Demystifying the Higgs Boson with Leonard Susskind

Stanford University the goal tonight is not to tell you why the Higgs boson is the best thing since flush toilets a griddle of that anybody who's here has probably read all of the height the excited reckless superlatives that that have hit the newspapers and so forth things bosons are explained the origin of the universe mr. Mack I'm not going to do that first of all before I say it the the excitement and the enthusiasm is justified it's not that it's not justified it is justified the history is fantastic it's an unbelievable event and so forth but that's not what my thing is it's not what I do well what I do well is explain how things work my goal tonight is going to show be to show you and so far as I can in one hour which is tough which is hard and may not work but as well as I can to explain to you the nuts and bolts of what Higgs physics is about one of my closest friends incidentally is named Francois and Blair and Francois and Blair would be appalled to do that I was sitting here talking about the Higgs that since francois hollande later was the discoverer of it so from time to time and we call it the drought on where kinks effect but I also may slip and slide and just call it a ticks effect because something to get like everybody else oh all right so how it works first of all there's a lot of moving parts a lot of pieces that I would have to lend this I would have to explain to you first to really do it right and I'm going to try to explain those pieces and move a little module shall we start on it's a highly quantum mechanical effect it cannot really be understood without quantum mechanics and so I would begin with a course of quantum mechanics and let's say the course in quantum mechanics consists of just one thing things are quantized quantized means that they come in discrete integer quantities the most famous example of this is angular momentum angular momentum the rotational properties of the rotational momentum of an object are quantized which means that kind of discrete steps and the discrete steps are one Klunk unit 1 unit of Planck's constant we'll take that as all we really need to know about for the most part about quantum mechanics tonight the next concept which is an easier one at least I think it's an easier one of the classical concept is the idea of a field a field is just a condition in space it could be the electric field it could be the magnetic field it could be a gravitational field whatever you think of as being present in space which characterizes the conceived behavior of space at that instant and in that place in in space and time not so steals the space can be filled with fields now ordinarily you imagine in empty space empty space is the thing we call a vacuum from a quantum mechanics point of view the vacuum is just a state of lowest energy nothing there no energy other than what quantum mechanics requires there to be there and so you ordinarily think that the fields that couldn't have its space are zero in empty space the electric field the magnetic field and so forth but there's no important requirement of physics that says that that should be the case let's imagine a world filled with electric field how could that electric field get there well it might be there because there are capacitor plates infinitely far away so far away we can't cover there there we will have a world in which there was an electric field will just be there empty space would have no electric field now when you think about fields of we're beginning a little bit too all right so what I was saying to the audience two things the first was that quantum mechanics we're going to summarize by one simple statement that things are quantized in quantum mechanics quantized means they come in discrete bits the most important example is angular momentum not the most important necessarily the most important example but the most important example for me tonight will be angular momentum and angular momentum is just to do with rotating objects and so forth angular momentum in quantum mechanics unlike classical mechanics comes in discrete units the unit is Planck's constant you can't have a tenth of a unit of angular momentum you can only have angular momentum 0 1 2 3 - 1 - 2 - 3 you can also have 1/2 integers but we're not going to worry about that tonight but no you can't have angular momentum PI only discrete integers that's the first fact that I want you to remember the other fact from quantum mechanics that we'll have to remember also is the uncertainty principle but we'll come to it now the other thing we spoke about was fields fields are things that can fill space electric field magnetic field gravitational field other kinds of fields that exist in physics they are functions of space they can vary from place to place and they affect for example the way things move an example would be an electric field affecting where charged particle moves now the other thing I said was you can imagine a world in which for all practical purposes empty space is filled with a field an example would be if I went out to Alpha Centauri at that end and oh excuse me I don't know what's outfit excuse me yeah whatever's out there and play some capacitor plates big capacitor plate out there big one out there make an electric field in between that's so far apart that we can't see them we would say that the world is a world that exists for the magnetic field and we would say that charged particles move in peculiar ways but that was just a fact of nature um generally speaking fields cost energy the space without an electric field has zero energy with an electric field it has energy and if we were to plot the energy of a field the typical field could be electrical be a magnetic could be something else generally we imagine that the field energy as a function of the field horizontally imagine the value of a field vertically it's energy zero field right here and we imagine exciting the field causing it to vibrate causing it to vibrate by giving it a push at some region of space and nearby the field will vibrate those vibrations are quanta of the field they are particles there are particles the quanta of vibration of the field of particles now you might have a situation where there is more than one field relevant let's call it Phi and Phi Prime or whatever you want to call it doesn't matter then instead of plotting the field is one dimensional we might plot it as two-dimensional now this is not space this is the value of some collection of fields and then the energy would depend on both fields here's an example an energy function which looks like that which simply says that no matter how you displace the field it costs energy now imagine that this upside-down paraboloid here or whatever it is was nice and symmetric nice and rotationally symmetric in the field space exactly like the top of my hat here as which as you can see is symmetric and so the field as a oh yes where's my little deaths that's it right the field as a function of position when most likely if the energy is