Abstract
The Standard Model of particle physics is currently our best framework for the description of all the known elementary particles and all the fundamental forces of Nature except gravity. It is the second fundamental ingredient of the Standard Model of cosmology. Here matter is described by fermions, spin-1/2 particles. Fermions are grouped into two classes, namely leptons and quarks, on the basis of their interaction properties. Leptons participate only in electroweak interactions, while quarks possess both types of interactions, electroweak and strong. Forces are described by gauge theories and are mediated by gauge bosons, spin-1 particles. Lastly, there is a spin-0 particle, the Higgs boson, which plays a special role and provides a mass to the other particles. During the last 50 years, the theoretical predictions of the Standard Model of particle physics have been deeply investigated in particle collider experiments, and in some cases the level of agreement with the measurements is astonishing. However, we have also clear indications of new physics.
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Notes
- 1.
We have to distinguish the MSM and a minimally extended model, which includes Supersymmetry, possibly Grand Unification, and sometimes even something more, see below.
- 2.
Particles with spin in the same (opposite) direction as their momentum are called right-handed (left-handed). In the case of massless particles, this is independent of the reference system. For very light particles, this classification is approximately valid too.
- 3.
We note that the Y in \(U_Y(1)\) stands for hyper-charge. It is used to distinguish the U(1) symmetry above the electroweak symmetry breaking from the U(1) symmetry below the electroweak symmetry breaking; the latter is indicated by \(U_{em}(1)\) and describes the usual Maxwell electrodynamics. The L in \(SU_L(2)\) is used to indicate that the SU(2) symmetry only acts on left-handed particles and right-handed antiparticles. In some extensions of the Standard Model there is also the symmetry \(SU_R(2)\), which acts on right-handed particles and left-handed antiparticles.
- 4.
Leptons, quarks, and massive gauge bosons are protected by the electroweak symmetry, which is restored at high energies and forbids mass terms.
References
D. Griffiths, Introduction to elementary particles, 2nd edn. (Wiley-VCH, Weinheim, 2008)
F. Mandl, G. Shaw, Quantum field theory, 2nd edn. (Wiley, Chichester, 2010)
K.A. Olive et al., Particle data group collaboration. Chin. Phys. C 38, 090001 (2014)
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Problems
3.1
Estimate the cross section of the scattering \(\nu _e \nu _\mu \rightarrow \nu _e \nu _\mu \), both in the case \(E \ll M_Z\) and \(E \gg M_Z\).
3.2
Draw the Feynman diagram for the muon decay. [Hint: since the lightest baryons and mesons are heavier than the muon, the latter can only decay into leptons.]
3.3
Draw the Feynman diagram for the neutron decay \(n \rightarrow p e^- \bar{\nu }_e\). [Hint: the neutron is a bound state udd and the proton is a bound state uud. A d quark in the neutron decays into a u quark with the emission of a virtual W-boson.]
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Bambi, C., Dolgov, A.D. (2016). The Standard Model of Particle Physics. In: Introduction to Particle Cosmology. UNITEXT for Physics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-48078-6_3
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DOI: https://doi.org/10.1007/978-3-662-48078-6_3
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