Oltre il Modello Standard: quello che non sappiamo del mondo subatomico

Added: 2026-05-22

[00:00:08]If people ask me today what do we know and how much do we know about the subatomic world about particle physics I'm going to tell you you know there are some problems there are some things that we are not totally sure about but in general we know a a fair amount And what I think is interesting is that if we go back in time to about 100 150 years and ask physicists the same question, they will probably give you a very similar answer. And in fact, I found this quote.

[00:00:45]>> Thank you. I found this quote from 1894 from Sir Michaelson which was a very important physicist. He measured the speed of light and won a Nobel Prize and in 1894 he said the more important fundamental laws and facts of physical science have all been discovered which I thought was quite funny because everything I will tell you today has been discovered after 184 and they didn't know as much as they thought back then. They didn't even know in fact that there was an electron, a proton, and a neutron. And they didn't have like a good picture of the atom.

[00:01:30]What we know now is that we have a nucleus made of proton and neutron at this at the core of the atom and then some electrons that orbitate uh around it. And thank you. Um, a better picture really of the atom is this one with uh the nucleus being very very small and the electron being more of a probability cloud.

[00:02:00]And I wanted to tell you uh how much we knew back in 1933. Why did I pick this year? It's not the best year for history.

[00:02:11]But but for physics it was a good year.

[00:02:15]Um and so we start by saying which particles we knew of in 1933. So of course we knew of the electron, the neutron, the proton but also they knew of a thing called the posetron which is an anti-electron.

[00:02:32]So what happened in those years was that uh a physicist named Dak found an equation that was supposed to describe particles and in particular he was trying to make it work for the electron and he had this equation went on and solved it and found two solutions one with positive energy and one with negative energy.

[00:03:00]So the one with positive energy all good that's the electron positive energy also means positive mass because we often consider in physics energy and mass as two sides of the same concept.

[00:03:14]But what about the negative energy solution?

[00:03:18]Some people thought it was just mathematical flow and you could just ignore it. But Durk thought that maybe that meant something.

[00:03:29]And so he went on with this concept and try eventually to uh link this negative energy solution with some kind of particle.

[00:03:40]And later on with uh more mathematical tools they discovered that that was another particle and it wasn't so much negative energy that was a problem they they had had with the theory. It was more a particle with opposite charge. So if the electron has negative charge, the posetron had positive charge.

[00:04:02]That's the name. And they found the positron. Uh this guy Carl Anderson found the positron in 1932.

[00:04:12]And the way that he found it was he had this kind of uh chamber, a cloud chamber. So you have this box with vapor inside. You want to keep the vapor u in a state so that it is almost almost almost almost uh water but not quite so that even just a single particle is enough to pass into that to make the grape condense and leave a trace. And this is what happens here.

[00:04:50]And so you have these uh trucks and you can have these machines. I don't have one here. Um but you could just like put it here and wait and from the space you will get particles and you will just see trucks and trucks and trucks from directions. But that doesn't give you enough. So what he did was to put this chamber inside a magnetic field because the catch is that a magnetic field makes charged particle um spin and this is why you see these trucks are are bending and the thing is that they spin with a radius that is connected to how massive they are. So these two tracks having the same radius means that they have the same mass and charged particles bend clockwise or anticlockwise depending on their charge. So here we have that one bends clockwise and one bends anticlockwise. So one is an electron and the other one is a positron and that's how they discovered the positron.

[00:06:03]And this was the first particle they discovered of antimatter. And later on they went on they discovered that basically every particle has an antiparticle.

[00:06:16]What else did they knew about particles in 1933?

[00:06:21]They knew that there may be something called dendrinum which really means something small and neutral. And they didn't know much more.

[00:06:33]They knew it was a particle that was supposed to be there maybe uh small, neutral, and very weakly interacting.

[00:06:42]And when I say weekly interacting, I mean very weekly interacting. Uh in fact, you could build a wall from here to the sun made of lead and make an entrino pass through it and it wouldn't interact. you would have to build a wall from here to I think one light here to make interrath.

[00:07:08]Um so who um postulated this particle actually apologized for it because he say done a terrible thing and postulated a particle that cannot be detected.

[00:07:22]But why did he have to hypothesize this particle?

[00:07:29]The reason is uh this process which is the beta decay. So already in these years they knew that there was um something that was happening in big nuclei uh in which in big nucle you can have like many neutrons and many protons. Uh these are not like small ones such as hydrogenous or helium. These are more like uranous. Um and you could have sometimes this happens that a neutron becomes a proton and emits an electron and this is what we call radioactivity.

[00:08:10]But the problem was that if you don't know about the nutrino, if you don't consider any ghost particle to be there, you have you have a neutron and you can imagine that the neutron is just steel. At some points it emits a particle, let's say in this direction. And then you have a proton that gets recoiled a little bit.

