Tom Wellton
январь 2017.
1130

# Why do we need just bigger and bigger particle accelerators?

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Particle physics is the study of the building blocks of nature, answering fundamental questions about the pieces that the world is made from and how they interact. For forty years the answers have been described by a set of mathematics known as the ‘Standard Model’. This theory contains a set of 12 fundamental particles, the quarks and leptons, and describes three forces: electromagnetism, and the weak and strong nuclear forces. It also predicted the existence of the Higgs boson, which was discovered at CERN’s LHC in 2012. But it leaves many open questions.

In the development of the Standard Model, many such questions were answered using Particle colliders. They allow us to do controlled experiments, colliding particles of known type and energy at a fixed place, where we build sophisticated detectors to observe the results. The energy in the collision can make new particles, where the famous equation E=mc2 tells us how much energy is required to produce a given mass. But a collider with a given energy has an upper limit on the mass of particles it can produce. We cannot study particles that are more massive than our highest energy accelerator can produce.

• The next generation particle accelerator might be built in Japan

In a circular proton collider, like the Large Hadron Collider (LHC), the most difficult challenge is making the particles bend. They are travelling at 0.99999998 of the speed of light, but have to reverse direction thousands of times a second. They are bent with the strongest magnets we can build. But if the energy of the particle beams is to be doubled we need twice as much bending power – and this means more magnets (or a breakthrough in building even stronger ones) So there is always an unknown frontier: perhaps a bigger, higher energy accelerator might allow us to study previously unknown particles? Historically, every time we have been able to increase the energy by a factor 10 in our colliders, we have discovered something new.

Take one example of an open question: Dark matter. Several observations of the Universe tell us most of the mass is some invisible unknown. We do not know what it is, but a very popular theory, called supersymmetry, predicts the existence of a particle with just the right properties. If this particle has a mass of the order of ten times more than the Higgs boson then the number in the Universe today is just about right to explain the dark matter observations. So building a collider which can look a factor ten beyond the Higgs boson has a real chance of explaining dark matter.

• Could the next generation particle accelerator explain dark matter?

There is another reason too. The Higgs boson is a particle of unique properties. It is the only fundamental particle which does not spin, which makes the equations describing its mass very odd. In the Standard Model, if there is some maximum energy to which the theory works, that energy is added to the Higgs mass. If there is no maximum the addition would be infinite! A natural conclusion is that the Standard Model stops being a complete description at about the Higgs mass we measure. For example, a new particle (e.g from super-symmetry), weighing maybe a few times more than the Higgs boson, could fix the problem.

Whether these hints are true or not we can only know by building a bigger collider and going to look.

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