Force carrier particles

The standard model includes three types of forces acting among particles: strong, weak and electromagnetic. Gravity is not yet part of the framework.
Forces are communicated between particles by the exchange of special "force-carrying particles" called bosons, which carry discrete amounts of energy from one particle to another. Each force has its own characteristic bosons: the gluon (strong force), the photon (electromagnetic force), the W and Z bosons (weak force).
A big success of the Standard Model is the unification of the electromagnetic and the weak forces into the so-called electroweak force. The achievement is comparable to the unification of the electric and the magnetic forces into a single electromagnetic theory by J.C. Maxwell in the 19th century.
Nowadays, physicists are also trying to include the strong force in a unified scheme called the Grand Unified Theory (GUT). They even contemplate the possibility of including gravity, thus unifying all the forces of Nature in a single "super force". However, much experimental and theoretical work is needed before such a goal is achieved
 

Why do physicists want to study particles?
Particle Physics Today: the Standard Model


The theories and discoveries of thousands of physicists over the past century have created a remarkable picture of the fundamental structure of matter: the Standard Model of Particles and Forces.
The model requires 12 matter particles and 4 force carrier particles to summarize all that we currently know about the most fundamental constituents of matter and their interactions.
The Standard Model is by now a well-tested physics theory, used to explain and exactly predict a vast variety of phenomena. High-precision experiments have repeatedly verified subtle predicted effects.
Nevertheless, physicists know that it can't be the end of the story, and that's way they are searching for 'new physics beyond the Standard Model', that will lead them to a complete "theory of everything".
Matter particles
There are two matter particles "families" - the quarks and the leptons - both point-like and without internal structure.
There are six quarks, which are usually grouped in three pairs because of their mass and charge proprieties: up/down, charm/strange, and top/bottom.
There are then six leptons, three with a charge and a mass - electron (e-), muon (µ) and tau (t) - and three neutral and with very little mass - electron-neutrino (?e), muon-neutrino (?µ) and tau-neutrino (?t). Again, as their name openly implies, they are grouped to form three pairs (because of some distinctive behaviour during the creation or decay processes).
The (e-/?e) and (up/down) have the lightest mass and are all that is needed to build up the stable matter in the Universe. They make up what is called the first generation of matter.
However, they are not all that was needed to build up the Universe; high energy processes produce a large variety of short-lived particles which require the existence of "heavier" pairs, or heavier "generations" of matter. We have then (µ/?µ) and (charm/strange) which make up the second generation, while (t/?t) and (top/bottom) the third generation.
Recent results from the LEP collider at CERN and from astrophysics confirm that there can be no more generations of this type. All second and third generation particles are unstable and quickly decay into stable first generation particles. That's why first generation particles are the only ones we observe in our everyday world.


What's the origin of the mass of particles?
Particles have a wide range of masses.
Photons and gluons are completely massless, while the W and Z particles each weigh as much as 80 to 90 protons or as much as a reasonably sized nucleus. The most massive fundamental particle found so far, the top quark, is twice as heavy as the W and Z particles, and weighs about the same as a nucleus of gold!
Why there is such a range of masses is one of the remaining puzzles of particle physics. Indeed, how particles get a mass at all is not yet properly understood.
In the Standard Model, particles gain a mass through the Higgs mechanism (named after theorist Peter Higgs). According to this theory, both matter particles and force carriers interact with a new particle, the Higgs boson. It is the strength of this interaction that gives rise to what we call mass: the stronger the interaction, the greater the mass.
Experiments have yet to show whether this theory is correct. The search for the Higgs boson has already begun at the LEP collider at CERN, and this work will continue into the 21st century with CERN's next machine, the Large Hadron Collider (LHC).
Can the electroweak and the strong forces be unified?
One of the major breakthroughs in particle physics in the 1970s was the merging of electricity, magnetism, light and radioactivity - the development of a unified description of the electromagnetic and weak forces. Now theorists are attempting a broader grand unification, which will also include the strong force.
Experiments show that the strong force becomes "weaker" in its effects as energies increase. This suggests that at very high energies, the strengths of the electromagnetic, weak and strong force are the same, and the forces are basically indistinguishable.
Unfortunately, the energies involved are a thousand million times greater than particle accelerators can reach; they would have existed only in the very early Universe, 10-34 seconds after the Big Bang.
But grand unified theories also have consequences at lower energies and can thus be tested with present day experiments. They require, for instance, a deep symmetry in the laws of nature, which in turn require the existence of special "superparticles". Some of these could be seen at the LHC.
What is "Dark matter" made of?
Measurements in astronomy imply that up to 90% or more of the Universe is not visible (i.e. does not emit electromagnetic radiation) . Scientists call this undetectable "stuff" dark matter.
Its presence is felt through the gravitational effects on the matter we can see. Stars in galaxies, for example, appear to be moving much faster than they would if they were influenced only by the visible matter in the galaxy.
The nature of dark matter and its role in the evolution of the universe are still unknown.
Probably it is made of several components, among which are neutrinos, dust, cold gas, and special particles predicted by the grand unification theories but not yet seen, the so called "superparticles".
Physicists hope to identify some of the elementary constituents of dark matter at the LHC.
Why are there three generations of matter?
All matter around us is built from only two types of quarks: "up" and "down", which form neutrons and protons. It also requires two types of leptons: the electron and the electron-neutrino. However, this pattern repeats itself in two heavier "generations", each with two quarks and two leptons.
Why are there three generations and why is it that the only one that forms our world is not enough? We do not know.
This puzzle though is linked to another, which is also part of the mysteries of the Universe: where did the antimatter go? Experiments in particle physics show that matter and antimatter are always created in equal quantities, indicating that this should also have happened at the Big Bang. If so, why did the antimatter not completely annihilate the matter, leaving only energy (photons) in the Universe?
It seems instead that there was some small but significant asymmetry between matter and antimatter in our early universe. This asymmetry could come from an effect called CP-violation. Present understanding of this effect is inextricably tied up with the existence of three generations.
So far, CP-violation has been seen affecting particles that contain quarks of the second-generation (strange), but the LHC should readily produce particles containing the heavier, third-generation "bottom" quark. If the theory is right, such particles should also reveal the symmetry breaking effect of CP violation.



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