[00:08:37]But if you add it up, it doesn't recoil in a way that makes sense. To make an example, if you clash two cars together and they go into pieces and then also pieces go and fly in the same direction, that would be physically very very strange. There is a thing that is important that is uh energy conservation and momentum conservation. And in this process uh they were they were measuring the neutron, they were measuring the electron and measuring the proton and adding it up the initial and final energy uh of these two uh states were the same.

[00:09:19]And so either you accept that either energy is not conserving or momentum is not conserving which is not something that physicists really want to do because physics is mostly energy conservation. Um or you do what PI did.

[00:09:36]So you invent a new particle. You invent a ghost particle that maybe gets emitted here and goes in the other direction.

[00:09:45]And so it makes sense that your final state is the one that you measured.

[00:09:52]And so this was the overall picture in particle physics in 1933. And this is where uh Eromy entered.

[00:10:02]We go on. Thank you. Um so Fermy attempted to solve uh the beta decay and this is known as FM's tentativo. his attempt and he thought, okay, let's let's put the pieces back together. We have a neutron, we have a proton, a neutrino, and an action. But he asked himself, what happens in the middle? What happens here? And he came out with this. That's cool.

[00:10:38]Impressive.

[00:10:42]So um without knowing anything else he just thought there must be a pointlike interaction between these four particles and these would be instantaneous and just a vertex interaction. He didn't he didn't know anything else about these particles. So it wasn't actually that that trivial of an idea because he also thought he also thought maybe the neutron and the prozol are just two states of the same particle and they can go from one to the other with an emission of a nutrino and an electron.

[00:11:21]It was it was a first try in understanding the beta decay and yeah when they started to collect data it wasn't that bad like it was predicting the data quite quite well. The problem was more on a mathematical side because in particle physics often what we predict and what we measure is a thing that will be the probability of interaction. So you calculate how likely it is that this interaction occurs and you can compute it with fermy theory and you can measure it at colliders or in experiments but you can measure it and you can uh compute it at different energies. Um what happened uh with Ferm's theory computing it with higher and higher energies was that this quantity which is a probability so it should always be less than one it was just like increasing linearly.

[00:12:30]So that means that at infinite energies you would have an infinite probability which doesn't make sense. In fact, it doesn't even make sense for a probability to be higher than one. So with this theory, you will get the unphysical prediction.

[00:12:49]And let's go.

[00:12:52]Um and so there yeah there were like some aspects that Fermy didn't get right. Two in particular. First one is protons and neutrons are not two states of the same particles. They are composite particles and they're made up of quarks. So a proton is made of two up quarks and a down quark. And a neutron is made of two down quarks and a up quark. And what happens in this process is that a up a down quark goes into an up quark thus making a neutron a proton.

[00:13:32]And the second thing that he got wrong because they didn't know it is that this interaction doesn't happen in a single point and it doesn't happen instantaneously but it's rather carried by a particle that it's now known as the double repos.

[00:13:50]So even with this though uh let's go thank you. uh even with this though as I said before at low energy the predictions of the firming theory of the experiments that they were running were quite good.

[00:14:07]It's only when you when you go at iron energy that you really had a problem but they didn't know about this.

[00:14:15]And so my question is is this a wrong theory or is it just an effective one?

[00:14:22]So is it just an approximation that works at some lower energy?

[00:14:28]And of course what we know now is that there's no point like interruption.

[00:14:32]There is a double bosom that carries the interaction.

[00:14:37]And also here you can see these are more like the allergies that uh the collider at CERN can probe now. And you can see that you have a peak in our current theory around the mass of the W bosom because what happens is that when you smash things together at around the energy or around the mass of the W bosom uh the nature goes like wait a minute I can make a W puzzle that's very convenient and so the probability of the process to happen is way higher and you can see that the problem that you had with the Fmy theory that this probability was just always crazy. You don't have it anymore because the real picture is that you get a bump and then it starts going down.

[00:15:28]Let's go on. Thank you. So this carries us when we are today. So this is the standard model of particle physics. So we have this big table of particles.

[00:15:42]Some of them we have mentioned already, some of them we haven't. Um but the actually the first if we just look at the first column we have mentioned them.

[00:15:54]So an up quark, a down quark, electron and neutrino. And then the second column and the third column are just like the first column but heavier. So second generation, third generation and then we have uh the interactions or orth carriers uh bzons. So I mentioned the W bosom that carries the beta decay.

[00:16:20]There's also a Z boson that's just like the W bosom but is not charged.

[00:16:26]A photon which is quite famous that carries the electromagnetic interaction.

