We Should Build a Muon Collider
Scientific Progress is and should be measured by what we actually learn, not by the difference between what we expect to find and what we did.
The Large Hadron Collider (LHC) has turned on again, pushing the collision energy between protons up to 13.6 TeV. This is its third major run, and plenty of good science has been done.
As this international facility approaches its design energy, some physicists are already setting up for the next big experiment.
As well they should. It takes an entire generation of physicists to build one.
The first official design document for the LHC was submitted late in 1995, just a few months after Fermilab announced the discovery of the top quark. The collider at Fermilab - the Tevatron - was officially approved in 1979.
The construction of these behemoth experiments has only gotten more intricate with time. For particle physics to progress, we need to start construction of the next big experiment now.
To quote Will Kinney - and frankly, a lot of other physicists:
Why build a new particle collider?
The LHC collides protons. Muon colliders can provide a much cleaner signal than a proton collider.
Protons are tiny bags of subnuclear goo in which three valance quarks live. While collisions between these constituent quarks certainly create a mess of new particles, these hadrons also have all the extra, gooey “shrapnel” left over from each collision.
They’re easier to accelerate than electrons, which are enormously inefficient at high energies1. So the trade off for reaching new energies is a giant mess provided at each proton collision.
For all their operational inefficiencies, electron-positron colliders provide a clean, relatively crisp signal because the particles themselves are simpler. For all we know, they’re elementary.
While proton colliders can push towards new knowledge by touching the edge of observable collision energies, the clarity afforded by lepton colliders typically compliments this approach by refining that knowledge through careful study.
What’s exciting about muon colliders is that they can potential offer both: the clarity at high energy scales is expected give sufficient precision to see new physics.
Last time we wrote an implicit if ambivalent eulogy for Weak Scale Supersymmetry. Our thesis was that we don’t need a radical redesign of the Standard Model of Particle Physics2.
Given that null hypothesis, why does it make sense to build yet another mammoth experiment?
Today I’ll give you four reasons, but first among those is simple:
There’s a lot more Science to do.
We don’t fully understand the Standard Model. Plenty of parameters aren’t precisely known3. We’re only beginning to understand how quarks interact with each other beyond the familiar meson and baryon representations. There are some mild peculiarities that violate time-reversal invariance.
And of course, we would do well to understand the interactions, decays and other details of the Higgs Boson a whole lot more! The value of the mass and its self-coupling is suspiciously close to a critical scale which itself demands an explanation4.
Technology Transfer to Medicine and Industry
We learn a lot by building and using new accelerators. Improving the performance of the accelerating magnets requires both developing new superconducting technologies as well as new ways to productionalize them. The LHC has over 1200 superconducting dipole magnets that each weigh 35 metric tons and produce an enormous 8.3 T magnetic field. 8.3T each!
By comparison, the giant magnetic fields in a typical MRI machine are 1.5 T. A typical refrigerator magnet is more like 0.01 T.
The technology transfer from collider physics to medicine alone has accomplished enormous amounts for diagnosing and treating cancer. Many hospitals and medical facilities have their own accelerators now to create radioisotopes for tracing. Where do you think they learned how to build, run and maintain those things?
And the superconducting material technology has applications far beyond just magnets. For example, superconducting coils can serve as industrial, factory-scale surge protectors.
Hardware aside, one of the biggest challenge of building detectors for the LHC was data collection. There was so much data coming in at such high velocity that it literally can not be written to disk fast enough. Between that data pipeline and the LSST, it’s pretty clear these large experiments were on the bleeding edge of the technologies that touch virtually every aspect of modern life.
Institutional Memory is Valuable
We have generations of physicists and an enormous collective experience in building these projects. The creative supply chain runs deep. And the applications to modern technology cannot be overstated. Business exist specifically to find new purposes for these technology. Ending our run at collider experiments now would destroy that intellectual and industrial ecosystem.
In short, if we don’t keep building colliders, we would “forget” how to build them. We’d lose our institutional memory, not to mention the massive blow to diplomacy that such an international collaboration can build5.
For a localized example, consider the Superconducting Super Collider (SSC). The SSC which was slated to finish running a decade before the LHC even turned on. It was expensive, sure, but since that project was canceled, the center of innovation in Collider Physics has moved the US to Europe. As we mentioned above, those downstream benefits to the economy of those kinds of expenditures suggest that such a price tag should have been rephrased as an investment. An investment we didn’t make6.
Compared to the 14 TeV maximum design energy of the LHC, the SSC would have topped out at 20 TeV. We would have found the Higgs - in Texas - a decade earlier.
Even more Scientific Research comes with it
And finally, particle physics has an excellent track record of delivering new science. Armchair laments about the failure of Weak Scale Supersymmtery aren’t meaningful policy conversations.
Scientific Progress is and should be measured by what we actually learn, not by the difference between what we expect to find and what we did.
Using this more appropriate metric we have learned an enormous amount from the LHC, and can expect to learn even more from the next big collider.
The aftermarket value of these colliders to other kinds of science might also surprise you. The Stanford Linear Collider at SLAC was repurposed as the world’s first hard X-ray laser, affording new insight into how molecules - some really big ones - are structured and move in real time.
So yes. There is every reason to continuing building particle colliders. To continue searching. There is still so much to learn.
So what’s next?
Hopefully many things. In particular, we should build a muon collider.
Next time, we’ll dig into the details of what a muon collider, their challenges and how they marry the best parts of hadron and lepton colliders.
We will discuss why this should, in detail, in our next post.
Not that SUSY phenomenology was particularly radical, it was a slight extension of the model to include a large number of other particles. Otherwise the model still pretty much worked as usual. Arguably the only ad-hoc assumption was R-parity. Of course, it wasn’t guaranteed to be a unique extension by any means.
One particularly exciting example is the magnetic moment of the muon.
Of course, it could just be sheer coincidence, although past experience has shown us to be suspicious of such coincidences.
The LHC currently has four large detectors , each with a huge international staff of scientists and engineers. Having two, competing general purpose experiments (ATLAS and CMS) not only fosters creativity, it also helps promote good Science through cross checking facts across difference hardware, software and analysis.
One of the main internal arguments against the SSC was for a reallocation of funds to other areas of physics. Weinberg argued passionately against this “scarcity” logic. While this might seem appropriate for dealing with the modern Federal government, it’s hard not to look at the public excitement over everything NASA does and not see abundance in the long term “yes, and…” approach to funding big Science.