Higgs Boson and Higgs Field

hello today we're going to talk about the Higgs boson and the associated higgs field and the first thing to say is that these may not exist they have not yet been found they are the subject of the experiments at the Large Hadron Collider but as of yet we do not know that they definitely exist before we get into the substance of the Higgs we first need to understand what we mean by fields let's take a room inside that room will be a temperature you could put a thermometer on the wall of the room and it would measure the temperature but is that the same temperature everywhere in the room well not necessarily it might be a bit cooler by the windows or by the draft of a door what you could do is to take this thermometer and measure the temperature at every single point in the room and for every point in the room you would have a separate temperature and that list of numbers which would represent the temperature every point is known as the temperature field as it is it just sits there and does nothing until something happens in with which it can interact for example if you were to introduce an ice cube then the ice cube would melt if the temperature of the room was significantly higher than the ice cube and consequently you would get water instead of ice in other words the temperature of the room the temperature of the air molecules would cause the ice to melt in certain circumstances but if there were no ice there then the temperature field simply sits there it's the same with an electric field if we take two plates and we put a battery such that this plate is positively charged and this plate is negatively charged we say that there is an electric field which runs between the plates and there's a value for that field everywhere between those plates it just sits there but if you were to put an electron in that electric field then the electron would accelerate towards the positively charged plate in other words when you put something in the field that interacts with the field then it causes that electron to move by contrast if you were to put say a neutron which is neutrally charged a neutron will not interact with the electric field and will simply sit there so the electric field has no effect so the question is what did Peter Higgs think he was doing when he created what is now known as the Higgs field and the Higgs boson back in 1964 well he was trying to answer the question what causes elementary particles to have mass and why does some have mass and some don't we have already looked at elementary particles in other videos they're set out in what is called the standard model here we have the standard model in a diagram form there are six quarks there are six leptons made up of three types of electrons and three types of neutrinos and this line here is the four so-called gauge bosons and those are what are called elementary particles there is nothing that is known to be smaller than them but if you look you'll notice that the photon and the gluon have no mass at all whereas the other particles have varying masses going from a new trainer which is this neutrino here is almost massless an electron nought point five over at any MeV an up quark 2 MeV right the way up to a top quark which is 171 GeV two of the bosons are massless but the Z boson has 90 GeV of mass the W boson has 80 GeV of mass and in case you're puzzle why we are describing mass in terms of energy this just comes from the term that e equals mc-squared and what you do is you take the mass which is very very small indeed multiply it by C squared to get a number which is strictly expressed in energy terms but these numbers are rather easier to understand than something that might be for example 10 to the minus 27 kilograms so why is it that some particles have mass have no mass and some particles have mass and different masses that's what the Higgs is about to explain I should just say however that the Higgs field does not explain why a proton has mass or indeed why I have mess those are quite different things the Higgs explains why elementary particles have mass if you think of a proton for example that consists of three quarks two up quarks and one down clock and we just had a table which showed that up quarks have two MeV of mass and down quarks have about four MeV of mass so the total mass of the quarks inside a proton is about 8 MeV and yet the total mass of a proton is about 938 MeV so how can a proton be that heavy when its constituent parts weigh only a fraction and the answer is to do with what is called quark confinement it's really based on the Heisenberg uncertainty principle if you take three quarks and you can strain them to be inside a proton which is no bigger than 10 to the fifth minus 15 meters then they will have a huge amount of energy associated with that confinement which by equals mc-squared is manifested in mass so the proton gets its mass and consequently I get my mass from quark confinement but what gives the quarks themselves mass that's what the Higgs explains massless particles must travel at the speed of light the reason for this is that when we did our videos on special relativity we derive the formula that e equals gamma MC squared gamma is the effectively the Lorentz transform it's one divided by the square root of one minus V squared over C squared where V is the velocity of the observer now what this suggests is that when M is zero the energy would be zero and for a photon for example which is massless you would expect all photons to have zero energy but we know that photons have energy Einstein said that energy comes in packets consisting of the Planck's constant multiplied by the frequency of the wave of which the photon is a part but how can a way a photon have an energy if it's mass is zero because this formula would tend to suggest that energy is also zero and the answer is that if and only if the particle is traveling at the speed of light which means that V is also C this term becomes zero and one divided by zero becomes infinity and then the energy equals gamma which is infinity times mass which is zero times C squared and that is mathematically undefined infinity times zero is not defined but it isn't zero and that's how a massless particle can have energy it must travel at the speed of light so now let's think a bit about the theory of the Higgs field the argument is that the Higgs field exists in all points in space throughout the universe it's what core is called a scalar field and the idea is that the particles which have mass interact with that field but particles without mass don't now just as with our room where which had temperature we would say that any point in the room you would have a number on the temperature scale which would represent the point of the temperature at that point in the room well the Higgs field is a little bit more complicated than that it has as it were two dimensions it has the field itself and then the potential energy associated with that field and the shape for mathematical reasons which is a bit complicated to go into but give you the idea of the principle of the thing the shape looks a bit like this it's actually a three-dimensional