1.5 - particle interactions

what changes the momentum of an object?

a single force acting on the object

what happens when a single force acts on an object?

it changes the momentum of the object

momentum equation

p = mv

p - momentum

m - mass

v - velocity

what is momentum?

the product of an object’s mass and velocity

what force do 2 interacting objects exert?

equal and opposite forces on each other

what happens to the momentum when 2 objects interact?

momentum is transferred between the objects by their equal and opposite forces, considering no other forces act on them

what is a virtual photon?

a photon that cannot be seen or destroyed. known as the ‘exchange particle’ or the ‘force carrier’

what is a virtual photon also known as?

• exchange particle

• force carrier

which direction do you move if you throw momentum forwards?

backwards

why do particles of the same charge repel?

they exchange virtual photons (known as electromagnetic force), transferring momentum, repelling them

what are the electromagnetic forces due to?

the exchange of virtual photons

what interactions are virtual photons involved in?

electromagnetic forces acting on each other (attraction and repulsion), interactions between charged particles

what interactions are W bosons involved in?

weak nuclear

what do weak nuclear forces do?

turn protons to neutrons, and vice versa

what interactions are weak nuclear forces involved in?

• β⁺ decay

• β^- decay

• positron emission

• neutron-neutrino interactions

• proton-antineutrino interacts

why is weak nuclear force weak?

as it works within the nucleus (since it works with nucleons), it needs to be weaker than the strong nuclear force holding the nucleus together

β⁺ decay

• weak nuclear force

• p → n

• W⁺ boson released to turn a proton to a neutron, and a neutrino to a β⁺ particle

which type of β decay has a proton turning to a neutron?

β⁺

what kind of photon is released in a β⁺ decay?

W⁺ boson

what kind of boson is released in β^- decay?

W^- boson

which type of β decay involves a neutrino?

β⁺ decay

which type of β decay involves an antineutrino?

β^- decay

in which β decay does a proton turn to a neutron?

β⁺ decay

in which β decay does a neutron turn to a proton?

β^- decay

β^- decay

• weak nuclear force

• n → p

• W^- boson released to turn a neutron to a proton, and an antineutrino to a to a β^-

feynman diagram for β⁺ decay

feynman diagram for β^- decay

what happens if a neutrino / antineutrino is not present to turn into a β particle in either β decay?

then the W boson will decay into the products instead, although there will always be a neutrino / antineutrino in an unstable nucleus as a supplier of energy

what is always present in an unstable nucleus as a supplier of energy?

neutrino / antineutrino

what are the 4 fundamental interactions?

gravitational attraction

• objects acting on each other based on mass

strong nuclear

• holds nucleus together

electromagnetic force

• between charged particles

• attract and repulsion

weak nuclear

• β decay

• positron emission

• neutron-neutrino interaction

• proton-antineutrino interaction

what does the strong nuclear force do?

hold the nucleus together

what is gravitational attraction force?

objects acting on each other based on mass

how reactive are neutrinos and antineutrinos?

very unreactive - they hardly interact with other particles. however sometimes interactions do happen

what are some neutrino / antineutrino interactions?

• neutron-neutrino

• proton-antineutrino

what kind of interaction is a neutron-neutrino interaction?

weak nuclear

what kind of interaction is a proton-antineutrino interaction?

weak nuclear

neutron-neutrino interaction

• neutrino interacts with a neutron

• W^- boson released

• interaction causes neutron to turn into a proton

• a β^- particle (electron) created and emitted as a result of the change

what interacts with what in a neutron-neutrino interaction?

neutron interacts with a neutrino

what kind of interaction is a neutron-neutrino interaction?

weak nuclear force, because it involves electrostatic particles turning into each other

which W boson is released in a neutron-neutrino interaction?

W^- boson

what does the neutron turn into in a neutron-neutrino interaction?

a proton

what is created and emitted in a neutron-neutrino interaction? why?

a β^- particle (electron), as a result of the neutron turning into a proton

feynman diagram of neutron-neutrino interaction

proton-antineutrino interaction

• antineutrino interacts with a proton

• W⁺ boson released

• interaction causes proton to turn into a neutron

• β+ particle (positron) created and released as a result of the change

what interacts with what in a proton-antineutrino interaction?

antineutrino interacts with a proton

which boson is released in a proton-antineutrino interaction?

W⁺ boson

what type of interaction is a proton-antineutrino interaction?

weak nuclear force, because it involves electrostatic particles turning into each other

what does the proton turn into in a proton-antineutrino interaction?

a neutron

what is created and emitted in a proton-antineutrino interaction? why?

a β⁺ particle (positron), as a result of the proton turning into a neutron

feynman diagram of a positron-antineutrino interaction

neutron-neutrino vs proton-antineutrino

β^- decay vs neutron-neutrino

β⁺ decay vs positron-antineutrino

what are the neutrino and antineutrino interactions similar yet different to?

β decays

which neutrino-antineutrino interaction involves a W^- boson?

neutron-neutrinon

which neutrino-antineutrino interaction involves a W⁺ boson?

proton-antineutrino

which neutrino-antineutrino interaction involves a β^- particle?

neutron-neutrino

which neutrino-antineutrino interaction involves a β⁺ particle?

proton-antineutrino

why are photons and W bosons called force carriers?

because they are exchanged when the electromagnetic force and weak nuclear force act (respectfully)

what are the exchange particles in weak nuclear interactions?

W bosons

what are the exchange particles in electromagnetic interactions?

photons

how are W bosons different to photons?

• weak nuclear

• non-zero rest mass

• have very short range, ~0.001 fm

• can be charged; positively or negatively

do W bosons have a rest mass?

yes, they have a non-zero rest mass

do photons have a rest mass?

no

which has a shorter range, W bosons or photons?

W bosons

what is the range of a W boson?

very short - no more than 0.001 fm

can photons be charged?

no

can W bosons be charged?

yes - they can be negatively or positively charged

what is electron capture?

in a proton-rich nucleus, a proton turns to a neutron via a weak nuclear interaction with an electron from an outer shell. a W⁺ is released to turn the proton to a neutron and an electron to a neutrino

in which kind of unstable nucleus does electron capture occur?

in a proton-rich nucleus

what interaction is electron capture?

weak nuclear, because it involves a proton turning to a neutron

what are the products in electron capture?

a neutron and neutrino

feynman diagram for electron capture

which W boson does the proton exchange to the electron in electron capture?

W⁺ boson

what does the proton turn into in electron capture?

a neutron

what does the electron turn into in electron capture?

a neutrino

does electron capture involve a neutrino or an antineutrino?

neutrino

when does electron capture occur?

• in a proton-rich unstable nucleus in which a proton needs to turn to a neutron

• very rarely when a proton and electron collide at a very high speed in a particle accelerator

• with a W^- exchange from the electron to the proton, if the electron has sufficient energy

how else can electron capture occur, other than an unstable proton-rich nucleus?

when protons and electrons collide at very high speeds in particle accelerators. this happens very rarely as a lot of energy is needed to overcome the electrostatic forces

why is electron capture via particle accelerator rare?

because a lot of kinetic energy is needed to overcome the electrostatic forces for the proton and electron to successfully collide

how else can electron capture happen if the electron has sufficient energy?

with the same overall change, except as a W^- boson exchange from the electron to the proton