Particle Physics: a brief review

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The goal of particle physics is to search for and examine the basic constituents of matter and to determine their fundamental interactions. The present knowledge is summarized by the Standard Model of particle physics, a field theory which consistently describes the known fundamental particles and their interactions at presently available energies of a few hundred GeV (1 GeV = 109electron volts). Only gravity needs still to be included. In recent years strong connections to cosmology have been developed, giving important constraints and hints for physics beyond the Standard Model (e.g. missing dark matter, baryon-antibaryon asymmetry). It is generally believed that the Standard Model is only a “low-energy” effective theory, which may have to be modified at TeV energies (1 TeV = 1012 electron volts). Various scenarios have been discussed how to modify the Standard Model in order to extend the validity to the Planck scale, where gravity cannot be neglected anymore. The experiments at the Large Hadron Collider (LHC) will provide tests of these ideas.

Magnification of the atom

In the current understanding of particle physics, the set of fundamental constituents of matter consists of six quarks and six leptons. The quarks are named up, down; charm, strange; top, bottom and the leptons consist of electrons, muons, taus and their corresponding neutrinos. They are all spin ½ fermions and are grouped in three families, each consisting of a quark pair and a lepton pair. It has been known for a long time that the quarks exhibit small quantum mechanical mixing among themselves, which allows weak decays between different quark families as well as time reversal-violating transitions. The masses, coupling constants and mixing angles are parameters of the Standard Model which have to be determined experimentally. Recently, neutrino oscillation experiments have obtained evidence, that also the neutrinos in the lepton sector have masses and mix among themselves. Next generation neutrino experiments aim at a measurement of the absolute neutrino masses, the mass hierarchy and will search for CP violation in neutrino interactions.

The quarks make up hadrons such as the protons, neutrons and pions and interact through the strong, weak and electromagnetic forces. The leptons interact only through the weak and, if charged, the electromagnetic force.

In the Standard Model, the strong, weak and electromagnetic interactions are mediated through the exchange of spin-one particles (vector bosons) in a way precisely prescribed by the gauge symmetry of the theory. Different vector bosons are responsible for the different types of interactions: Eight gluons for the strong interaction, described by quantum chromodynamics (QCD), W± and Z bosons for the weak interaction and photons for the electromagnetic interaction (quantum electrodynamics, QED). While the gluons and photons are found to be massless, the W and Z have large masses.

Leptons and quarks

In the Standard Model the fundamental particles are initially massless. The masses are then generated through interactions with hypothetical scalar fields, called Higgs fields, without violating the gauge symmetry. At least one of these Higgs fields should be visible as a massive scalar boson, called the Higgs particle (H). Very recently, a candidate for this Higgs particle with a mass of about 125 GeV was discovered at the LHC at CERN.

Processes involving weak and electromagnetic interactions can be calculated using perturbation theory. This allows precise predictions which can be compared with experiments. In particular, the impressive precision measurements at the electron-positron collider LEP have been matched by very careful evaluations of electro-weak radiative corrections to which particles contribute virtually, even if they cannot be produced at the actual energy of the experiment. In this way the mass of the top quark was predicted before it was found and bounds for the Higgs mass have been obtained. At high energies the strong interaction can also be treated perturbatively. However, the strong interaction has a special feature, the so-called confinement, which makes it impossible to create an isolated quark or gluon. What is observed as hadrons are bound states of quarks held together by gluons, and such states cannot be treated using perturbation theory. The only exact methods available in this non-perturbative regime at low energies are expansions relying on a particular symmetry of the theory (chiral perturbation theory) and numerical calculations in which the continuous space-time is replaced by a lattice. However, for many low energy phenomena the construction of phenomenological models is still an essential and useful tool.

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