ANDERS RYD: So it is my pleasure to introduce this whole Bethe Lecture here. So the Bethe Lecture is in memory of Hans Bethe. He had a profound impact on the Cornell Physics Department. He fled Nazi Germany in 1935 and came to Ithaca here at the age of 28. Before the Second World War, he published his famous reviews of nuclear physics and did his groundbreaking work on the theory for energy production in star, for which he got the 1967 Nobel Prize in Physics.
During the Second World War, he was the head of the Theory Division at Los Alamos. After the war, he brought some of the most brilliant physicists back to Cornell, in particular, Richard Feynman and Robert Wilson. He officially retired in 1975 after 40 years on the faculty here at Cornell, but remained active in research still for a much longer time. The Bethe Lectures were instituted in 1975 to honor and commemorate his services to Cornell and the nation.
So today it's my pleasure to introduce Fabiola Gianotti, who is the Fall 2013 Bethe Lecturer. Fabiola got her PhD from the University of Milan. After finishing her PhD she was an INFN Fellow before she started working for CERN in 1994, where she has been since.
Fabiola has been involved with the ATLAS Experiment since the very beginning, in fact, even before ATLAS officially was formed. Since her PhD, she has been one of the leaders in the development of liquid-argon calorimetry, which was the chosen technology for the ATLAS Experiment. While working on a development of the ATLAS calorimeter, she also joined the ALEPH Experiment at LEP. In 1999, she became Physics Coordinator for ATLAS, and then in 2004, Deputy Spokesperson.
In 2009, she was elected spokesperson for the 3,000 people strong ATLAS Collaboration. During the first three years of the LHC operation, she was in charge of ATLAS until she stepped down earlier this year. Of course, the first three years of the LHC operations saw the discovery of the Higgs, and we will hear more about that in Fabiola's talk today.
Fabiola has gotten numerous awards for her work. Earlier this year, for example, she received the Italian Enrico Fermi Prize. With, therefore, basic science intense media coverage of the LHC and the Higgs discovery, Fabiola got a lot of media attention and was one of the runners-up last year for the Time magazine's Person of the Year award. Of course in the end the competition was strong, and Barack Obama was selected. However, I am glad to say that she has done better than some of her fellow runners-up, like Mohammed Morsi.
With that, I'd like to introduce Fabiola.
FABIOLA GIANOTTI: Good afternoon. Thank you, Anders, for this very nice introduction. And really it's a pleasure. And I'm really honored to be here and to give this prestigious lecture at a very prestigious place, which has also has made a very strong contribution to development of accelerators and accelerator-based particle, particle physics. And I'm also happy that this lecture happens to be just a few weeks after the award, the announcement of the award of the Nobel Prize to Francois Englert and Peter Higgs for the theoretical discovery of the so-called Brout-Englert-Higgs mechanism and the theoretical prediction of the Higgs boson.
So the Higgs boson would actually be a large part of my presentation today, though not the only one. And here I will focus mainly on the ATLAS experiment because here at Cornell University you have a very strong group working on the other general purpose experiment, CMS. And of course they can tell you much better about the CMS features and physics goals and achievements and accomplishments. But from here, from now and then I will also show some results from the CMS experiment.
So first of all, the Higgs boson has been discovered at CERN at the Large Hadron Collider, the so-called LHC. So let me remind you that the LHC is one of the most ambitious projects in science ever. It's actually the most powerful accelerator ever built by mankind. It has the most high-tech and complex particle detectors, and it has required new, innovative concepts and many new technologies that were developed in order to meet the scientific goals and the technical requirements to achieve them and the effort, 20 years, or more than 20 years of effort of the worldwide international community.
Actually, operations started on November 2009, more than 20 years after the first discussions about the potential of such an accelerator. The first high-energy run took place between March 2010 and February 2013. This is called LHC Run 1.
And now we are in a period of shutdown, so-called LS1, long shutdown one, during which we are going to make some improvements, in particular to the accelerator, in order to be able to operate at higher energy in 2015. So the LHC, as many of you know, is a 27-kilometer accelerator wing, 100 meters underground. So this is an aerial view of the Geneva region. The accelerator actually is at the border-- is in the country between Switzerland and France. These little white dots here show the border between Switzerland, here, and France, there.
This is Geneva Airport. Lake Geneva is down here. I live here, but you don't care. OK. This is CERN main site, so here are the main buildings and offices of people working at CERN, and this circle here indicates the location of the LHC ring, 27 kilometers, 100 meters below ground, so we don't see it on the surface.
So over the last three years, two beams of protons have been accelerated in the two opposite directions and brought into collisions at four points, where these little yellow stars are located. And there, four big caverns have been excavated, where four big, giant particle detectors had been installed.
The B managing, until today has been up to four TeV for center of mass energy, so collision energy of 8 TeV, but in 2015, we will move up to close to the design energy, which is 40 TeV. So 14 TeV is seven times larger than the energy of the most powerful collider until-- before the LHC, this was the Tevatron Collider at Fermilab. And this, of course-- which makes the LHC the energy frontier machine today.
