Glossary

Background

When searching for signs of new physics, physicists compare what they observe to what theories predict they will observe. The background is the set of results scientists expect to see. If an experiment sees more instances of a certain type of event (see “Excess”) than they expect to see as part of the background, it might be evidence of new physics.
Beam

The particles in an accelerator are grouped together in a beam. Beams can contain billions of particles and can be divided into discrete portions called bunches. Each bunch is typically several centimetres long and just a few microns wide.
Confidence Level (CL)

Confidence Level (CL) is a statistical measure of the percentage of test results that can be expected to be within a specified range. For example, a Confidence Level of 95% means that the result of an action will probably meet expectations 95% of the time.
Cross-section

The cross-section is a measure of the probability for that process to occur during any proton-proton collision. Processes with larger cross-sections occur more often than processes with small cross-sections.
Dark Matter

Only 4.9% of the matter in the Universe is visible. The rest is known as dark matter (26.8%) and dark energy (68.3%). Finding out what dark matter consists of is a major challenge for modern science.
Decay channel

Particles decay into other particles over time. Decay channels are the possible transformations a particle can undergo as it decays. For example, the Higgs could decay into several different channels, such as two photons or two W bosons or two Z bosons.
End-cap

Detector placed at each end of a barrel-shaped detector to provide the most complete coverage in detecting particles.
Event

An event is a snapshot of a collision in the Large Hadron Collider (LHC).
Excess

When scientists observe more of a certain type of event than expected in a data plot, they call that an excess. Scientists measure the statistical significance (See “Standard deviation / Sigma”) of excesses to determine how certain they are that they result from new physics and not simply random fluctuations.
Exclusion

If a search for a particle reveals that it is statistically unlikely to exist with certain characteristics (e.g. a particular mass), a particle with those characteristics can be excluded. This narrows the search parameters within which the particle might be found. Establishing such exclusions is important in the search for undiscovered particles.
Ion

An ion is an atom with one or more electrons removed (positive ion) or added (negative ion).
Lorentz-contraction

Lorentz-contraction refers to the process of relativistic contracting of objects along the direction of motion. This is significant only for objects traveling at relativistic velocities.
Mass / Invariant mass

Particle physicists use the word "mass" to refer to the quantity (sometimes called "rest mass") which is proportional to the inertia of the particle when it is at rest. This is the "m" both in Newton's second law of motion, F=ma, and in Einstein's equation, E=mc2 (in which E must be interpreted as the energy of the particle at rest). When a particle decays and hence no longer exists, its mass before the decay can be calculated from the energies and momenta of the decay products. The inferred value of the mass is independent of the reference frame in which the energies and momenta are measured, so that that the mass called "invariant". The concept is frequently generalized, so that for any set of particles (e.g., two leptons emerging from a collision), one can apply the same formulas to obtain an "invariant mass" (also called the “effective mass”) of the set.
Mass spectrum

One of the important statistical tools in particle physics is to create a histogram of the invariant mass of a particle or a group of particles (thought to originate from the decay of something interesting) versus the frequency with which that particular mass was recorded. This plot is known as a mass spectrum, and is used to signify the presence of new particles and to establish their masses.
Model / Scientific model

Scientists construct and develop 'models' to describe a scientific theory in the context of related phenomena. In general, a model is based on a theory (a set of hypotheses), acting on a set of parameters obtained from actual experimental data and/or from observations. Computer simulations may sometimes be used to test the reliability of a model. If it was found to be reasonably reliable, the simulation can even be used to predict what would happen if the initial parameters were different.
Nucleon

The collective name for protons and neutrons.
Photon

The force carrier particle of electromagnetic interactions.
Positron

The antiparticle of the electron.
Pseudo-rapidity

A parameter (denoted by η) frequently used in colliding beam experiments to express angles with respect to the axis of the colliding beams. It has the value 0 for particle trajectories that are perpendicular to the beam, and positive or negative values for those at an angle to the beam. 
R-parity

Also known as "matter-parity". In many Supersymmetry (SUSY) models, R-parity ensures that protons – and hence all of the atoms in the universe – are unable to decay to other particles quickly by exchanging SUSY particles. Though this can also be prevented in models without R-parity conservation, introducing R-parity is often considered the simplest possibility.

The R-parity of a particle is defined by (-1)^3(B-L)+2s where B is baryon number, L is lepton number, and s is spin. R-parity has a value of (+1) for Standard Model particles and (-1) for their superpartners.
Resolution

Measure of the accuracy of a detector measurement, e.g. of energy or spatial position.
Spin

Spin is the intrinsic angular momentum of an elementary particle, measured in units of the reduced Planck’s constant ħ. In quantum field theory, the spin of a particle is related to its behaviour. For example, particles with integer spin (0, 1, 2…) are called bosons, and can occupy the same quantum state at the same time. In contrast, particles with half-integer spin (1/2, 3/2, 5/2…) cannot. The known elementary constituents of matter (electron, quarks, neutrinos…) are spin 1/2 particles, whereas the particles (photon, W/Z, gluon) which mediate the known interactions (respectively electromagnetic, weak, strong) are spin 1 particles. 

The Higgs boson has spin 0 (it is a so-called “scalar” boson) and positive parity as predicted by the Standard Model. It is the only elementary scalar particle to be observed in nature.
Standard deviation / Sigma

A standard deviation is a measure of how unusual a set of data is if a hypothesis is true. Physicists express standard deviations in units called sigma, σ. The higher the number of sigma, the more incompatible the data are with the hypothesis.

If the data are incompatible enough with a hypothesis that says the experiment will find only background, that could constitute a discovery. Typically, the more unexpected or important a discovery, the greater the number of sigma physicists will require to be fully convinced.

Five sigma significance is traditionally required to claim a discovery of a new particle; this was the threshold passed by the Higgs boson when its discovery was announced on 4 July 2012.
Technical Design Report (TDR)

The blueprint for a sub-detector system.
Trigger

An electronic system for spotting potentially interesting collisions in a particle detector and triggering the detector’s read-out system to record the data resulting from the collision.