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Particle physics (also known as high-energy physics) is a branch of physics that
studies the nature of the particles that form matter and radiation. Particle
physics investigates the smallest detectable particles and the fundamental
interactions necessary to explain their behaviour. It also looks for entirely
new particles and interactions.
Image: Wikimedia Commons: Palomca
It was thought at one time that atoms were the smallest fundamental particles. By "fundamental" we mean that they are not compsed of smaller
particles. We know now that
atoms have a tiny but dense, positive nucleus and a cloud of negative electrons (e-).
The nucleus consists of protons (p+), which are positively charged,
and neutrons (n), which have no charge. The protons and neutrons themselves are not fundamental particles.
They are composed of even smaller particles called quarks.
The theory called the Standard Model of Particle Physics (often just called the Standard Model) explains what the world is made up of and
what holds it together.
The fundamental parts of the model include:
* 6 quarks
* 6 leptons (the best-known lepton is the electron!)
* Force carrier particles corresponding to three of the four fundamental forces
* The Higgs boson (only just discovered in 2012)
All stable, known matter is made up of quarks and leptons. In fact, all of atomic matter is made up only of up and down quarks
(combinations of which make up protons and neutrons) and electrons.
Interactions between particles can be thought of in terms of exchanging force carrier particles. There are three fundamental forces in the Standard Model:
* Electromagnetism, combining electricity and magnetism, mediated by the photon
* Strong force, which holds together quarks and the nuclei, mediated by gluons
* Weak force, which govern particle decay, mediated by the W and Z boson
There is a fourth force, gravity, which is not included in the Standard Model. How gravity fits together with the other three forces is one of the fundamental questions in physics.
Quarks only exist in groups with other quarks and are never found alone.
The heaviest known fundamental particle is the top quark, which is as heavy as a gold nucleus.
Composite particles made of two or more quarks are called hadrons. Protons and neutrons are examples of hadrons.
The best-known lepton is the electron (e-). The other two charged leptons are the muon(μ)
and the tau(τ), which are charged like electrons but have more mass. The muon is heavier
than the electron and tau is heavier still.
The other leptons are the three types of neutrinos (ν).
They have no electrical charge, very little mass, and are not easily detected.
For every type of matter particle found, there also exists a corresponding **antimatter** particle, or **antiparticle**.
their masses are the same, but properties such as charge are opposite.
For example an electron is negatively charged whereas an anti-electron (called a positron) is positively charged.
When a matter particle meets its antiparticle they annihilate and convert into pure **ENERGY** in the form of photons.
Image: NASA, ESA, M. J. Jee and H. Ford et al. (Johns Hopkins Univ.)
The visible matter we are familiar with only makes up less than 5% of the universe.
The rest of the matter does not interact via the electromagnetic force. This means that this unknown matter does not absorb,
reflect or emit light, making it extremely hard to spot, and has therefore been given the name "dark matter".
Researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter.
Dark matter it and in of itself makes up only another 27% of the universe. Something called dark
energy is believed to make up the rest. So most of what makes up the universe is of an unknown nature and "dark".
One way particle physicist search for new particles and interactions is to smash other particles together
and to study what happens. This is an extremely simplified way to describe it, but it's not inaccurate.
A good analogy of how physicists study particles through colliding is the car crash example.
Imagine a person wanted to look inside cars. By crashing two cars together at very high speeds, we can break the
cars apart and see inside. In the same way, physicists crash two particles together in order to break them and study the inside.
Reality is even stranger, as what really could happen is that the pieces of the car combine to make entirely new things
that weren't even originally part of the car. It could be like crashing togther two cars head-on and producing a dinosaur!
It's best not to take this analogy too far though.
One way to explain this through the well-known equation E = mc2.
Mass is simply a form of energy. Matter can be converted into energy and vice-versa.
As stated before, if one combines an electron and a positron they annihilate and leave behind energy.
This energy can then be used to form new particles, but only with a combined mass up to the initial energy.
If one gives the initial particles more and more energy and collide them, then more and heavier particles
can be created from their collisions.
One way particle physicists look for dark matter is to search for entirely new particles, ones that
could possibly be dark matter.
The particles are given more and more energy in a process known as acceleration, in which the particles' speeds
are increased more and more until they can nearly (but never reach) the speed of light c.
At the points where the particles collide are detectors that record the "debris" from the collisions.
A detector (actually multiple detectors) is large complicated device that consist of layers of material that exploit the
different properties of particles to catch and measure the energy and momentum of each one. In general, they
can detect the stable particles left over from decays of unstable (and perhaps more interesting!) particles like W, Z, and Higgs bosons
that get created in the collisions.
The detector are like massive digital cameras that take snapshots of the collisions (which happen billions of times a second).
Each snapshot is called an event. Each collected event is analyzed and "reconstructed" to see what was produced in the event.
The most powerful accelerator ever built is the Large Hadron Collider (LHC) at CERN in Geneva,
accelerating protons and colliding them with a total energy of 13 TeV.
A few facts about the Large Hadron Collider:
* It is located 174 metres underground.
* It is 27 kilometres in circumference - so big it runs underneath the French-Swiss
border, near Geneva.
* It is filled with 2000 giant super-conducting magnets that are at 1.9 Kelvin. That's colder
than the space between the stars!
The Large Hadron Collider accelerates protons to nearly the speed of light, in clockwise and anti-clockwise directions,
and then collides them at four locations around its ring. At these points, detectors such as CMS detect and measure the
The "visible" particles from these collisions such as electrons, muons, and photons come from the decay of heavier unstable
particles such as W, Z, and Higgs bosons. Quarks are produced as well but quickly interact with other quarks to form "jets"
By measuring precisely as possible properties of these electrons, muon, photons, and jets we can reconstruct the properties
of the particles that produced them such as their mass. This includes measuring how much energy they have, their charge (where applicable),
where they were produced, and in which direction they went.
The protons in the collisions travel along the beam pipe (depicted in the image below; this is along the z axis of our coordinate system).
The detectors themselves form a "cylindrical onion"
of layers of detectors in order to detect all that comes out of the collision. One such layer (the electromagnetic calorimeter, which measures energy)
is shown in blue below.
Before each collision, the protons travel along the direction of the Large Hadron Collider beams, and not in
directions perpendicular to the beams (which are defined as the x and y directions). This means that their momenta in these perpendicular
directions – their "transverse momentum" – is zero. A fundamental principle of physics is that momentum is
conserved (constant) and so, after the collision, the sum of the transverse momenta of the products of the
collision should still be zero. Therefore, if we add up the transverse momenta of all the visible particles
produced in the collision and find it not to be zero, then this could be because we have missed the momentum
carried away by invisible particles.
Image: Wikimedia Commons: Maksim
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All waveform images: Wikimedia Commons: Omegatron
Image: Wikimedia Commons: Abdull
Image: Wikimedia Commons: Nicostella
Image: Wikimedia Commons: Nicostella
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Image: Wikimedia Commons: Niau33
Image: Flickr: smokeghost
Image: Flickr: pablosanz
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