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Glossary of terms in alphabetic order


Accelerator: a machine used to produce particles of very high energy, with speeds close to the speed of light

Antiparticles: – see FAQ – what are antiparticles?


Baryon (anti-baryon): is a hadron made of three quarks (anti-quarks) held together by gluons, the carriers of the strong force; for example, a proton is made of 3 quarks (uud); an anti-neutron is made of 3 anti-quarks (u-bar d-bar d-bar).

Beam: in the context of bubble chamber (and other 'fixed target') experiments, protons from the accelerator are used to produce 'secondary' beams – pions, kaons, antiprotons, for example – whose interactions are studied in the bubble chamber.

Bremsstrahlung (means `braking radiation' in German): all accelerating charged particles radiate electromagnetic radiation. (A radio transmitter consists essentially of charges running up and down!) The greater the acceleration, the greater the radiation. Imagine an electron and an antiproton (same charge q ) entering a bubble chamber with the same speed v . They will both experience the same force from atomic nuclei, but the electron will accelerate about 2000 times more because it has a mass 2000 times smaller than that of the antiproton – just a = F/m . So the electron will radiate and lose energy much more quickly. This is the origin of the characteristic electron (and antielectron) spirals in bubble chambers. For pictures, click here or here.

Bubble chambers: make use of ionization. The energy of the electrons from the ionized atoms initiates boiling locally in the liquid, creating a trail of tiny bubbles along the path of the particle causing the ionization. These bubbles are allowed to grow until they are about a millimetre across, and then photographed to give a permanent record.


Collision: an interaction between a beam particle and a target nucleus.

Compton electron: when an electron is struck by a high energy gamma ray, it appears in the bubble chamber as a lone spiralling electron or knock-on electron.

Conservation laws: are widely used in interpreting and classifying particles particle interactions in bubble chambers: in particular, energy (including E=mc2) and momentum; charge, baryon number, lepton number and strangeness.


Dalitz pair: A decays to two photons . Occasionally a photon 'converts internally‘ and appears in the bubble chamber as an pair coming from the collision.

Decay : particles that are unstable are said to 'decay' into lighter particles; if the decay results from the strong/electromagnetic/weak interaction the lifetime of the unstable can be very short indeed (about 10-23 s)/short (ranging from about 10-19 s for the to about 10-18 s for the )/short but long enough for the particle to travel a measurable distance before decaying . Particles in the last category were widely studied by bubble chambers. Neutral particle decays produce 'vees' in the bubble chamber; charged particle decays produce 'kinks'. – see FAQ – what is particle decay?.

Delta ray: see Knock-on electron.

Detector: a machine used to record the particles emerging from a collision between two particles (eg. a pion and a proton in a bubble chamber).


Electron volt (eV): the energy gained by a particle (of equal or opposite charge to that of the electron) when it is accelerated through a potential difference of 1 volt. Current accelerators produce particles with energies of many thousands of millions of eV; 1 GeV = 109 eV.


Fiducials: crosses marked on bubble chamber window or walls whose positions are accurately surveyed. They measured along with events and are essential to being able to reconstruct events in 3-dimensions. (From Latin word fiducia meaning truth.)



Hadron: A hadron is a strongly-interacting particle made of quarks and/or anti-quarks held together by gluons, the carriers of the strong force.


Interaction: see particle interactions

Ionization: : the removal of electrons from atoms.

Ionization density: energy lost per unit distance by a charged particle travelling through matter; in physicists‘ jargon, this is called dE/dx. It is a key measurement in most particle physics experiments because it yields information on the velocities of moving charged particles.
In bubble chamber physics, ionization density is measured in terms of the number of bubbles per centimetre or the darkness of a track compared with 'minimum-ionizing‘ beamtracks. The darker the track, the slower it is moving.


Joule:  the SI unit of energy.  See FAQ – What is an electron-volt?


Kink: the word describing the characteristic sudden change in curvature (from higher to lower curvature) seen when a charged particle decays; eg to .

Knock-on electron (also known as delta ray): the particle creating a trail of bubbles by ionization may, by chance, give an electron enough energy for it (the electron itself) to travel a measurable distance, making its own trail of bubbles; such trails produced by electrons are usually easy to recognize because the track spirals (due to a process known as bremsstrahlung).


Lepton:  a particle that interacts via the weak force; (eg. e-, νe,  - )

Lorentz force:  the force on a point charge in an electric and/or magnetic field, F = q(E+ v x B)


Mass: in particle physics is usually measured in MeV/c2 or GeV/c2.

Meson:  A meson is a hadron made of a quark and an antiquark held together by gluons; for example, a π+ is made of (u d-bar)

Momentum: the radius of curvature r of the track of a particle of charge q moving in a magnetic field B is proportional to its momentum p : specifically p = (Bq)r. In particle physics momentum is usually measured in GeV/c.

Muon:  a charged lepton.  See lepton.




Pair production: when a high energy photon travels through matter, it can, under the influence of the electric field of a nucleus, materialize into an electron-positron pair.

Prong: the number of charged particles emerging from a collision.


Quark:  a constituent of a hadron.  See FAQ – What is a hadron?



Strangeness: for a first level discussion click here.

Synchrotron radiation:  Electromagnetic radiation produced when charged particles are accelerated by a magnetic field.  See FAQ – What is Synchrotron Radiation?


Target: for bubble chambers, the targets are the nuclei of whatever liquid is being used (eg. protons in hydrogen; protons, neutrons or even the whole nucleus in a neon-hydrogen mixture).



Vee: the word describing the characteristic shape of the decay of a neutral – typically , and .


WIMPS:  weakly interacting massive particles are hypothetical particles serving as one possible solution to the dark matter problem.   See FAQ –What are WIMPS?