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glossary of used terms

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

Electron volt (eV): the energy gained by a particle (of equal or opposite charge to that of the electron) when it is accelarated through a potential difference of 1 volt. Current accelerators produce particles with energies of many thousands of millions of eV; 1 GeV = 10 9 eV.

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.

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).

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

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).

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

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 .

Ionization: : the removal of electrons from atoms.

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.

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 recognise because the track spirals (due to a process known as bremsstrahlung).

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 more 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 Bqv in the magnetic field B , 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.

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'.

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

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

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.