Stanford physicists propose a Higgs boson Party in the USA
This new linear accelerator may lead to new physics
This new linear accelerator may lead to new physics
Recently a proposal dropped on the physics pre-print repository arxiv.org announcing a bold new American supercollider. Authors from the SLAC National Accelerator Lab, Los Alamos, UCLA, and Fermi National Accelerator collaborated to put together a unique design that could extend the reach of the current world champion, Large Hadron Collider, in Switzerland and get the United States back in the game of pushing the boundaries of high energy physics.
The proposed accelerator, which relies on new conductive technologies to avoid expensive, ultra-cold superconducting magnets as in the LHC, could provide a way for high energy physicists to break through the stagnation that the field currently feels.
While a proposed $23 billion, 100 km Future Circular Collider (FCC) program at CERN, the home of the LHC, is still being debated, particle physicists are forced to rely on existing accelerators for their research.
The new collider, once funded, wouldn’t be operational for about 30 years meaning that many mid-career scientists would never be able to use it for their research, meaning further stagnation.
This is far from the status quo.
For the past 30 years, high energy particle physics has seen a boom in experimental confirmations of theoretical predictions. These include confirmation of W and Z bosons that mediate the weak force. The top quark and Higgs boson have been detected and shown to fit theoretical predictions. Flavor mixing and parity and Charge-Parity violations have been validated. In addition, the LHC has enabled observations of strong and weak interactions that match theoretical predictions in very subtle ways.
Thus, the Standard Model has gone from weakly to strongly established. Coincidentally, however, more and more physicists are persuaded that the model is incomplete.
While astrophysical problems like dark matter and dark energy may have particle explanations outside the model, more basic and fundamental questions are even more pressing:
Why do coupling constants, which mediate the interactions between particles, take on the values they have (e.g., the fine structure constant at about 1/137)
Why do quarks and leptons (like electrons) have such different masses?
Why does the Higgs field have a non-zero value in space, leading other elementary particles to have mass?
The authors argue that particle physics is a victim of its own success in that its reigning theory is “done” and there is little experimental evidence to indicate a new direction.
Such a state has not been seen since the late 19th century prior to the discover of quantum physics when electromagetism and the classical theory of the atom were the crowning glory of physics.
In order to push the boundaries of known physics, new detectors are needed, but the idea that an ambitious, 100 km, $23 billion, 30 year supercollider project is the only hope is a sad one indeed. Something has to fill the gap.
The main goal the authors have is to construct an electron-positron beam-based Higgs factory, a linear accelerator specifically built to produce Higgs bosons by firing electron and positron beams at one another. (A similar accelerator is proposed in the EU called CLIC.)
The Higgs, which gives all other particles mass, is the most mysterious particle in the Standard Model pantheon. Measuring its behavior and nailing down its properties to better than 1% of theoretical values would be of enormous benefit.
In order to achieve this and other dreams of particle physics, we need a collider capable of producing multiple TeV (trillions of electronvolts) of energy.
An electronvolt, a common unit of energy in accelerators, is the energy gained by accelerating a single electron through one volt of potential energy. It is a tiny unit of energy but concentrated in a small enough area, a TeV can produce the Higgs boson from the background Higgs field sufficiently for it to be studied. Higher TeV can produce multiple Higgs — so-called double and triple Higgs energies.
Right now the LHC, which is a proton synchrotron meaning it accelerates protons in a circle, has achieved 1.18 TeV per beam. (It has a total capacity of 14 TeV with multiple beams.) This means that it can just barely achieve the energy necessary to produce the Higgs.
Electron-positron colliders, by contrast, are much cleaner than proton colliders and enable more precise measurements of particle interactions in some cases. They are more precise but can also challenge the energy frontier if they are sufficiently powerful.
The next goal would be to reach 10 TeV which is the parton-parton center of mass energy. (A parton is a quark or gluon. Parton was the name proposed for the particles making up protons in the 1960s.) But the LHC cannot reach these energies in a single beam.
Meanwhile, the tunnel alone for the FCC costs as much as the LHC. Imagine spending billions to create a tunnel, with no new physics in sight?
The answer is what is called a Cold Copper Collider or C3. This new technology relies on normal as opposed to superconducting radio frequency accelerators that operate on short pulses at about 5.7 GHz. Short pulses with long rest times (called low-duty cycle in RF parlance) mean greatly simplified cooling requirements, which is a big cost driver in accelerator technology. These operate most efficiently at about 77 Kelvin or -196 Celcius which is about the boiling point of liquid nitrogen.
This technology can reach very high accelerating gradients using a new distributed coupling accelerator, which is a machined copper manifold with an optimized shape where power is distributed to individual cells rather than the manifold as a whole. This enables minute control over the electric and magnetic fields.
Each of these cavities allows charged particles to be accelerated through an iris aperture using a carefully controlled magnetic/electric field.
The cavities are machined out of copper slabs using computer numerical control machines and do not require additional machining, which makes them relatively cheap to produce.
The authors describe a 250 GeV and a 550 GeV linear (as opposed to circular) design. Linear accelerators must accelerate particles to their final energy by the end of the accelerator, unlike circular ones which can continue to accelerate them around and around until they reach a maximal centripetal force. The goal would be to enable 70 MeV per meter of acceleration for the 250 GeV one and 120 MeV per meter for the 550. For either they would need a 4 km tunnel at a cost of $4 billion for the entire complex.
The nice thing about linear accelerators, unlike circular, is that you can make them longer by extending their tunnel.
The increased conductivity of copper at 77 K (as opposed to room temperature) also makes operating refrigeration plants a no-brainer, just from the energy savings.
The proposed design, however, is just a demonstration. For higher energies that would allow the creation of a Higgs factory they propose a 3 TeV version by simply extending the length. Cost per TeV actually is lower because of fixed costs. There are some changes such as power requirements. The length of the accelerator would be 13.5 km for each side and the total complex would be 33 km long. There would be two wings, an electron and a positron wing, that would meet back at a point near the injector.
On the face of it, the proposal seems to be less ambitious than it at first claims. It is more of a downpayment on a solution than a real solution since only the 250 GeV and 550 GeV are really detailed. The 3 TeV is feasible if large. The authors believe that the C3 technology can achieve 10 TeV as well but there isn’t a concrete proposal for such a linear accelerator.
The 3 TeV would be the minimum necessary to be a true Higgs factory and 10 TeV could lead to new physics. (There is a possibility of finding a Higgsino at 1.5 TeV.)
It would certainly be nice to see the United States getting back into the high energy frontiers business. After the 1992 Texas supercollider fiasco, the USA has had to rely on Europe for ultra-high energy collisions. While it isn’t necessarily a race or a contest, there are a lot of experiments that cannot happen because they can’t get time on an accelerator with sufficient energy to perform them. Whether such an idea gains traction will depend on the political winds.
Bai, M., et al. “C3: A ‘Cool’ Route to the Higgs Boson and Beyond.” arXiv preprint arXiv:2110.15800 [hep-ex] (2021).
De Blas, J., et al. “The CLIC potential for new physics.” arXiv preprint arXiv:1812.02093 (2018).