Quark
Quarks ( [ kwɔrk ], [ kwɑ ː k] or [ kwɑrk ] ) are in the standard model of particle physics the elementary components (particles ) from which hadrons (such as the atomic core building blocks of protons and neutrons ) are made.
They carry a spin of 1/2 and are therefore fermions. Together with the leptons and gauge bosons, they are now seen as the fundamental building blocks from which all matter is made. So there are baryons (eg the proton ) of three quarks, mesons (eg the pion ) each composed of a quark and an antiquark.
1964 postulated the Caltech physicist Murray Gell-Mann, the existence of quarks. For this schematization of the hadronic " particle zoo " means the quarks, he received the 1969 Nobel Prize in Physics. Regardless George Zweig developed at CERN, a similar model whose fundamental building blocks he called " aces ". However, the publication of his manuscripts foundered on the opposition of his superiors. The classification of the known hadrons with the specially - unitary group SU (3) independently suggested also Juval Ne'eman 1962.
The experimental investigation of quarks by deep - inelastic electron - nucleon scattering began in the late 1960s. Evidence for the existence and the properties of quarks have been found here in the structure functions.
The fact that so far no free quarks could be measured, is one of the biggest unsolved problems of particle physics dar. This known as a confinement phenomenon is one of the Millennium problems (see Yang-Mills theory). While there is strong evidence that the theory of strong interactions, quantum chromodynamics ( QCD), leads to such a confinement of quarks, a rigorous mathematical proof is still out.
- 3.1 Up - Quark
- 3.2 Down - quark
- 3.3 Strange quark
- 3.4 charm quark
- 3.5 Bottom quark
- 3.6 Top Quark
- 5.1 The mass of the top quark
- 5.2 confinement
- 5.3 QCD phase diagram
Introduction
The quarks are the previous result of the attempt to find the basic building blocks of matter. With the advent of the atomic theory in the 19th century, the atoms were considered and these blocks first, of which the name testifies, considered indivisible. In Rutherford 's atomic model showed that the atom consists of nucleus and electron shell is composed. Nuclear physics then showed the structure of the atomic nucleus of protons and neutrons. With only five elementary - except protons, neutrons and electrons nor muons of cosmic rays and the beginning only indirectly detected neutrinos - a satisfactory image was apparently reached in the 1930s.
But the proof is always new mesons and baryons, first in the cosmic radiation, later with particle accelerators, which eventually " Particle " led to jocular expression, was an impetus to search for more fundamental particles that make up the hadrons, ie mesons and baryons are constructed. The other motivation were measurements of the form factor of the more stable hadrons that uniquely who demonstrated a spatial extent, while electrons and muons prove to the limits of measurability as a point.
Properties
There is an antiparticle with opposite electrical charge to all quarks. These antiparticles are called antiquarks. Only the quarks of the first generation form nucleons and thus the normal matter. The constituents of atomic nuclei, protons and neutrons are composed of down quarks and up quarks.
Subject quarks, in contrast to the leptons, all the fundamental forces of physics:
- Strong interaction
- Weak interaction
- Electromagnetic interaction
- Gravity
Color charge
Quarks carry a charge, called color charge. The color charge can be red, green and blue take the three values . If three quarks come together with one of these three values , then the resulting object is colorless. With the colors known from everyday life, the color charge has nothing to do. Anti-quarks carry color charge in accordance with the antired, anti- green or anti- blue.
The confinement hypothesis states that only colorless states can exist in isolation. This assumption was erected after you never could watch isolated individual quarks. You are always bound in hadrons. When baryons are included as combinations of three quarks. In antibaryons there are three anti-quarks. Other hand, mesons consist of a quark and an antiquark. Theoretically, they could also exist other colorless states. Examples would be four quarks and one anti - quark pentaquark or two quarks in combination with two anti-quarks. Whether such objects actually exist, is the subject of current research.
Using computer simulations, one can show that between two static quarks (pair creation is suppressed ) forms a potential which increases linearly with the distance. This is explained by the fact that the exchange particles of the strong interaction, the gluons themselves carry color charge ( a color - anti-color combination), bind to a strand whose energy increases with the length. A color to separate charged particles from the rest, would therefore require extremely high energy. A separation of the quarks from gluons is therefore only possible under certain conditions and for a very short time.