[00:16:33]And then we have a gluon that carries the strong force. That is a force that uh occurs uh in a in a proton between quarks that keeps a proton together.

[00:16:46]And so in 2008, LHC started. So like the large alone collider experiment started in Geneva. and what they were expecting when they started, which is uh a little funny to look back at interviews that they were doing it those year now because physicists were so happy about the large bedroom collider and they thought they were going to discover a bunch of particles. They were going to discover at least another particle for every particle you see that you see here. And they also thought they were going to discover the X bosom. They were they were very happy about that. and they in fact discover it in 2012 but they didn't discover any other particle after that. So we built this very large collider very large experiment to discover particle initially and to discover the expos in particular. We discovered the expos but then we didn't dis discover any other particles and so naturally uh physicists did what they could with the data that they had and so we are now using the large drone collider more as a precision tool. So we're trying to get the measurement to be more precise and to make calculations that are just better and see that by doing this eventually we can find something more uh telling than what it's in this table.

[00:18:25]>> Thank you. Um, so I talked a little bit about what we do know and maybe I mentioned or maybe I should highlight it more, but everything we see we've seen at CERN and other colliders has been very well calculated with this theory with the standard model. But there's also quite a bit of things that we do not know. First of all, matter antimatter symmetry. So I I say that there is antimatter.

[00:18:59]But uh why don't we see it? We are all made of matter. Everything is made of matter. Antimatter is just very very rare. And when it touches matter, they just anagulate. So antimatter is very rare and in our in our matter universe, it tends to disappear. Why is that? In theory, matter and antimatter are just the same.

[00:19:25]Why should should there be more matter than antimatter? We don't know.

[00:19:30]Nutrino masses. So if we can we go back sorry. Okay. So in this table for every particle you have a mass for neutrino you have that for example electron neutrino you have the mass is less than 0.8.

[00:19:49]So there's only a upper limit and we just know that it cannot be zero. We know that from experimental evidence and that there is an upper limit. Um but yeah in our model initially we didn't take into account any nutrino mass. So that's another uh little problem that we have. Thank you. And then we can go to bigger and more important problems. Um gravity. So gravity is not included in this model. The all gravity which is maybe the most important force that we experiment every day. We cannot include it at the moment because at particle level gravity is just so weak that we have no real experimental measurement of it.

[00:20:40]And then we have that in fact everything we know of and everything we're made of is allegedly only about 5% of the mass or the energy in our universe because we know from other measurements that there is a big chunk that is dark matter. So that means that is matter that does not interact with light. So it's just dark and we don't really know what it is. And then there is a 72% or around that that number that is dark energy. So that just means we don't know.

[00:21:21]Yeah. So we have a very good theory very precise uh but it does describe only 5% of our universe and this motivates why we're looking for physics beyond the standard model. Let's go. Thank you. Um so together with that table that I show you before apologies if this is a confusing Z. Together with that table that I show you before uh of all the particles we also have something like this which is all the possible ways in which these particles can interact with one another. So for example this is what we talked about before about the the beta decay. So you would have like an up quark, a down quirk and then a double bosom and so on and so forth.

[00:22:10]There are many types of interactions.

[00:22:13]So how do we move forward? The core idea is that maybe we just accept that this this theory is an effective theory. It's an approximation and there is going to be a true underlying theory somewhere.

[00:22:32]And so we yeah we accept this and we add to our current theory our current interactions that we know of all the possible interactions that could be there. We just add them and then we go on and calculate our our observables taking this into account and then we try to look for signs of these into our experimental results and see if some of these appears in them then we can go on and say oh what is this what does this mean at a higher energy level otherwise as it is happening today if all of these appear to be zero there is no hint of new physics Um, so yeah, this is this is a bit of what I'm working on, a bit of the the approach that we're trying to build to be more general, more systematic because you could also you could also say, oh, let's imagine that there is a given new particle at high energy. Let's invent it. Let's make the calculation and see if you can find it in the data. But that's >> But that's should should I eat? Should I scream?

[00:24:02]>> I think I think so.

[00:24:05]>> Yes, I think you do have to spoon.

[00:24:07]>> Okay. Can you hear me?

[00:24:10]>> Great. Um yeah, but that's some more uh hum not hum, but like it's it's some more um you're you're inserting a bias, a human bias doing that. Whereas in this way, you just you allow everything to be there and in a very general way and then if there is some new physics in in theory, you're able to see it with this approach. And this was all I wanted to like leave you with with a quote which is yes which is all mothers are wrong but some are useful.

[00:24:51]>> Thank you Marty.

[00:25:04]So yeah, if you have Yeah. Good. Yes.

[00:25:06]The Higs Bosen was a theory and then after an awful lot of effort they managed to find it. Do do we have a theory for another particle which we're looking for?