as low as possible just sit at the bottom of the potential energy of the field just to lower the energy as much as possible so we might think of the field at every point in space a little ball which can be made to oscillate back and forth and do things and those are just oscillations of the behavior of physics in the local region of space as I said they often correspond the quantum particles those oscillations but for the moment they're just oscillations now one of the things we could do if we had a field whose values will like the position in the Hat here would be to start it out displaced from the origin let's say up to here and then start it moving in a circle just in the same way you could take this ball and if it was if the Hat was really nice and smooth and symmetric give it a push in it we go around enough in a circle that circular motion of the field is very very similar in a way to angular momentum it's not angular momentum in space but it's a kind of angular momentum that exists in the field space that angular momentum like all angular momenta are quantized come in integer multiples of Planck's constant what do they correspond to they correspond to something else that is also quantized in nature the value for example of electric charge so in modern physics the way one thinks about the electric charge in a is that in some region of space a particle a charged particle a charged particle is viewed as an excitation of the field in which the field is made to spin around in the internal space of the field not in real space but in the internal space of the field that's one way in fact it's the main way that we think about charge as a kind of rotation in an internal space okay now what I want you to do is imagine taking the Hat and turning it over imagine that the potential energy was not turned it over this way excuse me this way is the way that the potential energy is minimum at the crown of the hat but if the potential energy really looked like that so that it was maximum at the top of the Hat then the top of the Hat would not be a position of equilibrium it would be a position of unstable equilibrium would look like this turning over the Hat the crown of the hat now this is the way that real hat looks like this and it doesn't okay let's just let's make the how's that that look like a hat yeah looks like a what kind of hat does it look like to you looks like a sombrero right looks like a Mexican hat physicists call this kind of potential energy function a Mexican hat believe it or not it's called a Mexican hat it turns back up the top is unstable if a pas ball was put at the top it would roll down and where would it go it would go to the brim of the Hat if for some reason the potential energy of a field was like this then the state of lowest energy would not be at zero field it would be out here now that's kind of interesting it would be a vacuum a world which had a field just like having an electric field except it's not an electric field and what's the value of the field at every point in space was not zero you might notice it how would you notice it well you might notice it because it might affect other things and indeed it does affect other things as we will see but there's now something interesting you can do that you couldn't do here over here if you wanted to set this thing into rotation you would have to displace the field a little bit because it doesn't mean anything to rotate right at the center if you wanted to set up a rotation you displace the field and then give it a flick so making the charge particle costs some energy here you can imagine setting this thing into rotation with just a little flick that costs no energy it costs no energy because you don't have to ride up the side of the hat in other words you could have a motion in which F naught got it you got it you got you understand huh you got have a motion in which that field slowly wound around the top of the potential in fact they could do it everywhere simultaneously not in real space but in this field space that would correspond again to a charge if rotation in this internal space corresponds to some kind of charge but now the whole world if the whole field was moving like that would have a little bit of charge in it a charge density charge filling space and essentially no cost of energy that's the nominal is called a condensate it's called spontaneous symmetry breaking but it's also called a condensate a condensate in space of charge now you might say okay look I want to find the lowest energy that the vacuum can have that empty space can have my best bet is to make the field not move with time just like a ball at the bottom of the sombrero hat here there's also kinetic energy of motion causing the field to move around in a circle like that would cost some energy so you would say the true lowest energy state of the world should be with a field either here or here or here it could be anywhere along the rim of the hat but it should be standing still right the problem with no angular momentum or no charge empty space should not have charged the problem with that is the uncertainty principle let me remind you what the uncertainty principle says it says that if you have a object and you're interested in its position X in ordinary space now and its momentum P velocity if you like the uncertainty principle says that the uncertainty in its position times the uncertainty in the momentum is greater than or equal to what Planck's constant you can't have something both standing still and having zero momentum if it's stand sorry you can't have something standing still namely no momentum and also localized at a point Delta P times Delta X is greater than h-bar same thing here if you know where the field is on this Mexican Hat if you know with great precision then it follows from the uncertainty principle that it must have a very large uncertainty in how fast it's moving around here ah that's interesting now I would say that you can't have empty space with no charge in it can't have empty space with no charge in it because if you lay the field down at this point you know where it is on the rim of the Hat and if you know where it is there's a necessary uncertainty in the charge the charge being like the angular momentum alright so where are we then if this were the case for electric charge for ordinary electric charge we would say that the vacuum empty space not only is filled with charge in a certain sense but a totally uncertain amount of charge totally uncertain and this is a quantum effect a totally uncertain amount of charge there would be equal probability let's take a little volume of space there would be equal probability that the charge was zero or that the charge was 1 or minus 1 or 2 or minus 2 3 minus 3 now this is truly odd this is not something you should try to visualize because you can't visualize an uncertain amount of charge but nevertheless that is what a region of space would look like if you measured its charge it could be anything from minus infinity to plus infinity ok now I want you to imagine that you have