The exact mechanisms of how this strand is formed depends on the interaction of gluons with each other or the interaction of gluons with vacuum fluctuations together and is the subject of current research. There are several scenarios, like this strand can form a unified picture has not yet been enforced.
As part of a thermodynamics of QCD, a state is predicted for quarks, in which the quarks as quasi- free particles behave, the quark -gluon plasma. The associated phase transition is expected at a temperature which corresponds to an energy of 200 MeV and one to three times the density of nuclei. A direct observation of the quark- gluon plasma is not yet possible; Experiments at CERN and BNL, however, provide clues to its existence.
Electric charge
The electric charge of the quarks is either -1 / 3 or 2 / 3 of the elementary charge. Consequently, the bound states ( baryons, mesons ) always have integer charges. Experimental ( eg Millikan experiment ) there is no evidence of fractional charges of isolated particles. The drittelzahligen charges of the quarks bound in hadrons but can be clearly deduced from scattering experiments.
Quark flavors
In the Standard Model of elementary particle physics include the down quark, the up quark, the electron and the electron neutrino to the first generation of particles. The six quarks, together with the leptons and gauge bosons to the basic building blocks of matter.
The following six different quark types are also known as quark flavors ( flavor, American. English flavor).
The quantum numbers of top - and bottom- quark are also available as truth or beauty known.
Dark Fields: u, c and t and their antiparticles are the up- type ( charge number 2 / 3, inverted at the antiparticle sign), Hellefelder: d, s and b are respectively the down - type ( charge number -1 / 3 ). Up, Down, and Strange quarks are collectively referred to as the light quarks.
The assignment of the compounds is not clear. A distinction is made in this connection constituent quarks ( "effective" quarks in hadrons ) and current quarks ( "bare " quarks ). The values given herein are the masses of the current quarks. Because quarks never alone, but always occur in groups, one can conclude that the individual components only from the mass of the group.
The naturally occurring mass eigenstates of the quarks q are not identical to the eigenstates of the weak interaction q '. Nicola Cabibbo showed how the physical down quark d can be described as a mixture of the weak down quark d ' and the weak strange quark s'. The mixture is stirred further parameterized by the so-called Cabibbo angle. This formalism has been extended to a mixture of the weak eigenstates of Down, Strange and bottom- quark to the physical eigenstates. Now it is required rather than a single four parameters that describe a 3 × 3 matrix, the so-called Cabibbo Kobayashi Maskawa matrix.
Up- quark
Up is English for upward. This name is based on one of the physical quantities, which are awarded to quarks: the isospin. The isospin corresponds in its mathematical description of a half-integer angular momentum ( spin), which can be oriented in analogy to this in two different ways, up or down ( These guidelines, however, have no relation to spatial directions ). It was originally proposed by Heisenberg, to represent the two core components of the proton and neutron spin as different settings of the same particle, the nucleon. This was motivated by the fact that behave protons and neutrons from the perspective of nuclear forces identical. In the constituent quark picture of the isospin of the nucleons is a direct consequence of the isospin of the involved up-and down - quarks. The up quark has an electric charge of 2 e / 3
Down quark
Down is English for down. The down - quark corresponds to the different setting of isospin: down. It has an electric charge of -1 / 3 e, an isospin of -1 / 2 and a mass of (5 ± 1) MeV.
Strange quark
After the construction of some baryons such as the, the and could not be explained by the based on up-and down - quark quark model, introduced a new Gell-Mann quark one, to these particles with the help of the quark- model to explain. He called this " strange" quark strange quark.
The strangeness (English: Strangeness ) S of a particle is equal and opposite of how many strange quarks. A single strange quark has therefore the strangeness -1.
Particles containing the strange quark, also called Strange Particles (English Strangelet or strange matter). To this end, under the mesons such as the kaons and phi resonance, as well as among the baryons include the hyperons.
Charm quark
The charm quark belongs to the second family of quarks and is thus a counterpart of the strange quarks. The charm quark corresponds to the charm quantum number C, which takes the value 1 for the charm quark. The charm quark was predicted in 1970, in 1974 it was first created artificially in an experiment. The mass is significantly larger than that of the three light quarks.
