If you were to take any everyday object,
say a coffee cup, and break it in half, then in half again, and keep carrying on,
where would you end up? Could you keep on going forever? Or would you find a set of
indivisible building blocks out of which everything is made? Physicists have found the latter- that
matter is made of fundamental particles, the smallest things in the universe. Particles interact with each other according
to a theory called the “Standard Model”. The Standard Model is a remarkably
elegant encapsulation of the strange quantum world of
indivisible, infinitely small particles. It also covers the forces that govern
how particles move, interact, and bind together to give shape
to the world around us. So how does it work? Zooming in on the fragments of the cup, we see molecules, made of atoms
bound up together. A molecule is the smallest unit
of any chemical compound. An atom is the smallest unit of any
element in the periodic table. But the atom is not the
smallest unit of matter. Experiments found that each atom
has a tiny, dense nucleus, surrounded by a cloud
of even tinier electrons. The electron is, as far as we know, one of the fundamental, indivisible
building blocks of the universe. It was the first Standard Model
particle ever discovered. Electrons are bound to an atom’s
nucleus by electromagnetism. They attract each other by exchanging
particles called photons, which are quanta of light that carry
the electromagnetic force, one of the fundamental
forces of the Standard Model. The nucleus has more secrets to reveal,
as it contains protons and neutrons. Though once thought to be fundamental
particles on their own, in 1968 physicists found that protons and neutrons
are actually made of quarks, which are indivisible. A proton contains two “up” quarks
and one “down” quark. A neutron contains two down
quarks and one up. The nucleus is held together
by the strong force, another fundamental force
of the Standard Model. Just as photons carry
the electromagnetic force, particles called gluons
carry the strong force. Electrons, together with up
and down quarks, seem to be all we need to build atoms
and therefore describe normal matter. However, high energy experiments reveal
that there are actually six quarks– down & up, strange & charm,
and bottom & top - and they come in a wide range of masses. The same was found for electrons, which have heavier siblings
called the muon and the tau. Why are there three (and only three)
different versions of each of these particles? This remains a mystery. These heavy particles are only produced,
for very brief moments, in high energy collisions,
and are not seen in everyday life. This is because they decay very
quickly into the lighter particles. Such decays involve the exchange
of force-carrying particles, called the W and Z, which
– unlike the photon – have mass. They carry the weak force,
the final force of the Standard Model. This same force allows protons and
neutrons to transform into each other, a vital part of the fusion interactions
that drive the Sun. To observe the W and Z directly, we needed the high energy collisions
provided by particle accelerators. There’s another kind of Standard Model
particle, called neutrinos. These only interact with other particles
through the weak force. Trillions of neutrinos, many generated
by the sun, fly through us every second. Measurements of weak interactions found that
there are different kinds of neutrinos associated with the electron,
muon, and tau. All these particles also have
antimatter versions, which have the opposite charge
but are otherwise identical. Matter and antimatter particles are
produced in pairs in high-energy collisions, and they annihilate each other
when they meet. The final particle of the Standard Model
is the Higgs boson – a quantum ripple in the background
energy field of the universe. Interacting with this field is how all the
fundamental matter particles acquire mass, according to the Standard Model. The ATLAS Experiment on
the Large Hadron Collider is studying the Standard Model in-depth. By taking precise measurements of the particles
and forces that make up the universe, ATLAS physicists can look for
answers to mysteries not explained by the Standard Model. For example, how does gravity fit in? What is the real relationship between
force carriers and matter particles? How can we describe “Dark Matter”, which makes up most of the mass in the
universe but remains unaccounted for? While the Standard Model provides a beautiful
explanation for the world around us, there is still a universe’s worth of
mysteries left to explore.