an extra charged particle an extra charged particle and you throw it in you don't know initially what the charge is but what does that do it displaces the charge by one unit let's suppose it was a positive charge you've displaced the charge by one unit and so if it was zero to begin with it's now warning if it was one to begin with it's now two if it was two to begin with it's three it was minus one it's 0 1 minus 1 minus 2 and so forth but that's exactly the same as what we started with we started with something which had an uncertain amount of charge equally likely for any value of charge and what did we end up with after we threw the charge in exactly the same thing what if we pluck the charge out of this thing same thing so a condensate is a funny configuration of space where with respect to whatever kind of charge we're talking about it's so uncertain that you wouldn't even realize it if you put an extra one in or pulled one out now the real world is not like that with respect to electric charge we know if we have a charge in space so it's not like that with respect to electric charge however there are materials that behave like this superconductors superconductors are exactly like this so it's not unheard of it's not a totally new thing to have a condensate of charge we're in a region the charge is completely uncertain okay that was module number one if you like condensates or what sometimes called the spontaneous breaking of symmetry modulo number two the standard model now we come the particle physics and I'll give you a short course in particle physics first of all particles have mass and the mass can be anywheres from zero we're talking about small particles now we're not talking about railroad engines or or stars we're talking about small particles we call an elementary particles but there's also a maximum mass that can have if they were bigger than that they would form a black hole if they were more massive than that if a point particle was more massive than something it would form a black hole and it would be something different so up to some maximum and that maximum is called the Planck mass it is not a very large mass it's neither a very large mass nor a very small mass it happens to be about one hundred thousandth of a gram a small dust mote but that is the heaviest of the chart of a.m. that an elementary particle can be without turning into a black hole and if you ask now where on this chart from zero this is called M plunk up to the maximum where are the ordinary particles the electrons of photons the quarks they are way way down here the largest mass of a known elementary particle is about 10 to the minus 17 of the Planck mass why are the particles so light well one answer is in order to detect massive particles you have to have a lot of energy in order to have a lot of energy you need a big accelerator we've only made accelerators up to some size and so for all we know the rest of this is filled with particles and that's probably true that's probably true but what is special about these particles well first of all let me name them and then I'll tell you what's special about them that makes them clump up at zero mass let's name them the particles of the standard model they come in two varieties it is not important that you know the difference well I'll give you a rough idea what the difference is they come in two varieties called fermions and bosons the fermions are all the particles that make up matter in the usual sense the electron which I'll just call e well the neutrino goes along with the electron that's a new electron the neutrino quarks there's a variety of different quarks incidentally there are several different kinds of electrons we call them electron muon tau it doesn't matter but they're very electron like and several kinds of neutrinos the electrons have the electric charge the neutrinos don't and then there are quarks a variety of different kinds of quarks up corpse down quarks this kind of quark that kind of quark and those quarks several different kinds of quarks you know what the role of them are they make up the proton and that's about it for her forum for fermions for bosons on the other hand is first of all the photon gamma gamma for a gamma ray photon there's an object called the gluon G it's very much like a photon it's very much like a photon but it doesn't have anything to do with atoms it has to do with nuclei and protons and neutrons it plays the same role in holding the nucleus or better yet the proton together as the photon plays in creating electrical fields inside an atom so there's the gluon and then there are two others called W bosons and Z bosons for the most part we won't be interested in any of them except the photon me here and there but mostly we'll be interested in the Z boson that's it that's the standard model that's all there is to it with one exception I've left something out it's the thing you came to find out about tonight okay so we'll come to it if there was no expose on then this would be it now what is special about this set of particles what's special about them is for reasons that I'm going to come to reasons that I will come to all of these particles in the standard model as I've laid it out here with nothing else in it would all have mass equal to zero they would be massless and I'll explain why that is in a little while we often hear that's the role of the Higgs boson to create mass for particles or to give the particles their mass that's the expression that I've heard over and over the Higgs gives particles a lot to put why the particles have to be given mass why can't they have mass of their own why do they have to be given mass well as it turns out for reasons we'll explain this set of particles is exactly the set of particles which would have no mass if this was all there was now in part that explains and part it explains why the particles why these particles are so very light it's because the mass lifts they have no mass well not quite we can't live with that because we know that particles really do have mass next question I'm going to draw some figures over here what do these particles do what kind of processes that they are they involved in at the basic process of the standard model this is an oversimplification but it's qualitatively right is that the fermions there's a Fermi on moving along and I will describe a Fermi on by a solid line solid because it's what makes up stuff solid line that's moving from one point in space-time to another point of space-time what the standard model does is it causes the emission of bosons a electron moving along can emit a photon electron moving along can emit a photon and that's connected with the electric charge any electrically charged particle can emit a photon a photon that's the first thing that the standard model does now this of course is just quantum electrodynamics it does not have to be the electron it could be any electrically charged particle next the quark let's see we have room here yeah we'll just do it the quark quark let's just call it Q the quark can emit a