During the gap between the first run of the Large Hadron Collider [LHC], which ended in 2012 and included the discovery of the Higgs particle (and the exclusion of quite a few other things), and its second run, which starts a year from now, there’s been a lot of talk about the future direction for particle physics. By far the most prominent option, both in China and in Europe, involves the long-term possibility of a (roughly) 100 TeV proton-proton collider — that is, a particle accelerator like the LHC, but with 5 to 15 times more energy per collision.

Do we need such a machine?

The answer is “Yes, Definitely”. Definitely, if human beings are to continue to explore the inner world of the elementary laws of nature with the same level of commitment with which they explore the outer world of our neighboring planets, the nearby stars and their own planets, and distant galaxies far-flung across the universe. If we can send the Curiosity rover to roam around the surface of the Red Planet and beam back pictures and scientific information — if we can send telescopes like Kepler into space whose sole purpose is to look for signs of planets around distant stars — then surely we can build a machine on Earth whose sole purpose is to help us understand the fundamental principles and elementary objects that underlie the natural world. That’s why we built the LHC, and machines before it; and the justification for a 100 TeV machine remains the same.

Definitely, also, if the exploration of the laws of nature is to continue as a healthy research field. We have a large number of experts who know how to build a big particle accelerator. If we were to postpone building such a machine for a generation, we would suffer some of the same problems suffered by the U.S. space program. All sorts of crucial knowledge of the craft of rocket building was lost when the U.S. failed to follow up on its several trips to the Moon. If we have a hiatus of a generation between the current machine and the next, we will find it much more difficult and expensive to build the next one when we finally decide to do it. So it makes sense to do maintain continuity, especially if it can be done at reasonable cost.

One thing that’s interesting to keep in mind is that a roughly 100 TeV machine is hardly a stretch for modern technology; it’s not going to be a machine with a significant risk of failure. The Superconducting SuperCollider (SSC), which was to be the U.S. flagship machine and was due to start running in the year 2000 (in which case it would definitely have discovered the Higgs particle many years ago — sadly, the U.S. congress canceled it, after it was well underway, in 1993), would have been a 40 TeV machine. The technological step from 40 TeV to 80 or 120 is not a big one. Moreover, the SSC would have been an easier machine to run than is the LHC, which has to strain with very high collision rates to make up for the fact that its energy per collision is a third of what the SSC would have been capable of. The main challenge for such an accelerator is that it has to be very large — which requires a very long tunnel (over 50 miles/80 km) and a very large number of powerful magnets.

It’s no wonder the Chinese are interested in potentially building this machine. With an economy growing rapidly enough to catch up with the other great nations of the world in the next decade or two, and with scientific prowess rapidly increasing (see here and here), some in China rightly see a 100 TeV proton-proton collider both as an opportunity to gain all sorts of technical and technological knowledge that they have previously lacked, and to establish themselves among the few nations that can be viewed as scientific superpowers. Yet it will not require them to go far out on a limb with technology that no one has ever attempted at all, and invent whole new methods that don’t currently exist. Moreover, some of the things that would be expensive or politically complex in the U.S. or Europe will be easier in China. They may be able to pay for and construct this machine themselves, with technical advice and personnel from other countries, but without being dependent on other nations’ political and financial challenges.

In fact, there’s another huge potential benefit along the way, even before the 100 TeV machine is built: a “Higgs factory”. One can potentially use this same tunnel to first build an accelerator that smashes electrons and positrons [i.e. anti-electrons] together, at an energy which isn’t that high, but is sufficient to make Higgs particles at a high rate — not as many Higgs particles as the LHC will produce, but in an environment where precise measurements are much easier to make. [Protons are messy, and all measurements in proton-proton collisions are very difficult due to huge collision rates and large backgrounds; electrons and positrons are simple objects and measurements tend to be much more straightforward. This comes at a cost: it is harder to get collisions at the highest energies physicists would ideally want.]

The value of a Higgs factory is obvious: a no-brainer. The Higgs particle is our main way of gaining insight into the nature of the all-important Higgs field, and moreover the Higgs particle might also, through its possible rare decays, illuminate a currently veiled world of unknown particles and forces. It’s a research effort whose importance no one can deny, and it serves as a technical stepping stone to a 100 TeV collider, complete with the realistic possibility of Nobel Prize-worthy discoveries in the near term. For China, it’s perfect.

Of course, the Chinese aren’t the only ones interested. My European colleagues, recognizing a good thing when they see it, and with the advantage that they built and ran the LHC, are also considering building such a machine. [Neither the U.S., which is expertly squandering its scientific leadership in many scientific fields (and pushing many of its best scientists toward the Chinese effort), nor Russia, which is busy starting a disastrous invasion of its neighbor, seem able to make any intelligent decisions at the moment, and surely aren’t going to be the leaders in such an effort.] For the moment, the scientists involved are all working together. Over recent years, any particle physicist worth his or her salt (including me) would spend some time at Europe’s CERN laboratory, which hosts the LHC. And now, many young U.S. experts in theoretical particle physics are planning to spend extended time at China’s “Center for the Future of High Energy Physics“. There was a time young Chinese geniuses like T.D. Lee, C.N. Yang and C.S. Wu did Nobel Prize-winning (or -deserving) work in the United States. Soon, perhaps, it will be the other way around.

But what, scientifically, is the justification for this machine?

Why build a 100 TeV collider?

