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Particle Physics Terminology


Annihilation: When matter/antimatter particle pairs meet and turn into energy.


Antiparticle: An antiparticle has the opposite properties of its particle, like charge, strangeness, etc. For example, an electron has negative charge and its antiparticle, the positron, has positive charge.


Baryon: A hadron with a half integer spin, such as 1/2 and 3/2.  All baryons are composed of three quarks.


Beta Decay: When a neutron decays into a proton, an electron and an antineutrino. The underlying process is a down quark changing into an up quark, an electron and an antineutrino. The weak interaction is responsible for beta decay.


Bosons:  The mediators or force carriers of the weak force.  These are heavy compared to the carriers of the other forces.  There are three bosons that exist, W+, W- and Z0, where the superscripts correspond to the particles’ electric charge.


CERN: In English CERN stands for the European Organization for Nuclear Research. It is located in Geneva, Switzerland and is the location of the LHC.


Collider: An accelerator in which two beam of particles travel in opposite directions and collide head-on.


CP Violation: The violation of the charge and parity transformations simultaneously.  If you were to look in a mirror and move your right arm, your mirror image would move its left arm.


Decay: The process of one particle becoming two or more particles.


Detector: Any device that can sense the presence of a particle and give information about one or more of its properties.


Electroweak force: Force that acts between all charged objects. It can be attractive or repulsive. This force can change one quark type into another. It is the only force affecting neutrinos.


Electron: A fundamental particle with negative electric charge. It is one of the three constituents of an atom, however not located inside the nucleus.


Electron Volt (eV):  This is a unit of energy.  Physicists use the charge of an electron as a base energy unit to describe the amounts of energy other particles have.  “A particle with a charge equal to that of an electron gains the energy of one eV when accelerated in an electric field whose potential different is one volt.”  Also, one electron volt is equivalent to 1.6 x 10-19 Joules.


Force: That which governs the interaction between particles. There are four known fundamental forces: strong, electromagnetic, weak and gravitational.  Recently, the electromagnetic and weak have been unified, to create the electroweak force.  The strength of each force decreases in the listed order.


Force carrier: The particle exchanged during interaction.


Gluon: The mediator or force carrier of the strong force.  It is a massless particle.


Graviton: The mediator or force carrier of the gravitational force.  However, there is no experimental proof that this exists just yet.


Gravitational force: The force that acts between all objects with mass.  It is always attractive.


Hadron: Any particle that experiences the strong force.  Baryons and mesons are types of hadrons.


Joule (J): A unit of energy in SI units. (SI is standard units used in the majority of the world.)


Kaon: A strange meson with about half the mass of a proton.


Lepton: Considered to be a fundamental particle. There are six known leptons: the electron, the muon, the tau, and three neutrinos: electron neutrino, muon neutrino, and tau neutrino.  Leptons do not experience the strong force.


LHC: Large Hadron Collider set to re-start in summer 2009 and is the world's largest particle accelerator.


LHCb: An experiment at the LHC, Large Hadron Collider beauty, which will investigate the decay of B mesons as well as search for new particles.


Mediator: See Force Carrier.


Meson: A hadron with an integer spin, such as zero or one.  Mesons contain either two quarks or the combination of one quark and one antiquark.


Muon: A fundamental lepton with a mass of about 200 times the mass of the electron.


Neutrino: A fundamental lepton that has no electric charge and little or no mass (still disputable today). There are three kinds of neutrinos: electron neutrino, muon neutrino and tau neutrino.


Neutron: A constituent of the nucleus. It has no electric charge and is made of three quarks: two down and one up.


Nucleus: The densest part of an atom, containing protons and neutrons.


Photon: The mediator or force carrier of the electromagnetic force.  It is a massless particle.


Pion: A meson with a mass of 1/7 times the mass of the proton.


Positron: The antiparticle of the electron.


Proton:A constituent of the nucleus. It has positive electric charge and is made of three quarks: two ups and one down.


Quark: Considered to be a fundamental particle according to the standard model. There are six flavors of quarks: up, down, charm, strange, bottom and top.  Quarks have antiparticles as well.


Relativistic: Referring to speeds of particles that approach the speed of light. 


Speed of Light: The exact measurement of this quantity is 299,792,458 meters per second.  This is equivalent to 983,571,056 feet per second and 670,616,629.2 miles per hour.  Only massless objects can travel at this speed.


Standard Model: A model of six quarks and six leptons as fundamental entities, as well as the 3 mediating force carriers.


Strong force: The force that acts between all nucleons. It is attractive for all combinations of protons and neutrons. Hadrons and quarks feel the strong force, but leptons do not.  


Tau: A fundamental lepton with a mass of about 3,500 times the electron mass.


(From: Griffiths, D. (1987). Introduction to Elementary Particles. Harper & Row Publishers, Inc. Philadelphia, PA.
Schwarz, C. (1997). A tour of the subatomic zoo: A guide to particle physics, 2nd ed. American Institute of Physics, Woodbury, NY. 118pp.)