Wing Sheung Chan

The Standard Model and lepton flavour violation 9 Table 1.3.: Properties of quarks [6] . Generation Quark Rest mass Electric charge [ e ] 1 u 2 . 16 +0 . 49 − 0 . 26 MeV 2 / 3 d 4 . 67 +0 . 48 − 0 . 17 MeV − 1 / 3 2 c 1 . 27 ± 0 . 02 GeV 2 / 3 s 93 +11 − 5 MeV − 1 / 3 3 t 172 . 9 ± 0 . 4 GeV 2 / 3 b 4 . 18 +0 . 03 − 0 . 02 GeV − 1 / 3 Table 1.4.: Properties of some commonly observed hadrons [6, 7] . The limit on the proton mean life is measured at 90% confidence level. Hadron Rest mass [MeV] Mean life [s] Major decay mode (branching fractio n † ) p 938 ± 3 × 10 − 7 > 6 . 6 × 10 36 n 940 ± 5 × 10 − 7 879 . 4 ± 0 . 6 pe − ¯ ν e (100.00%) π + 139 ± 2 . 4 × 10 − 4 (2 . 6033 ± 0 . 0005) × 10 − 8 µ + ν µ (99.98%) π 0 135 ± 5 × 10 − 4 (8 . 52 ± 0 . 18) × 10 − 17 γγ (98.82%) Hadrons can be further classified into baryons and mesons depending on the baryon number, which is defined as (number of quarks − number of antiquarks) / 3 . Baryons have a non-zero baryon number, while mesons have a zero baryon number. Each single quark can carry one of the three colour charges, arbitrarily labelled red, blue and green. A quark system is colour neutral if it has three quarks or three antiquarks that carry all the three different colour charges, or a quark and an antiquark that carry the same colour charge (but with opposite signs), or is a combination of these systems. In fact, an isolated quark has never been observed experimentally, and hadrons are only known to exist in colour-neutral states. This phenomenon is referred to as the colour confinement. Physicists could only rely on the spectroscopy of hadrons to provide evidence for the existence of quarks and indirectly measure their properties. Due to colour confinement, quarks produced in a collider experiment are never detected as isolated particles, but instead bunches of hadrons clustered together. These observed bunches of hadrons are called jets and the process of their production is called hadronisation. The proton ( p ), neutron ( n ), charged pion ( π ± ) and neutral pion ( π 0 ) are some of the most commonly observed hadrons in everyday environment as well as in a collider experiment. A summary of the properties of these hadrons [6, 7] is given in Table 1.4. † The branching fraction for a decay is the fraction of particles which decay by an individual decay mode with respect to the total number of particles which decay.