glue on precisely the same pattern the quark emits a glue on now the court can also emit a photon if it happens to be electrically charged and quarks are electrically charged but electrons cannot emit gluons gluons are the things that bind quarks together to hold them together into protons and neutrons and then there's one more important process for me tonight there are two more processes but I'll just write down one here and it is either an electron or incidentally a neutrino cannot emit a photon it has no electric charge it cannot emitted lawn it's not a quark okay both electrons and neutrinos and quarks for that matter can emit the Z boson whereas the Z boson here's the Z boson right here and when they do so the Z boson being electrically neutral the electric charge of whatever's here doesn't change so this is another process that the standard model describes now first of all why are the bosons massless well the photon is massless we know that it travels with the speed of light now could we make a theory in which the a photon had some mass yes we could but the more important thing is that we can make a theory in which the photon doesn't have a mass why because the photon doesn't have a mass using the same kind of Theory the Z boson would not have a mass and the gluon would not have a mass everything would be massless these will be the processes that could happen these would be the particles they would all be massless okay now how do fields how do fields give particles mass or better yet more simply I'm a simple example I'm going to show you a simple example now the simple example is how a field can affect the mass of a particle we'll come back in a moment to how it can give something which didn't have mass mass but let's take a more modest question how many fields affect the mass or better yet how might they make different masses for different particles so I'm going to show you an example this example is a little bit contrived but it's a real example a water molecule water molecules have the basic property that they're little dumbbells they have a plus end and a minus end electrically-charged plus and a minus end they're actually not the more likewise you know why were three ends but we can think of them as having a plus end dumbbells and a minus end now the mass of a water molecule water molecules have mass the mass of a more water molecule doesn't depend on its orientation if we turned it over and made a water molecule with its minus end here and the plus end here it would have exactly the same mass why it's the symmetry of space space is the same in every direction and so by symmetry we would say that the water molecule standing up straight has exactly the same mass as the water molecule standing on its head let's not worry for tonight about whether it's lying on the side quantum mechanics tells us we don't have to worry about anything but standing up straight and lying on its head all right so that's that's true about water molecules their mass is the same as their standing up straight and think of water molecules now as particles think of them just as particles we don't know what they are they're just little elementary particles we can't see them and so we have two kinds of particles the upstanding and the standing on his head particle with exactly the same mass now I thought I had a purple no purple I told them to put purple we'll have to use orange we're way over here underneath the board mean under here oh good alright I have my color coding in my notes here and if I blow it out terrible okay so it looks Brown to me it is brown okay let's create a region in which there's an electric field we're going to make a field it could be between two capacitor plates the capacitor plates could be far apart it doesn't matter but let's put them there capacitor plates here and here and inside that region let's create an electric field the electric field in this case pointing up that means it pushes plus charges up and minus charges down if I have my signs right and let's take one of these water molecules and insert it in here once I insert the water molecule in here the energy of the up standing water molecule and the upside down water molecule are different which one has less energy the one with the plus up has less energy and the one turned over has larger energy the water molecule itself is electrically neutral it has no electric charge but it's a little dipole it has a pair of charges and which one has more energy depends on the sign of the electric field okay so there we are we have two water molecules two types of water molecules two different particles we give them different names we can call it water and scotch and what a molecule has one one energy the scotch molecule has another energy and there they are well by e equals mc-squared this also tells us that the two molecules have different mass non practices would be a tiny different mass between them but they would have different mass so the same effect of this field which exerts itself on charged particles does something to neutral water molecules incidentally notice that it doesn't exert any net force on the water molecule the water molecule moves smoothly through it with no force no net force acting on it but there is a difference in the up in the two configurations of the water molecule and so it's as if we had particles of two different mass so this is just an example of how a field creates mass in this case it increases one mass and decreases the other mass incidentally if you read some of the literature and they'll tell you about how the Higgs field gives a mass I've read any number of places that it's something like space being filled with molasses it is not like space being filled with molasses the vacuum is not sticky and one of the things that molasses would do well the idea is that massive particles move slower than massless particles so the idea is that molasses slows them down but fields don't slow particles down if you give the particle a push in this direction it will just continue to move because there's no net force on it it'll just slide right through this thing frictionlessly no no impedance no no friction no molasses there's not the other the other analogy I once heard is that it was like trying to push a snow plow through a heavy snow in the Arctic it's nothing to do with it whatever that's a that's a that's a lazy way to explain it it's a wrong way to explain it okay so there we are but now let's think of this in a slightly different way the electric field in here can also be pictured in terms of photons a field is another way of talking about a collection a condensate of photons an electric field we can replace the electric field by a condensate the same kind of condensate the same kind of condensate of photons let's draw photons by just their little squiggly lines fill this up with photons how does it know which way the electric field is pointing well photons have a polarization they could be up or they could be down so just imagine this thing being filled with photons but not filled in the usual way but filled