Martinus Veltman (2012) - The LHC at CERN and the Higgs

Well, these are very exciting days if you are a high energy physicist. At CERN they threaten you almost every day with the latest news. In fact you have to buy the paper to be sure that you are up to date. And after 40 years that’s something. But to date there is no news from the Higgs. Ok. So, what really is going on? So, let me try to give you the story. The story that I am going to tell you I have prepared for something like an hour but I have only half an hour. So I have a problem which I will solve. So, let me first go with the introduction. The introduction is telling you about elementary particles. You start off with the helium atom in which you discover there are a proton and a neutron and an electron circling it. And then the proton and the neutron are made of quarks. And quarks come in 3 kinds, up, down and strange which you don’t see here yet. And then on top of it every quark comes in 3 colours. So there are 3 times 3 different quarks. That’s what we have today. There are more than 3 quarks actually. And then you have something called neutron decay and that’s where we saw, at least not we but Mr. Becquerel way back in 1895 or so or later for the first time had the neutrino. He didn’t know he had a neutrino because you can’t see the neutrino. But that happens when the neutron decays and he was looking at the first form of radioactivity at the time. And so he was basically looking to this decay where a neutron goes to a proton while emitting an electron and a neutrino. And it is in this relation more than anywhere else that Einstein’s famous relation E=m*c^2 is actually verified. That’s the relation that’s especially important in particle physics and not that important anywhere else. So, that’s interesting to note. Then this was actually, this was pre-1905. So the reaction that established the E=m*c^2 was something that came before the equation itself. Well, ordinary matter then is made up from these particles. There’s the up quark in 3 colours, the down quark in 3 colours and the electron neutrino and the electron. Now this is the first family and from that all matter as we know it is made up. But as you go on and as you research and the basic thing for research for us has been CERN in Europe. At CERN you can see here, you see that the airport from Geneva, downstairs, you see it gives you an idea how big the thing is. And this is used to make new particles. So I have to explain how you make new particles. You make new particles by creating which I will call... Well, I don’t know how to call it, a fireball or a superparticle or something. You create a point which has a lot of energy. And then you let that point, that energy, you let it decay. That’s the principle that governs the operation of big accelerators. You collide 2 particles, protons, electrons, whatever and then at a point of collision you wait till you see other particles coming out and then you can study those. So, that’s the basic tool that we have in studying elementary particle physics. And here you see a small list of all these particle machines that have come into action in the time since they were invented. The first one is the Cockcroft-Walton of 1932 and it had an energy of 1 MeV, 1 million eV, the same energy that you get if you let an electron go through a field of 1 million volts. And from there all the way on you see all the steps. There is the Van de Graaff accelerator which has played a big role in education since every university could have one. There is the cyclotron which is much bigger already. And that could go a lot higher to a 1,000 MeV. That was already a machine going beyond what static electricity can do. Then there’s the Cosmodrome, the first circular machine which was in Brookhaven in 1953. And it has 3,000 MeV. That was considered a tremendous energy in those days. Later on at Brookhaven and also at CERN, CERN got its first; the first machine that CERN got was actually a cyclotron. The first machine that CERN actually had was a cyclotron but later on, quite in the beginning, in 1960 CERN started having a bigger machine. That’s the CERN PS, the Proton Synchrotron. It had an energy of 30 GeV. This was about the time that I arrived at CERN. So I saw all the action there. And I saw how this machine was switched on. I can tell you such a machine switching on, that’s an exciting moment. In any case there have been more such exciting moments as time went on. But after CERN and Brookhaven did this there came Fermi Lab. And CERN is a new machine, 10 times bigger. It always goes through the factor of 10 somehow. And came this 300 GeV CERN machine, Fermi Lab machine, and many discoveries have been made with this machine. Fermi Lab went up by another factor of, well, something to 1 TeV, 1,000 GeV, in 1983. And CERN with the LHC is supposed to go to 7,000 GeV eventually. At this time it’s around 7 TeV and is supposed to go to 14 TeV which is... A tremendous amount of energy has been obtained at these accelerators. Ok, I will go through this. This is the inventor of the cyclotron, Mr. Lawrence. And this is the Cosmotron, the first circular machine. It’s already getting big. And here is... At the CERN is this machine of 30 GeV of 1960. And I think that was a big piece of progress and you can see here how that machine... how big it is. Now Fermi Lab, well, then came later on. We got the super machines which were yet the factor 10 bigger than this machine here. That was the Proton Synchrotron and later on Fermi Lab used superconductivity. The magnets which you see in the picture here actually became superconducting magnets which allowed for higher magnetic fields and therefore higher energy for the beam to go around. And so came the Tevatron which had an energy of 1,000 GeV. And that happened in 1983. And then the important discovery was made essentially by Mr. Touschek. Touschek is someone who was sitting in jail during a part of the war. And it’s there that he worked together with the Norwegian Wideroe. He built the first storage ring. At least on paper because in the jail it’s very hard to build anything. In any case they used to come together. Wideroe would come to jail and bring Touschek something that he needed, that is alcohol in fact. And together they started working on these things. Wideroe has had some trouble after the war proving that he was not a Nazi and Touschek went on to Italy to build his first ring, ADA. And because this was a storage ring you had the particles, And