**3. Basic neutrino physics**

The current view of elementary particle physics is embodied in the Standard Model of Particle Physics (Cottingham & Greenwood, 2007; and Griffiths, 2008) that assumes all matter is composed of three types of fundamental or elementary particles: leptons, quarks, and mediators of the fundamental interactions. Bevelacqua (2010b) provides a description of the Standard Model from a health physics perspective.

Leptons interact primarily through the weak interaction and electrically charged leptons also experience the effects of the electromagnetic force. They are not affected by the strong interaction. The leptons may be naturally grouped into three families or generations as ( *e* , *<sup>e</sup>* ), ( , ), and ( , ).

Neutrinos are neutral leptons, once believed to be massless, but now evidence suggests they have a non-zero mass (Particle Data Group, 2010). The electron and muon neutrinos are well studied, but less is known about tau neutrinos

94 Particle Physics

are usually attenuated to insignificant levels by the concrete and earth shielding outside the

From a health physics perspective, the energy loss of the circulating, accelerating electrons and positrons produces synchrotron radiation (photons). Given the mass of the electrons and positrons, their trajectories are easily altered. Therefore, synchrotron radiation is expected to be a large fraction of the available beam power. The synchrotron radiation requires shielding, and the extent of the shielding depends on the specific location within

The amount of synchrotron radiation depends on the specific design characteristics of the electron-positron collider. Dominant factors governing the production of synchrotron radiation are the beam power and radius of curvature of the accelerator ring. From a practical standpoint, radiation generated from the circulating electron and positron beams occurs within the unoccupied shielded ring and is not normally a health

The dominant contributors to the radiation environment at an electron-positron facility include electromagnetic cascade showers, external bremsstrahlung, photoneutrons, muons, and synchrotron radiation. Muon pair production in the Coulomb field of a nucleus is possible above a photon energy of about 211 MeV. This process is analogous to electronpositron pair production, but the muon pair production cross-sections are smaller by a factor of about 40,000 due to the differences in electron (0.511 MeV) and muon (105.7 MeV)

The dominant muon pair production process is coherent muon production. In coherent production, the target nucleus remains intact as it recoils from the photon interaction. In a few percent of the time, the nucleus breaks-up with the resultant emission of muons. Muons also result from the decay of photopions and photokaons. However, the number of muon decays in a conventional electron-positron collider is not sufficient to produce a neutrino effective dose concern. To understand the neutrino effective dose from a muon collider, it is

The current view of elementary particle physics is embodied in the Standard Model of Particle Physics (Cottingham & Greenwood, 2007; and Griffiths, 2008) that assumes all matter is composed of three types of fundamental or elementary particles: leptons, quarks, and mediators of the fundamental interactions. Bevelacqua (2010b) provides a description of

Leptons interact primarily through the weak interaction and electrically charged leptons also experience the effects of the electromagnetic force. They are not affected by the strong interaction. The leptons may be naturally grouped into three families or generations as

Neutrinos are neutral leptons, once believed to be massless, but now evidence suggests they have a non-zero mass (Particle Data Group, 2010). The electron and muon neutrinos are well

necessary to understand neutrino physics and neutrino interactions.

the Standard Model from a health physics perspective.

 , ).

studied, but less is known about tau neutrinos

accelerator tunnels containing the beam tubes.

the accelerator facility.

masses (Bevelacqua, 2008).

**3. Basic neutrino physics** 

( *e* , *<sup>e</sup>* ), ( , ), and (

physics issue.

To allow for massive neutrinos, the Standard Model must be modified and its assumptions altered. However, current experimental knowledge of neutrino properties does not permit the selection of a specific modification to the model. For example, it is not known if neutrino masses are to be interpreted as evidence of new, light, fermionic degrees of freedom (e.g., Dirac neutrinos), new, heavy, degrees of freedom (e.g., Majorana neutrinos), or whether a more complicated electroweak-symmetry-breaking interaction is present. However, the Standard Model is sufficient for the purposes of this chapter.

Within the Standard Model, neutrino effective doses are determined from the muon decay processes:

$$
\mu^- \to e^- + \nu\_\mu + \overline{\nu}\_e \tag{1}
$$

$$
\mu^+ \to e^+ + \overline{\nu}\_{\mu} + \nu\_e \tag{2}
$$

The neutrino effective doses depend on the number of muon decays, and the subsequent production of neutrinos. Specific effective dose relationships are provided in subsequent discussion.