in a condensate what does the condensate mean a condensate means that if I pull one out it doesn't make any difference if I put an extra one in it doesn't make any difference that's the meaning of a condensate so it's an indefinite number of photons that's what a field is indefinite and if you pull one out nothing happens and now let's reintroduce the water molecule let's just draw the bottom molecule moving through here now I'm going to make the water molecule that I've already blown my my color coding here's a water molecule moving through here and what is it going to do it has charged particles inside it the charged particles can emit and absorb photons they emit and absorb photons we've made the photons green now so it emits photons but when in the emits a photon putting an extra photon in doesn't matter and so we usually draw that by just putting a cross at the end a cross simply means that throwing an extra photon in doesn't affect anything photon is emitted and just is absorbed or is just disappears into the condensate as this object the dumbbell moves through the electric field it's constantly emitting and absorbing these photons which get lost in condensate that is another way of talking about how the field effects the particle and depending on whether the photons are polarized up or down this effect of constantly being absorbing and emitting photons will have the effect of shifting the energy of the two configurations of the of the dumbbell that's simply an example of how a field can affect the mass of a particle and how it can be thought of in terms of particles and condensates that's what I want you to keep in mind that picture okay now let's come to elementary particles not dumbbells not molecules first question is there any reason why a particle or an object just can't have a mass does it need an excuse to have a mass does it need anything called the Higgs phenomenon to have a mass well there are lots of things in nature that have mass and have nothing whatever to do with the Higgs phenomena we give you an example imagine you had a box and let's make that box out of extremely light stuff the lightest stuff you can think of but it's a box with good reflecting walls and fill it with lots of high-energy radiation bouncing off the walls but never getting out it's made out of massless stuff the photons are massless they have no mass the box who are imagining is made out of stuff which is exceedingly light doesn't have much mass but there's plenty of energy in there lots and lots of energy well e equals MC squared and so this box will behave exactly as if it had a mass we didn't need anything to give mass just energy that's all it took are there any particles which are like this which get mass having nothing to do with the Higgs or anything else yes the proton the proton is a particle which is made out of quarks quarks three quarks and a bunch of gluons jeez a bunch of gluons a large number of gluons quarks and gluons in the standard model are massless does that mean that the proton would be massless if the quarks and gluons are massless not at all if the quarks and gluons are massless the effect on the proton would be about a 1% or even less change in its mass not much at all where does its mass come from it comes from a kinetic energy of these massless particles rattling around in a box the Box being created by the proton so mass doesn't have to come from black holes or another example black holes have mass it doesn't come from the Higgs phenomenon doesn't have anything to do with Higgs so what is it about the models of the state of the particles of the standard model which require us to introduce a new ingredient so I'm going to concentrate on the electron let's concentrate on the electron we don't need all of this or I need to tell you about is that their act theory of electrons really we don't have to know very much about the Dirac theory all we have to know is that electrons have spin and furthermore if an electron was moving very fast down the axis here let's say with close to the speed of light we really accelerate that electron then there's two possibilities the spin of the electron can be right handed like that think of my thumb as a direction of motion of the electron it can be going that way like my right hand or could be going that way like my left hand oh I didn't realize it could do that now two kinds of electrons right-handed and left-handed now the right hand that electrons always stay right-handed can they flip and become left-handed in the right hand they'd become a left handed and left handed become a right handed yeah that's exactly what the Dirac theory says but if it was moving with the speed of light it couldn't why not because if a thing is moving with a speed of light time is infinitely slowed down and nothing can happen to the object it just moves along but nothing can happen internally to the object so if it's mass with zero it couldn't flip but in the Dirac theory this flipping back and forth between I tend to do it this way but that's not right this way this way this way this way that is intimately associated with the mass of a particle and in fact the mass of a Dirac particle is simply proportional to the rate at which it flips from left to right that's the Dirac theory in a nutshell mass is the rate for the electron to flip back and forth from left to right okay of course the faster it's going the slower it will flip but that's all right you take that into account so mass is left to right to left to right and we could draw the motion of an electron in the following way here's the electron moving down the axis at first it's right-handed so it's going this way and then it's left-handed it's going this way and then it's writing can you tell the difference maybe not but that's okay and in between it jumps from one to the other the probability or the rate at which it jumps is a measure of the mass of the electron so it jumps back and forth and back and forth now I'm going to ask you to believe something really crazy they remember the Z boson whereas the Z boson the Z boson was associated was emitted it could be emitted from electrons it could be emitted from neutrinos but let's concentrate on electrons it is not the same as the photon and the thing which emits it is not the same as the electric charge it is another kind of charge a completely separate kind of charge it's like charge but it emits Z bosons we need a name for it we don't have a name for it well we do have a name for it's a very awkward name it's called the weak hypercharge I don't like that because it's the thing which emits Z bosons I call it zilch zilch zilch is like electric charge but it's not electric charge when a particle which has zilch accelerates it emits a Z boson it may also emit a photon if it also happens to have electric charge now electrons both right-handed and left-handed have the same electric charge okay but left-handed and right-handed electrons do not have the same zilch in the standard model this is part of the mathematics of the standard model the left-handed and the