China's Proposed Particle Accelerator Will Be Double the Length of the LHC

China's proposed "Higgs factory" will have more in common with freeway beltways than anything else in scientific research. With 52 kilometers of tunnel, the yet unnamed and yet unfunded collider could fit every single one of its active particle-smashing kin inside of itself quite easily, including China's largest physics tool operating today, the Beijing Electron Positron Collider (BEPC).

The proposal, put forth by researchers at Bejing's Institute of High Energy Physics, was first reported this week in Physics World. If it secures government approval, the new collider would be ready to begin operation in 2028 with the aim of achieving an electron-positron collision energy of 250 GeV. For reference, the Large Electron–Positron Collider, the predecessor to the proton-smashing Large Hadron Collider, achieved energies up around 107 GeV before shutting down in 2001.

The current world leader in electron-positron collisions is the BEPC itself, one of only two currently active colliders dealing in electrons. It achieves a mere 3.7 GeV over a length of just 240 meters. Some research doesn't take that much power or space, so it's not like there's anything wrong with the BEPC. In fact, over the past few years there's been a marked migration of American scientists to Beijing as stateside electron/positron experiment facilities have shuttered, in particular Cornell's CLEO-c detector, which closed down in 2008 after almost 30 years in operation. Meanwhile, the Russian electron-positron collider project VEP 2000 has been collecting data on low-energy hadrons, which give us information about the strong nuclear force, one of the four fundamental forces and the thing that keeps matter from falling apart.

The new collider's object of study is the Higgs boson, which can be produced in a number of different ways by smashing different combinations of particles. In a sense, energy is energy, and energy is how we can even describe particles. In this realm of very small physics, as energy and mass become interchangeable, we start talking about things in terms of electronvolts rather than kilograms (or very tiny fractions of kilograms, that is).

As the coupling of the Higgs boson to other particles depends on those particles' masses, proton collisions like those produced in the Large Hadron Collider become particularly helpful. (Proton masses are around 1,700 times larger than those of electrons.)

Electron-positron collisions are different though. Proton-proton collisions depend on the subsequent recombination of pairs of the force-carrying particles that hold protons and neutrons together, called gluons, into "loops" of virtual particles (particles that freely pop into and out of existence, thanks to quantum uncertainty), which then decay into Higgs bosons. Because of nature's statistical preference for disorder, those Higgs bosons then decay themselves into pairs of photons or pairs of quarks or pairs of W or Z particles (always pairs).

Proton collisions are handy because of the higher energies (masses) possible, but these sorts of collisions are also very messy—imagine shattering a bowling ball versus shattering a marble. In an electron-positron collisions, we're dealing with the latter, trading cleaner collision results for lower energy collisions. Lower energies, however, don't exclude the possibility of a Higgs detection. They do make it less likely, though. The Large Electron–Positron Collider (LEP) in its last days managed a Higgs boson detection with 91 percent accuracy, leading to a several month extension of the experiment before it was finally shut down to allow construction of the Large Hadron Collider to commence.

"Protons are very complicated objects, a 'bag' containing quarks and gluons, so when they collide it is rather like colliding two oranges together," Brian Foster, a physicist at Oxford University and the European regional director for the planned International Linear Collider, told me. "You want to see the collisions of the pips but everything is masked by juice and pulp from the complicated oranges. So it is very difficult to do precision measurements with proton-proton collisions because all the extra particles that are produced can mask subtle effects."

"Electrons by contrast are simple point-like particles," Foster said. "In the orange analogy like pips on their own with no accompanying rind, juice etc. They are therefore much simpler to interpret and complicated and subtle effects masked by proton-proton collisions can be clearly seen in electron-positron collisions. This is particularly true for the Higgs, where to understand its nature we need to make studies at very high precision."

So, the appeal of a superpowerful electron-positron collider like the one being proposed in Beijing should be clear. This is how we arrive at our "Higgs factory," a continuous light-speed assembly line of particle annihilation: with the high energies of the Large Hadron Collider, coupled with the clean results of lepton (electrons, positrons) collisions.

Such an accelerator would make for something of a partner to the International Linear Collider, the proposed successor (in spirit) to the LHC, which would initially achieve collision energies near 500 GeV and, later, 1,000 Gev, easily dwarfing not only the proposed Beijing collider but anything yet built or even imagined.

"The Chinese proposal is limited to energies which are just enough to produce the Higgs particle," Foster said. "Although many interesting things can be done here, in order to fully understand the Higgs particle and also access other interesting processes such as top-quark production, higher energies are required, up to 350 GeV as a minimum."

The Beijing collider, while being less powerful, would be circular as opposed to the ILC's straight-arrow. The advantage of a circular collider is that it's possible to get a more continuous stream of events and, thus, a great deal more data. Meanwhile, linear colliders fire their particle bunches down a long, straight tube and once a load of particles is unleashed, that's it. The straight-arrow approach nonetheless has its advantages.

"The great advantage of linear colliders is that it is easy to make them longer and therefore reach higher and higher energy by adding more and more accelerating structures," Foster explained. "Once you build a circular collider, you are stuck. You can't make a circle any longer, and hence reach higher energies, with our starting again from scratch."

It's fair to wonder why researchers want to double down on a particle whose existence has already been validated to the tune of $9 billion (for the LHC alone). There are other vexing mysteries after all—like dark matter, the universe's missing antimatter, and-or the cosmic photon underproduction crisis. Why then a Higgs factory?

Despite "god particle" overhyping, the Higgs boson is in some ways undersold as a square on the physics bingo card of the Standard Model. It's the thing needed to make all of the other stuff in that model—the most fundamental bits and pieces of matter and forces—work right. But this perspective points at some completeness or potential completeness that just isn't there. The Standard Model itself is a placeholder or square on the bingo card that is a full and complete description of reality, and the Higgs boson is just one potential way to access the next square just by virtue of contradicting or confirming other emerging physics.

For example, there's the apparent contradiction between Higgs boson physics and, well, existence. The Higgs field as observed would should have led to universal collapse quickly after the Big Bang, rather than the cool, structured cosmos we see. This is one of many avenues leading toward the realm of new physics or post-Standard Model physics, which is in a way a much more exciting realm, where physics pushes beyond confirmation and into relatively uncharted waters. The Higgs fits our theories by not fitting, in a sense.

"It is a particle that is not related to any other particles we know about…," Michael Peskin, a professor of physics at the SLAC National Accelerator Laboratory, told Symmetry Breaking a few weeks ago. "The mass of the Higgs boson tells us something, but theorists are having a big debate about what it tells us … new discoveries are coming. Knowing the Higgs exists is an important milestone, but now we need to move to the next step."

So a Higgs factory is one way of getting to that next step, by further defining and describing the particle that allows "things" to exist in the very first place. In the process, physicists can further refine our notions of new, deeper physics—where it is and isn't, and just how big future experiments will have to grow in order to get there.