From today’s paper: The 50-year search for the Higgs boson – the elusive particle that attributes mass to matter – is months from completion. Physics will never be the same
The Large Hadron Collider’s search for the Higgs boson – the theoretical particle that is believed to give all matter in the universe mass – is, according to physicists, entering its last phase. At some point in the next few months, perhaps as soon as December, we will know: either it exists in the form that is predicted, or it does not. Either way, it will have profound implications for our understanding of physics. Why haven’t we found it? And what will it mean when we do – or if we don’t?
Modern particle physics is based around the so-called “Standard Model”, which describes all known subatomic particles and how they interact. The Standard Model explains why certain particles have qualities, such as electromagnetic charge, which distinguish those particles from mere empty space. But why some particles have mass – why heavy things are harder to push – is less clear.
Peter Higgs, a University of Edinburgh physicist, argued in the 1960s that mass was the product of a field which permeates the whole universe, even empty space. The field has a viscous effect on other particles, grabbing at them stickily, making them hard to move: giving them mass. Quantum theory dictates that any field must have an associated particle, in the way that electromagnetism – light – has the photon. For Higgs’s hypothesis to work, a Higgs particle must exist.
Unfortunately, so far, while every other particle predicted by the Standard Model has been detected in experiments, the Higgs remains stubbornly unfound.
Tom Whyntie, a physicist at the LHC, says that this is a real problem for the Standard Model. “We say that it works like this, and this is how mass is generated, but we’ve got no experimental evidence for it whatsoever. And that really is a deal-breaker on how we think matter works at the fundamental level.”
That is where particle accelerators such as the LHC come in. By smashing particles together at high speeds, new particles are created, according to how much energy the collision contains: mass and energy are the same thing, as in Einstein’s equation E=mc². It is not known what the mass of the Higgs is, but if two particles smash together at the right speed, it will be created, and its brief existence and decay can be recorded in the LHC’s detectors. So all the LHC team have to do is smash together particles at all the mass-energy levels that the Higgs could theoretically be, and it will be found.
Except, as Whyntie points out, it’s not as simple as that. “Smashing together protons is a mess,” he says. Each proton is made up of three smaller particles called quarks, and you never know quite what the energy of the two quarks in any particular collision will be. Earlier, lower-energy colliders used smaller, elementary particles, allowing for precision, but they were limited in power, and soon reached their upper limit without finding the Higgs. “The LHC is more powerful but messier – it’s the difference between a sniper rifle and a shotgun,” says Whyntie.
This messiness makes the search for the Higgs a more complicated procedure. Each result the LHC finds has to be examined carefully and repeated, to ensure that nothing was missed. Even so, during the relatively short time the accelerator has been working, it has searched a large swath of the area that the Higgs might have been found in.
The energy of the LHC’s collisions – and therefore the mass of the Higgs that will hopefully be created – is measured in gigaelectronvolts (GeV). Earlier colliders ruled out a Higgs with a mass below 114GeV. The LHC has ruled out one between 135 and 500GeV.
Prof John Ellis, a Cern physicist who has been involved in the search for the Higgs for nearly four decades, says that whatever the LHC finds, physics will need some significant rewriting. “The fact that we have not yet found the Higgs is a fantastic success for the Standard Model,” he says. “The region that remains to be explored [between 114 and 135GeV] is precisely the region where, according to the Standard Model, we would expect the Higgs to be. But what the LHC has shown so far is that there must be physics beyond the Standard Model.”
There are three scenarios, he says. “Either the Higgs weighs less than 135, or it weighs more than 500, or between those two figures there is something that is not the Higgs.” All three scenarios lead to difficult questions.
If it’s a heavy Higgs, says Prof Ellis, not only would it fail to match data established elsewhere, the theory would also break down at high energies. “You’d need to find some way of accommodating that, so you’d need new physics,” he says. And if nothing like the Higgs is found, or if it’s something slightly Higgs-like in the medium range, “we know it cannot be a Standard Model Higgs, so there must be some new ingredient, at the minimum, to make a sensible theory”.
Even finding the Higgs exactly where we expect it to be will not be the end of the matter. “It would be a victory for the Standard Model, but that victory comes at a price,” says Prof Ellis. At high energies, the light Higgs leads, according to current understanding, to another breakdown of the theory. Again, new physics would be required to repair it.
Whyntie agrees: “Finding the Higgs and nothing else would be unsatisfactory,” he says. “To make the equations work, you have to do some really dodgy accounting.”
A possible theoretical solution would be “supersymmetry”, a model in which every particle has a so-called “superparticle” partner, which would help the maths make more sense. But, so far, there is no evidence to support it.
The really interesting questions, though, will arise if no Higgs is found at all. While the Higgs has dominated thinking for decades – so much, says Whyntie, that it may be “unhealthy for science; a sort of Cult of Higgs” – alternative theories do exist. One involves a “composite Higgs”, with two smaller particles doing the job of the previously theorised one.
Another draws on string theory, positing a universe with more dimensions of space than our usual three, and with particles “wrapping themselves around” those dimensions and leaking energy into them. In it, there would be no Higgs-like particle at all, just particles behaving differently depending on where they are in these exotic dimensions.
Whatever happens, it’s an extraordinary time to be a particle physicist, says Prof Ellis. “I wrote my first paper on the Higgs in 1975, so it’s been a while. But finally, we’re going to get closure. Either we’re going to find it, or we’re going to prove it doesn’t exist. One way or another, we’ll have an answer.”
Perhaps surprisingly, despite having looked for the Higgs himself for so long, he hopes they don’t find it – the really dramatic breakthroughs will come if the conventional wisdom is proved to be wrong. “The most exciting possibility is that the Higgs doesn’t exist. But sometimes in physics there are results which are just too exciting, too incredible to believe.”
He points to last week’s apparent finding, also at Cern, of faster-than-light neutrinos, as an example of such an unlikely result. “Likewise with Higgs. It would be great if we don’t find it. But it’s more likely that we’ll find it where we thought it would be.”