right-handed electrons have different zilch the left-handed electron has zilch o plus 1 and the right-handed electron has zero zilch I didn't make this up in fact my friend Steve Weinberg didn't make it up if anybody made it up he's up there or down there I don't know where but and it is just the way it is it is the way the mathematics of the standard model works that the left-handed and the right-handed particles have different zilch and now we have a puzzle when the electron moves along and it flips from left to right that means the zilch goes from plus 1 to 0 but zilch is like electric charge it's conserved how can the zilch go from 0 to 1 it can't it can't and that's the reason that the electron in the standard model doesn't have a mass because the left-handed in the right-handed have different value of a conserved quantity so left can't go to right period no mass how do we get around this we get around this by introducing a new ingredient and the new ingredient is called the zigs boson it's not the Higgs boson not yet we haven't gotten to the Higgs boson yet we've gotten to the zigs boson the zigs boson is one new ingredient it is closely connected with this Mexican half the type configuration here it's a kind of particle but it forms a condensate you can't tell how many of there you can put one in you can take one out and so forth without changing the vacuum so we have one more ingredient it's a condensate that space is filled with and the nature of the condensate is it doesn't have electric charge it has zilch and it's a condensate meaning that if you put a zilch in nothing happens if you take one out nothing happens and let's ask now what that means the left-handed electron coming in has a zilch of one let's call it a Z of one the right-handed has Z equals zero back to the left-handed Z equals one is that possible only if you emit something at this point which carries off that Z equals one Ziggs zigs boson gets emitted it carries Z equals one but what happens to it where does it go it goes into the condensate it gets lost in the condensate you put as you put one in and it just gets absorbed into the condensate and so the electron goes on its merry way the condensate absorb the zilch and it goes one to zero but then it can borrow a particle back from the condensate borrow one back it doesn't even have to borrow it if you pull one out nothing changes again and so it goes on its merry way from left-handed to right-hander from left-handed or right-handed everytime it switches it emits a particle carrying this zilch quantum number which then just gets absorbed into the condensate that's the mechanism by which a field and in this case it's a field which forms a condensate by itself it doesn't require capacitor plates it just requires the energy to be such that the field naturally gets shifted and that's the mechanism by which electrons quarks and the various partners of those particles the MU particle the Tau lepton all those ordinary ordinary and extraordinary particles the fermions get there masked by this phenomenon here phenomenon doesn't really have a name it's called the spontaneous breaking of chiral symmetry but it does have a name but this is what it is okay what about the Z boson I told you before the Z boson is like a photon photons are massless how does the Z boson get a mass so I'll just show you something very similar happens to the Z boson let's remind ourselves what a Z boson can do it can take any particle which has a zilch and in particular this green zigs particle it can take the zigs particle and the zigs particle can emit a Z boson it has charge not real charge but zilch and zilch emits Z bosons all right so now let's ask what that means that means that a Z boson moving along can do something a little bit similar to this it can absorb some zilch out of the condensate condensate but now it has zilch originally it was just a Z boson Z bosons don't have zilch it absorbs some zilch and it becomes a zigs Z boson becomes a zigs but then it can emit zigs which gets lost in the condensate again and the Z boson just moves on its merry way constantly going back and forth from being a Z boson to being one of these imaginary not imaginary Ziggs particles that's the nature of the way that particles get mass from fields this phenomenon of the Z boson getting a mass is called the Brout on glair Higgs phenomenon this is the one that's called the Higgs phenomenon the Z boson getting a mass now this could have happened to the photon had there been a condensate of ordinary charged particles the photon would have become massive we would all be dead if that were the case massive photons would not be healthy for us and so we are very lucky that the that this phenomenon here did not apply to ordinary electric charge will we ever discover the zigs particle sure we discovered it long ago it's just part of the Z boson Z boson was discovered means it was postulated 1967 but or even before that by of many people but it was discovered I don't even remember when 1980 I forgot when the when the experiment but slack first discovered the existence experimentally but when it was discovered that there was a Z boson that had had a mass and that when its properties was studied the properties were not only consistent but required that it was a thing which went back and forth and back and forth and back and forth between pure Z boson and the zigs particle so they've existed we're not in doubt about them and we never were at least not for many years so far I have not mentioned the Higgs boson so what is the Higgs boson well the Higgs boson has to do with this condensate it has to do with this condensate but it's a different kind of excitation than sliding around the the edge of the sombrero here does not have to move it's not something which has to do with sliding around here it has to do of putting in two different ways to think about it you have a condensate and you can imagine the condensate has a density a density of these fictitious particles in the condensate imagine something which changes the density of them kind of like a sound wave a compression wave of some kind which squeezes them closer and further and closer apart makes more and more less dense that kind of vibration is what a Higgs volar Higgs boson is another way to think about it is that it doesn't have to do with sliding around the periphery of the sombrero it's you go to a place in space and start the field oscillating this way in and out this way the further away a tie is the stronger the condensate the closer to the center the weaker the condensate so it sloshes back and forth it's kind of a compressional wave in the condensate that mode that phenomena that oscillation is what is called a Higgs boson the Higgs boson is like the sound wave propagating through the through the condensate the reason it has been so important is because it was the one element that had not yet been discovered as I said the zigs was discovered long ago the Z and the W the electrons and all the others were discovered long ago and so the next question which I'll try