July 4th in 2012 is destined to be a special day in the history of humankind. On that day, physicists working with the world’s largest scientific facility, the Large Hadron Collider (LHC), at the European particle physics laboratory CERN, in Switzerland, announced that they have discovered a particle that looks a lot like the long-sought Higgs boson – the final missing piece in the Standard Model that describes fundamental particles and forces. The Higgs boson is the key to the explanation of how all the other fundamental particles get their masses. A few months later, this particle was confirmed to be a Higgs particle.
Discovering this new particle at the LHC is a triumph. After two successful years of operation at the LHC, the next step in our understanding of the Universe has been revealed. Now that the scientists have found a Higgs boson, many more years of follow-up research will be needed to verify its full identity. For a particle to be exactly the Higgs as originally conceived, all of its properties must be measured with great accuracy; a tough job indeed.
The energy of the Higgs particle recently discovered at CERN is well within the range of the ILC. It is too soon to know exactly what additional information will be uncovered from the LHC experiments, but even without this information the potential for exploiting Higgs physics at the ILC is enormous. At the ILC, Higgs particles will be created in electron-positron collisions and their properties measured: e.g. mass, the strength of their interactions with all other elementary particles with unprecedented precision and without assumptions. Will the Higgs properties be as predicted by the Standard Model? Or will it be just the first of a family? Will nature be more complicated than a single “minimal” Higgs boson? The precision of the measurements that can be made at the ILC allow us to estimate at what energy new particles may appear. There is agreement in the high-energy physics community that a linear collider like the ILC is the ideal facility to make these vital measurements.
The Standard Model of particle physics is a theory that describes the known particles that are the constituents of matter and three of the four known fundamental interactions between them. These interactions, or forces, are the electromagnetic force (which we experience every day when we turn on the light, the TV, use wireless communication etc), the strong force (which holds quarks together inside the protons and neutrons in the atomic nucleus, thereby forming the plethora of elements, from helium to iron to uranium, that make up our world) and the weak force (which is responsible for the Sun shining, without which life on Earth would be impossible, as well as for many radioactive decays).The Standard Model works extremely well, but we know that it cannot be the complete theory if for no other reason than that it is incomplete; it does not incorporate gravity. It describes beautifully the ordinary matter of which we, and the entire visible universe, are made. It does not describe the invisible 95 % of the universe that we know to be there, but which has thus far evaded detection. The Standard Model has nevertheless been tested to exquisite precision over a wide range of energies. It must therefore be a good approximation to a final, unified, theory.
The Standard Model successfully describes all of the elementary particles we know to exist and how they interact with one another. But one piece is missing. The Standard Model cannot yet answer one basic question: why do most of these elementary particles have mass?
Theoretical physicists Robert Brout, François Englert, Peter Higgs, Gerald Guralnik, Carl Hagen and Tom Kibble proposed a mechanism that would explain how particles get their mass. This mechanism postulates a medium that exists everywhere in space. Particles gain mass by interacting with this medium, or “field”. Peter Higgs pointed out that the mechanism required the existence of a particle unseen until now, which we now call the Higgs boson after its inventor.
The Higgs mechanism predicts the Higgs boson to be a fundamental scalar, meaning a spinless particle. No other fundamental spinless particles exist in nature. Its spinless nature allows the Higgs to condense and fill the vacuum much like steam condenses to form the sea. The Higgs discovery raises a variety of new questions on the supposed nature of this boson and opens up a very important area of research.
The ILC is complementary to the LHC’s proton-proton collisions. The LHC, a circular proton-proton synchrotron, operates at the highest energies any particle accelerator has ever achieved. The International Linear Collider will explore the same phenomena using a different approach. By colliding electrons with positrons, the ILC would allow us to home in with exquisite precision on the new landscape that the LHC will reveal. It will expand on the discoveries made by the LHC and investigate new laws of nature. Apart from its spinless property, the Higgs boson’s coupling strength to other particles is its second unique feature, which is ultimately responsible for generating these particles’ masses. Measuring the strength with which the Higgs boson interacts with particles having different masses will investigate whether the predicted relative strengths are correct.
The many precisely measured Higgs events at the ILC will produce quantitative measurements of the different coupling strengths that will enable us to distinguish among possible different types of Higgs bosons. Another unique feature of the Higgs boson is its coupling to itself. The Standard Model precisely describes how the Higgs boson couples to other particles, including itself. With its precision, the ILC enables an accurate measurement of the Higgs’ self-coupling and determines its potential, confirming or disproving in a completely model-independent way whether it is the Standard-Model Higgs boson.