that is also what he saw for the first time happening in Frascati, with the ADA. That was a great piece of progress and it made the energy much bigger. Because shooting a beam on a target just really makes for not too high energy. But if you can collide them, that makes all the difference. So, that’s what happened. And ever since people have been busy trying to make storage rings, first at CERN and later elsewhere. And in this way we got a very effective energy that became very big. The effective energy this day if you go to LEP is about 200 GeV. That seemed at the scale of things I’m talking now not to be that much but you see it was all effective. It was one of the better machines. And the great thing about those beams as well: they are going around and around, you can make them collide all the time. The disadvantage of beams like that, storage rings, is that the density of these rings is not too high. So you have difficulty in getting events. All that is quite complicated and it’s not for me to discuss that. The important point is that in the process of making all these machines, progress was being made in understanding particles. And the important thing is... Now, let’s see. The proton has a mass of 1 GeV. Cyclotrons cannot make particles heavier than a proton. You can only make a K meson and so on. But when you go to higher and higher energy you can make more and more new particles. And so by the end of the previous century, I mean 1900 something, 1990 or so, many new particles were found. And so all of these particles were essentially created artificially by these machines. And out came something which was utterly surprising. And this I need to emphasise. We are facing things and in the noise about the Higgs we have a tendency to forget those problems. But this is not right in my feeling. The first problem that we are facing is the problem of the 3 families. When you have something like this... This is a particle that we discovered. The up quark, the down quark, these are the first family. It’s the family that makes up for ordinary matter as we know it. The second family is something that you don’t see around but you can make it sort of easily. At least some of it. And then the third family it gets hard and that was discovered and completed, the scheme of things was completed, only in 1995 or 6 or so when they discovered the top quark. You realise that if you look to this picture and leave out the top quark you can make a prediction. There’s people who make that kind of prediction. So the top quark was predicted in that sense. Ok, they were all discovered and that is what we stand with at the year 2000. And there is just nobody, nobody who knows why there are 3 families. All kinds of things have been tried. I remember thinking of this supersymmetry and supersymmetry held a promise of explaining why there are 3 generations but it never fulfilled that promise. So to this day we don’t know. There is one piece that you have to add to this puzzle here. The question is now if you see 3 you might be tempted to think there should be 4, 5, 6, 7. How do you know? After all when you look to the last family there, the top quark is very, very heavy, very difficult to produce. So you could imagine that the next family is even more difficult to prove, to make. But that doesn’t hold for electrons. And then there came something else. That’s where you need the theorists. And the theorists discovered something at a time. They discovered there was a measurement relating somehow, buried deep in all the data, which gave you an idea. And that is very specific of the kind of theory that we have. In the theory we have locations, points where you can actually see what happens up there whereby up there I mean for high masses. And so we know at this point because of the agreement of the data with the best standard model there is only 3 generations. We know from that that in all likelihood unless there is some sort of funny conspiracy. In all likelihood this is all. And now this gets a problem. So, I want to sketch a little bit more to let you know where our problems are. If you have something like this, 3 particles, if those 3 particles would come about in a sense like we have seen the system of atoms and so on from Mendeleev. You know that you have to have many elementary things than the table of elements, you know, hydrogen, helium and so on. So, if you see a thing like that and first of all if you think there could be more, then it would be natural to think that those particles are the result of some bound states. That’s the very natural assumption. Well the first blow you get when they tell you that there is no more than 3 because if you have bound states you expect there to be more than 3. And so here is the first problem. In thinking of bound states you cannot explain this business by bound states or you would have to invent a trick whereby making a bound state is not something that you can make things of heavier and heavier, larger and larger mass. So no bound states. So, here you see you have 3 sets and you cannot make them bound states of something. So, this is where things get real hairy. But we don’t know about that and to this day that is a problem that we do not know how to attack. What I am saying right now is almost literally the same as something that I said at a conference in 1979 at Fermi Lab. And that just shows you how much progress there has been. In 30 years we have been facing this problem here and no one knows the solution to this particular problem. And this problem, if you want my opinion, I consider to be something which is more strange, further out than such a thing as a Higgs particle. Ok. Now, if you look to all those particles, there are forces between those particles. Some of them we know like gravitation and electromagnetism. But we know a bit less about the strong forces the forces that keep the nuclei together. But then there are the weak forces. That’s the first force which makes the neutron decay. And then there are the Higgs forces which at this point completely theoretical things. So, let’s see how that goes. With every force is associated a particle. And on the other hand, the mass of such a particle gives actually an idea of the reach of that interaction. Now, gravitation and electromagnetism have essentially an infinite range. Gravitation goes as far as our solar system and beyond and we suppose that it also goes... if it creates and holds galaxies, although we don’t know for sure. So that is gravitation