Three jet event
Encyclopedia
In particle physics
, a three-jet event is an event
with many particles in final state that appear to be clustered in three jet
s. A single jet consists of particles that fly off in roughly the same direction. One can draw three cones from the interaction point, corresponding to the jets, and most particles created in the reaction will appear to belong to one of these cones. These events are currently the most direct available evidence for the existence of gluon
s, and were first observed by the TASSO
experiment at the PETRA
accelerator at the DESY
laboratory.
Since jets are ordinarily produced when quark
s hadronize, and quarks are produced only in pairs
, an additional particle is required to explain events containing an odd number of jets. Quantum chromodynamics
indicates that this particle is a particularly energetic gluon, radiated
by one of the quarks, which hadronizes much as a quark does.
A particularly striking feature of these events, which were first observed at DESY
and studied in great detail by experiments at the LEP collider
, is their consistency with the Lund string model
. The model indicates that "strings" of low-energy gluons will form most strongly between the quarks and the high-energy gluons, and that the "breaking" of these strings into new quark–antiquark pairs (part of the hadronization process) will result in some "stray" hadron
s between the jets (and in the same plane). Since the quark-gluon interaction is stronger than the quark-quark interaction, such hadrons will be observed much less frequently between the two quark jets. As a result, the model predicts that stray hadrons will not appear between two of the jets, but will appear between each of them and the third. This is precisely what is observed.
As a check, physicists have also considered events with a photon
produced in a similar process. In this case, the quark–quark interaction is the only strong interaction
, so a "string" forms between the two quarks, and stray hadrons now appear between the corresponding jets. This difference between the three-jet events and the two-jet events with a high-energy photon, which indicates that the third jet has unique properties under the strong interaction, can only be explained by the original particle in that jet being a gluon.
The line of reasoning is illustrated below. Please note that the drawings are not Feynman diagram
s; they are "snapshots" in time and show two spatial dimensions.
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...
, a three-jet event is an event
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....
with many particles in final state that appear to be clustered in three jet
Jet (particle physics)
A jet is a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics or heavy ion experiment. Because of QCD confinement, particles carrying a color charge, such as quarks, cannot exist in free form. Therefore they fragment into hadrons...
s. A single jet consists of particles that fly off in roughly the same direction. One can draw three cones from the interaction point, corresponding to the jets, and most particles created in the reaction will appear to belong to one of these cones. These events are currently the most direct available evidence for the existence of 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....
s, and were first observed by the TASSO
Tasso
-People:*Torquato Tasso, the famous Italian 16th-century poet, author of Gerusalemme liberata**Tasso, Lament and Triumph, a symphonic poem by Franz Liszt based on the poet*Bernardo Tasso, his father, also a poet...
experiment at the PETRA
PETRA
PETRA is one of the particle accelerators at DESY in Hamburg, Germany. From 1978 to 1986 it was used to study electron–positron collisions. It was here that the TASSO collaboration found the first direct evidence for gluons in three jet events...
accelerator at the DESY
DESY
The DESY is the biggest German research center for particle physics, with sites in Hamburg and Zeuthen....
laboratory.
Since jets are ordinarily produced when quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s hadronize, and quarks are produced only in pairs
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
, an additional particle is required to explain events containing an odd number of jets. Quantum chromodynamics
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...
indicates that this particle is a particularly energetic gluon, radiated
Radiative process
In particle physics, a radiative process refers to one elementary particle emitting another and continuing to exist. This typically happens when a fermion emits a boson such as a gluon or photon....
by one of the quarks, which hadronizes much as a quark does.
A particularly striking feature of these events, which were first observed at DESY
DESY
The DESY is the biggest German research center for particle physics, with sites in Hamburg and Zeuthen....
and studied in great detail by experiments at the LEP 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:...
, is their consistency with the Lund string model
Lund string model
In particle physics, the Lund string model is a phenomenological model of hadronization. It treats all but the highest-energy gluons as field lines, which are attracted to each other due to the gluon self-interaction and so form a narrow tube of strong color field...
. The model indicates that "strings" of low-energy gluons will form most strongly between the quarks and the high-energy gluons, and that the "breaking" of these strings into new quark–antiquark pairs (part of the hadronization process) will result in some "stray" hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s between the jets (and in the same plane). Since the quark-gluon interaction is stronger than the quark-quark interaction, such hadrons will be observed much less frequently between the two quark jets. As a result, the model predicts that stray hadrons will not appear between two of the jets, but will appear between each of them and the third. This is precisely what is observed.
As a check, physicists have also considered events with a 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...
produced in a similar process. In this case, the quark–quark interaction is the only strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
, so a "string" forms between the two quarks, and stray hadrons now appear between the corresponding jets. This difference between the three-jet events and the two-jet events with a high-energy photon, which indicates that the third jet has unique properties under the strong interaction, can only be explained by the original particle in that jet being a gluon.
The line of reasoning is illustrated below. Please note that the drawings are not Feynman 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...
s; they are "snapshots" in time and show two spatial dimensions.