Why is there mass

The mystery of the crowd


The search for the Higgs particle

Milan Kunderad found the “lightness of being” “unbearable”. In contrast, physicists from all over the world perceive the “existence of gravity” as “incomprehensible”. No, this is not about the melancholy dispositions of a work-overloaded researcher class. Rather, what is addressed is a fundamental question that basic physical research has been grappling with for more than 40 years.

A question that sounds so simple that even Sesame Street-socialized “why, why, why” children would hardly ever think of asking their parents at all. "Why is there mass?" - "Weight", the layman would say - is now puzzling the international scientific community of particle physics in the third generation of researchers. Since the 1960s there has been a hypothesis as to what it actually is: 'mass'. And also how it comes about. However, physics still owes the proof to this day. That could change in the coming years. The particle physicists want to confirm their theory, which is four decades old, by tracking down a tiny particle of matter of subatomic size. Thousands of physicists from all over the world are gathering for this purpose at the European Laboratory for Particle Physics CERN, equipped with a gigantic machine: the 'Large Hadron Collider', or 'LHC' for short. From the end of 2007 the particle accelerator will be ready to hunt down the phantom. The so-called 'Higgs particle' is searched for.
With the Large Hadron Collider, natural science opens the door to a fascinating world of the smallest order of magnitude and the highest energy scales at the beginning of the new millennium. With the LHC, particle physicists are penetrating the phenomenal realm of the microcosm deeper than ever before. So far, physics has only been able to access these areas in alternative, sometimes contradicting theories. The LHC is now also to provide access experimentally. It is currently being completed in Geneva at CERN - the world's largest laboratory for particle physics. The experimental machine is a ring accelerator that will be the most powerful of its kind for years to come. With the LHC, the CERN physicists are expected to accelerate packets of protons on a 27 km long circular path to almost the speed of light and collide with an energy of two times seven tera-electron volts. The new particle accelerator makes alchemist dreams come true - even if only in the smallest detail: in the collision, the energy of the proton beams is transformed into new matter. Thousands of particles are created, which are picked up by detectors - huge measuring devices.
The physicists at CERN have installed detectors around all the collision points, a total of four, which, as a kind of camera, measure and absorb the resulting particles in different detector layers, depending on their properties and penetration capacity. At the same time, they provide countless measurement data that provide researchers with answers to a multitude of questions about the structure of the world in the subatomic area. The detectors are experimental measuring instruments of the highest complexity. They are something like the eyes with which physicists look into the deepest depths of the microcosm. Two of the four detectors under construction, ATLAS and CMS - measuring devices the size of a five-story house, consisting of billions of circuits, microchips and super magnets - are dedicated to a wide range of fundamental physical issues. At the center of the research work is, among others, the question of the origin of the mass of the elementary particles.

The mass problem and the Higgs mechanism

The standard model of particle physics, the model in which the accumulated knowledge of particle physics research of the last decades flows together, was developed in the 1960s and has since been subjected to a large number of precision tests - with great success. The researchers were unable to observe any discrepancies between theoretical predictions and experimental findings. The joy is clouded, however. Because there is a not insignificant problem: the model in its original form can only describe massless elementary particles.
You don't need to have studied physics to recognize that science is in need of explanation at this point. If the matter of our universe is made up of elementary particles, how can there be elementary particles without mass? That sounds implausible: we are confronted every day with our own body weight and the weight of the objects around us - sometimes in an uncomfortable way. Experiments on older particle accelerators have also confirmed what everyday experience has always suggested: a specific mass can be determined for almost all known particles; the heaviest subatomic particles are the top quark and the exchange particles of the weak nuclear force, which correspond approximately to the mass or half the mass of a gold atom.

