The Future Of Particle Accelerators, Beyond The Standard Model

Beginning in the 1930’s, physicists have used particle accelerators to smash elementary particles together, helping them explore the building blocks of the universe. Since then, they have gotten far larger and much more powerful, allowing them to delve even deeper into the basic particles and the forces that govern them, all of which have been organized into the Standard Model of particle physics. However, after the Large Hadron Collider (LHC) famously found evidence for the Higgs boson in July 2012, many have wondered what the next step is. Some have even suggested that particle accelerators no longer have a purpose worthy of the billions of dollars it takes to build and maintain them.

Fortunately, some physicists have come to their defense and, in fact, believe there’s a lot left for them to accomplish. In particular, they have put forth the idea of a new fundamental force and particle, possibly opening up a new and exciting field of physics. Other physicists have pointed to unexplained phenomenon such as matter/anti-matter asymmetry, neutrino oscillations, dark energy, dark matter, etc. as new possible avenues for them to explore. Therefore, the larger, next generation of particle accelerators are being planned, with some of them already under construction. Also exciting is that some physicists are creating accelerators that are much smaller, making them more accessible to researchers and opening up new applications for them.

The Standard Model Of Particle Physics And Beyond

The Standard Model organizes the known elementary particles, much like the periodic table does for the elements. These particles are categorized into fermions and bosons. Fermions include quarks, which come in six “flavors” and combine to form hadrons, such as neutrons and protons. Fermions also include electrons and neutrinos.

Bosons, on the other hand, carry forces. That is, photons are responsible for electromagetic interactions. W and Z bosons are responsible for the weak interaction, which is the force behind radioactive decay. The eight types of gluons are responsible for the strong interaction, which binds quarks and holds neutrons and protons together. The gravitational force is carried by the graviton, although this has yet to be experimentally confirmed. These are considered to be the four fundamental forces, as they are thought to explain all known interactions. However, some physicists believe a fifth force is needed to explain dark matter and/or dark energy. Bosons also include the Higgs boson, which is responsible for giving all of the particles mass.

Particle accelerators have played an integral role in providing evidence for the Standard Model. By slamming particles together at higher and higher speeds, physicists can reach high enough energy levels to detect different types of particles. For example, to detect the Higgs boson, which has a life span of only 10^-22 second, physicists at CERN had to increase the energy to 8 TeV, a record level. This was accomplished by using powerful electric fields to accelerate and powerful magnetic fields to guide protons around a 27 km track, colliding them at nearly the speed of light.

Finding evidence for new particles like the graviton or probing the nature of unexplained phenomenon would require not just larger and more powerful accelerators but new types. However, the cost of these would dwarf the $5 billion or so it took to develop and build the LHC. Despite this, plans for these are in the works, along with plans for smaller and more accessible designs, including ones that can fit on a microchip.

Mini-Particle Accelerators

Physicists at Stanford University and SLAC National Accelerator Laboratory recently announced they created an accelerator that can fit on a microchip. By using lasers, they were able to accelerate electrons down a nano-path carved into silicon. Although, the speeds possible in this setup are nowhere near those in a large-scale accelerator, the authors of the paper published in the journal Nature believe this technology opens the door for research into numerous fields.

The authors claim their micro-accelerator can “perform cutting-edge experiments in chemistry, materials science and biological discovery that don’t require the power of a massive accelerator.” The idea is that these micro-accelerators are much cheaper and easier to build, allowing them to be used by researchers that do not have access to large accelerators like the LHC.

For example, the authors believe this technology could be used in more precise radiation treatment for cancerous tumors. X-ray machines are large and difficult to focus, meaning there is inevitable damage to surrounding tissue. However, a mini-accelerator can target the tumors while minimizing damage to healthy tissue.