and that has, if not a long, an infinite range. It has a very long range. The other one is electromagnetism. Associated with that is... That’s also long range force. And associated with those 2 fields are the particle that we call, from quantum theory, we call them the graviton and the photon. So the graviton has a mass now 0 which means it has infinite range. And the photon has mass 0 which means that electromagnetism, Coulomb field and so on also have an infinite range. Now the graviton has never been seen really. It is experimentally not a thing that you can easily get at. The photon we know of course and of the photon we know with rather large precision that its mass is 0. You can argue about to what extent. There’s an amusing story about that, it has to do with a lecture that Feynman once gave. But I won’t go into that. So the photon we think is, well, about massless. As good as it can. Now, the weak forces, that’s another story. The weak forces have several particles. And one of them called Z. With masses of 80 and 90 GeV. So these are really very massive particles. These particles of the weak interactions have a mass that is like about 100 times those of the proton of the neutron. That in itself is already remarkable. The orders of magnitude when you do this particle physic business they really stagger you. So the W and the Z are both very high mass particles. So it took a while before they got discovered, before they got experimentally demonstrated. And in fact I would say that only came through after we got the more precise idea how heavy they were. Well, at this point dealing with these massive particles is something that for theoretical physics is very difficult to do. So at this moment theoretical physics ended. And now we have to ask ourselves what happens when you have forces with a heavy mass, intermediated by particles with heavy mass. How does that work on the scale of elementary particles? Our first example that I look at is the muon decay. The muon is a particle of about 100 MeV, its 0.1 GeV. And it decays all by itself into 2 types of neutrinos and an electron. But it does that through the intermediary of a W which has a mass of 80 GeV. And here we see a feature of quantum mechanics. You can have a very heavy particle play a role, given that it doesn’t take too much time. Just observed the relation at work. So the intermediary particle, the W, can play a role as long as you don’t try to actually make it because then it would exist for a long time. And that doesn’t work, that’s too much energy. So it is this type of reaction that allows you to get these particles to be active also at low energy. Now if you do it that way then you get also a thing like this. If you have a particle that is an intermediary particle for a short time and which cannot really exist because it’s too heavy, you call it a virtual particle. You can always call anything, anything but. So, a particle like that is called a virtual particle. And also other reactions can occur. And now we have here a reaction that is important to us. You can have a mu meson. It can split up for a small moment into a neutrino of the muon type as well as a W-. And recombine quickly. It has to be recombined quickly but this is a possibility. And in fact when you think about it, you discover that this is a quantum equivalent of a particle sitting in its own field, in this case the field of the weak interactions of the Ws. So here sits a particle in its own field and that generates this diagram and you can translate it in a contribution to the mass of that particle. And then it turns out, that’s the second step, that this mass is infinite. So here theory makes progress. But then we hit a wall and the wall is that the calculation that you want to believe in actually is giving infinite as a result. So you get an electron of infinite mass. And what the hell can we do with that? Well, no one can do anything with it. An electron sitting in its own field for all we know there is an infinity there. And we don’t know how to deal with it. Well we know how to deal with it but we don’t know anything about that particular infinity. So that energy, the self-energy is infinite. But on the other hand we know that that energy cannot be infinite because the electron has a mass of half an MeV which is not infinite. No one has solved this particular problem. And there has only been a partial solution to that problem due to a theoretical physicist from Leiden, Holland, Mr Kramers. And in 1948 he came up with the idea: Look, if you see something that you cannot solve, leave it. That’s what he did. That’s not such an un-logical statement. So what he did, he said: "Ok, as long as in a given theory the number of infinities like this one sketched here is finite, what you can do is you can take the experimental values for those quantities, substitute them for the infinities which of course is an ugly thing to do and then make predictions for other observables." So that’s the procedure proposed by Kramers. It’s since then become official. It’s called renormalisation. And the theory where you can do that... You can do that with the theory only if that theory has a limited number of infinities. Because if you have an infinite number of them, you have to fit an infinite number of data and that doesn’t go. So the renormalisable theories are the ones that allow you to make this procedure. And in 1948 it was established that quantum electrodynamics, the quantum version of Maxwell’s theory, was actually renormalisable. And has created a lot of excitement and produced a few predictions. The prediction where the anomalous magnetic moment of the electron and the muon. And there was also something called the Lamb shift. So the theory following the recipe of Kramers produced actual results. And this was a great moment in particle physics. So, that’s extended the life of field theories. Field theories in particle physics have had really difficulty getting there. I mean it’s like a difficult child that grows up and you have a difficulty every time around the corner. Now, in this case the difficulties was the one of the infinites. Before there were other problems. But by 1948 we got through that hurdle. We still were facing another hurdle but anyway that’s how far we got done. Weak interactions, the ones involving Ws and Zs, they remained mysterious at that time. But in 1964 it was discovered that the certain type of theory with much internal symmetry it has a new element coming in it. Namely you got