The Higgs boson: Evolution or revolution?

The search for the Higgs boson puts particle physics on the threshold of a new era.

Discovering the Higgs particle at the Large Hadron Collider (LHC) would be a triumph, but showing that it doesn’t exist could be at least as exciting, perhaps heralding a revolution in our understanding of nature at a fundamental level. After two good years of operation at the LHC, the moment of truth is drawing near. By the end of the 2012 LHC run at the latest, we’ll know whether the simplest incarnation of the Higgs particle is real, or just a chimera. Whatever the case, many more years of research at the LHC will be needed to fully get to grips with the consequences.

Finding the Higgs particle, or definitively ruling out its existence, is one of the top priorities for research at the LHC. The Higgs particle is associated with the simplest realization of a mechanism, proposed in the mid-1960s by Robert Brout, François Englert, Peter Higgs and others, which was put forward to explain why one of nature’s fundamental forces has a very short range, while another similar force has an infinite range. The forces in question are the electromagnetic force, which carries light to us from the stars, drives electricity around our homes, and gives structure to the atoms and molecules from which we are all made, and the weak force, which drives the energy generating processes of the stars. Today we know that the electromagnetic force is carried by particles called photons, which have no mass, whereas the weak force is carried by particles called W and Z, which do have mass. Rather like people passing a ball, interacting particles exchange these force carriers. The heavier the ball, the shorter the distance it can be thrown; the heavier the force carrier, the shorter its range. The W and Z particles were discovered in a Nobel prize winning enterprise at CERN in the 1980s, but the mechanism that gives rise to their mass has not yet been experimentally identified, and that’s where the Higgs particle comes in.

The Higgs mechanism in its basic form is the simplest theoretical model that could account for the mass difference between photons and the W and Z particles, and by extension could account for the masses of a range of fundamental particles. But the simplest form of the Higgs mechanism is not the only possible explanation. There are many others, linked to theories such as supersymmetry, which could account for the mysterious dark matter of the universe, or theories predicting extra dimensions of space, which, if verified, would truly revolutionise our understanding of the universe we live in. These searches in turn are just a part of the very wide programme of research that is ongoing at the LHC, which also includes looking for the subtle imbalance in nature between matter and antimatter that has allowed the matter we are made of to exist, and studying matter as it would have been in the first instants of the universe’s life.

The basic form of the Higgs mechanism forms part of the Standard Model of particle physics, the theory developed in the 1960s and 70s that describes the behaviour of fundamental particles and has since been thoroughly tested at laboratories such as CERN. The Standard Model works extremely well, but we know that it cannot be a complete theory. It describes beautifully the ordinary matter from which we, and the entire visible universe, are made. But it does not describe the invisible 96% of the universe that we know to be there, but which has thus far evaded detection. The Standard Model is nevertheless such a good theory that it will always remain valid over the range it has been tested. Today’s scientists are therefore looking for a theory that builds on the Standard Model, rather like Einstein’s theory of gravity, general relativity, builds on Newtonian gravity. That’s why finding an alternative to the Standard Model Higgs particle would be so exciting.

The Standard Model Higgs particle, if it exists, has well-defined properties that depend only on its mass. That’s why it will be possible to confirm or refute its existence before the end of 2012. Some of the possible non-Standard Model Higgs particles would look very much like the Standard Model variety, but could emerge more rarely from LHC collisions and therefore take longer to find. Others would be heavier than the LHC’s current reach, and require more energy to produce. If such particles are nature’s choice, we’ll have to wait until the LHC moves to its full design energy to find out.

Whatever the case, the LHC will make a discovery about the nature of the masses of the fundamental particles. That’s because another shortcoming of the Standard Model is that without the Higgs particle, or something that does the same job, its calculations of particle processes break down at the energies the LHC will reach in its second phase of running starting around 2014. That means that if the LHC does not discover the Higgs particle by 2012, it is heading towards a discovery later. Whatever mechanism nature uses, the LHC will bring us insights.

The status of the search for the Standard Model Higgs particle at the end of the 2011 LHC proton run in October was based on experimental work involving scientists from around the world. Direct searches from CERN’s previous flagship research facility, the Large Electron Positron collider, LEP, had excluded the mass range up to 114 GeV. Results from the Tevatron collider at Fermilab in the USA, and from the LHC, had excluded the range from 141 GeV to 476 GeV.  Indirect searches, in which scientists try to detect tell tale signs that a Higgs particle has influenced their measurements rather than looking for the particle directly, exclude the range above 200 GeV or so. That left just the region 114-141 GeV, which is precisely where theoretical and experimental considerations say a Standard Model Higgs particle is most likely to be. By December 2011, analyses by the ATLAS and CMS collaborations had further narrowed the range of masses available for the Standard Model Higgs particle to just 116-127 GeV, with both experiments seeing tantalising signs that that a Standard Model Higgs particle might be starting to emerge in the region of 124-126 GeV. Only time will tell.

All of this augurs very well for the long-term future of the LHC programme, since whatever form the Higgs particle takes, studying its properties, or examining its absence, will require considerable amounts of data. A Standard Model-like Higgs particle could yet point the way to new physics through subtleties in its behaviour that would only emerge after studying a large number of Higgs particles. A Standard Model-like Higgs particle might also be one of several types of Higgs particles, pointing the way to new physics, and this would only become apparent after detailed scrutiny. A non-Standard Model Higgs particle linked to a theory like supersymmetry that goes beyond the Standard Model would immediately open the door to new physics. And finally, if a Standard Model Higgs particle were definitively ruled out at the LHC’s current operating energy, that would point either to a non-Standard Higgs particle that could be discovered with more luminosity or to the existence of new physics at the LHC’s full design energy where the Standard Model without the Higgs particle starts to break down.

Whatever form the Higgs particle takes, or whatever mechanism drives the differences in fundamental particle masses, finding it is not a simple case of spotting the telltale signs and shouting Eureka! It is a painstaking process of statistical analyses based on measuring specific configurations of particles emerging from collisions. For example, one of the ways a Higgs particle can decay is into two photons, which would be detected. However, there are many other processes that also produce two photons, so the searches compare the number of so-called two-photon events measured with the number expected from already known processes. They do this for all the possible decay modes, and only when they see a statistically significant excess can scientists claim a discovery. In particle physics, people talk of 95% confidence levels, which means that a given signal, such as that for a Higgs particle decaying to two photons, has only a 5% chance of being due to a statistical fluctuation. However, 95% confidence is not enough to claim a discovery, for that, the probability of a statistical fluctuation being responsible for the measurement has to be much smaller, say, than one in a million. This is what physicists call a five-sigma effect. Discovery or exclusion of a Standard Model Higgs particle to that level of confidence is what’s on the cards for 2012 at the latest.

A non-Standard Model Higgs particle could take longer to be discovered, but would certainly be worth the wait. Either way, there will be much work ahead for the LHC scientists to fully understand the new physics that is just over the horizon.

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