ago

Haha, I actually read about an article, or probably saw a vid that was titled this!! And i was like.....SEEE YAAA EINSTEIN!!!!

- 5 years, 6 months ago

Quite right. Even I had the same thought after seeing it. I almost fell unconscious! $\huge\ddot\smile$

- 5 years, 6 months ago

This is the "faster-than-light neutrino anomaly" at CERN that caused a brief stir in 2011, but a faulty fiber optic was the cause. The OPERA experiment, designed to track neutrino "flavor", seemed to show faster-than-light travel by neutrinos. While top theoretical physicists doubted the initial reports, some were actually excited at the possibility of anything like that happening, because that's what theoretical physicists do--look for unexpected violations. When the flaw was finally found, life got boring again.

- 5 years, 6 months ago

Hahaha, Well explained @Michael Mendrin !!!:D:D

- 5 years, 6 months ago

Oh . . . ho! So, the flaw made life boring again. Bad news $\huge\ddot\frown$

- 5 years, 6 months ago

I'm surprised this hasn't been posted yet: Lethal Neutrinos - xkcd

- 5 years, 6 months ago

This is great! "Lethal neutrinos!"

- 5 years, 6 months ago

Cool, thanks @Raj Magesh...But, my actual doubt was that what was the main reason that caused the neutrinos in the freon plasma to interact, while neutrinos don't interact so easily...

- 5 years, 6 months ago

where did u get to know all this from

- 5 years, 6 months ago

I was watching some vids On quantum physics by MIT professor Alan Adams and he shows this pic in his class...I researched on it, and read all this...

- 5 years, 6 months ago

I too watched the first few lectures but I couldn't understand the Fourier transform at the time. So I decided to stop there and figure out what it was. But I never did go back there :p

- 5 years, 6 months ago

Oh well, i went through the fourier transform part, and i did get it...It was basically a mathematical theorem that was used in a way to establish a relation between the wave function for 'x' and its amplitude...but i would ask the math geniuses on here to help me as well as u through it!:)

- 5 years, 6 months ago

I realized that I really don't want to start on quantum before I thoughroughly get a good grasp on the classical part of physics. I don't know about you, but school totally ruined the experience of physics. I almost gave up on the subject!

- 5 years, 6 months ago

Haha, aww, well i feel you, but luckily, my physics teacher is pretty awesome and he invites independent thinking along with classic conventional numericals. Plus, as far as i have seen in my school life, knowledge matters a lot more than just marks, so, well i try my best to keep the former to level of the latter:)

- 5 years, 6 months ago

Well, I don't have any information on neutrinos. But here's my guess. Maybe that is because neutrinos are (maybe) Fermi particle(or fermions, whatever you may say). As what I have learnt, they "interact with a negative sign"(this is quite misleading but a good way to remember). Here is where Exclusion principle comes and all the Fermi particles follow it, due to which it is "impossible" to find them in the same state. In that plasma experiment, maybe the energy is made so high that the neutrinos are in an entirely different state than "the electron neutrino" and hence they can interact.

This is just a guess. So, there is a very high probability for it to be wrong(just like the probability for a Bose particle to be found with another Bose particle than at any other place, in an undisturbed space).

- 5 years, 6 months ago

That is actually true...Fermions are the particles with half integer spins, like an electron and proton...and neutrino IS a fermion. But my question actually is, will this amazing phenomenon work for other fermions like the proton as well..If not, then what is so special about a neutrino. Moreover a term Z-boson has been coming up a lot and i'd like someone familiar with it, to explain that!

Thanks!!:)

- 5 years, 6 months ago

Wait, a proton is a baryon, right?

- 5 years, 6 months ago

Yea, but baryons can be fermions, cuz a proton has a spin of +1/2 so...it is a fermion!

- 5 years, 6 months ago

Well, then I fail completely at giving a right answer. Now, it is becoming too complex and a beginner at QM like me, would just not be the right person(as you have already said).

- 5 years, 6 months ago

Anyways thanks for your help Kartik!

- 5 years, 6 months ago