Physics Highlights from the DØ Experiment (1992--1999)
Fermi National Accelerator Laboratory, Batavia, Illinois, U.S.A.

6. PHYSICS OF THE BOTTOM QUARK

Within the family of known quarks, the bottom (or b) quark is characterized by a set of rather peculiar and often intriguing properties, sufficiently so as to warrant dedicated facilities for its study. Discovered in an experiment at Fermilab in 1977, its unexpected appearance created an imbalance in the internal organization of the existing quarks. The absence of a "weak isospin" partner represented a theoretical discomfort that was only dispelled with the later discovery of its missing companion, the top quark (see Section 3).

When confronted with its earlier known siblings, the bottom quark is considered heavy, with a mass about four times that of its next heaviest colleague, the charm quark. Such relatively high mass grants the bottom quark special status in the studies of QCD. Bottom quarks are produced in proton-antiproton collisions dominantly by the strong QCD interactions of gluons and light quarks that reside within the colliding beam particles.

The large value of as and the non-abelian nature of QCD are responsible for the difficulty of making quantitative predictions. However, the higher the mass of the involved quark, the more reliable are the calculations. The mass of the bottom quark is high enough for obtaining reliable QCD calculations, but still low enough to have copious production at the Tevatron. This balance is one of the aspects that single out bottom quarks as an excellent source of data for confrontation with theory, a true "laboratory" for QCD studies. Consequently, one of the ways we test the reliability of QCD in DØ is by measuring the rate at which bottom quarks are produced. An added bonus of heavy quark production is that the dependence of the production rates has a direct correlation to the internal gluon distributions within the colliding protons, which are not well measured, and can be extracted from such data.

DØ has measured the production of bottom quarks in various kinematic regimes, and through the observation of different reactions and final configurations. DØ is especially well equipped for such studies, partly because of its extensive angular coverage. Once produced, free colored quarks do not exist for very long, but immediately initiate a process of pulling light quarks from the vacuum and "dressing" themselves into colorless bound-state hadrons. Bottom quark hadronization usually leads to the production of an unstable B hadron that subsequently decays. Muons are produced in such decays about 11% of the time, and can be used to tag b quarks. DØ has a good muon detector, and the extended muon coverage near the incident beams, the so-called forward rapidity region, is unique to DØ, and has provided measurements of bottom-quark production in new kinematic regions.

The process starts with a selection of collisions that contain one or more muons, a promising signature of something interesting having happened in that event. Weeding out background leaves a sample that can be classified according to the number of muons present in the final state, and how they relate to each other (if two are present) and to the remainder of the collision products. For example, a muon moderately close to the hadrons comprising a b jet provides a signature for a b quark.

Such studies have yielded a wealth of valuable measurements. Resonant and non-resonant final states, in different physical configurations and kinematic regions, have been traced back to their origins in bottom-quark production, enabling a multifaceted focus on production rates, correlations, and confrontations with predictions of QCD.

The results of such measurements are intriguing. While the general aspects of the QCD predictions are in agreement with DØ observations, the calculated production rates systematically fall short of the observed yields by roughly a factor of three. The data from several related studies are shown in Fig. 9, and indicate the level of agreement between theory and experiment as a function of transverse momentum. Similar results have been obtained by CDF. Although there are uncertainties in theory and experiment, the present status represents an exciting challenge that is currently being addressed by theorists, and motivates the program of increasingly accurate measurements for the next Tevatron run.

Fig. 9: The DØ inclusive b-quark cross section compared to theoretical calculations.

We noted that the bottom quark is a heavy object when compared with its earlier known siblings; in striking contrast, when confronted with its companion top it is in fact remarkably light. This delicate placement in the mass scale, together with the tendency of quarks to interact mainly with their weak isospin partners, conspire to give the bottom quark yet another set of very welcome properties. The b quark has an unusually long lifetime (hadrons containing b quark travel typically a few millimeters before decay), and clear signatures associated with its decay products. Once an experiment is equipped to observe and analyze specific bottom-quark decay modes, another entirely new and rich chapter of physics is opened, which includes such fundamental topics as CP violation, and windows of exploration into particle physics phenomena beyond the scope of the Standard Model.

The installation of a superconducting solenoid and precision tracking sensors in its interior, are two important features of the upgraded DØ detector for the next Tevatron run. They will give us access to specific bottom quark decay modes and an opportunity to focus on some of these new topics.

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