It’s important to distinguish two types of scientific enterprises: exploratory and targeted. Exploratory refers to when you’re doing a search, in a plausible place, for anything unexpected — perhaps for something whose existence you might suspect, but perhaps more broadly. Targeted refers to doing a search or study where you know roughly, or even exactly, what you’re looking for.

Often a targeted enterprise is also exploratory; while looking for one thing, you can always stumble on something else. Many scientific discoveries, such as X-rays, have been made while doing or preparing experiments with a completely different purpose. On the other hand, an exploratory enterprise may not have any targets at all, or at best, only a very vague target. Sometimes we go searching just because we can. When Galileo pointed his first telescopes at the moon and the planets and the stars, he had no idea what he would find; he just knew he had a great opportunity to discover something.

The LHC was built as a clearly targeted machine: its main goal was to find the Higgs particle (or particles) if it (or they) existed, or whatever replaced them if they did not. Well, now we know that one Higgs particle exists, and it resembles the simplest possible type of Higgs particle, which is termed a “Standard Model Higgs”. But much remains to learn. Is this Higgs particle really Standard Model-like, not just at the 30% level but at the 3% level and better? Are there other Higgs particles? Are there other as-yet unknown particles being produced at the LHC? Are there new forces beyond the ones we’re aware of? Other than the detailed study of the new Higgs particle, these questions are mostly exploratory. In short, though the LHC was built as a targeted machine with a near-guarantee of success, its mission has now shifted toward exploration of the unknown, with no guarantee of further discoveries. But it’s also important to understand that a lack of discoveries will be just as important to our understanding of nature as discoveries would be, for reasons I’ll return to in my next post.

Now what about the 100 TeV machine? Will it be a targeted experimental facility, or an exploratory one?

For the moment, the answer is: we don’t know. Currently, there is no clear target; more precisely, there are lots of possible targets, but none that we know could emerge to be a major, central one. But this machine won’t be built and completed for a couple of decades, and things could change dramatically by then. If the LHC discovers something not predicted by the Standard Model (the equations used to describe the known elementary particles and forces), then clarifying this new discovery will become a major target, and possibly the main target, of the 100 TeV machine.

This highlights one of the challenges with large experimental projects. One has to start thinking about them far in advance, long before it’s entirely clear what their precise use will be. When the SSC and the LHC were first proposed, they did have a proposed target — finding the Higgs particle or particles. But if the recently discovered Higgs particle’s mass had been, say, half of what it actually is, it would have been discovered some years before the SSC or LHC were completed… in which case, the target of the SSC and LHC would have significantly shifted. So we have to start considering, proposing, and perhaps even building the 100 TeV machine before it’s completely clear whether it will have a prominent and definite target, or whether it will be mainly exploratory. That ambiguity is something we just have to live with.

In contrast to the 100 TeV machine, which currently has to be viewed as exploratory, the Higgs factory that would precede it in the same tunnel is much more sharply targeted… targeted at detailed study of the Higgs particle. There are some other targeted and exploratory activities that it can be involved in, including more detailed investigation of the Z particle, W particle and top quark, but its main focus is the Higgs.

However, even if no prominent target for the 100 TeV collider shows up before it is built, its justification as an exploratory machine is clear. In quantum field theory, collisions at higher energy and momentum allow you to probe physics at shorter times and distances — for “particles” are really quanta, i.e., ripples in quantum fields, and a higher-energy quantum has a shorter wavelength and a faster frequency. And we’ve learned time and time again that one way (though not the only one) to discover new things about the world is to examine its behavior on shorter times and shorter distances than we’ve previously been capable of. This enterprise has been going on for generations; first microscopes discovered bacteria and other cells; then these were found, with more powerful experiments, to be made of molecules, in turn made from atoms; yet more powerful experiments showed first that the atoms contain electrons and atomic nuclei, then that the nuclei are made from protons and neutrons, and then that these in turn are made from quarks and gluons. All of this has been discovered by probing the world with ever more powerful particle collisions of one form or another. So building a higher energy accelerator is to take another step along a well-trodden path.

However, it’s not the only path, nor has it ever been.

Is this the most promising path to explore?

The LHC is still in its adolescence, and we can’t predict its future discoveries. At this point the LHC experiments have collected a few percent of the data they’ll collect over the next decade, and they have done so with proton-proton collisions whose energy is only about 60% of what we expect to see in the next few years. Moreover, even the existing data set, collected in 2011-2012, hasn’t been fully analyzed; this data could still yield discoveries (but only if the experimenters choose to make the relevant measurements.) So we certainly can’t know yet whether the LHC will produce a new target for the 100 TeV machine. If it does, then it will be much clearer what to do next and how to use the 100 TeV machine. If it doesn’t… well, that’s something that deserves a bit more discussion.

Suppose that, after the LHC’s last run, nothing other than the Higgs particle’s been found, with properties that are consistent, to a few percent, with a Standard Model Higgs. While this sounds dull at first glance, it’s actually among the most radical possible outcomes of the LHC. That’s because of the “naturalness puzzle”, which I discussed in some detail in this article. Never before in nature, in any generic context, have we come across a low-mass spin-zero particle (i.e. something like the Higgs particle) without other particles associated with it. In this sense, the Standard Model is an extraordinarily non-generic theory, at least from our current point of view and understanding. It will be quite shocking if it completely describes all LHC data.

But maybe it does. If it does, what does this potentially imply about nature? And what would be the implications for our future explorations of nature at its most elementary level? I’ll address this issue in my next post.