a theory, you got an infinity there and you made something else that gave the infinity with an opposite sign. And this kind of procedure is what became attractive at the time. That was created for the weak interaction. So we got a symmetry guaranteeing renormalisability, that’s what happened. And such a theory is called Yang-Mills theory. The first particles in that theory, the equivalent of the photon and the graviton, also here were massless. And so the problem the remained was to give a mass to those particles. With massive particles the theory became non-renormalisable. It was a big disappointment. And in 1971 it was discovered that the renormalisability of the theory could be restored at a cost introducing 1 new particle, the Higgs particle named after Peter Higgs who proposed a particle in 1964 but did do nothing about renormalisability. So that’s the situation that we got in ’71. We could make the theory of weak interactions renormalisable, but at a cost of a new particle. And that shows you something about the Higgs. It’s really a theoretical figment. It’s very hard for you to grasp why we need the Higgs. It is there to cancel an infinity. How would you visualise that? I know only 1 way of doing it, where you have a scattering process. Let’s say a proton of an electron or 1 quark of an electron; you have between them the Coulomb law. That goes like 1 over R so obviously that is going to explode when these particles come very near. Then the way to solve that problem is to create another force which was the force due to W and Z which becomes effective only at very short range. And you make that have the opposite sign. So they scatter and by the time you get very close the weak interactions and electromagnetism conspire to cancel each other. But if you do that you discover theoretically that you cannot do the actual cancellation too in the largest detail. You have to make something more. And then comes in the Higgs field which also is an exchange between the two. And it sort of cancels the last piece. That’s a complicated construction that the theorists have made of that theory. And that’s the Higgs particle. And so we need it for theoretical reasons but you have a hard time visualising why you need that particle because you have to dig into the calculation of infinities and things like that. Now, this Higgs particle which by itself is not a prediction of the theory, the mass of this we don’t know. The action of cancelling force is not something that you can get the mass from. Whether it happens at small distance or large distance doesn’t really matter. And that has been looked for ever since 1972 or ’71, roughly there. And I remember every year the limit on the mass, that’s the lower limit, used to go up. So, it started off at about 500 MeV, went to 1 GeV, went to 10 GeV. Then there were people saying it was 30 GeV and so on and so forth. So I myself contributed to the mass by adding that, and that was a safe bet since they have seen it not since now. So, maybe it’s not there at all. So I contributed the idea that it wouldn't be there. Of course that gives then the problem. You get the problems of remaining infinities. Well, you have to deal with that. So now at CERN the latest news is that they might have seen the Higgs with a mass of about 125 GeV. It comes about Thursday we will know for sure. Well, for sure? A little bit sure. So the Higgs is coming up. What does it mean? And that’s the thing that I have to tell you. What does it mean if they discover the Higgs at the LHC? First of all, it completes the standard model. Unfortunately completing that standard model is like closing a door. You close a door on the standard model and you stand in a room of which you don’t see a door anymore. So the difficulty with that solution, namely that the Higgs is there, is that we have no idea what experiment to do to better understand the whole thing of particles which I just explained to you has big problems such as the family structure, the masses and so on. So in a sense discovering the Higgs is sort of a bad thing. Well, it’s sad but true. And so here we are then, if the Higgs indeed is found, and the fact is that then they can go on at CERN for another couple of years establishing the properties of that particle. They have to do that, they have to show that that particle has all the properties that you expect of it. Which is very precise, we know precisely what it should do. And after that we can say: "Let’s close the stuff and let’s go home (laugh). You want me to go home now." So that’s the situation and I wanted to go a little bit further than that and so I won’t. There’s the Higgs decay. How can it decay? That’s how you can see it. And then I’ve got on to another problem. That is cosmology. Because the Higgs has its problems we suspect with cosmology. The Higgs field is something that is there in space everywhere around. And if that’s the case gravity is the first one to detect it and a short calculation tells you that gravity would, when there was such a Higgs, make our universe about this size. This is against experimental data. So, it has a disastrous consequence for cosmology. But of course cosmology who have an intellectual... how you can say that ...an intellectual capability of solving all problems, they can solve this one too. What they do is they assume that the universe before the Higgs comes into being is actually curved the other way around. And then comes the Higgs and now comes out flat with a precision of running god knows how many 10 to something order, pretty unbelievable. But that’s the situation that we have. And as the cosmologists go on and start discovering the universe and coming up with new explanation every time there happens to be one. We have to wait. But it is a real difficulty I would say. There’s the cosmological constant and that can be solved in 2 ways which I insist on saying. It can either be, and that will be there is something wrong with gravitation, there is something wrong with the Higgs system or there can be something wrong with gravitation. With our understanding of gravitation. Now that’s real serious. So when they come out at CERN with the announcement please keep this in mind that we are in fat trouble somewhere else. And here’s Mr. Peter Higgs trying to tell that to me. He says it’s 125 GeV. And I say I don’t believe you and then he’s right for god sake. And this is Mr. Einstein and the theory of relativity has been suffering all kinds of difficulties. Here he has a problem.