So the four experiments are called ATLAS and CMS-- these are the so-called general purpose experiments. These are the biggest experiments used to explore the new energy frontier, and these are the two experiments that have announced the discovery of the Higgs boson. And then there are two smaller experiments, but not less important, called LHCb and ALICE, devoted to some specific topics like the study of B-meson, which is an important tool, laboratory, to understand the matter-antimatter symmetry in the universe, and ALICE, devoted to the study of heavy ion collisions.
Now, the US laboratories and universities have made very strong and crucial contributions to the four experiments, and to the accelerator. And in particular, as I mentioned before, there is a strong group here at Cornell University, consisting of about 30 physicists and also many, many students, who have contributed to the CMS experiment, working on development, detector construction, Trigger, which is the online selection of the event software computing physics, of course, exploiting the physics, the data, and analyzing the data, and exploiting the physics potential, and now on the upgrade of the detector to operate at higher beam intensity.
So the single most challenging component of the LHC, and the one that has allowed the LHC to achieve higher energy compared to its predecessor, is a set of 1,200 high-tech superconducting magnets working at very low temperature. So in a proton-proton accelerator, the difficult thing is not to accelerate the protons, but to guide them and keep them inside a ring of limited dimension. As you know from Lawrence Formula, if you give a given accelerator ring of a given radius-- and the LHC ring tunnel preexisted. This is the tunnel used for the previous CERN collider, the [INAUDIBLE] [? plus or minor ?] accelerator, so the radius was four kilometers. The ring existed already.
And so the limit, the maximum energy of the proton, and so the design energy-- 7 TeV, for 14 TeV in the collision energy-- this limit was set, essentially, by the bending power, by the technological possibilities of reaching the largest possible magnetic field. And at the time, and I think still today, the maximum achievable field was at the level of 8 Tesla. Today, of course, there exist magnets that are able to go beyond 8 Tesla, 14, 16 Tesla, but at the level of small prototypes, not at the level of filling a full ring of 27 kilometers with magnets, which needs to be built by industry and the cost has to be, of course, contained, et cetera.
So the breakthrough was the ability of building these 1,200 superconducting magnets, providing a field of 8 Tesla, which gave a maximum beam energy of 7 TeV. This is why we have chosen 7 TeV per beam. It's not just a random number, but it's essentially the limit coming from the available technology.
So these magnets have to be superconducting, because to provide such a high field, we need a lot of current. So typically, they use a current of 12 kiloamps. So they are superconducting. They are made of almost 8,000 kilometers of [? niobium-Ti ?] superconducting cables. Superconducting magnets work at low temperature, and so the LHC magnets work at 1.9 Kelvin. This is 1.9 degrees from absolute zero, minus 271 degrees, which makes also the LHC, by the way, the coldest place of the universe, because the universe today is at a temperature of 3 Kelvin, whereas the LHC is at 1.9 Kelvin. So we say it's the coldest, but also the coolest place, to be.
And these magnets are immersed in a bath of 120 tons of superfluid helium. So this blue tube that you have seen many times-- so this is a view of the LHC underground tunnel-- these blue tubes contain the superconducting magnets, which are quite high-tech objects with a lot of pipes and several important components. And actually, you may remember that in 2008, after switching off-- a few days after switching on some of the accelator, there was a big accident due to the fact that one of the superconducting interconnections between two of these magnets broke, essentially, because the soldering, the welding, was not well-done.
And this caused a stop of one year, and when operation restarted one year later, so in 2009, it was decided to run the accelerator at lower energy to avoid these kind of problems. And so now, during this shutdown, we are repairing and consolidating all these superconducting, and also the [? warm ?] connections between the magnets, in order to be able in 2015 to go to the design energy. So this is what we are doing at the moment.
OK, the detectors are also unprecedented in terms of complexity, technology, and performance. For instance, this is ATLAS. You can see it's-- well, you cannot see, but I tell you, it's 45 meters long, 25 meters high. And if you want to have an idea of the scale, here it is compared with two human beings, a woman and a man, because we promote equal opportunity in ATLAS, so we put always women and men in our drawings.
So why so big? These detectors are so big because, of course, we want to absorb and measure with precision the very high-energy particles produced in the collision, and of course, you cannot measure with precision a TeV or 700 GeV particle in just a few centimeters of material, so you need very big volumes. At the same time, we need to measure the thousands of particles produced in the collision with very high precision at the level of a few microns, where a micron is one millionth of a meter, as you know. And so we need a very high granularity. It is a very huge number of individual sensors-- most of them are actually pixel sensors-- in order to measure, ideally, each particle produced in a collision with very high precision to construct their trajectories, measure their energy, and therefore, have a full picture of the collision event.
Also, these detectors have to be very fast, because the two proton beams collide, at design operation, 40 million times a second. So the detectors typically need to have a fast response on the level of 25 to 50 nanoseconds in order to cope with this very high interaction rate. And also, the computing resources are a challenge, because every year these detectors, each experiment produces typically 10 petabyte of data, which of course is an enormous amount, and needs to be stored, distributed, et cetera. So it's a big challenge.
