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 *n*q 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

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**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.

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**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.

* number of strangeness = -( number of strange quarks - number of antistrange quarks)

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.

* 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.

In our modern understanding, strangeness is **conserved during the strong and the electromagnetic interactions, but not during the weak interactions.**