Another type is plasma wakefield accelerators, which are designed to be only a few meters. As the name suggests, these types work by creating a wake in plasma. Plasma is the fourth state of matter and is created by stripping atoms of their electrons usually with a laser. With either a beam of particles or a laser, physicists can create a wake in a plasma, much like a boat going across a body of water. Particles in this wake are accelerated at far greater speeds than other designs as they provide a powerful acceleration gradient. Physicists from the UK claim “The acceleration gradients generated in a laser- or beam-driven plasma wakefield accelerator are typically three orders of magnitude greater than those produced by a conventional accelerator.”

However, this gradient is unstable and difficult to harness, meaning it will be a few years before they become operational, although many are being planned.

Bigger Particle Accelerators

Proposed in 2012, the Circular Electron Positron Collider (CEPC) will have a circumference of 80 km, more than twice the size of the LHC. As the name suggests, the CEPC will collide electrons with its antimatter equivalent, positrons. This enables physicists to create millions of Higgs bosons, which can be measured with far higher precision. Other goals for this accelerator include measuring the decay of Z bosons and exploring supersymmetry. Construction begins 2021, with the first experiments starting in 2030.

The Compact Linear Collider is an update to the LHC. It will use electron-positron collisions and operate in 3 stages, each at different energy levels: 380 GeV, 1.5 TeV and 3 TeV. When it is operational in 2035, physicists will be able to make more precise measurements of how the Higgs boson interacts with itself and other particles, such as possible couplings with fermions and bosons. Physicists will also be able to measure specific aspects of the top quark, such as how it interacts with the Z boson and photons and how it changes its “flavor” through decay. It is also thought that it might detect new particles such as those predicted by supersymmetry like the charginos, neutralinos, and sleptons.

The High-Luminosity Large Hadron Collider is another update to the LHC that was started in 2018 and will be finished in 2027. This update allows physicists to increase the luminosity of the LHC by a factor of 10 by reducing the size of the particle beam at the point of collision and by tweaking the size of the group of particles. Physicists will be able to collect far more data than before and increase the amount of Higgs bosons created to 15 million per year, as opposed to the 3 million created in 2017.

Autonomous Accelerators

Furthermore, some physicists are trying to implement algorithms to allow particle accelerators to run themselves.

For example, Dan Ratner at the SLAC National Accelerator Laboratory has put troubleshooting into the hands of machine learning algorithms, freeing up hundreds of man-hours. When a magnet, for example, goes down the system can be calibrated to compensate, although this is overwhelmingly complex for a human to do, due to the many interconnected parts. These specialized algorithms can handle a problem like this in a fraction of the time, without human input.

These types of algorithms can also predict problems before they begin, such as predicting the changes caused by movements of the Earth, which can be seriously disruptive to such sensitive equipment. Jean-Paul Carneiro at the Fermi National Accelerator Laboratory in Illinois is diligently working implementing these algorithms and he believes it will drastically reduce the accelerator’s downtime.

Researchers are also creating systems that automatically run simulations to help optimize particle beam placement. This was used at the Fermilab Accelerator Science and Technology facility, where it completed a task in less than a millisecond that would have taken over 20 minutes if done by a person.

Is It Worth The Money?

With any big scientific project, questions abound about its usefulness as well as concerns over taking already limited money away from other areas. These questions are certainly being asked about the above projects, just as they were asked about the LHC, the thousands of other particle accelerators, the Human Genome Project, the Very Large Array, the International Space Station, Spallation Neutron Source, among many, many other large collaborations. What opponents don’t realize is that these projects are not only trying to accomplish their stated purpose, but they are doing untold amounts of basic research.

Basic research is essential for all of the sciences, as well as virtually every industry, as it provides the theoretical framework on which applied science is done.

Dr. George Smoot from the Lawrence Berkeley National Laboratory explained it best when he said “People cannot foresee the future well enough to predict what’s going to develop from basic research. If we only did applied research, we would still be making better spears.”

Therefore, the next generation of particle accelerators is essential for pushing all fields of science forward, not just physics, which inevitably trickles into industry and the daily lives of average people.

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