It has been also a big challenge from the engineering point of view to install these big detectors in the underground cavern. So for instance, here you can see the ATLAS underground cavern, 100 meters below ground, as it was in June 2003, when the civil engineering work finished and the cavern was handed over to the experimental collaboration order to install the detector. So you'll see that the cavern is empty, there are a few people doing some survey.
And then we started to install [? the fits ?] of the experiments. And then, one by one, you can see these big objects here, these are eight superconducting coils, providing very strongly magnets in order to measure high-energy muons with high precision, and each one of these superconducting coils is 25 meters long and has a weight of 100 tons, so it's a very-- sorry, did something stupid. It's a very big enterprise to install. It was a very big enterprise to install this quite impressive toroidal magnet system in the cavern. And you see at the end of the installation of this system, you see again this nice picture, and the dimensions of this detector compared to that of a human being.
The LHC experiments had also thousands of quality controls of the individual components, because of course these detectors are under huge radiation, so we had to check every single connector and every single piece of detector that it could stand the high radiation. Many years of tests of the detector components with beams, prototype models, and then the final components of the detector. 20 years of detector simulation and studies of the physics potential, a lot, a lot of paperwork.
We wrote in ATLAS, and similarly, CMS, 20 technical design reports to convince committees and various bodies that we would be able to build these detectors, and eight years of worldwide computing data challenges. So all in all, a lot of work. So the question is, why?
So the reason is the following. Let's look at the world of elementary particles before the LHC started to operate, and before a very important day, the 4th of July, 2012, when the Higgs discovery was announced. So by that time, we knew the particles, the elementary particles-- elementary particles are particles that cannot be broken into smaller pieces, smaller components-- so we knew the world of elementary particles and their interactions thanks to almost 100 years of theoretical work and almost as many of experimental work. Many elementary particles have been discovered over the last 40, 50 years at accelerators from all over the world. Their interactions had been studied in detail.
And we have a theory called the Standard Model which is able to describe the elementary particles and their interaction with very high precision. High precision means, first of all, that all the particles predicted by the Standard Model, except one at this date-- so before the 4th of July, except the Higgs boson-- had been discovered experimentally. So the theory was correct and had a very high predictive value, because most of these particles had been predicted before they were experimentally observed.
Second, all of the predictions of the Standard Model-- in terms of, for instance, interaction of these particles, the way they behave, their features-- had been measured with very high precision, and verified with very high precision, by many experiments at various accelerators. So a big success for the theory. Until the 4th of July, 2012, the world of elementary particles consisted of 16 particles, shown here, which can be divided into two main classes.
This part here, indicated in green, these particles are called matter particles. And they're called like that because the particles belong into-- you see there are three different generations, first, second, and third generations. They are made of quarks, which are the elementary-- the constituents of the neutrons and protons of the nuclei, and leptons. And they are called matter particles because those are the first generation, up quark, down quark, and electrons, are the fundamental constituents of the atom.
OK, an atom is made of a nucleus containing neutrons and protons, and neutrons and protons are made mainly of up and down quarks, and then the nucleus is surrounded by a cloud of electrons. So these particles are the fundamental consitituents of ordinary matter. That's why this class of particles is called matter particles. They have the feature of having [? spin ?] equal to [INAUDIBLE], and you know the [? spin ?] is intrinsic quantum number of particles, in particular of elementary particles. So these particles are [INAUDIBLE].
Class number two, the force carriers. So those particles that at the microscopic level, at the level of quantum interaction, quantum mechanics, are responsible for transmitting the force between two elementary particles. So for instance, the photon transmits the electromagnetic force between two electrons. Likewise, the W and Z bosons, which were discovered at CERN in the beginning of the '80s, are responsible for transmitting the weak force between, for instance, quarks and between leptons, and that is possible of the weak force which acts in radioactive decays or in thermonuclear reactions in the sun.
These particles, called force carriers, are characterized by having spin one, so they are vector bosons, as we have said. So the picture looks quite OK. We have a theory that describes these particles very well. All of those predicted by the theory have be discovered. But if you look at this picture a bit closer, we found already-- we find some problems.
First of all, why are there three generations of matter particles if matter, ordinary matter, is made only in the first generation? Why is nature so [INAUDIBLE]? Why three replicas of this particle?
Second, if you look at the force carriers, why some of them, like the W and the Z boson, are massive, typically 100 GeV, 100 times the mass of the proton, whereas the photon is massless? Third question, why is gravity so weak? So the forces among elementary particles that are important are the electromagnetic force, the strong force which binds the quarks inside the neutrons and protons, and the weak force? And gravity is extremely weak at the level of elementary particles, so what is special with gravity? So these are some of the questions that we-- and we still, some of them, we still have. Some others-- one of them has been solved.
So in addition to this question, there are a certain number of observations, experimental observations, that tell us that the Standard Model is not a complete theory. It's a correct theory in the sense that in the phenomena observed so far related to elementary particles, the Standard Model seems to work extremely well. But on the other hand, we know that there are some other phenomena to which the Standard Model is not able to give an answer.
I mentioned already in the previous slides the problem of the particle masses; how do elementary particles acquire a mass? In particular, why the photon is massless, whereas the W and Z bosons are massive? This is related to the Higgs boson, so I will come back to this later.