shape so that is a kind of circle and if you can imagine it it's like an inverted Mexican hat now where is the state of lowest energy everything always wants to get to its lowest energy the answer is at the bottom of this curve or the bottom of this curve but since its three-dimensional it's essentially anywhere on this circle that is where the energy is at its lowest and the theory is that massless particles occupy this lowest energy state but if you oscillate about that point there which of course you can do all the way around it's like a kind of gutter if you oscillate in that section there that oscillation is what gives rise to mass and it's the interaction of the particle with the field that causes that oscillation that gives the particle mass think about light or for a moment light is governed by the formula that the speed of light is equal to the frequency times its wavelength or if you like C divided by lambda equals F so if we plot the frequency against 1 over lambda we are going to get a straight line and the gradient of that line will simply be C now what happens if we multiply it both this term and this term by H Planck's constant H F is what Einstein described as the energy of the photon so this now becomes energy H over lambda is the de Broglie equation and that is momentum so we now have exactly the same graph but now it's energy against momentum and the straight line shows that the velocity is C and what this shows is that when the energy is zero the momentum is zero but for a massive particle we showed in the videos on special relativity that the energy is the square root of C squared P squared plus M Squared C to the fourth where m is the mass of the particle P is the momentum C is the speed of light and E is the energy now that will give rise to a graph that looks like this here we are plotting energy against momentum and even when the momentum is zero there is still some energy and that energy is the rest mass energy and you can see from this formula that if the mass is zero then you I use a photon then e equals simply PC the square root of P squared C squared whereas if it's a massive particle with no momentum in other words it's stationary then e is simply the square root of M Squared C to the fourth which of course is MC squared which is this rest mass here so conceptually how does the Higgs field work well let's consider a room and I am walking across that room and I can walk across it very easily nothing really impedes me or is fine now let's suppose that we fill that room half full with water I will now struggle to walk across that room I will be significantly impeded as I try to walk across the room there will be far greater resistance by contrast a little fish in that water will dart about very quickly without any problem and this conceptually is what the Higgs field is all about I represent a massive particle I struggle to get through the Higgs field and that's how I acquire mass the fish is the photon it doesn't have any problem zipping about in the higgs field and therefore it doesn't get any resistance now we've seen in earlier videos that wherever you have a field it is what they call mediated by a gauge boson for example when we said that two electrons interact we used a Fineman diagram here's an electron coming in here's an electron going out the two electrons essentially just coming together and then repelling but what we said was that it actually repels because there is some communication between the two that is done by a gauge boson in this case a photon the photon is responsible for the exchange of information between the two electrons to cause them to repel and when you get a Higgs field that is mediated by the Higgs boson how then can you find a Higgs boson you can't photograph it you can't even directly detect it because you don't actually know what you're looking for but the Higgs boson is predicted to have a hefty mass and it will decay into other particles and it's those particles that you're looking for you're looking for interactions which happen which generate more of one type of particle than you were expecting which will give you a hint that the Higgs boson has decayed into those particles theoretical physicists have narrowed the field down for the Higgs boson to have a mass somewhere between a hundred and 500 ge V the higgs-boson needs to be pretty massive in order to give mass to the heaviest of the elementary particles by contrast of course a proton has a mass of approximately 1 GeV so the Higgs boson is at least a hundred times bigger than the mass of a proton now the question is what does a Higgs boson decay into and the answer is well it depends how big it is or how massive it is because it will decay into elementary particles which are broadly of the same mass that it it is so if for example the Higgs is only a hundred GeV then it's likely to decay into a bottom quark if it's about here it might decay into a bottom quark or a W boson here it's probably just going to decay into a W boson here it can decay into a W or a Zed and at the top range you would expect it to decay into a W or a Zed or a top quark so depending on the mass you will find that the decay products differ now many experiments have already been done and directly or indirectly a huge range of this has already been ruled out to be what they call 95% confidence level that means we're 95% confident that the Higgs boson is an anywhere in this range or in this range and that leaves a very narrow range of 115 to 127 GeV which is the range where you're likely to get something like a bottom quark or a W boson so in the Large Hadron Collider in CERN what is happening is that two protons are being collided together at very very very close to the speed of light each resulting in a cataclysmic kind of smashing together with all sorts of things being produced and what the scientists are looking for is evidence of a Higgs boson in other words they're probably looking for slightly more bottom quarks or W bosons than they might expect how does this manifest itself well it's probably not as glamorous as you might think the Higgs boson doesn't just appear and say hello Here I am what is typically likely to happen is that you're going to have some kind of graph with a distribution showing emitted particles and that would be all sorts of different particles and what you're looking for is some kind of blip on the side of the graph that suggests that they were slightly more than you were expecting and that could be evidence of the existence of the Higgs boson because this suggests that there are more decayed particles the new were otherwise expected however it could be evidence of something completely different which is why the experiment needs to be done in several places and at several different times to verify in order to be able to give you the 95% confidence level that you need to be sure that you've found the Higgs boson that is responsible for giving maths to elementary particles

Channel: DrPhysicsA

Views: 155,608

Rating: 4.9088254 out of 5

Keywords: Higgs, Field, Mass, Boson, Standard, Model

Id: JY_F606E268

Length: 18min 59sec (1139 seconds)

Published: Mon Feb 27 2012