In
particle physics
Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and ...
, underlying event (UE) refers to the additional interactions of two particle beams at a collision point beyond the main collision under study. Specifically, the term is used for
hadron collider A hadron collider is a very large particle accelerator built to test the predictions of various theories in particle physics, high-energy physics or nuclear physics by colliding hadrons. A hadron collider uses tunnels to accelerate, store, and colli ...
events which do not originate from the primary hard
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
(high energy, high momentum impact) process. The term was first defined in 2002.
Further explanation
Underlying events can be thought of as the remnants of scattering
interaction
Interaction is action that occurs between two or more objects, with broad use in philosophy and the sciences. It may refer to:
Science
* Interaction hypothesis, a theory of second language acquisition
* Interaction (statistics)
* Interaction ...
s.
[
The UE may involve contributions from both "hard" and "soft" processes (here “soft” refers to interactions with low p-T, i.e. transverse momentum, transfer).
These are important both in the ]simulation
A simulation is the imitation of the operation of a real-world process or system over time. Simulations require the use of models; the model represents the key characteristics or behaviors of the selected system or process, whereas the ...
of particle experiments (often using event generators 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 the early universe.
Events come in different types ...
); and interpretation and analysis
Analysis ( : analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (3 ...
of data so as to filter out the desired signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
s.[ Wilhelm and Else Heraeus-Seminar on “New Event Generators for Particle Physics Experiments”]
Features
Contents of UE include initial and final state radiation In quantum field theory, initial and final state radiation refers to certain kinds of radiative emissions that are not due to particle annihilation.noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
.
See also
* Minimum bias event Minimum bias (MB) events are inelastic events selected by a high-energy experiment's loose (minimum bias) trigger with as little bias as possible. MB events can include both non-diffractive and diffractive processes although the precise definition ...
* Drell–Yan process and the underlying event
References
{{reflist
External links
CMS measures the ‘underlying event’ in pp collisions
Particle physics
Scattering