But you know very well that today, we know that 96% of the universe is dark, meaning that we only know and understand and see 4% of the universe. 4% is made of the ordinary matter I was describing before, atoms, elements, light elements, et cetera, chemical elements; but the rest is made of a form of matter at the level of 23%, or a form of energy at the level of 73%, which we don't know. We can only-- we don't even see with our instruments. We can infer their existence indirectly from the observation of some phenomena.
Now, concerning dark matter, none of the particles that I've shown before-- sorry, I'm going in the wrong direction. None of these particles have the good feature to explain dark matter and its features as we observe it by some, for instance, astronomical observations, or from the motion of the galaxies, and so on and so forth. So clearly, the Standard Model cannot explain everything, and therefore, there must be physics beyond the Standard Model. There must be new particles accounting for dark matter and there must be an explanation to various phenomena.
And experimental data and theoretical arguments actually indicate that these new physics should lie, should manifest itself, should be at the TeV energy scale being explored at the LHC. And this was the main motivation to conceive and build the LHC. We knew that the Standard Model was not complete, was not the ultimate theory of particle physics, and that the new physics explaining all of some of the questions to which we didn't have an answer within the Standard Model should manifest itself at the TeV scale, so let's build a machine, go there, and see what we can find.
So the adventure actually started in April 2010, when first beams of unprecedented energy started to circulate in the LHC, thereby opening a new era of exploration of a new energy frontier. And since then, we have made quite a few things. First of all, the fundamental component of this project, the accelerator, of course, the detectors, and also the computing infrastructure, the massive computing infrastructure, performed very well, beyond the most optimistic expectations.
So in only three years of data taking, the so-called Run One, we were able to do a lot. We have recorded a huge amount of data and analyzed them, so the ATLAS and CMS experiments, the two general purpose experiments, have recorded about five billion collision, proton-proton collisions, each. With this data, we have, first of all, rediscovered, if you want, the Standard Model. Most of the particles that you've seen a couple of slides ago have been observed again in a different energy regime. They have be measured again in a different energy regime, and we found that the Standard Model works extremely well.
And we have also started to look for new physics which could explain-- answer some of the questions I was mentioning before. But for the time being, we have found nothing. And of course, also finding nothing is a very good-- is very important in research, of course, and we were able to exclude several scenarios. So for instance, this is actually a collection of results from the CMS experiment that shows the mass limits on several scenarios of physics beyond the Standard Model [? composite ?] [? that have ?] [? heavy ?] [? resonances, ?] long-lived particles, [? contact ?] interaction, extra dimension, et cetera. This vertical black line indicates the 1 TeV limit region. And you'll see that in many cases, in many scenarios, CMS and ATLAS, similarly, has been able to exclude the prediction of these new phenomena well below the TeV scale.
So no [? ins ?] for new physics yet, so should we be desperate? Well, I think no, first of all because, of course, we have a lot of data recorded, and so we will continue to search. Of course, we have excluded the most obvious topologies and the easiest things, but one never knows. We have to look in more detail in the parameter space.
We have to still do a lot of work with the present data to see if new physics hide somewhere in the less obvious places. But also, the energy at which we have been operated until now is 1.7 times smaller than the design value of the LHC, so 8 TeV compared to 14 TeV, and the data sample is a factor of 10 smaller than the design data sample of the LHC, not to count-- not to include and mention the possible upgrade. So there is still room, of course, for finding new physics.
But clearly, the highlight of the operation in [INAUDIBLE] has been the discovery of a new particle which we called Higgs-like on the 4th of July, 2012, with mass 125 GeV, meaning about 130 times the mass of the protons. And today we have crossed out the "like," because after about one year of additional investigation of this particle, we can state that this particle is a Higgs boson. So we like it, but the "like" has been removed because we think it's a Higgs boson.
So let me try to say in a few and simple words the importance of this particle and of the Higgs mechanism. So until the 4th of July of last year, we didn't know how the elementary particles could get a mass. So as I said before, we didn't know why the photon is massless pure energy whereas the W and Z bosons are a massive particle. We didn't know why the heaviest elementary particle observed ever, the top quark, which was discovered at the Tevatron Collider in 1995, has a mass compatible with a gold atom, whereas the electron is 350,000 times less massive.
And this is because in the original Standard Model theory, all particles are massless, they are pure energy, and this contradicted experimental observation. So in order to make, essentially-- in order to introduce the possibility of giving masses to elementary particles, in 1964, three physicists-- not only them, but there were also other people who had, of course, contributed to the development of this field in particle physics.
But in 1964 in particular, Robert Brout, Francois Englert, and Peter Higgs, introduced a mechanism called Brout-Englert-Higgs, on which I will now give you a very, very simplified view. Englert and Higgs got Nobel Prize this year. Brout did not, because unfortunately he passed away in 2011. By the way, Robert Brout was an American physicist, but he joined Francois Englert in Belgium, so actually spent most of his life in Brussels.