In particle detectors to detect hadrons containing charm quarks to their relatively long life span of about 10-12 seconds. This is due to the fact that the charm quarks can decay only via the weak interaction into strange quarks or down quarks.
The charm quark is for example part of the D- mesons and the J / ψ meson.
Bottom- quark
The bottom- quark (also called beauty quark ) together with the top quark, the tauon and the tauon - neutrino, the third generation of particles of the Standard Model. The first particle containing a bottom quark, was discovered in 1977 at the Fermi National Accelerator Laboratory.
The bottom quark is part of the so-called B- mesons and the Υ meson.
The assigned to it flavor quantum number is the bottom Ness B ' (even beauty ), the bottom- quark has B ' = -1.
Top quark
The top quark (also called Truth quark ) is the heaviest quark and the partner of the bottom quarks. Because his life is only 4.2 × 10-25 seconds, it can be in nature no hadronic bound states form ( hadronization occurs after approximately 10-23 s ). The top quark decays, therefore, in contrast to all other quarks well before the time that is needed to form hadrons. There are thus neither meson nor baryon containing a top quark.
Another special feature is that it is extremely difficult to 173.2 ± 0.9 GeV / c ², which is on the order of a gold atom. It was proven experimentally due to the immense energy required to produce only 18 years after his partner in 1995 ( by the CDF at the Fermi National Accelerator Laboratory), though it has been postulated theoretically in 1977 with the discovery of the bottom quarks.
The top- quark associated flavor quantum number is the Topness T ( also Truth ), the top quark has T = 1.
History
The concept of quarks was independently developed in the early 1960s branch by Murray Gell- Mann and George. This scheme grouped the particles with definite isospin and strangeness after a certain unitary symmetry, the herleitete from the current algebra. Today, this global SU ( 3) -flavor symmetry (not to be confused with the gauge symmetry of QCD ) is known as part of the approximation used chiral symmetry of QCD.
In this scheme, the lightest mesons ( spin 0 ) and baryons (spin 1/2) were grouped in octets of the flavor symmetry. A classification of Spin-3/2-Baryonen forming a decuplet, leading to the prediction of a new elementary particle, the Ω - led. With the discovery of the Ω - in 1964, the quark model has been widely accepted.
Gell-Mann called this scheme Eightfold Way, a term that combines the octets of the model with the Eightfold Path of Buddhism. He also coined the name quark, which he took from the phrase " Three quarks for Muster Mark " from James Joyce's novel Finnegans Wake. Joyce had turn in transit at the farmers market in Freiburg im Breisgau heard the word, as market women offered their dairy products.
Since individual quarks have never been observed in experiments, described Gell-Mann himself it in his early publications still as a mathematical fiction, which he then did not stand alone.
From the analysis of certain properties at high-energy reactions of hadrons postulated Richard Feynman in 1969 a substructure of hadrons, the partons. A scaling of deep inelastic scattering cross sections, the herleitete James Bjorken of the current algebra, could also be explained by the partons. When the Bjorken scaling in 1969 by the experiments of Jerome I. Friedman, Henry W. Kendall and Richard E. Taylor ( Nobel Prize for Physics in 1990) has been demonstrated, it was clear that partons and quarks could be the same. With the proof of the asymptotic freedom of QCD in 1973 by David Gross, David Politzer and Frank Wilczek (Nobel Prize for Physics in 2004), this concept is further established.
The charm quark was established in 1970 by Sheldon Glashow, John Iliopoulos and Luciano Maiani postulated ( GIM mechanism) to prevent hitherto unobserved flavor change in decays through the weak interaction (so-called " flavor - changing neutral currents "); otherwise such a flavor change would occur in the standard model. This was in 1974 with the discovery of the J / ψ meson, which consists of a charm quark and its antiquark, confirmed.
The existence of a third generation of quarks was predicted in 1973 by Makoto Kobayashi and Toshihide Maskawa (Nobel Prize for Physics in 2008). They noted that the CP violation is not with the standard model with two quark generations by neutral kaons explainable. The bottom- quark and the top quark was discovered at Fermilab in 1977 and 1995.
Current research interests
The mass of the top quark
A collaboration of scientists at Fermilab (Illinois / USA) it was not until 2004, to determine the mass of the top quark with good accuracy and thus enable a better prediction of the mass of the predicted by the standard model, but until then undiscovered Higgs boson.
