What kind of machine would you use to find new particle physics?
As discussed in the previous section there are currently a number of unanswered questions in particle physics. In general, for each question, a number of possible explanations have been proposed by theoretical physicists. Each theoretical explanation predicts new physical phenomena, which can be widely varying in many aspects including: their interaction with the particles of the SM (meaning it could be difficult to detect with apparatus made out of regular particles), and the energy scale at which they exist (meaning that we might not be able to generate energies large enough to produce the new phenomena). As experimental particle physicists, it is our task to determine which theoretical explanation is the one that actually describes nature. So how can we make something that can generate and observe such a wide range of different phenomena?
The first step is to actually produce the new phenomena, which is usually in the form of a new heavy particle. In particle physics this is performed by 'smashing' two lighter particles together to form one or more heavier particles. In this way, heavy particles can be created by converting the kinetic energy from the initial particles into mass energy of the final particles, as described by the famous equation E=mc². This is done in a particle accelerator, where beams of particles are accelerated up to large energies and then collided together. Since the aim is to probe the largest range of physics possible, it is highly advantageous to be able to produce new particles in a large mass range. Hadronic colliders (that collide protons and/or antiprotons) are the best at doing this for two reasons:
- Hadrons can be accelerated to much higher energies than other particles.
- Hadrons are not fundamental particles, they are made up of quarks and gluons, each of which carries a small variable fraction of the total energy. When the hadrons collide only a few of the quarks and gluons will interact, leading to a large range of effective collision energies.
These two properties are the reason that hadron colliders are chosen when searching for new physics.
The second step is to observe the phenomena, as it is all very well to produce something, but useless unless you are able to observe it. Observing new particles however, is not as simple as just clicking your camera at the right time. Apart from the fact that the particles are extremely tiny (<10-15m), since they are heavy they decay extremely quickly into other particles. As a result, the only way of observing them is by measuring the particles they decay into, and reconstructing the particles to determine what it was that they came from. To do this you need a detector system that is very good at not only measuring the energy and direction of particles, but furthermore distinguishing between the various possible types of decay particles. The specifics of detector design and functionality will be described in the section The ATLAS Experiment
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