OK, so in order to introduce-- to give mass to the elementary particles, these three gentlemen added to the equation of motion, say, to the standard model equation-- to the Standard Model Lagrangian, as we call it in technical terms-- a potential, a potential made in this way where phi is a field. OK, you know very well the electromagnetic potential. The difference is that this field is a scalar field. [? It has ?] spin zero. OK, and this is the shape of the potential that they put, by hand, in the theory.
Lambda is a parameter which has a positive value. Mu squared, the sine of mu squared, is very, very critical. So what actually we have learned today from this mechanism, with the discovery of the Higgs boson, is the following. Just after the Big Bang, the universe had origin from a big explosion about 14 billion years ago, mu squared was positive. And if mu squared is positive, the ground state, that is the minimum of the potential in which the universe finds itself, corresponded to phi equal zero, and the value of the potential was equal to zero. So a parabola like this one. So there was no field like that one at the beginning of the universe.
With time, the temperature goes down, the universe expands, and then at some point of the history of the university, and this some point has been located 10 to the minus 11 seconds after the Big Bang, the temperature goes below a critical value and a phase transition occurs. Mu squared changes sign. At this point, with a negative mu squared, the minimum of the potential is not zero any longer. The minimum of the potential occurs for a value of phi of the field of 250 GeV.
So the universe now is permeated by a medium, say a field, the Higgs field, and particles interacting with the Higgs field acquire a mass. So essentially, in the question in the Langrangian of the Standard Model, the additional potential introduced an interaction with particles, and this produces massive terms for the W and the Z particles, and for fermions. So this is the Higgs mechanism.
A couple of things to note. First of all, since then, since the time this potential and the Higgs field has come into action, the vacuum is not empty any longer. There is a Higgs field everywhere. There is a Higgs field in this room. Otherwise the electrons and quarks in your body, and also my body, would not have mass, and so we would not exist.
And this phenomenon is also called electroweak symmetry breaking. Why? Because in the Standard Model, in the picture that I showed at the beginning, the weak and electromagnetic interaction behave in a very similar way. The weak and electromagnetic interaction behave in a very similar way, the only difference being that the particle exchanging the electromagnetic force, the photon, is massless, and the particles exchanging the weak force, the W and the Z particles, are massive.
So this breaks the symmetry, because the symmetry between the two interactions is not perfect. It's a broken symmetry, it is not a perfect symmetry, which has also some macroscopic consequences, because the range of the two forces is very different. The electromagnetic force has an infinite range, [? because ?] [? like ?] 1 over r squared, whereas the weak force, which is transmitted by massive particles, has a very short range. It's confined to, for instance, the size of a nucleus.
The consequences of this mechanism, Brout-Englert-Higgs, as you know, is that as a consequence of this potential put in the equations, a new particle pops up, the Higgs boson. And you know this particle has been looked for for almost 50 years, since the time the theory was developed, and finally it has been found. I don't need to tell you the importance of the Higgs discovery also so for our life, in some sense, because a world without the Higgs mechanism, or something that plays the role of the Higgs boson and the Brout-Englert-Higgs mechanism, will not be a normal universe and a normal world, because without mass, the atoms would not exist. You know that the radius of the atoms goes like 1 over the mass of the electron. If the mass of the electron is zero, then the atom has an infinite radius, so atoms will not stick together.
But also to note is that if the masses of the elementary particles would not be exactly what they are, the universe would not be the same. The protons could decay. If the proton decays, then there are no atoms, no chemistry, no nothing. So we are what we are because the masses of the elementary particles are exactly what they are, and this was not understood until one year ago.
Actually, it's not really true that we have understood everything, because the Higgs mechanism itself explains very well the masses of the W and the Z bosons, and give us some indication, and it's important-- a necessary step to understand the mass of the other particles or the fermions. But we need another step, further step, to understand the masses of the fermions and the problem of the values, families, and [? flavor. ?] Anyway, I don't want to go into details.
So let's go now to the experimental observation of-- 15 minutes? OK, 10 minutes. The experimental observation of the Higgs boson and the discovery itself, I will try to be quite quick and also quite simple in a few slides. So the Higgs boson at a hadron collider like the LHC is produced through four main mechanisms. Actually, two of them are the dominant ones.
First of all, the interaction between two gluons, one from a proton and another one from the other protons-- the gluons are the particles who are responsible for making the strong force which keeps the quarks bound together inside the neutrons and the protons. So two gluons interact, and through a loop of quarks dominated by the top, so a quark top loop, produce the Higgs boson. So this is by far the dominant process.
Another important process is what is called vector boson scattering. So the incoming quarks from the two protons irradiate a W, mainly, or a Z boson, and the two fusion to produce a Higgs boson. And the Higgs can also be produced in association with a W, a Z boson, a top quark pair. Anyway, these two are the dominant mechanisms. The cross-section is large, however, most of this cross-section cannot be used at hadron colliders because in most cases, the Higgs boson, the case [INAUDIBLE]. For instance, to a pair of big quarks, and these final states cannot be observed on top of the huge background coming from QCD processes, like interactions of the two protons through the strong force.
So in this region where the Higgs boson has been observed, a mass around 125 GeV, the most sensitive, experimentally most sensitive channels, are the decay into two photos, the decay into four leptons through a pair of Z bosons, and the decay into two leptons and two neutrinos through a pair of W bosons, followed then by two more difficult channels, decay to [? tau-tau ?] and to [INAUDIBLE] where the Higgs is produced in association with the W and the Z bosons.
