Event generator
Encyclopedia
Event generators are software libraries
that generate simulated high-energy particle physics
events
.
They randomly generate events as those produced in particle accelerators, collider
experiments or during the initial phases of the Universe creation.
Events come in different types called processes as discussed in the Automatic calculation of particle interaction or decay
article.
Despite the simple structure of the tree-level perturbative
quantum field theory
description of the collision
and decay
processes in an event, the observed high-energy process usually contains significant amount of modifications, like photon
and gluon
bremsstrahlung
or loop diagram
corrections, that usually are too complex to be easily evaluated in real calculations directly on the diagrammatic level. Furthermore, the non-perturbative nature of QCD
bound state
s makes it necessary to include information that is beyond the reach of perturbative quantum field theory, and well beyond present ability of computation in lattice QCD
. And in collisional systems more complex than a few lepton
s and hadron
s (e.g. heavy-ion collisions), the collective behavior of the system would involve a phenomenological description that also cannot be easily obtained from the fundamental field theory by a simple calculus.
Any realistic test of the underlying physical process in a particle accelerator
experiment
, therefore, requires an adequate inclusion of these complex behaviors surrounding the actual process. Based on the fact that in most processes a factorization
of the full process into individual problems is possible (which means a negligible effect from interference
), these individual processes are calculated separately, and the probabilistic
branching between them are performed using Monte Carlo method
s.
The final-state particles generated by event generators can be fed into the detector simulation, allowing a precise prediction and verification for the entire system of experimental setup. However, as the detector simulation is usually a complex and computationally expensive task, simple event analysis techniques are also performed directly on event generator results.
A typical hadronic event generator simulates the following subprocesses:
A typical heavy-ion event generator usually can be less strict in simulating the rare and rather negligible processes found in a hadronic generator, but would need to simulate the following subprocesses, in addition to those in a hadronic generator:
Partly due to historic reasons, most event generators are written in FORTRAN 77, with a few C++
generators slowly emerging in recent years. The Particle Data Group
maintains a standard
for designating Standard Model
particles and resonances with integer
code
s in event generators (also known as the "PDG code").
Hadronic event generators
Heavy ion event generators
Neutrino event generators
Specialized event generators
"Meta-generator"
Library (computer science)
In computer science, a library is a collection of resources used to develop software. These may include pre-written code and subroutines, classes, values or type specifications....
that generate simulated high-energy particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
events
Event (particle physics)
An event in particle physics describes one set of particle interactions occurring in a brief span of time, typically recorded together. At modern particle accelerators this refers to the interactions that occur as a result of one beam crossing inside a detector....
.
They randomly generate events as those produced in particle accelerators, collider
Collider
A collider is a type of a particle accelerator involving directed beams of particles.Colliders may either be ring accelerators or linear accelerators.-Explanation:...
experiments or during the initial phases of the Universe creation.
Events come in different types called processes as discussed in the Automatic calculation of particle interaction or decay
Automatic calculation of particle interaction or decay
The automatic calculation of particle interaction or decay is part of the computational particle physics branch. It refers to computing tools that help calculating the complex particle interactions as studied in high-energy physics, astroparticle physics and cosmology...
article.
Despite the simple structure of the tree-level perturbative
Perturbation theory
Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem...
quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...
description of the collision
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
and decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
processes in an event, the observed high-energy process usually contains significant amount of modifications, like photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
and gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
or loop diagram
Feynman diagram
Feynman diagrams are a pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, first developed by the Nobel Prize-winning American physicist Richard Feynman, and first introduced in 1948...
corrections, that usually are too complex to be easily evaluated in real calculations directly on the diagrammatic level. Furthermore, the non-perturbative nature of QCD
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
bound state
Bound state
In physics, a bound state describes a system where a particle is subject to a potential such that the particle has a tendency to remain localised in one or more regions of space...
s makes it necessary to include information that is beyond the reach of perturbative quantum field theory, and well beyond present ability of computation in lattice QCD
Lattice QCD
Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
. And in collisional systems more complex than a few lepton
Lepton
A lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
s and hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s (e.g. heavy-ion collisions), the collective behavior of the system would involve a phenomenological description that also cannot be easily obtained from the fundamental field theory by a simple calculus.
