In particle physics, a three-jet event is an event 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 gluons, and were first observed by the TASSO experiment at the PETRA accelerator at the DESY laboratory.
Since jets are ordinarily produced when quarks 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" hadrons 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. The drawings are not Feynman diagrams; they are "snapshots" in time and show two spatial dimensions.
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, 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 PETRA electron–positron storage ring at DESY, physicists determined that the gluon has spin one rather than spin zero or spin two. Subsequent experiments at the LEP storage ring at CERN confirmed this result.
A gluon is an elementary particle that acts as the exchange particle for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. In layman's terms, they "glue" quarks together, forming hadrons such as protons and neutrons.
In particle physics, a hadron is a subatomic composite particle made of two or more quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron.
Quarks are 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. Due to a phenomenon known as color confinement, quarks are never found in isolation; they can be found only within hadrons, which include baryons and mesons, or in quark–gluon plasmas. For this reason, much of what is known about quarks has been drawn from observations of hadrons.
The up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, forms the neutrons and protons of atomic nuclei. It is part of the first generation of matter, has an electric charge of +2/3 e and a bare mass of 2.2+0.5
−0.4 MeV/c2.. Like all quarks, the up quark is an elementary fermion with spin 1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the up quark is the up antiquark, which differs from it only in that some of its properties, such as charge have equal magnitude but opposite sign.
The down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. Together with the up quark, it forms the neutrons and protons of atomic nuclei. It is part of the first generation of matter, has an electric charge of −1/3 e and a bare mass of 4.7+0.5
−0.3 MeV/c2. Like all quarks, the down quark is an elementary fermion with spin 1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the down quark is the down antiquark, which differs from it only in that some of its properties have equal magnitude but opposite sign.
The top quark, sometimes also referred to as the truth quark, is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs Boson. This coupling is very close to unity; in the Standard Model of particle physics, it is the largest (strongest) coupling at the scale of the weak interactions and above. The top quark was discovered in 1995 by the CDF and DØ experiments at Fermilab.
Hadronization 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 of quark-gluon plasma into hadrons is studied in lattice QCD numerical simulations, which are explored in relativistic heavy-ion experiments. Quark-gluon plasma hadronization occurred shortly after the Big Bang when the quark–gluon plasma cooled down to the Hagedorn temperature when free quarks and gluons cannot exist. In string breaking new hadrons are forming out of quarks, antiquarks and some times gluons, spontaneously created from the vacuum.
Two-photon physics, also called gamma–gamma physics, is a branch of particle physics that describes the interactions between two photons. Normally, beams of light pass through each other unperturbed. Inside an optical material, and if the intensity of the beams is high enough, the beams may affect each other through a variety of non-linear effects. In pure vacuum, some weak scattering of light by light exists as well. Also, above some threshold of this center-of-mass energy of the system of the two photons, matter can be created.
This is a timeline of subatomic particle discoveries, including all particles thus far discovered which appear to be elementary given the best available evidence. It also includes the discovery of composite particles and antiparticles that were of particular historical importance.
In particle physics phenomenology, chiral color is a speculative model which extends quantum chromodynamics (QCD), the generally accepted theory for the strong interactions of quarks. QCD is a gauge field theory based on a gauge group known as color SU(3)C with an octet of colored gluons acting as the force carriers between a triplet of colored quarks.
Quark–gluon plasma or QGP is an interacting localized assembly of quarks and gluons at thermal and chemical (abundance) equilibrium. The word plasma signals that free color charges are allowed. In a 1987-summary, Léon van Hove pointed out the equivalence of the three terms: quark gluon plasma, quark matter and a new state of matter. Since the temperature is above the Hagedorn temperature—and thus above the scale of light u,d-quark mass—the pressure exhibits the relativistic Stefan-Boltzmann format governed by fourth power of temperature and many practically mass free quark and gluon constituents. We can say that QGP emerges to be the new phase of strongly interacting matter which manifests its physical properties in terms of nearly free dynamics of practically massless gluons and quarks. Both quarks and gluons, must be present in conditions near chemical (yield) equilibrium with their colour charge open for a new state of matter to be referred to as QGP.
Marek Gaździcki is a Polish high-energy nuclear physicist, and the initiator and spokesperson of the NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS).
Strangeness production in relativistic heavy ion collisions is a signature and a diagnostic tool of quark–gluon plasma (QGP) formation and properties. Unlike up and down quarks, from which everyday matter is made, heavier quark flavors such as strangeness and charm typically approach chemical equilibrium in a dynamic evolution process. QGP is an interacting localized assembly of quarks and gluons at thermal (kinetic) and not necessarily chemical (abundance) equilibrium. The word plasma signals that color charged particles are able to move in the volume occupied by the plasma. The abundance of strange quarks is formed in pair-production processes in collisions between constituents of the plasma, creating the chemical abundance equilibrium. The dominant mechanism of production involves gluons only present when matter has become a quark–gluon plasma. When quark–gluon plasma disassembles into hadrons in a breakup process, the high availability of strange antiquarks helps to produce antimatter containing multiple strange quarks, which is otherwise rarely made. Similar considerations are at present made for the heavier charm flavor, which is made at the beginning of the collision process in the first interactions and is only abundant in the high-energy environments of CERN's Large Hadron Collider.
PLUTO, constructed at DESY laboratories in Hamburg in 1973-1974 and substantially upgraded in 1977-1978, was an experimental detector for high energy particle physics.
The proton spin crisis is a theoretical crisis precipitated by an experiment in 1987 which tried to determine the spin configuration of the proton. The experiment was carried out by the European Muon Collaboration (EMC).
The photon structure function, in quantum field theory, describes the quark content of the photon. While the photon is a massless boson, through certain processes its energy can be converted into the mass of massive fermions. The function is defined by the process e + γ → e + hadrons. It is uniquely characterized by the linear increase in the logarithm of the electronic momentum transfer logQ2 and by the approximately linear rise in x, the fraction of the quark momenta within the photon. These characteristics are borne out by the experimental analyses of the photon structure function.
Stephan Narison is a Malagasy theoretical high-energy physicist specialized in quantum chromodynamics (QCD), the gauge theory of strong interactions. He is the founder of the Series of International Conferences in Quantum Chromodynamics (QCD-Montpellier) and of the Series of International Conferences in High-Energy Physics (HEPMAD-Madagascar).
In quantum chromodynamics, heavy quark effective theory (HQET) is an effective field theory describing the physics of heavy quarks. It is used in studying the properties of hadrons containing a single charm or bottom quark. The effective theory was formalised in 1990 by Howard Georgi, Estia Eichten and Christopher Hill, building upon the works of Nathan Isgur and Mark Wise, Voloshin and Shifman, and others.
Luigi Di Lella is an Italian experimental particle physicist. He has been a staff member at CERN for over 40 years, and has played an important role in major experiments at CERN such as CAST and UA2. From 1986 to 1990 he acted as spokesperson for the UA2 Collaboration, which, together with the UA1 Collaboration, discovered the W and Z bosons in 1983.
Pierre Darriulat is a French experimental particle physicist. As staff member at CERN, he contributed in several prestigious experiments. He was the spokesperson of the UA2 collaboration from 1981 to 1986, during which time the UA2 collaboration, together with the UA1 collaboration, discovered the W and Z bosons in 1983.