Computer simulation of particle traces from an LHC collision in which a Higgs Boson is produced. (c) CERN. Image credit: Lucas Taylor
The need for the Higgs
Our Universe seems to be described by four fundamental forces: gravity, electromagnetism, the weak force (which regulates nuclear phenomena like fusion within stars) and the strong force (which manifests on the scale of atomic nuclei). From around the turn of the last century, physicists have tried to unify these forces under a single, overarching theory. A major breakthrough came nearly five decades ago when physicists realized that there are very close ties between the weak force and the electromagnetic force. These two forces can be described by a single theory based on a fundamental symmetry between them, and this ‘unification’ implies that electricity, magnetism, light and some types of radioactivity are all manifestations of a single underlying force called the electroweak force.
The theory of electroweak interactions and Quantum Chromodynamics (the theory of the strong force) form the basis of the Standard Model. The Standard Model successfully describes all of the elementary particles we know and how they interact with one another. But our understanding of Nature is incomplete. In particular, the Standard Model as originally conceived cannot answer one basic question: Why do most of these elementary particles have masses?
The symmetry responsible for electroweak unification requires the force-carrying particles involved to have no mass. The photon, carrier of the electromagnetic force, fulfils this requirement; however, the W and Z bosons, carriers of the weak force, have non-zero masses. The fact that the W and Z are massive breaks the fundamental electroweak symmetry. It also leads, without corrections, to nonsensical predictions – for example, interactions with probabilities greater than 100%.
Needing a way out of this conundrum, several physicists proposed a mechanism to explain the broken symmetry. Once it was incorporated into the equations, this electroweak-symmetry breaking mechanism would allow particles to have mass. The mechanism also explains why the weak interactions appear to be weak at low energies; the force carriers are massive and therefore the force is short ranged. Peter Higgs pointed out that the mechanism required the existence of an unseen particle, which we now call the Higgs boson.
According to our current understanding, all particles were massless just after the Big Bang. As the Universe cooled and the temperature fell below a critical value, an invisible field called the ‘Higgs field’ was formed; this field prevails throughout the cosmos. Particles such as the W and Z acquire mass through their interaction with this field – the more intensely they interact, the heavier they become. The existence of such a field preserves the underlying symmetry of the electroweak theory, whilst explaining the broken symmetry we observe in Nature today. Other force-carrying particles – the photon and the gluon – do not feel any interaction with the Higgs field and remain massless. The Higgs boson is the quantum particle associated with the Higgs field just as the photon is the quantum particle associated with electromagnetic field. Since the field cannot be observed directly, the LHC experiments search for the particle, discovering which would prove the existence of the field.
Where does all mass come from?
Interactions with the Higgs field are not just reserved for force-carrying particles. The theory can also explain how all other fundamental particles – such as the electron, or quarks inside protons and neutrons – acquire their rest mass. Composite particles however, such as the protons and neutrons themselves, gain mass mainly through the binding energy holding them together. Without mass, the universe would be a very different place. For example, if the electron had no mass, there would be no atoms. Hence there would be no matter as we know it, no molecules, no chemistry, no biology and no people.
The particle hunt
The Higgs boson is the only fundamental particle predicted by the Standard Model that has not yet been seen by experiments. The technical problem is that the theory does not predict the exact mass of the Higgs boson itself, which makes the particle more difficult to identify. We have to look for it by systematically searching over a very large range of masses. Fortunately, depending on its mass, the Higgs boson would leave a characteristic footprint. So we know what to look for and would be able to calculate its mass from the particles seen in the detector. If it turns out that we cannot find it, this will leave the field wide open for physicists to develop a completely new theory to explain the origin of particle mass. The Higgs boson has been at the top of physicists’ most-wanted list for more than four decades. However, in its most basic form, incorporating the Higgs field into the Standard Model is not completely satisfying. It does the job of explaining how the symmetry between electromagnetic and weak force-carriers is broken and it accounts for how force-carriers acquire their mass; but it does not predict or explain the degree of interaction with the field and hence the relative masses of these particles. Moreover, it does not explain why symmetry is broken in this way, nor does it predict the pattern of masses of quarks and leptons.
Other bosons – Looking beyond the Standard Model
We might find that the Higgs boson is different from the simplest version the Standard Model predicts. Many theories that describe physics beyond the Standard Model, such as Supersymmetry and composite models, suggest the existence of a zoo of new particles, including different kinds of Higgs bosons. CMS is a general-purpose detector. This means that not only is it designed with particular hypotheses in mind, but also with the aim to study whatever happens when particles collide at high energies, even if the results are like nothing we expect. If unexpected phenomena do occur, we plan to be ready for them. On the other hand, finding no Higgs boson at the LHC would give credence to other classes of theories that explain the symmetry-breaking mechanism in different ways. If we instead see new, interesting and different phenomena, this could launch a revolution in physics, sending theorists back to the drawing board and challenging our ideas about the world at the most basic level. In either case, it seems we are looking at just the visible tip of an iceberg – hidden below the Standard Model must be a deeper, more fundamental theory that gives reason to what we see on the surface.
 F. Englert and R. Brout, “Broken symmetry and the mass of gauge vector mesons”, Phys.Rev. Lett. 13 (1964) 321–323, doi:10.1103/PhysRevLett.13.321. P.W. Higgs, “Broken symmetries and the masses of gauge bosons”, Phys. Rev. Lett. 13 (1964) 508–509, doi:10.1103/PhysRevLett.13.508. G. Guralnik, C. Hagen, and T. W. B. Kibble, “Global conservation laws and massless particles”, Phys. Rev. Lett. 13 (1964) 585–587, doi:10.1103/PhysRevLett.13.585. It is also of note that Landau and Ginzburg had proposed a field giving the photon a mass in a superconductor, the maths of which is identical to the “Higgs mechanism” and predates it by several years. Credit: this text is based on a CERN "backgrounder" article and a CERN Bulletin article.