International Linear Collider Matters

Yes, it does matter. And it does matter:

(Simulated ILC collision by Norman Graf, showing production and decay products of the Z particles that areso important in electroweak physics.)

The high energy physics community is excited abut the Large Hadron Collider’s upcoming switch-on, since we hope to get new information about the physics of the structure of matter at the most fundamental level that we currently understand. It is chiefly designed to explore what is called the Electroweak symmetry breaking mechanism. The buzzwords that are used in place of those words usually go along the lines of “it’s going to probe the origin of all mass”, which is partly true, in a sense. The symmetry breaking aspect refers to fact that at higher energy scales that existed a long time ago in our universe (conditions that the machine will essentially recreate in order to study) the well known force of electromagnetism (that you use everyday in so many ways) and the less well known (but explicitly used in hospitals a lot – it controls certain radioactive decay used for radiological tracking, for example) weak nuclear force were combined into one force known as the electroweak force. The “symmetry” refers in essence to that fact that the forces -very distinct in our current world- were on the same footing back then. We say that there was a “symmetry” between them, in a very precise sense. In dropping below those energy scales to get to the familiar world we know now, the symmetry between the forces was “broken”, in the sense that they don’t look much like each other any more. In the process of that happening, all the particles that interact with those forces (quarks, neutrinos, electrons…) -which were grouped into families within which they were on the same footing as each other- broke out of those family groupings somewhat and received the range of different masses that we see today. Since all that we see is made up of those sorts of particles, knowing how they got their masses is often equated with learning “the origin of mass”¹. The mechanism that is associated to this process is often called the “Higgs” mechanism and there is a particle (or family of particles) called the “Higgs” that realizes the mechanism explicitly, it is believed. I say “it is believed”, because this is where we are now in terms of direct experimental work on this central story of our origins. One of the things the LHC will look for directly is evidence of the Higgs. The simplest versions of the story point to one Higgs, and that’s it. The excellent work of so many experiments of the years have pointed us to a narrow window of where to look for this, and this is what the LHC is primarily going to do.

The simplest thing might not be what Nature has chosen to do however. There are several hints and hopes that there’s more going on than just the simple Higgs scenario. There are lots of other questions connected to that area of energy scale and not far beyond, and the LHC may begin to see some of this broader/richer physics. We simply don’t know. The questions are huge as well, and involve not just the origin of the masses, but possibly the (currently unknown) nature and origins of 83% of the matter in our universe (the Dark Matter), the fate of the further unification of the known forces, the weakness of gravity, the true nature of the Planck scale, and so forth². These are currently some of the huge questions of our field, and involve theoretically intricate (sometimes elegant and simple, sometimes not) ideas and frameworks such as supersymmetry, extra dimensions, string theory, and more. And this is why it is such a wonderful time to be involved in High Energy Physics. We don’t know what is going to come up at the LHC, and we don’t know which (if any) of these detailed ideas make connection to what Nature actually chose to do. Further, we need guidance from the LHC (and beyond) to further develop some of those ideas and frameworks into stronger, testable theories.

So what about the International Linear Collider? There’s a recent New York Times article by Dennis Overbye, a press release, and a very splendid article from JoAnne Hewett for you to read. The LHC was not designed to explore much of the new physics (beyond a simple Higgs) that could show up. So in short, we may well need the ILC to move forward as a field. The LHC can find hints of things like supersymmetry, but it will be unlikely to be able to study it in enough detail to understand what’s going on. This is the purpose of the next big project, whatever that might be. It looks like it will be the ILC, and serious planning and costing (and discussion about locations) is underway. It is very exciting – particularly so because there could well be a big change in the landscape of who the key players are on the world stage of high energy physics. Since the USA dropped the ball many years ago with the SSC’s cancellation³, the focus was set to be very firmly in central Europe for some time, and that is where it is going to be for a while, with the LHC (it is at CERN, on the French/Swiss border). The ILC could well broaden things out quite a bit more, bringing Japan and China (among other asian countries) into the forefront of the effort. There is even talk of locating the machine there, as an alternative to locations in the USA, or in Europe (at CERN).

This broadening out of the international effort is, on balance⁴, exciting in the scheme of things, and from a pragmatic point of view is undoubtedly healthy for the future of our field.

This is going to be just great!

-cvj
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1. The reason it’s not really true -but it is a nice buzz-phrase- is that to get the mass of an everyday object, you must take into account not just the mass of the individual constituents, but also the energy contained in binding them together… but I’m perhaps being over-fastidious here. [return]
2. I’m talking about things like gauge coupling unification, fine tuning and the hierarchy problem, the fact that the lightest superpartner in supersymmetric scenarios are very good candidates for Dark Matter constituents, the fact that the Planck scale (where quantum gravity becomes impossible to ignore) may not be as high as naive dimensional analysis suggests – it could be just around the corner (in a sense), and so on. Some careful googling on these matters will turn up more information. Start by looking at the websites of CERN and Fermilab though. [return]
3. See for example this Wikipedia article, particularly the references. [return]
4. Yes, there are huge concerns about the future of high energy physics in the USA. The climate of funding for lots of fundamental science is worrying, and this is part of that strand. When I say “on balance”, I’m placing my eyes on the broader question of the physics goals we want to achieve. I’m part of the community of physics in the USA, and of course have serious concerns about the “local” picture. [return]