this video is brought to you by brilliant click the link in the description to sign up for free and support this channel you probably heard the big news in 2012 titles like physicists find elusive particle seen as the key to the universe or God Particle found it was a big day in particle physics and one of the biggest scientific triumphs in decades the last piece of the standard model was discovered the higs ban a unique particle responsible for giving fundamental particles their Mass with all the hoopla you would assume the scientist must have seen or somehow measured the particle but no such thing happened it was never seen nor measured what how is that possible yes it's true but if scientists can declare the discovery of a particle without ever having seen or measured it then we have to ask the question what is a measurement anyway in this video I'll tell you the truth about particle discoveries and how we can't really see most of the particles of the standard model but we know their existence how can we be so sure I'm going to explain that coming up right [Music] now the chart that you're looking at represents the standard model of particle physics it lists all the fundamental particles that we know of everything in the universe that we can see is made up of them this chart along with all the equations representing how these particles interact represents the best theory we have about how the universe works but here's a hard truth about these particles we have never actually seen or measured most of them the standard model is based on something called Quantum field Theory according to this Theory a particle is nothing but an excitation or a kind of wave in a Quantum field that permeates all of SpaceTime each particle is an excitation or a Quant quta of energy in its own field so for example an electron would be a quanta of energy equal to 0. 511 Mega electron volts or me the rest mass of an electron in the electron field similarly an up Quark would be an excitation of 2. 2 meev in the up Quark field if you have a field and add a Quant of energy you get a particle if you add another Quant you get two particles in the field and so on since the higs particle has a mass of 125 gig electron volts or gev you must add 125 gev worth of energy in the Hig field to form a Hig particle note that this is a relatively very high energy level equivalent to the rest mass of about 244,000 electrons So based on this you might logically conclude that all we need to do is add 125 gev to the hick field and boom we can discovered the higs particle right in principle yes that's all you need to do in practice this is not easy because the higs particle is extremely heavy compared to other particles of the standard model in fact only the top cork is heavier at 173 G the issue is that most heavy particles of the standard model are not stable because they Decay to lower Mass particles this is also the case for the higs particle why because everything in the universe is intrinsically l e and wants to go to its minimum state of energy I have a video on why the universe is this way and how entropy plays a role here if you want to know the details the upshot of this is that as I said heavy particles will be unstable and tend to Decay into lighter particles so the rules of particle physics is pretty simple if a particle can Decay into something lighter it will do So eventually and the greater the mass difference which is also the energy difference the faster the particle will Decay for example the heavier cousin of the electron the TOA particle has a lifetime of around 2.
9 * 10 -13 seconds the slightly lighter cousin of the electron the muon particle has a lifetime of 2. 2 * 106 seconds but the electron is stable simply because it's the lightest of its type there are some details to this that we're ignoring right now but in short there's no lighter particle which the electron can turn into so it's stable now the issue is that since they're many particles of the standard model that are lighter than the hix particle there are many energy paths that lead to more stable particles if you start out with the higs particle consider for example the electron its mass is almost like I said 250,000 less than the higs mass in protons which are stable we have up and down quirks which have masses of around 2 and five MAV this is about 5 and 10 times heavier than the electron respectively but but still nowhere close to the higs mass the other heavier quirks are all less stable and are thus not readily available in the universe and neutrinos are even lighter and almost never interact with any other particle so they're completely hopeless to work with in this context at this point you might think how the heck can we ever even hope to make such a heavy particle as the higs if the only stable thing we have to work with are particles thousands of times lighter well here we're rescued by the mass energy equivalence principle recall that mass is only part of the energy of a particle so if you want to create a hix particle from electrons you would need to give it a lot of momentum or kinetic energy such that you add the equivalent of about 250,000 times its rest Mass such that its total energy is about 125 gev now this is not the full story you can't just turn an electron into a higs particle but the general idea is that we can form heavy particles from lighter particles by accelerating them and smashing them into each other and by combining potential energy from the mass and kinetic energy from Movement we can have enough energy to make something heavier this is the principle behind particle accelerators like the large hydron collider at CERN