In heavy-ion collisions such as lead-on-lead, the produced Quark-Gluon Plasma (QGP) region may be spatially eccentric. This means it can be physically longer along one axis than another.
The high pressure at the centre of the QGP decreases to zero pressure in the vacuum outside the QGP region. The pressure gradient is therefore larger along the short axis than along the longer axis. This larger gradient leads to a stronger hydrodynamic flow for the particles emitted along the short axis than those along the long axis, increasing their momentum.
As a result, measurements of particles at fixed momentum find that there is a higher yield of particles in a quadrupole pattern along the short QGP axis. This is quantified as a second-order Fourier coefficient v2, known as the azimuthal momentum anisotropy.
The illustration shows the evolving energy density of the QGP created in a non-central collision. Pressure gradients act on the initial geometrical anisotropy to create a final velocity field (arrows), which may be decomposed into elliptic (yellow), triangular (teal) and higher order components. (Image: MUSIC/arXiv:1209.6330)
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.
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) 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.
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.
A two dimensional plot used to describe the kinematics of a three-body decay. One typical usage is for the axes of the plot to be the squares of the invariant masses of two pairs of the decay products. If there are no angular correlations between the decay products then the distribution of these variables appears even.
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.
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. Physicists can calculate how long a particle should last and the ways it should decay. Knowing a particle’s decay channels can help physicists spot new particles created in ATLAS collisions, even if the particle decays before a detector can capture it. If experimentalists can’t see the particle itself, they can see the products of its decay.
Detector placed at each end of a barrel-shaped detector to provide the most complete coverage in detecting particles.
An event is a snapshot of a collision in the Large Hadron Collider (LHC).
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.
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.
Two gluons, one from each of the incoming LHC protons, interact or “fuse” to create a particle, such as a Higgs boson. The figure shows a Feynman diagram illustrating the process.
An ion is an atom with one or more electrons removed (positive ion) or added (negative ion).
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.
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.
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.
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.
The collective name for protons and neutrons.
The force carrier particle of electromagnetic interactions.
The antiparticle of the electron.
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.
When four gauge bosons interact with each other, the strength of this interaction is known as the "quartic gauge boson coupling". This parameter is represented in Feynman diagrams by a vertex where four gauge boson lines meet.
The figure shows quark-quark interaction, where one virtual W boson decays into three W bosons; the convergence of the interaction lines is the “quartic gauge boson coupling”.
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.
Conservation of R-parity prevents proton decay via exchange of a superpartner into a positron and a neutral pion. Conservation of R-parity also prevents the decay of the lightest superpartner into standard model particles, and thus the lightest superpartner becomes an appealing candidate for dark matter.
While conservation of R-parity accomplishes its purpose of preventing proton decay, other choices are possible. Extending the Standard Model with a new gauge symmetry with charge (B–L) would give a natural way to suppress unwanted baryon- and lepton-number-violating interactions, and would allow the lightest superpartner to decay.
Measure of the accuracy of a detector measurement, e.g. of energy or spatial position.
Simplified template cross sections (STXS) are an approach to categorise the Higgs-boson candidate events according to the properties associated with the Higgs production mode. This allows physicists to characterise the Higgs boson independently of its decay channel.
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.
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.
The blueprint for a sub-detector system.
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.
A quark from each of the incoming LHC protons radiates off a heavy vector boson. These bosons interact or “fuse” to produce a particle, such as a Higgs boson. The initial quarks that first radiated the vector bosons are deflected only slightly and travel roughly along their initial directions. They are then detected as particle "jets" in the different hemispheres of the detector. The figure shows a Feynman diagram of this process.