Important terms in Higgs research

Background
When scientists search for new physics, they 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.

Confidence Level (CL)
Confidence level 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.

Decay channel
Most massive particles like the Higgs boson are unstable and decay into other particles over time. Just as a vending machine might return the same amount of change using different combinations of coins, a particle can decay into different combinations of particles. These sets of secondary particles are called decay channels. If the Standard-Model Higgs boson exists, it could decay into several different channels, such as two photons or two W bosons or two Z bosons. Physicists have calculated how often a Standard Model Higgs boson would decay into these different channels depending on its mass. They search for excesses of events in those decay channels, which could indicate the presence of a Higgs.

Event
An event is a snapshot of a collision in the LHC. Because energy is equivalent to mass, highly energetic collisions can create particles more massive than those involved in the collisions (protons, in the case of the LHC). These massive particles quickly decay into lighter, stable particles (see “Decay channel”). Physicists study the decay products of collisions to determine what more massive particles were created in the events.

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 searches for a particle reveal statistically that it is 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.

Look-elsewhere effect (LEE)
When physicists see more of a certain type of event than predicted, they must consider the look-elsewhere effect in determining the statistical significance of that excess of events. To calculate the look-elsewhere effect for a certain observation, scientists take into account the probability of seeing something similar in any particular spot over the mass range in question. The chances of a statistical fluctuation causing an excess of events at one point on a plot are lower than the chances of a statistical fluctuation leading to an excess of events at any point in a range of points on a plot. Other things being equal, the smaller the range one considers, the smaller the look-elsewhere effect.

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.” 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 (See “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.

Standard Model
The Standard Model is a collection of theories that embodies all of our current understanding about the behaviour of fundamental particles.