The Scot Peter Higgs and other colleagues at the same time developed a 'mathematical trick' based on ideas from Philip Anderson in solid-state physics, which can solve the explanatory problem of the standard model, initially only on paper: the 'Higgs mechanism' named after its inventor. This makes it possible to give the elementary building blocks of matter (electrons and quarks) and the force particles an effective mass, and yet to keep the theory self-consistent. Now the physically educated may object that the mass of our environment is mainly based on the masses of protons and neutrons in the atomic nuclei, which for the most part comes from the kinetic energy and other effects of strong nuclear power. A mechanism that the Standard Model has always been able to describe without any problems. Accordingly, the mass of matter is based on the masses of the quarks and electrons only in a smaller percentage range - the explanatory gap that the Higgs mechanism is supposed to close could appear negligible from this perspective. However, the tiny electron mass that the Higgs mechanism is about determines the length scale of our world. Without electron mass there is no atomic bond and therefore no more complex structures such as plants, animals or humans. The specific mass of the atomic nucleus, the electrons, is by no means arbitrary. If the electron mass were to be increased by a factor of ten, we humans would - provided evolution had succeeded - suddenly only eight inches tall and the daylight would be in the X-ray range. Similar chains of arguments that prove the importance of the Higgs mechanism can be drawn up for the masses of the quarks. Conclusion: Elementary particles have mass and that is not only a good thing, it is even existentially necessary for life on earth!

What a theoretical trick do Peter Higgs et al. the mass of electrons and quarks? The Higgs mechanism can be vividly described using the following analogy: if we consider the movements of a person fleeing the dreariness of the German winter, turning his laps through a pool landscape in southern Spain - we would be more correct in assuming that the bather is not swimming but walking moved across the pelvic floor. We can see that the holidaymaker can only move more slowly in the water than the lifeguard who walks along the edge of the pool. If one were to trace this trivial observation back to its causes, there would be two possible explanations.
First: one neglects the existence of water for the explanation. Why does the winter refugee suddenly move more slowly in the pool than the supervisor at the edge of the pool with the same muscle strength? The only explanation: its weight - the physicist would say more precisely: its mass - must have suddenly increased so that the muscle power can only bring the body forward more slowly.
Second possible explanation: one includes the water in the explanation. Then it can plausibly be said that the holidaymaker has to start against the resistance of the water; the water 'slows down' the vacationer, so that he can only progress more slowly with the same exertion. What we called mass in the first justification, we would now call 'frictional resistance'.
With regard to the mass production of elementary particles, our little episode from the warm regions of the sunny south resembles the particle physicists' image of nature. The Higgs mechanism claims the existence of an omnipresent background field. Like water the pool, the Higgs field fills the universe homogeneously and isotropically.
If there were no Higgs background field (ether), all particles would move through space at the speed of light - the vacationer would be in an empty swimming pool, so to speak. If the Higgs ether now exists, the inertia of the elementary particles can be explained in two ways, analogous to the history of Spain: First: one ignores the ether and claims that all particles have mass (as is common practice in everyday language) or, secondly, according to the suggestion by Peter Higgs: the aether is taken into account and the interaction of the particles with the background field is described as the effect of 'frictional forces'. The effective mass of the elementary particles then depends on two factors. On the one hand from the 'viscosity of the ether' or, in the language of elementary particle physics, the 'vacuum expectation value' of the Higgs field; on the other hand from the 'coefficient of friction' of the specific type of particle or, in the language of physics, the 'coupling constants' of the interaction between Higgs field and particles. One particle is therefore more massive than the other or translated: one vacationer is 'heavier' in the water than the other.
The question now arises: what did you gain by introducing the Higgs field? The answer: The explanation gap of the Standard Model closes; the overall model retains its validity and informative value even for elementary particles that have mass. The Higgs mechanism thus strengthens the predictive capabilities of the standard model. In principle, it allows any precise predictions for the outcome of experiments with any high collision energies.
However, there is a price to be paid: the self-consistency of the theory requires another particle - the 'Higgs particle', also called the 'Higgs boson'. This elementary particle postulated by the theory appears as an energetic excitation of the Higgs field; In our comparison with the swimming pool, it would correspond to a vortex in the water. On the one hand, the Higgs particle is the necessary companion of the omnipresent ether. On the other hand, it also helps to make the theory experimentally verifiable and, if necessary, to falsify it. The omnipresent, homogeneous ether cannot be directly detected. The Higgs particle thus forms the last, still missing building block in the radiant theoretical building of the standard model of particle physics; What the 'Philosopher's Stone' was to the alchemists of the Middle Ages is the Higgs particle to some modern physicists. Some, like Nobel Prize winner Leon Lederman, even go so far as to call it a 'Part of God'.
Detecting and measuring the Higgs boson would not only be the missing piece of the puzzle to complete the Standard Model - it would also be the culmination of the physical research efforts of thousands of scientists over the past decades. However, all attempts to locate the Higgs boson experimentally have so far failed. It could not be detected with the "Large Electron-Positron Collider", "LEP" for short, the predecessor model of the LHC at CERN (1989 to 2000), or with the TEVATRON accelerator (2000 to today) at the Fermilab in the USA. The new particle accelerator, the LHC, will be more powerful than all previous models. With it, the Higgs particle should finally be located - and the hunt should end well after 40 years.