These decay modes are all very challenging from the experimental point of view, and discovering the Higgs boson was not easy at all, for various reasons, either because their rates are quite tiny, like in the Higgs to four leptons. This gives rise to a handful of events totally today. After analyzing the full data simple recorded by ATLAS and CMS, in these final states, ATLAS and CMS have each about 15 Higgs signal events in our data sample, in this mode, or because the signal-to-background ratio is small, like in the gamma-gamma case, where the signal is quite huge, a few hundred events, but the background is much larger, 30 times larger. So the signal-to-background ratio is not very good, or because the production of the Higgs boson, followed by its decay, give rise to very complex final states containing many leptons, many [? jets ?] coming from the quarks containing missing energy, et cetera. So challenging. In any case, the observation of this particle was not easy.
So I show you a couple of event displays, because of course, they give the impression how this Higgs boson looks in the detector. So this is a candidate Higgs boson into two photons. This is a nice event recorded by the CMS experiment.
So the two beams come from the two directions. They collide here. You can see the particles produced in the collision. And these two green lines indicate the energy deposited by the two photons produced in the decay, either of the Higgs boson or the ground process, in the calorimeter of CMS. By the way, this is the detector in which the Cornell team has been involved, which is used to measure the energy of the photons.
And this is another gorgeous event recorded by ATLAS. It's a Higgs candidate going into two electrons and two muons. You can see here the two electrons in green, recorded, again, and measured in the carolimeter, and in red, the two muons. And you see here that all four particles are emitted in one direction. So in these events, most likely, the Higgs [INAUDIBLE] was recoiling against something else, and so its decay products are all emitted in the same direction.
So in these three pictures, you can see the present signal that ATLAS has in the full data sample that we have recorded until now in Run One for the three main channels, gamma-gamma, four leptons, and two leptons and two neutrinos. So you can see, if you look at the gamma-gamma spectrums-- you look at events containing two photons, and then you plot the mass spectrum-- you see a steeply falling spectrum, as you expect, coming from various background, and then you see here, already [INAUDIBLE] you can see a bump. And this bump is the Higgs boson, which is even more clear if you subtract the background. You can see here a nice signal at the given mass due to the production of a new particle.
Likewise, if you look at the events that contain four leptons in different states-- four electrons or four muons, or two electrons and two muons-- and you compare them with the expectation from processes other than the Higgs boson, this is shown here. You see the mass of the four leptons in black, the experimental points of the data, in blue the prediction of the Standard Model without the Higgs mechanism. And you can see that the data reproduced very well the Standard Model expectation without Higgs, except in this region here, where you see a clear peak which is compatible with the signal expected for a Standard Model Higgs boson in red.
And finally, in the case of the two leptons and two neutrino final states, again, the data points show a signal-- show a rate which is much larger than what's expected from the background, here in blue, and you see that the difference between data and background is really filled by a signal here and here, right? And this is the signal expected from the Higgs boson. When you subtract the background from the data, you can see that there is a nice peak here, as expected, from the Higgs boson.
If you put together all these channels plus all the others, and you ask yourself, what is the probability that what I observe comes from a fluctuation of the background and not from a new signal, a signal of a new particle, the answer is 10 to the minus 24 for ATLAS alone, and the same for CMS, so clearly we have discovered a new particle. So since, actually, already several months ago, since the 4th of July, the emphasis has shifted from discovery to measurement of the properties of this new particle in order to understand, first of all, if it is a Higgs boson or something else, and then to know it better and better.
Ah, here you have an animation. So here you can see how the signal has come up from the data as a function of time. This is for CMS, Higgs to four leptons. This is for ATLAS Higgs to lepton-neutrino, [? lepton-neutrino. ?] So in black, the data points, you see how-- while the data accumulated and registered how the signals come out. And you see in blue the background, and you see that here there is a peak which is not-- cannot-- oops, CMS has started again. OK, sorry.
But anyway, you see how the peak came out and how at the end-- ATLAS now is starting. OK, sorry, I'm not very good with this animation. Anyway, I think you've seen the principle of all of this.
OK, so the first think that we started to do after the discovery of the new particle and the excitement of the 4th of July, we started to do-- went back to serious work. Sorry, I have a bit of a cold. Excuse me. And the first question we wanted to-- so clearly, this particle really looked like a Higgs boson. It was called Higgs-like because it was decaying and it was produced as we expected for a Higgs boson. But we had to test in order to be sure to it is a Higgs boson. We had to test the two fingerprints of a particle of the Higgs boson types, and the two fingerprints are the following.
First of all, to accomplish its job, which is giving mass to the other particles, these object has to interact, to couple with the other particles, with strength proportional to their masses. So we have measured the coupling of the Higgs boson to other particles in various final states, in various decay modes. And you can see here nice results from CMS, which shows the measured coupling with the various particles, with the tau lepton, b quark, W and Z boson, and top quark as a function of their mass. And you can see the striking proportionality between mass and coupling over two orders of magnitude, because we go from the mass of the tau leptons, 1.8 GeV, all the way up to the mass of the top quark, 172 GeV. So indeed, these particles interact with the other particles in a way proportional to their mass.