Martinus Veltman (2012)

The LHC at CERN and the Higgs

Martinus Veltman (2012)

The LHC at CERN and the Higgs

Abstract

The LHC at CERN and the Higgs.
Lecture by Martinus Veltman, Lindau, July 1-7, 2010.

Particle physics mainly developed after World War II. It has its roots in the
first half of the previous century, when it became clear that all matter is
made up from atoms, and the atoms in turn were found to contain a nucleus
surrounded by electrons. The nuclei were found to be bound states of neutrons
and protons, and together with the idea of the photon (introduced by Einstein
in 1905) all could be understood in terms of a few particles, namely
neutrons, protons, electrons and photons. That was the situation just before
WW II.

During WW II and directly thereafter information on the particle structure of
the Universe came mainly through the investigation of Cosmic rays. These
Cosmic rays were discovered by Wulf (1909) through measurements on the top of
the Eiffel tower and Hess (1911) through balloon flights. It took a long time
before the nature of these cosmic rays became clear; just after WW II a new
particle was discovered by Conversi, Piccioni and Pancini. This particle had
a mass of 105.65 MeV (compare the mass of the electron, 0.511 MeV and the
mass of the proton, 938.272 MeV). The development of photographic emulsions
led in 1947 to the discovery of another particle, the charged pion (mass
139.57 MeV), by Perkins. In subsequent years yet more particles were
discovered, notably the K-mesons and the "strange baryons" such as the Lambda
(mass 1115,683 MeV). Gradually the phenomenology of all these particles
developed, new quantum numbers were invented and classification schemes
developed. At the same time, the development of new devices and methods
greatly furthered the knowledge of elementary particles. The most important
of these are the particle accelerators, the cyclotron and developments
thereoff, and the detection instruments such as bubble chamber and spark
chamber.

