Saturday, October 29, 2011

Though fundamentally different Bosons and Fermions sometimes overlap

  Fermions are matter, bosons are just force carriers however some fermions (coupled electrons collecting into the same energy state at low temperature) can exhibit characteristics normally seen only in bosons (Bose–Einstein statistics, idential particles occupying the same energy states at the same time). Diagram: Neutron atom's only down quark decaying into an up quark (to conserve charge, also remember that the process produces a proton which requires the down quark ---> up) and a W boson which then goes on to further decay into an electron and electron neutrino. Also shown is the muon and muon neutrino (produced only through decay)

Baryon - Are those particles composed of 3 quarks (compared to mesons which have 2 quarks). They participate in the strong interaction (strong interaction is the force that keeps components of the nucleus together for example quarks within protons but also protons/neutrons within the nucleus; there's also a residual force mediated by the mesonic gluons that transmits the nuclear force). Baryons are fermions (1/2 integer spin as opposed to Bosons with integer spin) while mesons are bosons. Differ from bosons since they obey the pauli exclusion principle. A neutron is 1 up quark (+2/3 charge) and 2 down quarks (down quark has a charge of -1/3). In the case of neutron decay a down quark ----> up quark, what the W boson does is take -1 charge away in the process turning the down quark into an up quark -1/3---->+2/3 = net gain of +1 charge turning it into a proton.

Hadron family (quarks makes them distinct) - baryon (3 quarks), protons, neutrons, mesons (pi, rho). Composed of quarks. Quarks/Hadrons/Baryons are associated with matter. All hadrons have integer charges but quarks have fractional charges meaning the number of quarks/antiquarks used to create baryons (3) or mesons (quark+antiquark) must add up to integral charge. Quarks can't exist on their own, only in baryons and mesons.

Bosons - Associated with force carrying; ones with the same energy can occupy the same quantum state as opposed to the pauli exclusion principle. Photons, Mesons are examples, they all have integer spin (0, 1, 2, etc). Because they can occupy the same state, at low temperatures they can collect into the same energy state which is called condensation; that is illustrated for example in superfluidity. W bosons can decay into up and down quark (meson) or a lepton and a neutrino. The W boson plays a key role in muonic decay Unlike photons, bosons have mass which means the force they transmit (weak nuclear) operates on a smaller distance. W bosons - flavour and charge changes between quarks and leptons (quark/anti-quark or lepton + neutrino/anti-neutrino). In the case of Z bosons (no change in flavour or charge) the boson decays into lepton/anti-lepton pairs.

Muon - Elementary particle (lepton) that's heavier than the electron. Muons also help W bosons escape the nucleus, neutron during beta decay. The rate at which muons transition into lighter particles during decay is governed by the available energy of the reaction (depends also on how heavy the products are). Muons and electrons both carry the same negative electric charge. W bosons play a key part in muonic transitions, they are absorbed and emitted by muons and that's what gives the muons the extra charge needed to transition into muon neutrinos.

Fermion - Electrons, Protons. Associated with matter (only one kind can occupy a particular space, if more than one they must differ in terms of their properties). Half integer spin. 24 different fermions 12 of them being anti particles of quarks and leptons. Fermions are also produced through Z boson decay (into a fermion and its anti particle).

Leptons - Six main types of elementary particles categorized based on their flavours/generations (generations are how elementary particles are grouped). Only electrons are charged because electrons are the lighest (mass) they are the most stable. Many of the other leptons can only be produced in particle accelerators due to their instability. They are subject to all forces (weak inter - decay, EM force) except the strongest one (strong nuclear force). Though when paired, coupled electrons can be made to act like bosons, in which case they can be made to display zero electrical resistance (happens when they collect into the same energy state). Electrons, muons and tau differ significantly in terms of their mass (tau is 15X muon, muon is 200X electron).

Neutrinos - 1/2 integer spin (fermionic), no mass, no charge, subject only to the weak force. Electron antineutrinos are produced through beta decay (beta + and - decay so called based on the type of emission (+=positron, - =electron). Feynman diagrams are used to illustrate the process by putting into visual form the decay mechanism and its associated energy transmission & products.

Weak Interaction - This is where beta decay happens (heavy particles either absorb or emit bosons in the process altering their states). For example, leptons absorb bosons producing neutrinos; quarks absorb or emit bosons (W or Z) and convert into a different kind of quark (up and down are only 2 of the 6 different kinds of quarks however they have the least mass and are the most stable and so are more commonly encountered). Responsible for radioactive decay through the exchange of W and Z bosons which converts one quark (up) into another (down).

Strong Nuclear Force - Gives integrity to the structure of protons and neutrons. It is this force which creates color attraction between quarks of different charges. Force is carried by gluons. The strong force is associated with the color force.

Gravity - The only force that every particle is subject to. Also differs in the sense that different sources of gravity can reinfornce one another to create a bigger effect.

Some great sources for more info: The Four Forces and Hadrons, baryons, mesons
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