The search for the Higgs boson with ATLAS at the LHC

Search for the Higgs particle: who, how, what? First of all, the 'what': the Higgs particle cannot be directly grasped as such. At the moment of its creation, it breaks down again into so-called "secondary particles". It can only be finally identified via these secondary particles. However, only the mass of the Higgs particle is unknown. If one assumes a value for this, then all other properties are defined in the standard model and its profile can be precisely calculated. Depending on the mass range in which the Higgs particle occurs, the decay products are also different. Accordingly, different detection technologies have to be connected in series in the ATLAS experiment in order to be able to determine and measure all possible types of decays.
That leads us to 'how' and 'who'. ATLAS stands for 'A Toroidal LHC Apparatus' and describes, on the one hand, the huge detection apparatus and, on the other hand, the corresponding collaboration of around 2000 people from 153 universities and research institutions from 34 countries around the world. Hundreds of physicists have been studying and developing the design and technologies that are aimed at for the ATLAS experiment since the early 1990s. The construction of the detector in the cavern 100 meters below the surface of the earth at CERN and the development of the software for reading out and reconstructing the data are currently being completed.
In the 44 meter long ATLAS detector, starting from the collision point, a wide variety of measuring layers - track detectors, calorimeters, muon spectrometers - build up around the beam axis of the LHC accelerator up to a diameter of 22 meters. Each has its own specific function as its own sub-detector. Only when the ATLAS sub-detectors work together harmoniously like in a symphony orchestra can the numerous ATLAS physicists determine and analyze the types of particles that arise in the collision and approach the observation of the Higgs particle.

Markus Schumacher is part of this science thriller. The professor for experimental particle physics from the University of Siegen has been going on a Higgs safari for twelve years. Even after more than a decade of painstaking research, his passion to confront the tiny is still unbroken. Years of continuous research are made possible by funding from the BMBF and the German-Israeli Project Cooperation (DIP).
He is currently coordinating the 200-strong international working group at ATLAS that is responsible for the search for the Higgs particle. From the beginning of 2008 onwards, Schumacher and his colleagues from all over the world will sift through the immense mountains of data that ATLAS will deliver for decades - with the main interest in finally finding that ghostly particle whose existence Peter Higgs had predicted back in 1963. The group has been training emergencies for years with elaborate simulations. “With ATLAS we will finally find the Higgs phantom,” says Schumacher confidently. And adds a little more quietly: “If it actually exists.” Schumacher expresses the consensus in the international research scene. The particle physicists worldwide agree that if the Higgs particle actually exists in nature, it will be discovered within the next decade with the almost completed LHC accelerator and the connected ATLAS and CMS detectors.

"Discover"; the seemingly harmless term, however, obscures the complex reality of the search. If you look at the details of the 'Higgs-Boson' company, it quickly becomes clear that the expedition into the realm of the very tiniest need not fear comparison with any of mankind's larger journeys of discovery. On top of that. To the chagrin of the Argonauts of the microcosm, the Higgs particle is an extremely camera-shy being. The probability of generating the Higgs boson in the collision of two protons at the LHC is very low. Therefore one has to try to achieve as many proton-proton collisions as possible per unit of time. In the large experiments at the LHC, two packets of 100 billion protons each will cross each other 40 million times per second. If the accelerator meets these requirements, a detectable Higgs particle is generated about once per minute. So far so good! However, under these conditions about a billion - at least for the Higgs seeker - uninteresting collisions take place per second.Now the work of the ATLAS detector and its experimenters begins: How do you select the one so-called Higgs event in the 100 billion collisions or how do you find the 'Higgs' needle in the 'underground' haystack? If you wanted to write the information from the 140 million readout cells of the ATLAS detector to storage media every 25 nanoseconds, the resulting flood of data would be around a million gigabytes per second. This corresponds to the data rate of 100 billion telephone calls. Such a flood of numbers cannot be mastered by any computer system in the world.
A three-stage intelligent filter system - the so-called 'trigger' of the ATLAS experiment - recognizes practically instantaneously whether the event is of interest for further data analysis. The first stage filter makes the decision in less than two millionths of a second and reduces the event rate to a level of 100,000 events per second, which is reduced by the final two filters to a storable amount of 100 events per second. Despite this rigorous selection process, in which 99.9995 percent of all events are already discarded, the LHC delivers one million gigabytes of data every year or one CD per second through ATLAS alone. So much for the first technical step in removing the haystack. The actual gardening work of the Higgs troop is only just beginning here.