If the LHC does not find anything that hints at a deviation from the Standard Model, scientists would have to test the energy scale up to which the Standard Model can be valid. One way to do this is to check the stability of the theory. This is determined by values of the Higgs mass and the mass of the top quark. Whether the theory – its “vacuum stability”, as it is called – is absolutely stable or not depends critically on the precise value of the top mass. The ILC can measure the mass to unprecedented precision and decide the fate of the Standard Model.
Most of the matter in the universe is dark. Without dark matter, galaxies and stars would not have formed and life would not exist. It holds the universe together. What is it? It is only in the last 10 to 15 years that scientists have made substantial progress in understanding the properties of dark matter, mostly by establishing what it is not. Recent observations of the effect of dark matter on the structure of the universe have shown that it is unlike any form of matter that we have discovered or measured in the laboratory. At the same time, new theories have emerged that may tell us what dark matter actually is. Searches for candidate dark matter particles are underway at present-day colliders. If these particles have masses at the TeV scale, they will surely be discovered at the LHC. However, verifying that these new particles are indeed related to dark matter will require a linear collider to characterise their properties. The International Linear Collider can measure their mass, spin and parity with extremely high precision. These results will permit calculation of the present-day cosmic abundance of dark matter and comparison to cosmological observations.
If the values agree, it will be a great triumph for both particle physics and cosmology and will extend the understanding of the evolution of the universe after the big bang.
The theory of supersymmetry says that all known particles have heavier superpartners, new particles that bring a new dimension to the subatomic world. The lightest superpartner is a likely candidate to be dark matter, and could thus also explain the structure of the cosmos. A linear collider would be best suited for producing the lighter superpartners. Linear-collider experiments could focus on one type of superpartner at a time, measuring their properties precisely enough to detect the symmetry of supersymmetry, and to reveal the supersymmetric nature of dark matter. In this way, physicists could discover how supersymmetry shapes both the inner workings and the grand designs of the universe. Designed with great accuracy and precision, the ILC becomes the perfect machine to conduct the search for darkmatter particles with unprecedented precision; we have good reasons to anticipate other exciting discoveries along the way.
Many theories, such as Superstrings, that try to unify gravity with the other forces require the Universe to have additional dimensions to those of space and time that are familiar to us. Such theories attach additional spatial dimensions to each point in space. The extra dimensions must be very tiny or otherwise hidden from view since none of our experiments have so far given any evidence that they exist. Matter might be made of particles that already live in extra dimensions and feel their effects. A particle moving in an extra dimension would have extra energy, making it look like a heavier version of itself. Measurement of the mass and other properties of these travelers would show what the additional dimensions look like. If new dimensions exist at the Terascale, then the LHC should discover them; experiments will look for high-energy collisions in which particles literally disappear into an extra dimension. The ILC would be able to reveal the detailed structure of these extra dimensions and their associated particles and might detect signs for others that cannot be seen by the LHC.
“New directions in science are launched by new tools much more often than by new concepts. The effect of a concept-driven revolution is to explain old things in new ways. The effect of a tool-driven revolution is to discover new things that have to be explained.”
Freeman Dyson, Imagined Worlds
With the discovery of a Higgs boson, the ILC has a guaranteed, rich physics programme to explore. If the new particle is truly a spinless fundamental particle, it is the only such particle that we know about. It adds a completely new dimension to our understanding of the fabric of space-time. The ILC and its detectors are precision instruments allowing the properties of the Higgs boson to be studied with laserlike focus. The impact of the ILC, however, reaches far beyond the Higgs. With its variable center of mass energy, it can, as future measurements might require, carry out a programme of ultra-precise electroweak measurements of the Z-boson, study the top quark in great depth and study the self-coupling of the Higgs boson at its highest centre-of-mass energy. Furthermore, it can make measurements which do not rely on any theoretical assumptions, thereby investigating the internal consistency of new theories. The ILC will be a tool of unprecedented versatility. As Freeman Dyson once said, “New directions in science are launched by new tools much more often than by new concepts. The effect of a concept-driven revolution is to explain old things in new ways. The effect of a tool-driven revolution is to discover new things that have to be explained”. The ILC is such a tool!