Any realistic test of the underlying physical process in a particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
experiment
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
, therefore, requires an adequate inclusion of these complex behaviors surrounding the actual process. Based on the fact that in most processes a factorization
Factorization
In mathematics, factorization or factoring is the decomposition of an object into a product of other objects, or factors, which when multiplied together give the original...
of the full process into individual problems is possible (which means a negligible effect from interference
Interference
In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater or lower amplitude. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have...
), these individual processes are calculated separately, and the probabilistic
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
branching between them are performed using Monte Carlo method
Monte Carlo method
Monte Carlo methods are a class of computational algorithms that rely on repeated random sampling to compute their results. Monte Carlo methods are often used in computer simulations of physical and mathematical systems...
s.
The final-state particles generated by event generators can be fed into the detector simulation, allowing a precise prediction and verification for the entire system of experimental setup. However, as the detector simulation is usually a complex and computationally expensive task, simple event analysis techniques are also performed directly on event generator results.
A typical hadronic event generator simulates the following subprocesses:
- Initial-state composition and substructure
- Initial-state showerParticle showerIn particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts in the same way, a process that...
s - The hard process
- ResonanceResonanceIn physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
decay - Final-state showers
- Accompanying semi-hard processes
- HadronizationHadronizationIn particle physics, hadronization is the process of the formation of hadrons out of quarks and gluons. This occurs after high-energy collisions in a particle collider in which free quarks or gluons are created. Due to postulated colour confinement, these cannot exist individually...
and further decay
A typical heavy-ion event generator usually can be less strict in simulating the rare and rather negligible processes found in a hadronic generator, but would need to simulate the following subprocesses, in addition to those in a hadronic generator:
- NuclearAtomic nucleusThe nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
initial-state - High multiplicity, soft processes
- In-medium energy loss
- Collective behavior of the medium (not handled properly by any generators sofar)
Partly due to historic reasons, most event generators are written in FORTRAN 77, with a few C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
generators slowly emerging in recent years. The Particle Data Group
Particle Data Group
The Particle Data Group is an international collaboration of particle physicists that compiles and reanalyzes published results related to the properties of particles and fundamental interactions. It also publishes reviews of theoretical results that are phenomenologically relevant, including...
maintains a standard
Standardization
Standardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
for designating Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
particles and resonances with integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...
code
Code
A code is a rule for converting a piece of information into another form or representation , not necessarily of the same type....
s in event generators (also known as the "PDG code").
List of event generators
The major event generators that are used by current experiments are:Hadronic event generators
- PYTHIAPYTHIAPYTHIA is a computer simulation program for particle collisions at very high energies in particle accelerators.-External links:* *...
(formerly Pythia/Jetset) - HERWIG
- ISAJET
- SHERPA
Heavy ion event generators
Neutrino event generators
Specialized event generators
- AcerMC – LHCLarge Hadron ColliderThe Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
background processes - ALPGEN – multiple partonParton (particle physics)In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions. It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons...
processes - Ariadne – QCD cascade with Color Dipole Model
- MC@NLO – parton shower with next-to-leading-order QCD matrix elements
- JIMMY – multiple parton processes
"Meta-generator"
- CompHEPCompHEPis a software package for automatic computations in High Energy Physics from Lagrangians to collision events or particle decays.CompHEP is based on quantum theory of gauge fields, namely it uses the technique of squared Feynman diagrams at the tree level approximation...
– automatic evaluation of tree level matrix elements for event generation or export into other event generators
External links
- 2006 Monte Carlo Number Scheme, from the 2006 Review of Particle Physics.
- List of Monte Carlo Programs – from DESYDESYThe DESY is the biggest German research center for particle physics, with sites in Hamburg and Zeuthen....