[00:25:17]>> Uh >> do we think there's something beyond the hexos? There are theories. There are lots of theories, >> you know, and at some point that was the problem that and that was like the the reason why we we started doing this um more general approach because there are many many theories. There are people that think that maybe the standard the the exposone is not the standard model expos. So maybe it's a composite particle. So there are people looking into that. There are many people that look into what dark matter could be. So there are people that think that it's a a weekly interacting particle and they're looking into that. There are models of um yeah that there are there are many many theories uh out there but the the problem is that they don't seem to show any track at the energy that we can access right now through the the particle accelerator that we're at right now.

[00:26:28]Um, so my knowledge of the Dopp and Zed Bose was that there scary things to do with radioactivity of the nuclear force. Is that correct?

[00:26:37]>> Yes.

[00:26:38]>> Do I ever need to update that or can I just real hope to look at everything that's not the weak nuclear force?

[00:26:44]>> Uh, no, no, no. That's that's that's exactly it. Like they are the carries of the weak nuclear force. Yeah. um actually they I don't know if this is a bit too technical but there there has been like uh they they have been unified with the electromagnetic force. So actually there is a bigger theory inside the standard model that explains very well like the electromagnetism together with uh the weak force and so the Z and W possessons are as the are just another side of the photon in a way they just they they come from the same phenomena in a way >> I'm not a weak nucle before scarce.

[00:27:37]>> And you mentioned that like atoms were around 5% dark matter 23 uh yeah the dark energy around 72. How was that measured?

[00:27:46]>> Yeah, that's a very good question. Um that's that's a cosmological measurement. So there are people that uh study our universe and how it's evolving and that's cosmology and they have they have models on of how the universe is evolving. What they found is that the universe it's expanding and it's accelerating its expansion even and so they have equations and they have parameters and these parameters are like the the percentage of dark matter, dark energy and matter and you twiggle these parameters and to get the universe that we are seeing today you have to use these percentages.

[00:28:32]>> And that's another one on the same point. Then as you said because the universe is expanding is it changing the ratios or is it constant as it's expanding? It's still always going to be around 5% atoms.

[00:28:44]>> No no I think it is changing and it was also different at the start of the universe compared to now but I'm not an expert on this.

[00:28:55]>> Yes question about mass. You mentioned the neutrino mass presumably that was the electron neutrino.

[00:29:03]Yeah, old neutrinos should have a massive.

[00:29:05]>> Yeah.

[00:29:06]>> Do we know yet whether new and tam neutrinos have the same mass as the electro one or they heavier?

[00:29:15]>> Uh so I think that from the measure they have um they I think they measure them together. So I think they have like a upper limit of the sum of the three and then what will be logical is that the electron mass is lower like that they go in the same order as everything else in that table but I think actually we we don't go that for sure >> and and so with a similar sort of thing posetron mass is that found to be exactly the same was there a possibility we're being slightly different >> I think that's exactly the same as the as the electron Thank you.

[00:29:56]>> Do we well like I I assume that the second third generation Bernons are a lot rarer. Do they like appear at all in nature or are they very we make them in particle colliders and >> um so some of them really appear in nature. For example, muons. Um muons like come from space all the time. And that that was that is like a famous example that proved Einstein's relativity because they have um an average life that's shorter than how much distance they can travel to if that makes sense. So like in in on average the time that they live will not be enough for how far they go in the in hurt and that's why um like they travel very very fast and so their time it's gets dra >> but as well sorry I I I'll add to that um can we go can we go to the to the side of the standard model >> this one? Yes. Yes. So another thing is that I made a simplification when I say that for example a froen is made of two up quarks and a down quark because in reality yes it's made of two up quarks and a down quark but it's a it's a strong force system and the strong force is strong and thus the strong force makes so that constantly These two um can interact with a gluon and produce maybe another couple of these um of this even this is too massive to be produced in a in a proton but yes in protons you also have uh on like in a small percentage also these other quirks.

[00:31:56]>> Cool. That's pretty cool.

[00:32:01]How on earth did Charm and Strange get their names?

[00:32:08]>> Because as physicists, we didn't get a marketing team.

[00:32:17]>> Yeah. And so a lot of the names are just terrible or even just like weak force, strong force. Yeah. One is weak and one is Yeah. I know. Yeah. It's just Yeah.

[00:32:28]The strange I mean the strange one it's because um it makes up particles that were behaving in a strange way. And so they saw these particles that were behaving in a strange way because they were being creating create they were being created very fast and they were decaying very slow relative to the times of particle physics. And so they thought that something strange was going on there and they later found out that it was because they contained a strange cork.

[00:33:05]>> So sadly we don't have time for any more questions. We have a big quiz right now going on. So let's thank Martina one more times.