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) an ...

, a three-jet event is an

event Event may refer to: Gatherings of people * Ceremony, an event of ritual significance, performed on a special occasion * Convention (meeting), a gathering of individuals engaged in some common interest * Event management, the organization of eve ...

with many particles in final state that appear to be clustered in three jets. 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 A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bind q ...

s, and were first observed by the

TASSO TASSO (Two Arm Spectrometer SOlenoid) was a particle detector at the PETRA particle accelerator at the German national laboratory DESY. The TASSO collaboration is best known for having discovered the gluon, the mediator of the strong interaction an ...

experiment at the

PETRA Petra ( ar, ٱلْبَتْرَاء, Al-Batrāʾ; grc, Πέτρα, "Rock", Nabataean Aramaic, Nabataean: ), originally known to its inhabitants as Raqmu or Raqēmō, is an historic and archaeological city in southern Jordan. It is adjacent to t ...

accelerator at the

DESY The Deutsches Elektronen-Synchrotron (English ''German Electron Synchrotron''), commonly referred to by the abbreviation DESY, is a national research center in Germany. It operates particle accelerators used to investigate the structure of matt ...

laboratory. Since jets are ordinarily produced when

quark A quark () is a type of 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. All commonly o ...

hadronize Hadronization (or hadronisation) is the process of the formation of hadrons out of quarks and gluons. There are two main branches of hadronization: quark-gluon plasma (QGP) transformation and colour string decay into hadrons. The transformation o ...

, and quarks are produced only in

pairs Concentration, also known as Memory, Shinkei-suijaku (Japanese meaning "nervous breakdown"), Matching Pairs, Match Match, Match Up, Pelmanism, Pexeso or simply Pairs, is a card game in which all of the cards are laid face down on a surface and tw ...

, an additional particle is required to explain events containing an odd number of jets.

Quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type ...

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 A collider is a type of particle accelerator which brings two opposing particle beams together such that the particles collide. Colliders may either be ring accelerators or linear accelerators. Colliders are used as a research tool in particle ...

, is their consistency with the

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 ( ...

. 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 In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...

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 A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

produced in a similar process. In this case, the quark–quark interaction is the only

strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...

, 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. The drawings are ''not''

Feynman diagram In theoretical physics, a Feynman diagram is a pictorial representation of the mathematical expressions describing the behavior and interaction of subatomic particles. The scheme is named after American physicist Richard Feynman, who introduc ...

s; they are "snapshots" in time and show two spatial dimensions. Image:Three jet gluon string.png, Two

s (solid lines) and a

(curly line) fly apart, with the

strings String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other objects Arts, entertainment, and media Films * ''Strings'' (1991 film), a Canadian anim ...

(red bars) primarily between the gluon and each quark. Image:Three jet gluon jets.png, As a result, three jets (cones) form, with extra

s (arrows) found where the strings formed. Image:Three jet photon string.png, For comparison, physicists looked at events with two quarks and a

(wavy line). Here the string forms only between quarks. Image:Three jet photon jets.png, Therefore extra hadrons are found only between the two jets, which is inconsistent with observations.

Ellis–Karliner angle

The Ellis–Karliner angle is the kinematic angle between the highest energy jets in a three-jet event. The angle is not measured in the

lab frame In theoretical physics, a local reference frame (local frame) refers to a coordinate system or frame of reference that is only expected to function over a small region or a restricted region of space or spacetime. The term is most often used in t ...

, but in a frame boosted along the energy of the highest energy jet so that the second and third jets are back-to-back. By measuring the distribution of the Ellis–Karliner angle at the

electron–positron storage ring at

, physicists determined that the

has

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

one rather than spin zero or spin two. Subsequent experiments at the LEP storage ring at

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...

confirmed this result.

Ellis–Karliner angle

References

Further reading