The SLD QCD group:
The SLD QCD working group is organized by:
The QCD group is devoted to studying properties of the strong interaction, the force that acts between quarks and gluons. The prevalent theory of strong interations is Quantum Chromodynamics (QCD), in which quarks are postulated to be fundamental particles in nature and to carry one of three "color" charges. Gluons are the carriers of the force and couple to the color charge. Each gluon carries a color and a (different) anti-color, so gluons can couple to each other as well as to quarks. A feature of strong interactions is that of confinement: quarks do not appear alone in nature, but are found only in bound states, such as the familiar proton and neutron, as well as a plethora of other particles collectively called hadrons. Hadrons can be classified into two types: mesons are composed of a quark and an antiquark; (anti)baryons are composed of three (anti)quarks.
At SLD we study the process of electron-positron annihilation into hadrons at the Z0 resonance:
e+e- --> Z0 --> qq --> hadrons
qqg --> hadrons
qqgg ... --> hadrons
At these high energies, quarks and gluons appear in the detector as jets of hadrons. An example is shown of a 3-jet (qqg) event as seen in our detector [Gary] The green lines represent charged hadrons reconstructed in our tracking chambers, the rectangles represent energy deposited by charged and neutral hadrons in our calorimeter.
In e+e- experiments such as this we seek to understand two particular aspects of the strong interaction. The first is the structure of events in terms of jets, from which we can extract the strong coupling alphas, the quantum numbers of quarks and gluons, and test predictions of the theory in terms of numbers of jets and the distributions of angles between them. The second is the "fragmentation" or "hadronization process", by which quarks and gluons materialize as hadrons. A schematic model of hadronization is:
[frag picture from e.g. Tom's or Ken's thesis](descriptive text to be provided by Dave)
In addition to fundamental inclusive tests of QCD, we at SLD are particularly interested in new fragmentation studies involving the dependence on the flavor of the primary quark, differences between quark and antiquark jets, and dependence on the mass of the final state hadrons. There are five flavors of quarks that can be produced in Z0 decays: we classify three of them, up, down and strange, as "light" flavors and separate these from the heavier charm and bottom flavors using the SLD vertex detector. Many QCD calculations assume massless quarks, and we can test these calculations using the light-quark sample. Some other calculations predict interesting differences between heavy and light-quark jets. The polarized electron beam at the SLC causes the quark jet in Z-->qq events to point preferentially in a certain direction. In this way we can separate quark jets from antiquark jets and compare their properties. By identifying the types of the final state hadrons we can study the mass-dependence of the fragmentation process and try to determine how the quantum numbers of the intial quark are transmitted to the final state. SLD has excellent identification capabilities for charged pions, kaons and protons using Cerenkov ring-imaging, allowing the study of these hadrons as well as the reconstruction of many unstable higher mass states with good a signal to noise ratio.
For details and some examples of recent analysis, see:
Some recent topics of particular interest include: