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Simulation of a detection of the Higgs boson in the CMS experiment

The minuscule challenge

Journey into the microworld

The infinitesimal scale of particle physics is mind-blowing and rather abstract to imagine. If we enlarge an atom to the size of the Earth, then the protons and neutrons that make up the nucleus of the atom would each measure the length of an Olympic stadium. Smaller still are the quarks. If we consider our hypothetical atom blown up to the size of the Earth, then a quark would be smaller than a tennis ball.

However, this does not give us a very good idea of the size of the atom itself. So staying with the same analogy, but scaling things in the opposite direction, if the atom was the size of the Earth, then an amoeba would be as big as our solar system. Going even further, the distance from the centre of Geneva to CERN (about 10 km) would stretch across the entire Milky Way galaxy.

So no matter how hard physicists squint, they'll never see a quark with the naked eye. The key to studying the subatomic world is to use accelerators to boost the energy of particles before a collision, then make the results indirectly visible using detectors. But to appreciate why, we need to understand the Es and the Ms...

Swapping Es and Ms

Imagine if you threw a ball at a target without knowing what type of ball it was, but wanted to find out. Given some information on the target that was hit, the speed with which the ball bounced back, and the track it made when doing so, you might be able to deduce, for example, whether the ball was likely to be a small rubber ball, or a heavy cannon ball. Using the same principle, a particle accelerator 'throws balls' (particles) with great speed and energy, while detectors provide information on the characteristics of the collisions. Physicists analyse the results of many collisions to gain an insight into the nature of the particles being studied, despite not being able to observe them directly.

In practice, this is not an easy task and sometimes the apparatus can be huge and complex. To add to the challenge, in the rather bizarre world of particles, give the 'balls' enough of an energy boost and they may turn into something else entirely. That could be like shooting a ping-pong ball at a target to find it transforming into a truck-load of watermelons and a handful of beads! This phenomenon is described by Einstein's famous equation E=mc2, which says that matter is a very concentrated form of energy and the two are interchangeable.

Sensing the subatomic is a job for giants

The more energy you put into the particles, the more ‘stuff’ (mass) can be created in a collision. Particle physicists are interested in studying the particles created in high-energy collisions, because some rare and unusual ones may appear for a fleeting moment.

Something that may appear strange about particle physics experiments is the paradox between the sizes of the equipment and the subjects being investigated. Why does it take the largest machines to investigate something so unimaginably tiny?

To study particles that have never been investigated before often means building larger accelerators to reach higher energies. The history of particle physics may be traced back through a succession of accelerators and detectors of increasing sizes.

In addition to large accelerators, CERN also builds giant detectors to analyse the results of the experiments. General-purpose detectors, such as ATLAS and CMS, are designed to detect as wide a range of particles as possible. Their huge sizes are dictated by the need to contain all the energetic particles produced in the collisions, and by the methods of identifying them.

How small is infinitesimally small?

Particles are so minute that if we were to write out their sizes, the numbers would require many zeros after the decimal place. So how do physicists determine the sizes of particles?

In fact, the accelerators and detectors are used as ‘subatomic rulers’. Our eyes can see objects because of visible light reflecting off their surfaces. The reason that we cannot see particles with the naked eye is because their sizes are smaller than one ‘unit’ (wavelength) of visible light. Early in the 20th century, it was discovered that moving particles of matter can also be considered as waves. More interestingly, the wavelengths of these moving particles become shorter with increasing energy. This means that to study details a billion times smaller, we need to give particles energies a billion times higher. This forms the basic principle of how an accelerator can be used to measure the subatomic world.