The Higgs boson has eluded detection for the moment, but theorists remain busy reconsidering what form or forms this object of much desire might take. The latest thinking was aired at the International Conference on High Energy Physics, held last month in Amsterdam.
The existence of the Higgs particle was first proposed in the 1960s. It is the manifestation of the Higgs mechanism that explains why fundamental particles, such as electrons and quarks, have mass. Mathematically, the particle can be formulated as part of the standard model of high-energy physics, or it could take a more exotic form that goes 'beyond the standard model' (BSM). But attempts to find evidence that the Higgs — any Higgs — exists have so far failed. An exhaustive search at CERN's Large Electron Positron (LEP) collider in Geneva ended in 2000 with the tantalizing hint of a signal, such as the candidate Higgs decay pictured here: the particle tracks shown in green and yellow may originate from a Higgs particle, created in an electron–positron collision at the centre. But no unequivocal signature has been identified.
So what now? Well, the experimentalists haven't given up. They will continue the search at the Tevatron collider, currently up and running at Fermilab, Chicago; and CERN's Large Hadron Collider (LHC) will take up the baton in 2007. Some theorists, however, have gone back to the drawing-board. The basis of the Higgs mechanism remains, but tweaks to the model in the light of experimental data are altering what we might expect to see.
At the Amsterdam meeting, Martin Schmaltz (Boston Univ.) reported the results of a 'straw poll' he conducted among a gathering of theorists: what, he asked them, is the most exciting development in BSM theory? The clear winner was 'Little Higgs' theory, which many of the voters admitted to working on themselves. According to this theory, there should be one or more Higgs particles with masses around the 200-GeV range — the Higgs must be heavier than 114.4 GeV, according to the LEP data (http://lepewwg.web.cern.ch/LEPEWWG), and is probably lighter than a few hundred GeV. Moreover, the Little Higgs prediction is that there are other new particles to be discovered at about the 1,000-GeV scale. One of them could be a candidate particle for dark matter, the substantial mass that astronomical observations tell us is out there in the Universe, but is unseen.
So the goalposts have moved, but what is the significance of the new thinking? The theory of particles and interactions is beset with divergences — calculations that do not converge to a meaningful value. One solution is to invoke 'supersymmetry', a theory that predicts that every fundamental particle has a heavier, 'superparticle' partner. The Higgs mechanism and supersymmetry sit alongside each other quite happily, and physicists have even searched for a supersymmetric Higgs. But the Little Higgs theory can stand alone. It solves the problem of divergences in the standard model, without the need for supersymmetry.
A 200-GeV Higgs particle is an enticing prize, which Fermilab will no doubt race to find before CERN's LHC project comes into operation. The partner particles at 1,000 GeV are food for thought for proponents of the Linear Collider — a next-generation machine that has yet to get the go-ahead but that physicists hope will be operational within the next decade.
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Wright, A. Vote for Little Higgs. Nature 418, 738 (2002). https://doi.org/10.1038/418738a