The second fingerprint, important feature, to which I alluded already before, the Higgs boson is a particle which has zero spin, so it's a scalar particle. As I mentioned before, none of the particles observed experimentally until the 4th of July had spin zero. Matter particles are fermions, they have spin one-half, and force carriers are vector bosons, they have spin one. So actually this is the first "elementary particle," quote-unquote-- I will come to this later-- with spin zero.
So how do we test the spin of the new particle? Well, the spin can be determined, can be inferred, by analyzing, inspecting the angular distribution of the decay products in the final states. For instance, the two photons in final states containing two photons, containing Higgs two gamma-gamma decays. Because the angular distribution of the decay products keeps the memory of the spin of the mother particle. So we looked, for instance, at Higgs two gamma-gamma events. We have looked at the angular distribution of the two photons, and we have compared the data with expectation for this angular distribution for various spin hypotheses.
Here you can see the ATLAS data for Higgs two gamma-gamma. In red and in blue, the data, and the points are the data. And the curve shows the expectation for spin two particles in this case, and a spin zero particle in this case. You can see already by eye, without doing any statistical interpretation, [INAUDIBLE] root, or these sort of things, that actually the data fit better the zero spin hypothesis than the spin two hypothesis.
So we have done this work and this analysis for all-- for the most important final states, using also statistical techniques, and we have been able to reject a spin hypothesis rather than spin [INAUDIBLE] zero plus, which is the one foreseen for a Higgs boson, with very high level of confidence, more than 99-point-something percent in most cases. So this particle looks really like a scalar, which of course has very important consequences also for our understanding of the evolution of the universe. If this particle the court is indeed an elementary scalar-- we are sure it's scalar today.
We also think it's elementary and not composite, but of course we will continue to measure its coupling to see if it's really elementary. But assuming it is the first elementary scalar we observed in nature, this is a revolutionary observation. Because, as you perhaps know, we think today-- based not only on cosmological theory and prejudices, but also on observation-- that the universe just after the Big Bang went through a phase, a very fast expansion phase, an exponential expansion phase, called inflation.
And inflation was triggered by a scalar field called the inflaton, in this case a particle called the inflaton. Now, I'm not saying that the Higgs boson is the inflaton, but the fact that we have demonstrated that in nature scalar fields do exist-- and we had no evidence until the 4th of July that scalar fields do exist, because no such field had been observed before-- is, of course, an important step in that direction. OK, sorry, another [INAUDIBLE].
OK, so what are the next steps? With the data recorded until now, we will continue to measure of the properties of the Higgs boson with trying to achieve higher precision. Of course, we cannot increase the data today, but we can become more clever in analysis technique, and so measure the Higgs couplings, spin, et cetera, with higher precision. As I mentioned before, in 2015, after the repairs and the consolidation of the interconnects between magnets-- and about 20% to 30% of them have to be redone or consolidated-- we will be able to move to close to 14 TeV, which is the design energy of the LHC, and also increase the intensity of the two beams, the so-called luminosity.
So by the end of this decade, we should be able, through also another improvement of the-- increase in intensity of the beam, to record an amount of data which is 10 times larger than today. And then, through another upgrade phase, we should be able, thanks to another upgrade phase by the end of 2030, to multiply by 100 the ATLAS and CMS data sample. And with this data, we should be able to measure, by 2030 or so, so by the end of the next decade, we should be able to measure the Higgs couplings to the various particles with a precision of between 2% and 5%. Today we are at the level of 10%, 20%, so we should be able to decrease the experimental error to 2% to 5%, and also to extend the sensitivity of new physics-- the plot that I showed at the beginning with all the scenarios, et cetera-- by a factor of two, in terms of [? mass ?] [? reach ?] for new particles. To make an important step forward in understanding-- in exploring new physics scenarios.
And some of the big questions that we will have to address-- at least here I'm only listing the questions in the Higgs sector, regarding the Higgs boson. I already mentioned other questions like dark matter at the beginning. So the questions that we will have to address are the following. Today we know that this particle, the new particle, is a Higgs boson. It has all the features of a Higgs boson, the two fingerprints I was mentioning before.
However, we don't know yet if it is the Higgs boson of the minimal Standard Model Theory. So we'll have to measure its behavior, its coupling, its interaction, with much higher precision than today. Today what we measure is compatible with the Standard Model Higgs boson, but experimental [? precision ?] is not good enough to exclude that actually this particle is not the Standard Model Higgs boson, but a Higgs boson belonging to a more general theory beyond the Standard Model, like supersymmetry.
So we'll have to understand, by measuring the coupling, if it is elementary or a composite. If there are any deviations, even the tiniest deviation, from the Standard Model expectation, we will have to see that there are new sources of new physics contributing to its production or its decay, if it decays to invisible particles, if it is the only Higgs boson or there are other, heavier Higgs bosons belonging to a more complex theory that may become accessible when we go to higher energy, et cetera. We have to understand a very important problem, in which I don't want to go into details, but we don't understand today, in the Standard Model, why the Higgs boson is so light. OK, so this is called naturalness problem, and calls for new physics at the TeV scale. And we have to see if this Higgs boson also fixes some problems with the Standard Model of high energy that would become very, very important and dramatic if the Higgs boson were not there.
