John Ellis discusses the Higgs, the lack of the Higgs, and extra dimensions

On 13 September, CERN will be hosting a colloquium to mark John Ellis' 65th birthday. The colloquium comes as John ends his long career as a distinguished CERN staff member and makes a transition to Clerk Maxwell Professor of Theoretical Physics at King’s College London. The Bulletin took the opportunity to ask John to share his expectations from the LHC during this long-awaited data-taking phase...


John Ellis  in his office (July 2011).

So, let’s start from the Higgs boson: does it exist and where is it? The million-dollar question…

Sometime in the next few months, I think we will finally get clarity on the Higgs boson. While it has been with us as a hypothesis since 1964, I think we are finally closing in on it now. However, in every possible discovery scenario, clarity on the Higgs will lead to new physics. Let me explain...

Experiments have left only three possible mass ranges where the Higgs – or a Higgs-like particle – could be: between 114 GeV and 135 GeV, over 500 GeV, or between 135 GeV and 500 GeV.

In the first scenario (114-135 GeV), we could be looking at a Standard Model Higgs boson. This range has been refined experimentally: recent LHC results presented in Mumbai excluded the Standard Model Higgs from about 135 GeV to about 500 GeV, while LEP had previously excluded it up to 114GeV. That leaves a narrow low-mass range of about 20 GeV where it could lie. But if found in this range, the Standard Model theory would still be incomplete; the present electroweak vacuum would be unstable for such a light Higgs in the Standard Model, so we would have to come up with new physics to stabilise it.

The second option is that the Higgs boson is found to be heavier than 500 GeV, in which case it could also be a Standard Model Higgs. However, the theory defining it would become so strongly coupled that we wouldn't be able to calculate it reliably. In that case, I believe we would need some new physics in order to “domesticate” or “tame” this heavy Higgs.

The third option is that the Higgs boson lies somewhere between 135 GeV and 500 GeV. In that case, we would not have the Standard Model Higgs couplings – they would have to be weaker by some amount. And if such a “Higgs” were weaker, then it would not do the same job as the Standard Model Higgs and, again, we would need some sort of new physics.

I think that there is every chance that within the next few months, at most one year, we will know which of these possibilities is the case. Of course, there is also a fourth option: that there is no Higgs!


What if the LHC finds nothing at all?

Well, that might mean that there is nothing like a Standard Model Higgs boson, or that it is so heavy that it has to behave in a way that we cannot calculate (at the moment, of course!). I have always said that the most exciting result for theorists would be if there were no Higgs boson at all. It would really force us to throw away the ideas we have been playing with for the last 47 years.


And if there is no Higgs, what else should we expect?

There are already some ideas out there exploring the possibility that there is no Higgs boson. One of the most appealing, for me, is the extra dimension theory. The job of the Higgs boson is often explained as giving mass to particles. But another way of thinking about it is that it has to give masses to some particles and not to others, it has to discriminate between different types of particles. In the language of a particle physicist that means it has to break the symmetry between, say, a W boson on the one hand, and a photon on the other.

So how do you break this symmetry? We can try writing equations that are not symmetric – that have never had the symmetry to break in the first place. But that would leave us with a theory that is, literally, incalculable. The only possibility is to have symmetrical equations that somehow give different results for different particles.

But how do you force symmetric equations to have such asymmetric solutions? One option is to put the Higgs field into the vacuum, what we naively think of as “empty space”. The Higgs field will break the symmetry, discriminating between different types of particles depending on how strongly those particles interact with it. That is the option chosen in the Standard Model.

But there is another way, potentially, of breaking symmetry: by using boundary conditions. When you solve a differential equation, the solution depends on what boundary conditions you impose on the calculation at the edge of space. In the three-dimensional universe we see around us, space seems to extend to infinity, and there are no edges where different boundary conditions could be imposed for different particles. But if space had small additional dimensions, at their edges we could impose different boundary conditions for different types of particles, giving us different masses for different particles. I think that would be the most exciting alternative: to discover that there is no Higgs, but that there are more dimensions of space.


How will we be able to test all these options?

In principle, there are several ways of testing such a theory: by looking for deviations from Standard Model predictions in the scattering of W particles, for example. There are a couple of experimental options you might want to follow if there turns out to be no Higgs. In such a scenario, there will be a premium on exploring even higher energies. In order to look at W-particle scattering, for example, we could increase the intensity of the energy in the LHC or we could build a very high-energy positron-electron collider, like the CLIC accelerator that is currently on the drawing board.


The second part of the John Ellis interview, in which he discusses the future of CERN and particle physics, will be published in the 23 September 2011 issue of the Bulletin.


Information about the Colloquium to celebrate John Ellis' Birthday is available here.

by Katarina Anthony