Markus Schumacher explains the problem: "The signal characteristics of the Higgs event differ only minimally from the other events - also simply called 'underground'. In recent years, our group has mainly been concerned with developing optimal methods with which we can separate the 'wheat from the chaff' or the subsoil from the Higgs events. ”The aim is to use selection criteria to determine the unfavorable ratio of Enrich signal-to-background from one to a few million after the trigger to a ratio in the range of one to one. Since there is no data yet, the physicists simulate the expected reality in complex and detailed simulations of the expected physics and the detector's response behavior. The last popular parameter for deciding whether it is a candidate for a Higgs event is the invariant mass of all decay products, which can be calculated from the measured directions and energy deposition of the secondary particles in the ATLAS detector. In the hoped-for ideal case, a mountain of additional events rises above a flat surface, which are then assigned to the Higgs particle. By convention, a discovery is used when the probability that it is a false alarm due to statistical fluctuations is only 0.0000029 percent or less.

But how well do the developed simulation programs actually describe the rate of underground events, their characteristics and the response behavior of the ATLAS detector? How exactly do imagination and reality match? The trust of the physicists is limited, as they are breaking new ground with the LHC with energies never reached before. For this reason, detailed strategies are already being developed as to how and with what accuracy one can later extract the subsurface from the data itself, beyond all simulations. The 200 Higgs hunters at ATLAS are currently busy with this task. The studies should be completed by summer 2007 and published in a new report. The Higgs working group, led by Markus Schumacher and his French colleague Louis Fayard, meets once a month for one or two days at CERN. This is where the team members working all over the world present their work results. Here it is determined which course should be taken in the future. It is sometimes lively: now and then the methods and thus the results of one physicist initially differ from those of the other. Accordingly, the opinions about the further procedure and the right strategies then differ. As the coordinator, it is then up to Schumacher to smooth things over and, in agreement with all colleagues, to develop guidelines that hold the large international team together. “It's not always easy to please everyone,” Schumacher ponders with a serious expression. In the next moment his face brightens up again. With a smile, Schumacher emphasizes that he definitely didn't want to miss the experience of going on a search together with so many different mentalities and cultures. “Ultimately, all of our colleagues are highly motivated. The common goal, the discovery of the Higgs particle, welds together and usually helps to resolve differences very quickly, ”says Schumacher, describing his previous experience.

200 companions have set out to search the haystack of ATLAS data for the Higgs particle. You will still have to master many and unexpected challenges on your unique journey of discovery. Ultimately, however, ten times more people are necessary to lead the 'Higgs-Boson' company to success. 2000 scientists from all over the world have been working for many years to build the ship with which the adventure trip is to be undertaken. Schumacher never tires of emphasizing that the work of those who designed and installed the ATLAS detector, those who maintain and calibrate the detector, and those who write the necessary software is at least as important as the work of the Higgs Teams. Therefore, in the event of a discovery, all 2000 employees will sign the publication in alphabetical order, which proclaims the long-term success. It will be difficult with the Nobel Prize: so far, it can only be awarded to a maximum of three people.

The Higgs hunters are looking forward to the start of data collection with eager anticipation. “However, it will take some time before the quality of the data is understood and the first signs of a Higgs particle can show up,” says Schumacher, dampening hopes for a quick breakthrough. After three years of successful and proven data collection, the time has come: “Then we will know whether the Higgs particle of the Standard Model actually exists in nature or not. The discovery would be wonderful, but only the first step, ”says Schumacher. “Then the fun really starts. The newly discovered particle and its properties have to be measured in order to be able to finally clarify whether it is the Higgs particle we are expecting. "

All questions answered or still open !?