in Geneva the world's largest and most powerful collider in principle you could make the higs by Smashing an electron and an anti-electron but you'd have to pump an awful lot of energy to do this the LHC in fact actually accelerates protons not electrons to do this because it's a bit easier since a proton is much heavier at around one gev so it needs less kinetic energy to create the higs particle but now the question is how do you detect the higs once it's made this is the tricky part because you can't detect the particle itself directly there are two reasons for this first the the protons collide with the same energy but in opposite directions the combined momentum is roughly zero this means that if you create a Hig boson in the accelerator it will be roughly stationary in the particle beam it's difficult to detect something that doesn't move because the detectors only pick up particles that fly away from the Collision the detector is built around the beam it cannot be built within the beam because the energy of the beam would destroy the detector so anything formed within the beam is not really detected only things that fly away furthermore because the higs is really heavy it's not stable its lifetime is incredibly short at around 1. 5 * 10 to the -22 seconds so it decays effectively instantly these two things make the higs bosan practically impossible to detect so it should be no surprise then that we have never actually detected it but on top of that the higs is not a Charged part particle and since we generally rely on some electromagnetic interaction to physically detect something it's not clear how you would detect it even if it could reach the detector if all that is true you have to ask the question then what's the fuss all about regarding the so-called discovery of the higs as it turns out you don't need to measure it to know that it's there think of the dinosaurs we don't see them today but by looking at fossils we can learn a lot about them and we can certainly conclude that they once existed we can do something similar with the higs by looking at its Decay products the idea is something like this if you smash two protons together and get an event where the sum of the Decay products adds up to the mass of the higs then we can reasonably conclude that the event likely created a higs particle but you might ask what if the event created random interactions that just happen to yield Decay products equal to the mass of the higs yes that that could happen and this is called background noise but the trick is that if you make many multiple measurements over a long period of time then you can eliminate the possibility of just random interactions that just happen to add up to the same mass and in such cases you can make a reasonable conclusion that the consistent signal you're detecting in the form of an unexplainable Spike represents in fact the mass of the Hig particle and it's not just random noise and in the case of the 2012 announcement this bike achieved five Sigma significance which is the gold standard in particle physics for determining that a new particle was detected so the guys working at the LHC declared the discovery of the higs bosan not because they had actually measured the higs particle itself but because they measured events that could only make sense if the higs particle had been there so it's thus a statistically significant Discovery and given that it's 5 Sigma the chance of it being just a statistical fluke is 1 and 3.
5 million which is pretty low today in fact the measurement has achieved a higher than Six Sigma significance so there's really no longer any doubt that the particle is there and it turns out that there are many other particles that we have also never actually directly measured because of similar limitations for example the quarks and gluons that make up the protons and neutrons cannot because of the nature of the strong force ever be detected directly yet scientists still claim we discovered them they can make this claim because the procedure for their Discovery is similar to that of the Hicks we smash things together and end up with a result and from looking at the Decay products of the process we declare that it statistically only makes sense if these particles were in fact present as part of the process we measure now this next part of the video is going to be the most interesting part to some of you who love details I'm going to talk about exactly how the higs particles was created at the LHC and how it was detected but before I do that if you guys want to learn this material in Greater depth than ever before hop on over to brilliant. org our sponsor they have two of the best fundamental courses in quantum mechanics I've come across I would start with the course Quantum objects which Begins by explaining how quantum mechanics is fundamentally different than classical mechanics and why it's necessary and over the the course of 18 fascinating lessons it ends with explaining what I think is the most important equation to really understanding quantum mechanics and that is their Schrodinger equation the second course was developed with the help of my friend and fellow YouTuber Sabina hossenfelder it gets into explaining some of the more confusing elements of quantum mechanics including superpositions entanglement and bells theem the best part of Brilliance courses is that they make it fun to learn by using Graphics interactive quizzes and simulations this kind of Hands-On Interactive Learning is in my opinion the best way to learn new things and retain the information long term brilliant has a special offer for Arvin Ash feers right now go to brilliant.
