This could be it. The Large Hadron Collider finally has enough data to explore every nook and cranny where the elusive Higgs boson could be hiding. LHC physicists will announce the results of their latest hunt on Tuesday at CERN in Switzerland.
What if they find nothing? New Scientist takes a look at alternatives to the Higgs.
What is the Higgs boson?
It is the last undiscovered member of the standard model of particle physics, the leading theory describing how particles and forces interact. The mysterious particle is thought to give all other particles mass, but the standard model can't predict what the Higgs itself weighs.
Where might the Higgs be hiding?
The Higgs may be produced fleetingly when particles smash into each other at high speeds, and for years physicists have been looking for evidence of it at various particle colliders. They have gradually ruled out its existence at different masses, but there is still a narrow mass range, between 115 and 141 gigaelectronvolts, where the simplest version of the Higgs could take refuge.
What will LHC physicists report next week?
Rumour has it they have found hints of the Higgs at a mass of 125 gigaelectronvolts, about 133 times the mass of a proton. What is known for sure, though, is that researchers from the LHC's main detectors, ATLAS and CMS, will separately present the past year's worth of data from the proton collider. That represents more than 300 trillion high-speed particle collisions, more than twice the amount of data reported at a conference in August. That is still not enough data to be able to rule the Higgs definitively in or out, but it should be enough to show hints of the Higgs if it exists in the mass range that had previously not been scrutinised.
What if there is still no sign of the Higgs?
This time, if nothing materialises, physicists will really start giving up. "If we witness a lack of events in the full mass range, then clearly we will start disfavouring the presence of the standard model Higgs boson in LHC data," says CMS spokesperson Guido Tonelli. "To really exclude it we would need additional data. But if in this amount of data we don't see any indication that something is happening, the most likely hypothesis is that we have to look for another solution."
Are there other solutions?
"I think there are alternatives to the Higgs," says Nobel laureate Steven Weinberg of the University of Texas at Austin, who, together with Sheldon Glashow of Boston University and Abdus Salam of Imperial College London, wrote the standard model in the 1960s. Giving up on the Higgs boson opens the door for more exotic kinds of physics, including extra particles and extra forces.
Do we need a Higgs boson to give things mass?
No, says theorist Matt Strassler of Rutgers University in New Jersey. The Higgs boson is just a ripple in the so-called Higgs field, which is really what is thought to give all other particles mass. "The poor Higgs field labors in obscurity, protecting the universe from catastrophe but getting none of its deserved credit," Strassler writes in his blog.
Physicists are only looking for the Higgs particle because it is the easiest way to access the field. If they don't see it, then it suggests the field is different from the one predicted by the standard model. Normally, particles in fields are like ripples in ponds – photons are ripples in the electromagnetic field, for example. But if the field is more like molasses than water, then the ripples die away too quickly for us to detect.
That means matter might get its mass from a thick Higgs-like field that has no associated particle. To get such a goopy field, theorists need to add in more exotic possibilities – such as new particles or forces of nature. "You can't get the situation where there's no Higgs particle there unless you add something else," Strassler told New Scientist.
What about more exotic possibilities?
The existence of a new force, called technicolour, could also give particles mass without the need for a Higgs boson. Technicolour would act like a heavy-duty version of the strong nuclear force, which binds quarks together in the nuclei of atoms. The technicolour force would fill space with pairs of still more new particles, which would form a soup through which other particles would travel, gaining mass in the process.
"That would be an outstanding alternative if the Higgs isn't there," Weinberg says. "In that case there would be a whole host of other particles, probably at higher energy, that the LHC might discover. But it wouldn't find the Higgs. There wouldn't be a Higgs, in the usual sense."
Are there even more exotic ideas?
The existence of a fourth dimension of space, beyond the three we experience, could explain why particles have different masses – a fact that is usually attributed to the Higgs boson.
Different types of particles are thought to have different masses because they interact with the Higgs field with varying strengths. But if there is an extra dimension of space, particles can have higher-energy modes that show up only in the extra dimension. In ordinary particle physics, particles have a baseline energy called the ground state and can have more energetic "excited" states above that. Because energy is linked to mass through E=mc2, the excited states weigh slightly more than the ground state.
In a world with four spatial dimensions, particles have a full range of excited states, but most of their energy is trapped in the extra dimension, where we can't see it. The unseen states of a single type of particle end up looking like different particles to us in our 3D world. You still need a Higgs-like field to give the ground state mass, but an extra dimension can explain why we see particles of different masses without invoking the usual Higgs boson.
"This would be a way of discriminating between particles and giving them different masses, which doesn't make any reference to any Higgs boson or anything at all," says CERN theorist John Ellis.
Why take these alternative ideas seriously?
"It's the job of theoretical physicists to game out all the different possibilities, so that the experimentalists have all the tools that they need when they eventually discover or don't discover whatever it is the LHC will or will not reveal," says Ellis.
Whatever the LHC finds, the prospect of getting such a flood of data is thrilling, says Tonelli. "This is the atmosphere here. People are feeling that we are really touching something important."
The story has been updated to reflect the fact that having a ripple-less Higgs field requires physics beyond the standard model. The section involving a fourth spatial dimension was also added in the updated version.
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