10 - H1 - Desy

2 Theoretical framework
Gene- Quarks Leptons
ration Flavour Q M [GeV] Flavour Q M [GeV]
1 st u (up) +2/3 2.55 +0.75
−1.05 × 10−3 e (electron) −1 5.1 × 10 −4
d (down) −1/3 5.04 +0.96
−1.54 × 10 −3 ν e (e-neutrino) 0 < 1 × 10 −8
2 nd c (charm) +2/3 1.27 +0.07
−0.11 × 10 0 µ (muon) −1 0.105
s (strange) −1/3 104 +26
−34 × 10−3 ν µ (µ-neutrino) 0 < 2 × 10 −4
3 rd t (top) +2/3 171.2 +2.1
−2.1 × 100 τ (tau) −1 1.776
b (bottom) −1/3 4.20 +0.17
−0.07 × 10 0 ν τ (τ-neutrino) 0 < 2 × 10 −2
Table 1.1: The properties of the fundamental fermions (quarks and leptons, spin = 1/2)
of the SM [1,2]. The anti-particle partners of these fermions (not included in the table)
have the same mass (M), but opposite electric charge (Q). Q is given in units of the proton
charge.
quarks, there are 3 known generations which differ from each other only in mass and
flavour. The electron (e), muon (µ) and tau (τ) particles each have an associated low
mass, chargeless neutrino. Leptons are also group into singlets and doublets:
⎛
⎝ e ν e
⎞
⎠
,
⎛
⎝ µ ν µ
⎞
⎠
,
⎛
⎝ τ ν τ
⎞
⎠
, e R , µ R , τ R (1.2)
L
L
L
The neutrinos, neutral leptons, are considered to be massless within the SM. The observation
of neutrino mixing [3–7] however, has shown that in fact they can not be massles.
The electron, like the proton, is a stable particle and is present in almost all matter. The
µ and τ particles are unstable and are found primarily in cosmic rays.
Important ingredients of the SM are the intermediate gauge bosons, or the carriers of
force. Table 1.2 lists the fundamental forces and their carriers. The gauge bosons transmit
three of the four fundamental forces through which matter interacts. The gluon (g) is
responsible for the strong force, which binds together quarks inside protons and neutrons,
and holds together protons and neutrons inside atomic nuclei. The photon (γ) is the
electromagnetic force carrier that governs electron orbits and chemical processes. The
photon couples to the electric charge. Lastly, the weak force is mediated by W ± and Z 0
bosons responsible for radioactive decays. The weak force couples to quarks as well as
to leptons. Due to the lack of colour and electric charge, neutrinos do not interact via
the strong or the electromagnetic force, and therefore interact with matter only via the
weak interactions. Within the SM, theories of electromagnetic and of weak interactions
are unified to the Electroweak theory by Glashow, Salam and Weinberg [8].
The SM includes the strong, electromagnetic and weak forces and all their carrier particles,
and describes how these forces act on all the matter particles. However, the fourth force,