particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

, 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 a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...

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 (; "Rock"), originally known to its inhabitants as Raqmu (Nabataean Aramaic, Nabataean: or , *''Raqēmō''), is an ancient city and archaeological site in southern Jordan. Famous for its rock-cut architecture and water conduit systems, P ...

accelerator at the

DESY DESY, short for Deutsches Elektronen-Synchrotron (English: ''German Electron Synchrotron''), is a national research centre for fundamental science located in Hamburg and Zeuthen near Berlin in Germany. It operates particle accelerators used to ...

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 nucleus, atomic nuclei ...

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 In theoretical physics, quantum chromodynamics (QCD) is the study 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 of ...

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 that brings two opposing particle beams together such that the particles collide. Compared to other particle accelerators in which the moving particles collide with a stationary matter target, collid ...

, 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 In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

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 particles that can ...

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

strong interaction In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, 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 introduced ...

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 (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 ...

, 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: * Spin (physics) or particle spin, a fundamental property of elementary particles * Spin quantum number, a number which defines the value of a particle's spin * Spinning (textiles), the creation of yarn or thr ...

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 Meyrin, western suburb of Gene ...

confirmed this result.

Ellis–Karliner angle

References

Further reading