In the beginning sixties Gell-Mann and Zweig came up with the idea of
elementary constituents called quarks. These quarks did have unusual
properties, the main one being that they did have non-integer charge, in
contrast to all particles known at the time that did have integer charge
(such as the electron and muon with a charge of -1). For this reason the
quarks were not immediately accepted by the community. In addition, as we
know now, they can only occur in certain bound states such that the charge of
these bound states is integer. Thus the quarks by themselves are confined to
bound states. The reason for this confinement became clear much later, around
1972.

The theory of the forces seen to be active between these particles is quantum
field theory (QFT), a theory of such complexity that its development
stretched over many years. Around 1930 Dirac, Heisenberg and Pauli formulated
the foundations of QFT, but it was soon discovered that the theory as known
then was very defective, giving rise to infinite answers to well defined
physical processes. Fermi was the first to apply QFT to weak interactions,
notably neutron decay. The theory developed by Fermi was a perturbation
theory, with answers given in terms of a power series development with
respect to some small constant, the coupling constant. The lowest order
approximation of Fermi's theory was quite successful, but any attempt to go
beyond the lowest order met with failure. In any case, Fermi's theory
involving the then hypothetical neutrino postuled by Pauli, was successful
enough to cement acceptance of that particle.

A breakthrough was due to Kramers, who already before WW II discovered that
QFT implied certain corrections to the atomic spectra. Experiments by Lamb
actually measured such corrections (Lamb shift), and Kramers ideas found
acceptance by the community. In addition, Kramers introduced the idea of
renormalization, a procedure whereby the infinities of QFT were localized,
and where outside these isolated parts perfectly precise calculations could
be done. Feynman, Schwinger and others took up these ideas and developed the
QFT of electromagnetic interactions, allowing very precise calculations of
the Lamb shift and other corrections, commonly called today radiative
corrections. These developments, including very successful experimental
confirmations, took place around 1948.

The development of QFT of the weak interactions was very difficult and lasted
till aout 1971. A new idea, the interplay of forces arranged in a very
careful manner such as to avoid the occurrence of infinities, was developed.
This is known under the name of gauge theories. In such a theory there is a
multitude of forces and particles such that all irreparable bad features
cancel out. Thus the theory thereby predicted the existence of certain new
particles, necessary to complete the complex structure of balancing
infinities. The actual discovery of these particles, notably the Z0 and the
charmed quark, topped by the discovery of the top quark in 1995, has firmly
established the gauge theory of weak interactions.

The strong interactions, the forces responsible for the interactions between
quarks and notably supposedly responsible for quark confinement, profited
from the development of gauge theories. In the wake of the gauge theory of
weak interactions also a gauge theory of strong interactions was formulated
and investigated. An important step was taken with the establishment of
asymptotic freedom for the gauge theory of strong interactions. By 1980 the
Standard Model of Weak, em and strong interactions was settled; the Higgs
sector of that model remains still to be tested, which hopefully will be done
at least partially using the new machine L(arge) H(adron) C(ollider) at CERN,
now running.

Meanwhile, CERN has been producing results. These include events that could
be interpreted as evidence for a Higgs particle of approximately 125 GeV,
that is about 133 times as massive as the proton. The relevamt events observed
could be interpreted as one of the following three types:

- Higgs decay into two photons;
- Higgs decay into two Z (the neutral vector boson of the weak interactions)
of which one is virtual;
- Higgs decay into two W (the charged vector boson of the weak interactions)
of which one is virtual.

More data will be needed before any firm conclusions can be drawn; that could
be somewhere during the next year. The latest results will be discussed.

Cite


Specify width: px

Share

COPYRIGHT

Cite


Specify width: px

Share

COPYRIGHT


Related Content