Particle Interactions

Syllabus

142. understand how to use laws of conservation of charge, baryon number and lepton number to determine whether a particle interaction is possible

Properties that must be conserved (from the CGP textbook)
 * Energy and momentum
 * Charge
 * Baryon number
 * Lepton number (Standard Model)

Baryon number

☀From Wikipedia

In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as

where  n q  is the number of quarks, and nq is the number of antiquarks. Baryons (three quarks) have a baryon number of +1, mesons (one quark, one antiquark) have a baryon number of 0, and antibaryons (three antiquarks) have a baryon number of −1. Exotic hadrons like pentaquarks (four quarks, one antiquark) and tetraquarks (two quarks, two antiquarks) are also classified as baryons and mesons depending on their baryon number
 * Baryon number for a quark = 1/3 * (1-0) = 1/3


 * Baryon number for an antiquark = 1/3 * (0-1) = -1/3

Lepton number

☀From Wikipedia

In particle physics, lepton number (historically also called lepton charge[1]) is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction.

Lepton number = number of leptons - number of antileptons

Lepton flavor conservation (i.e. numbers that must be 0 in an interaction.) ————————————————————————————————————————————————
 * electron lepton number
 * = number of the electrons and the electron neutrinos - number of their antiparticles
 * muon lepton number
 * = number of the muons and the muon neutrinos - number of their antiparticles
 * tau lepton number
 * = number of the taus and the tau neutrinos - number of their antiparticles

Violations of the lepton number conservation laws

(Something not for the A-level exam:p)

☀From Wikipedia

Lepton flavor is only approximately conserved, and is notably not conserved in neutrino oscillation.[4] However, total lepton number is still conserved in the Standard Model.

Numerous searches for physics beyond the Standard Model incorporate searches for lepton number or lepton flavor violation, such as the decays a muon minus to an electron and a photon. Experiments such as MEGA and SINDRUM have searched for lepton number violation in muon decays to electrons; MEG set the current branching limit of order 10−13 and plans to lower to limit to 10−14 after 2016.[6] Some theories beyond the Standard Model, such as supersymmetry, predict branching ratios of order 10−12 to 10−14.[5] The Mu2e experiment, in construction as of 2017, has a planned sensitivity of order 10−17.[7]

Because the lepton number conservation law in fact is violated by chiral anomalies, there are problems applying this symmetry universally over all energy scales. However, the quantum number B − L is commonly conserved in Grand Unified Theory models.

————————————————————————————————————————————————

Strangeness

☀From Wikipedia

In particle physics, strangeness ("S") is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactionswhich occur in a short period of time. The minus sign is used so that the flavour charge (e.g. strangeness) and the electric charge of a quark have the same sign. With this, any flavour carried by a charged meson has the same sign as its charge. In our modern understanding, strangeness is conserved during the strong and the electromagnetic interactions, but not during the weak interactions.
 * number of strangeness = -( number of strange quarks - number of antistrange quarks)
 * If a meson with a strange quark(-1/3) is negatively charged, its strangeness is negative.
 * If a meson with an antistrange quark(+1/3) is positively charged, its strangeness is positive.