At the end of the 19th century. A young high school graduate visits the physics faculty of the University of Munich to ask the physics professor Phillipp von Jolly for advice. The school leaver is considering starting a physics degree. He asks Jolly whether that makes sense. This advises against the high school graduate. Jolly was of the opinion that almost everything in this science had already been researched and that only a few insignificant gaps had to be closed - many contemporaries shared Jolly's conviction at the time. The name of the young high school graduate: Max Planck. The later Nobel Prize winner studied physics against all common sense at the time and founded a century of physics with Albert Einstein. What does the physical world look like in the 21st century. Let's take a look into the future; the Higgs boson has been discovered and the Higgs mechanism has now taken its place in the Standard Model. Will high school graduates be advised against studying physics again, as they did 130 years before, since “everything has possibly already been researched”?

The answer is a clear no". Many questions remain unanswered even after research into the Higgs particle in particle physics and cosmology (of course also in other areas of physics). A brief outlook: Even if physics had solved the mystery of mass using the Higgs mechanism, that would still not understand the universe as a whole. In particular, the “energy balance” of the universe continues to pose many puzzles. Because the universe consists of only five percent visible matter, the material from which all stars, planets and also we humans are built and which the Higgs mechanism contributes to understanding. 95 percent of the universe remains in the dark - in the truest sense of the word. Because, according to current research, the universe consists of 25 percent so-called 'dark matter' and 70 percent of 'dark energy'. Two mysterious substances that, according to their name, are completely invisible. It is only through their work that researchers can infer the existence of these substances: 'Dark matter' makes itself felt through its gravitational effects, 'Dark energy' causes our universe to expand at an accelerated rate today. There are many candidates for 'dark matter' - e.g. the lightest object from the ranks of supersymmetrical partner particles. The theory of supersymmetry postulates the existence of partner particles for all bosons and fermions, which differ from their otherwise identical twins only in terms of their intrinsic angular momentum and mass. If there are supersymmetric particles, the chances are good that they can be discovered with the new LHC. What the 'dark energy' consists of, however, is completely unclear. The “naive” model of the Higgs ether and the value of its “viscosity” can partially contribute to the explanation of the workings of 'dark energy'. Unfortunately, the contribution has the wrong sign; in addition, it is too large by a factor of 1050. According to this explanation, the universe would only have reached the size of a soccer ball and then collapsed again. Even after the discovery of the Higgs boson, the Standard Model will probably remain only one stage on the way to a comprehensive physical explanation of the universe as a whole. Theories of 'supersymmetry' and of so-called 'extra dimensions' are available beyond the standard model to close explanatory gaps in fundamental questions. Is there a primal force in the early universe and are our four forces today just different manifestations of this one primal force at lower energies? What is the structure of spacetime; do we actually live in more than three spatial dimensions that are just too small for us to experience? Even after the discovery of the Higgs boson at the LHC, there are other major challenges for particle physics in the 21st century, which the LHC will hopefully provide the first aid to cope with.


The standard model in the 'nutshell'

The matter surrounding us consists of atoms, these in turn from the fundamental electrons and the composite nuclei, which are made up of the nucleons 'protons' and 'neutrons'. Each nucleon is in turn a bond state made up of three quarks. The proton consists of two up quarks and one down quark, the neutron consists of one up quark and two down quarks. In accelerator experiments and in cosmic rays, two heavier partners each were discovered for the up quark, the down quark and the electron. In addition, there is one neutrino for each of the three leptons, the electron and its two relatives.
All of these twelve fundamental particles have half-integer angular momentum and thus belong to the group of fermions. Their number is significant for the development of the early universe. The fermions can be divided into three families according to increasing mass, each consisting of two quarks and two leptons.
There are a total of six quarks up (u), down (d); charm (c), strange (s); top (t), bottom (b) six fundamental leptons (electrons (ε), muons (μ) and tauons (τ) with their respective neutrinos (νε, νμ, ντ)).
Forces - the physicist speaks of interaction here - between the elementary fermions are described by the exchange of vector bosons. These have their own angular momentum one. The electromagnetic interaction is mediated by the photon (γ), the strong 'color' interaction by gluons (g) and the weak interaction by weakons (W + and W-) and by the neutral vector boson Z. Only those fermions that have the corresponding charge take part in a certain interaction. Quarks have color, electromagnetic and weak charges, charged leptons (ε, μ, τ) have electromagnetic and weak charges, and neutrinos only have weak charges.
The mass of the fundamental matter particles as well as some exchange particles (W +, W-, Z) is given by the interaction with the Higgs field. The necessary companion of this Higgs mechanism is the Higgs boson, which is the only elementary particle with zero angular momentum.