to answer in a couple five minutes is why it was so hard to discover the Higgs what we discovered about it and very very quickly what the future might or might not bring try to do this in a couple of minutes okay so what kind of thing does the Higgs boson itself through now we're talking about the Higgs boson not the zigs boson not the Z boson the Higgs itself the one them the one that's been so elusive all these years it's called H and what it can do with some probability is for example create we read this from left to right the Higgs boson moving along in time time is now to the left can create an electron and a positron it can create a pair of quarks it can also create other things a new particle or a top quark or a bottom quark all of the different quarks electrons also neutrinos all of Area's fermions can be created in pairs when a Higgs boson decays you say yes if it's like a sound wave why does it decay well believe me sound waves decay if they didn't decay you continue to hear my voice ring forever and ever wouldn't do so sound waves do decay and it is possible to think of sound waves as decaying by creating particles so the Higgs boson decays it the case quickly if it exists if it really exists at the case quickly either into an electron positron or a pair of quarks or maybe some other of the fermions that exist in nature you can read this diagram in two different ways Oh incidentally the probability that the Higgs decays like this is proportional to the mass of the particle that it decays into the heavier the mass the more strongly that particle is coupled to the Higgs boson so heavy particles are favored and life particles are not favored now you can read this diagram in either direction you can say the Higgs boson decays but you can also say an electron and a positron confuse together to make a Higgs boson well if we want to make Higgs bosons and see them in the laboratory we want to read the diagram from right to left and we want to say this is a process whereby a pair of electrons can come together and make a Higgs boson we've been colliding electrons and positrons for a long long time almost as long as I've been a physicist not quite we've been colliding electrons and positrons together and nobody was ever able to discover the Higgs now one reason in the early days is it turns out that the Higgs is a fairly heavy particle I will tell you what its mass is but it's a fairly heavy particle and unless you have enough energy you don't have enough energy to make the Higgs boson but there's a more important reason in fact slack in the later days of slacks or life had plenty of energy to make the Higgs the problem was the weakness of the coupling the smallness of the mass of the electron translated into a very weak improbable cross-section too small in effect too unlikely to make the Higgs and so when you collide electrons together at high energy electrons are just not favourable they're too light and because they're light they tend to not make Higgs with any appreciable probability well how about quirks we can collide quarks together the usual quarks that make up the proton and neutron are also very light and because they are light also unlikely to ever make a Higgs boson well you know I'm sure they were made in slack but never in appreciable numbers that there was possible to to detect them so that was the main difficulty the lightness of these particles was a thing that essentially prohibited us from making Higgs is in abundance at SLAC or in other laboratories where collisions took place what is the most favorable particle most likely particle for the Higgs to decay in the heaviest the heaviest of the fermions and the heaviest of the fermions is called the top quark the top quark is hundreds and hundreds thousands of times heavier than the electron many thousand many many times over many thousands of times heavier than the electron and the Higgs preferentially will decay into top quarks so we'll just call those the quarks they are quarks but they're very heavy 170 times the mass of a proton basically which is heavy top and anti top top quarks and antiquarks so you say well look now it's easy to make the Higgs boson you just oh it actually in fact not possible for the Higgs to the cater to top quarks because the two top quarks are too heavy but if you read it the other way and you take a pair of top quarks and collide them together you can make a Higgs so it's easy we just go in the laboratory take a pair of top quarks collide them together and make a Higgs well the problem is that it's not so easy to find top quarks in nature why not they decay very rapidly to the other quarks they're not sitting or you can't put them into the accelerator and accelerate them they disappear in a tiny fraction of a second there are no top quarks sitting around not even buried inside protons and so forth not even buried inside other kinds of particles there are no top quarks around so we have to make the top quark somehow in the collision how do you make a top quark card so here's a way to make a top quark gluon can come along this is a gluon now and remember what gluons do they coupled two quarks one possibility is that the gluon can make a top quark and an anti top quark well as plenty of gluons around as we'll see in a moment so why don't we just take a gluon and make a top quark and the anti top quark out of it the reason is because gluons are very light they're almost there almost massless they don't weigh very much top quarks are very heavy there's simply not enough energy in the gluon to create a pair of top quarks so what we have to do is we have to take a pair of gluons now here's a process that you can imagine take a pair of gluons with a lot of energy moving toward each other with a huge speed plenty of energy let one of them make a pair of top quarks for a short period of time and then let the other one come and be absorbed by one of the top quarks there we have it a pair of top quarks created by a pair of gluons a pair of high-energy gluons smashed together and make a pair of top quarks once we've created those pair of top quarks the top quarks can come together and make our Higgs boson this is the way we usually draw this is to just draw glue on glue on and then a triangle Higgs is a top quarks going around the loop here that's the most efficient process for making for making Higgs bosons but where do you get gluons from gluons are in floating around well yes they are the proton is filled with gluons the proton mass of the protons maybe 50% energy from gluons or something like that it's filled with gluons and quarks you take two protons and you collide them together and the gluons inside the protons can collide during the collision and do this that was what was detected at LHC LHC is a proton proton Collider it collides protons together and when protons a very indirect way two protons collide together or gluon from each one of them scatter collide create a pair of top quarks and then the top quarks then have plenty of come together and create the Higgs boson that's the process that was