Quarks can not be observed experimentally individually: You always occur in combinations of two or three quarks (see below ) and are only indirectly detected by the use of certain transformations. Only in 1995 were two working groups independently of each other at Fermilab announce the detection of the top quark, which had arisen there as quark-antiquark pairs in proton -antiproton collisions. The sought after particle pair decays extremely short 10-24 seconds in W - bosons and lighter quarks, the latter almost always are bottom quarks. This in turn immediately bind other quarks in itself, a process which is called hadronization. This results in so-called jets. The mass of the top quark can be determined by a detailed analysis of the energy and momentum balance of these decays. The evaluation of such complex events resulted in the CDF experiment and DØ experiment (pronounced D -Zero ) high mass greater than 170 GeV / c ², much heavier than the other quarks; the measurement uncertainty was at that time, however, 10%.
After successful upgrade of the Fermilab as well as improvement in the detection of the measuring detectors operating at a collision energy of 1.8 TeV continued in 1999. A resulting higher production rate of top quarks allowed a more detailed analysis of particle jets. A new measurement of both working groups, the CDF and the DØ collaboration has, in 2009 clarified the mass of the top quark based on new measurement data and by means of a refined evaluation process and corrects the currently official value of 173.1 ± 1.6 GeV / c ². The Particle Data Group is the current value (as of 7/2012 ) with 173.5 ± 0.6 ( stat.) ± 0.8 ( Sys ) GeV / c ² to.
The extremely large mass of the top quark suggests that it is fundamentally different from the five lighter quarks. On the basis of a precise measurement of its mass can be on the theory that statements on the possible mass of the not yet finally proven Higgs boson win. This particle that was predicted in 1964 by the English physicist Peter Higgs, interacts with other elementary particles and thereby gives them their mass. It completes the standard model. The value for the mass of this Higgs particle could be determined by the two located at the LHC experiments ATLAS and CMS. According to the ATLAS experiment, the mass of the Higgs boson is 125.5 ± 0.2 ( stat.) 0,5-0,6 ( Sys ) GeV / c ², according to CMS at 125.3 ± 0.4 ( stat. ) ± 0.5 ( Sys ) GeV / c ².
The great mass of the top quark also makes its decays into a fertile field for the search for new particles, such as particles of supersymmetry, a possible extension of the Standard Model. With the production of top quark pairs at higher collision energies can perhaps answer the question whether it really is structureless, fundamental particles at the quarks the question. Therefore, New results on top quark also come from the Large Hadron Collider ( LHC) at CERN, which was taken in early September 2008. There are two beams of protons with an energy of 3.5 to 4 TeV per proton to be brought into collision.
Confinement
The theoretical explanation of the confinement problem is one of the major challenges of theoretical particle physics. Various models have been developed which have been theoretically studied in recent years. One possibility is the formation of a Gluonkondensates, which can then contain nontrivial topological objects ( chromo - magnetic monopoles, center vortices, Dyonen ), another idea is confinement by instantons, ie to explain tunneling processes. In recent years, individual Green's functions of QCD have been studied by different methods. Of particular interest here is the Gluonpropagator, provide for its behavior in the infrared range of different methods, different results. This problem has been and is much discussed and is currently (Jan 2011 ) are not yet completely solved. From the infrared behavior of the Gluonpropagators, there are indications on the validity of various Confinementszenarien.
QCD phase diagram
A further focus of research in recent years, at a theoretical level, the behavior of quarks at finite temperatures and densities. We know from experiments that adjusts at extremely high densities, a new phase, the quark -gluon plasma. The theoretical description of this state and the description of the phase transition is of great theoretical interest. Firstly, the quarks are quasi- free, so the confinement hypothesis is no longer valid, and one speaks of a confinement - deconfinement transition. Also at high temperatures and densities chiral symmetry is restored ( to the explicit refraction through the curd mass flow ). A correlation of these two phase transitions is considered highly probable and the transition temperatures for both transitions seem to agree. How exactly is the connection is given, of what order is the phase transition and whether but may, in certain areas the transition temperatures can not be different, as predicted by some researchers, but has not yet been released and probably can only be answered by experimental measurement.