So in conclusion, the first LHC run, so-called Run One, between 2010 and 2013, has been an extraordinary success. The accelerator, the experiments, and the computing have performed very well, beyond design specification, and even people. We were so much under stress and under, of course, a lot of pressure, these three years have been extremely demanding but also extremely exciting. Among the achievements, of course, many achievements, measurement of the Standard Model with precision in a new energy regime, a first look at new physics in a new energy regime.
But of course, the main achievement is the crucial discovery of the Higgs boson. I hope that I've convinced you that this particle is a very special one, and the era of precise measurement of this new particle has indeed already started, and will continue in the years to come. So after almost 100 years of superb theoretical, experimental work, the Standard Model of particle physics is now complete. All the particles predicted by the theory have been discovered, and all the predictions of the Standard Model have been verified with very high precision.
Nevertheless, as was mentioned before, we know also with similar confidence and similar precision, that this theory is not the ultimate theory of particle physics, as many unanswered questions remain, like the Higgs boson mass, the dark universe, the matter-antimatter symmetry in the universe, the difference between the three particles that are important on the level of-- the three forces that are very important on the level of elementary particles, and gravity, et cetera. And so in the 10 to 20 years to come, the LHC and its upgrade will, of course, help address some of these and other questions. But perhaps most importantly, the LHC will tell us what are the right questions to ask, and how to continue, and what are the next steps. Thank you.
SPEAKER 1: OK, we have time for a few questions. Any questions? If not, I'll start with my question here. So on your slides of the ATLAS Higgs signals, you didn't talk about the masses that you saw [INAUDIBLE] which are [? strikingly ?] different [INAUDIBLE] comment on that?
FABIOLA GIANOTTI: Yes, I can comment. Well, [? we ?] measure the mass of the Higgs boson in both experiments using the most precise final state. These are gamma-gamma and four lepton. [INAUDIBLE] perhaps I have a slide where I can give you also some numerical values [INAUDIBLE]. I was not really planning to go into all the-- oops, here we go. No, sorry. Here we go.
So we measure the masses in the two final states, Higgs to gamma-gamma and Higgs to four leptons, at this level with a very high precision. And in both experiments, ATLAS and CMS, measured a mass around 125 GeV, with an uncertainty at the level of, say, a few hundred [? maybe ?] so already at the [INAUDIBLE] level. So the mass difference-- ATLAS observed a mass difference between the two channels, gamma-gamma and four lepton, at the level of 2.3 GeV, with an error of about 1 GeV. So the question that you might ask yourself, and I think this is the question Anders was alluding to, is, what is the probability that if I measure this mass difference, this mass difference is compatible with coming from the same particles, and the signal I see in the two final states does not come from two different particles?
Well, the probability that they come from the same particle is at the level of between 1.2% and 8%, depending on how you treat the systematic uncertainty-- which, of course, is something that happens. A small probability is compatible with what we would expect, also given the very large statistical uncertainty in the four lepton channel. So we will see, we need more statistics, in particular from the four lepton channel, to understand if this difference remains or not. But it's on the level between 1% and 8%, depending on how you treat the systematic uncertainty in the global fit, in the best fit to the two channels.
SPEAKER 1: OK, any other questions [INAUDIBLE]?
FABIOLA GIANOTTI: Well, perhaps I should also mention when you fit overall the signal, you get one peak in the distributional data or the p value, so the probability that it's-- I mean, you don't fit two different signals, but just one. The p value gives you just one peak. There was a hand there.
SPEAKER 2: Given your precision knowledge of the mass and the Standard Model, I think the prediction for the branching ratios to the different products are pretty tight. So can you say now whether the observations are in agreement with the predicted branching ratios only for the Standard Model, or are there invisible decays?
FABIOLA GIANOTTI: No. So first of all, the theoretical uncertainty [INAUDIBLE] ratio themself are on the level of 4% when the mass has been fixed. Our measurement of the branching ratio and of the rates in the value's final states, assuming the production is well-known, are on the level of 10% to 20%, and there are upper limits [? on ?] [? an ?] invisible decay at the level of 70% today, or 70% of the branching ratio. Of course, we hope we'd upgrade to go much, much lower than that.
SPEAKER 1: OK, any other questions?
FABIOLA GIANOTTI: No?
SPEAKER 1: If not, let's thank Fabiola--
FABIOLA GIANOTTI: Thank you.
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Physicist Fabiola Gianotti presented the physics colloquium, "Challenges and Accomplishments of the ATLAS Experiment at the Large Hadron Collider," Nov. 11, 2013 as part of the Hans Bethe Lecture Series at Cornell.
The Hans Bethe Lectures, established by the Department of Physics and the College of Arts and Sciences, honor Bethe, a Cornell professor of physics from 1936 until his death in 2005 who won the Nobel Prize in physics in 1967 for his description of the nuclear processes that power the sun.