discovered at the LHC and it took a long time to get there it was a hard thing to do it was a very very hard thing to do but now it's done we know the mass of the Higgs boson it's 125 GeV about 127 times the mass of the proton and that's I think a finished fact before I quit let's talk about the near future what have we learned we've learned that the standard model is essentially correct we've learned the standard model is essentially correct everything seems to fit together the Higgs boson fitting together nothing it's not the Higgs boson really that gives the particles their mass it's the zigs boson but the Higgs boson is just what's left over when you think of these density oscillations it was the last remaining piece it is now in place it's finished but is everything fitting together exactly right quantitatively right well that we don't know we don't know there's one hint one hint of a discrepancy and I'll tell you what the hint of that discrepancy is let's uh here's I drew this picture let me draw it again over here it's the process of creating a Higgs by two gluons coming together glue on glue on top quark going around the loop and Higgs now this same process once the Higgs is created also allows the Higgs to decay but it's not so easy to see gluons in the laboratory they're difficult to work with that's not the best process for looking for the Higgs boson after you've created it the best process is to replace the gluons by photons I don't have to even change the picture photons it's exactly the same process except with photons out here once the Higgs is created by whatever it can create it it can decay into two photons it's an intricate process it involves a lot of theory and a lot of calculation a fine ling diagram not easy to calculate but you can calculate it and it depends on the properties of the top quark going around here at the moment at the moment and I'm not an expert at this I can only quote what I'm told at a month the moment the Higgs boson that was produced in the laboratory appears to decay into two photons a little too quickly about one and a half times too quickly now everybody agrees that that is not a statistically really significant fact yet but what will it mean if it persists it doesn't seem like a big deal one of the half times too fast but the point is the theorists have the ability to calculate that rate very accurately a one and a half times too big a rate is serious it means something is going on the most likely thing that would be going on is that there's another kind of particle in addition to the top quark that has not discovered yet that can also participate in the same kind of it's called a triangle diagram some other kind of particle that of course would be big news if there's something there that is not described by the standard model that would be big nose it could be a supersymmetric particle it could be anything all kinds of things if this this is something to watch for now the buzz words are the decay of the Higgs into a pair of photons and a excess of about one and a half I think it's a two Sigma effect whatever that means means something the statisticians it means that it's not so robust but it could be right it turns out to be right it means that we've discovered something unexpected well it might be Haven something that's expected but something new beyond the standard model remember the standard model is over 50 years old well over 50 years old and so 1967 am i right season 77 97 97 2007 no I'm not getting on 50 years old so discovering the Higgs boson wasn't really discovering anything was confirming something if this should be off by a factor of one and a half one will have discovered something absolutely new so if you want to watch if you know want to be a spectator in the sport and you want to watch what happens this is the thing to watch for next whether the Higgs decays are consistent with the standard model okay that's a really finish thank you very much and I hope you all one or two questions what would cause do different fermions have different rates of viral oscillation good the answer is going to be an unsatisfying one the answer is that the fermions have dipped what would cause different Fermi onstaff different masses different masses essentially different different also as different rates of oscillation are the same as different masses the coupling strength the coupling constant that couples the relevant particle to the to the Higgs field what happens the particle moves chicken yes each one has a separate coupling constant emits this what did I call it the zigs midst the cigs which gets lost there's a coefficient here which is basically a probability each one and we don't know why they are what they are we know how to parameterize it but we don't know how to explain it for each kind of particle let's say the electron or the mew particle or whatever it happens to be there's a different constant there and that constant is the constant which determines the rate than the mass it's the same constant which comes into telling you how rapidly the Higgs decays into these particles and therefore the heavier the particle the stronger the decay that's a good question I'd like to say something about it I forgot we know the value really that the message yeah we've known that for a long time 240 GeV the value of the expectation value is from one language it's simply the displacement of the field and another language it's the density of the condensate can think of it either way the density of the condensate or as the value of the displacement of the field and that's why oscillation in the magnitude of the field is the same as a density fluctuation what's that apparently not apparently not well yes I think it does but there are many many other things that give it an energy density and for whatever reason they almost all cancel out this is one of the great mysteries of yeah yeah right so that's that's a very good question too which we don't have an answer at the moment right okay I hope you got something out of that I had fun preparing it and figuring out how to try to explain it some of you you probably got something others are just mystified and sitting why are they talking about but that's it for more please visit us at stanford.edu

Channel: Stanford

Views: 706,092

Rating: 4.877274 out of 5

Keywords: Science, Quantum Physics, Mathematics, Mechanics, Electric Field, Motion, Vectors, Electromagnetic Wave, Relativity, Locality, Higgs Physics, Proton, Neutron, Photon, Gamma, Energy, Particle

Id: JqNg819PiZY

Length: 75min 8sec (4508 seconds)

Published: Thu Aug 16 2012

Reddit Comments

[u/[deleted]]

There are tons of Susskind's lectures at stanford up on youtube. If you have got the time, I would highly recommend 'sitting in' on at least a few. Such an amazing world we live in where we can hear people like him speak from anywhere we want.

[u/[deleted]]

His brow is perpetually furrowed.

[u/cratylus]

Any pointers to reading on how the rate of chiral flipping relates to electron mass?

[ this seemed to be missing link in explanatory chain ]